200 Questions and Answers on Practical Civil Engineering Works _ICE_July 2007

7/21/2019 200 Questions and Answers on Practical Civil Engineering Works _ICE_July 2007

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200 Questions and Answers on

Practical Civil Engineering Works


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Vincent T. H. CHU

CONTENTS

1. Bridge Works Q1-26 P4-14

2. Concrete Structures Q1-24 P15-2 3

3. Drainage Works Q1-19 P24-32

4. Eart!orks Q1-1" P33-3 6

5. Piers and #arine Structures Q1-1$ P3%-42

6. &oad!orks Q1-22 P43-5"

%. Pu'(ing Station Q1-1" P51-54

$. &ec)a'ation Q1-11 P55-5$

9. Water &etaining Structures and Water!orks Q1-16 P59-63

1". Pi(e *acking and #icrotune))ing Q1-6 P64 -65

11. Pi)es and +oundation Q1-3" P66-%5

12. ,enera) Q1-14 P%6-$"

&eerence P$1-$3


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Preface

This book is intended primarily to arouse the interests of graduate engineers, assistant

engineers and engineers in the technical aspect of civil engineering works. The content ofthe book mainly focuses on providing the reasons of adoption of the various current practicesof civil engineering. By understanding the underlying principles of engineering practices,graduate engineers/assistant engineers/engineers may develop an interest in civil engineeringworks. It is also intended that the book will serve as a useful source of reference forpracticing engineers.

Some of these questions are selected from the book and published in the column The !ivil"#$% in the monthly &ournal e /ong 0ong Engineer under the 'ong (ong Institution of)ngineer. *ther than this book, I have written another book called !ivi l )ngineering+ractical otes #-% which contains similar format and targets to provide quick and concise

answers to frequently asked questions raised by engineers during their day-to-day work. "orreader who have any queries or feedback, the author can be contacted at

freewing012ya hoo.com.h k.

2"" Questions and ns!ers on Practica) Cii) Engineering Works incent . /. C/
mailto:[email protected]:[email protected]:[email protected]


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3incent T. '. !'45uly 6778


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Chapter ! "ridge Works

1. #nder what situation shall engineers use $acking at one end onl% and &ro' (othends in prestressing work)

:uring prestressing operation at one end, frictional losses will occur and the prestressingforce decreases along the length of tendon until reaching the other end. These frictionallosses include the friction induced due to a change of curvature of tendon duct and also thewobble effect due to deviation of duct alignment from the centerline. Therefore, theprestress force in the mid-span or at the other end will be greatly reduced in case thefrictional loss is high. !onsequently, prestressing, from both ends for a single span i.e.prestres sing one-half of total tendons at one end and the remaining half at the other end iscarried out to enable a even distribution and to provide symmetry of prestress force along thestructure.

In fact, stressing at one end only has the potential advantage of lower cost when comparedwith stressing from both ends. "or multiple spans ;e.g. two spans< with unequal span length,&acking is usually carried out at the end of the longer span so as to provide a higherprestress force at the location of ma=imum positive moment. *n the contrary, &acking fromthe end of the shorter span would be conducted if the negative moment at the intermediatesupport controls the prestress force. 'owever, if the total span length is sufficiently long,&acking from both ends should be considered.

2. What is *preset+ during installation o& (ridge (earings)

+reset% is a method to reduce the si>e of upper plates of sliding bearings in order to save

the material cost. The normal length of a upper bearing plate should be composed of thefollowing components? length of bearing @ 6 = irreversible movement @ 6 = reversiblemovement. Initially the bearing is placed at the mid-point of the upper bearing plate withoutconsidering the directional effect of irreversible movement. 'owever, as irreversiblemovement normally takes place at one direction only, the bearing is displaced/presetted adistance of ;irreversible movement/6< from the mid-point of bearing in which the length ofupper plate length is equal to the length of bearing @ irreversible movement @ 6 = reversiblemovement. In this arrangement, the si>e of upper plate is minimi>ed in which irreversiblemovement takes place in one direction only and there is no need to include the componentof two irreversible movements in the upper plate.

ote? +reset% refers to the displacement of a certain distance of sliding bearings with respect to upper bearingplates during installation of bearings.

3. ,n incre'ental launching 'ethod o& (ridge construction- what are the 'easuresadopted to enhance su&&icient resistance o& the superstructure during thelaunching process)

(i) :uring the launching process the leading edge of the superstructure is sub&ect to alarge hogging moment. In this connection, steel launching nose typically about 7.A-7.A8times span length is provided at the leading edge to reduce the cantilever moment.Sometimes, instead of using launching nose a tower and stay system are designed

which serves the same purpose.

(ii) The superstructure continually e=periences alternative sagging and hogging moments


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during incremental launching. ormally, a central prestress is provided in which thecompressive stress at all points of bridge cross section is equal. In this way, it caters forthe possible occurrence of tensile stresses in upper and lower part of the cross sectionwhen sub&ect to hogging and sagging moment respectively. ater when the wholesuperstructure is completely launched, continuity prestressing is performed in which the

location and design of continuity tendons are based on the bending moments in finalcompleted bridge condition and its provision is supplementary to the central prestress.

;iiie of fi=ed piers will be large and massive. In thisconnection, for better aesthetic appearance, the selection of abutment as fi=ed pierscould accommodate the large si>e and massiveness of piers. ormally abutments are

relatively short in height and for the same hori>ontal force, the bending momentinduced is smaller.


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;i i< "or the central pier to be selected as the fi=ed pier, the bridge deck is allowed to movestarting from the central pier to the end of the bridge. 'owever, i f the fi=ed pier islocated at the abutment, the amount of movement to be incorporated in each bearingdue to temperature variation, shrinkage, etc. is more than that when the fi=ed pier islocated at central pier. Therefore, the si>e of movement &oints can be reduced

significantly.

6. So'eti'es the side o& concrete (ridges is o(served to turn (lack in colour/ Whatis the reason &or this pheno'enon)

In some cases, it may be due to the accumulation of dust and dirt. 'owever, for the ma&orityof such phenomenon, it is due to fungus or algae growth on concrete bridges. #fter rainfall,the bridge surface absorbs water and retains it for a certain period of time. 'ence, thisprovides a good habitat for fungus or algae to grow. Doreover, atmospheric pollution andpro=imity of plants provide nutrients for their growth. Improvement in drainage details andapplication of painting and coating to bridges help to solve this problem. Eeference is made

to Sandberg !onsulting )ngineers Eeport 0F F7/G/70.

7. ,n prestressing work- i& 'ore than one wire or strand is included in the sa'educt- wh% should all wiresstrands (e stressed at the sa'e ti'e)

I f wires/strands are stressed individually inside the same duct, then those stressedstrand/wires will bear against those unstressed ones and trap them. Therefore, the friction ofthe trapped wires is high and is undesirable.

8. ,n the design o& elasto'eric (earings- wh% are steel plates inserted inside the

(earings)

"or elastomeric bearing to function as a soft spring, the bearing should be allowed forbulging laterally and the compression stiffness can be increased by limiting the amount oflateral bulging. To increase the compression stiffness of elastomeric bearings, metal platesare inserted. #fter the addition of steel plates, the freedom to bulge is restricted and thedeflection is reduced when compared with bearings without any steel plates under the sameload. Tensile stresses are induced in these steel plates during their action in limiting thebulging of the elastomer. This in turn would limit the thickness of the steel plates.

'owever, the presence of metal plates does not affect the shear stiffness of the elastomeric

bearings.


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1ig/ / )ffect of steel plate in elastomeric bearing.

9. ow to deter'ine the si3e o& elasto'eric (earings)

"or elastomeric bearing, the vertical load is resisted by its compression while shearresistance of the bearing controls the hori>ontal movements. The design of elastomericbearings are based on striking a balance between the provision of sufficient stiffness to resisthigh compressive force and the fle=ibility to allow for translation and rotation movement.

The cross sectional area is normally determined by the allowable pressure on the bearingsupport. Sometimes, the plan area of bearings is controlled by the ma=imum allowablecompressive stress arising from the consideration of delamination of elastomer from steelplates. In addition, the si>e of elastomeric bearings is also influenced by considering theseparation between the structure and the edge of bearing which may occur in rotation

because tensile stresses deriving from separation may cause delamination. The thickness ofbearings is designed based on the limitation of its hori>ontal stiffness and is controlled bymovement requirements. The shear strain should be less than a certain limit to avoid theoccurrence of rolling over and fatigue damage. The vertical stiffness of bearings is obtained byinserting sufficient number of steel plates.

10. ,n a curved prestressed (ridge- how should the guided (earings in piers o& thecurved region (e oriented with respect to the &i.ed (earing in a(ut'ent)

To determine the orientation of guided bearings, one should understand the movement of

curved region of a prestressed bridge. Dovement of prestress and creep are tangential to thecurvature of the bridge ;or along longitudinal a=is< while the movement due to temperatureand shrinkage effects are in a direction towards the fi=ed pier. I f the direction of guidedbearings is aligned towards the fi=ed bearing in the abutment, the difference in direction ofpretress and creep movement and the guided direction towards fi=ed bearing would generatea locked-in force in the bridge system. The magnitude of the lock-in force is dependent on thestiffness of deck and supports. I f the force is small, it can be designed as additional forceacting on the support and deck. 'owever, i f the force is large, temporary freedom ofmovement at the guided bearings has to be provided during construction.


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1ig/ /2 The diagram showing how the guided bearings in piers of the curved region isoriented with respect to the fi=ed bearing in abutment.

11. ,n the construction o& a two4span (ridge 5span length 6 78 (% using span4(%4spanconstruction- wh% is a length o& a(out /297 (ridge seg'ent is constructed in the &irstphase o& construction)

Basically, there are mainly three reasons for this arrangement?

(i) The permanent structure is a statically indeterminate structure. :uring constructionby using span-by-span construction, if the first phase of construction consists of thefirst span length only, then the sagging moment in the mid span of the partiallycompleted bridge is larger than that of completed two-span permanent structure. Toavoid such occurrence, 7.68 of bridge segment is e=tended further from the

second pier which provides a counteracting moment, thereby reducing the mid-spanmoment of the partially completed bridge.

(ii) The position of 0.68 countering from the first pier is the appro=imate location ofpoint of contrafle=ure ;assume that the two-span bridge is uniformly loaded< inwhich the bridge moment is about >ero in the event of future loaded bridge.Therefore, the design of construction &oint in this particular location has the leastadverse effect on the structural performance of the bridge.

(iii) In case of a prestressed bridge, prestressing work has to be carried out after theconstruction of first segment of the bridge. I f the prestressing work is conducted atthe first pier which is heavily reinforced with reinforcement, it is undesirable whencompared with the prestressing location at 0 .68 from the first pier where there isrelatively more space to accommodate prestressing works.

ote? Span-by-span construction means that a bridge is constructed from one bridge span to another until itscompletion.

12. What are the advantages o& piers constructed 'onolithicall% with the (ridge deckover usage o& (earings)

Basically, piers constructed monolithically with the bridge deck are advantageous in thefollowing ways?

;i< Dovement of the bridge deck is achieved by the bending deformation of long and


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slender piers. In this way, it saves the construction cost of bearings by usingmonolithic construction between bridge deck and piers. Doreover, it is not necessaryto spend e=tra effort to design for drainage details and access for bearing replacement.*n the other hand, in maintenance aspect substantial cost and time savings could beobtained by using monolithic construction instead of using bearings as bridge

articulation.;ii< Donolithic construction possesses the shortest effective )uler buckling length for

piers because they are fi=ed supports at the interface between bridge deck and piers.

ote? Donolithic construction means that piers are connected to bridge decks without any &oints and bearings.

13. Are diaphrag's necessar% in the design o& concrete (o. girder (ridges)

:iaphragms are adopted in concrete bo= girder bridges to transfer loads from bridge decksto bearings. Since the depth of diaphragms normally e=ceeds the width by two times, theyare usually designed as deep beams. 'owever, diaphragms may not be necessary in case

bridge bearings are placed directly under the webs because loads in bridge decks can bedirectly transferred to the bearings based on 5org Schlaich H 'artmut Scheef ;0CF6


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"rom aesthetic point of view, an odd number of spans with a decrease in length in thedirection of abutment is desirable. Doreover, spans of equal length are found to be boring.'owever, the arrangement of irregular span lengths is not recommended because it gives afeeling of uneasiness.

"rom structural point of view, for a multi-span bridge with equal span length, the sagging

moment at the mid-span of the end span/approach span is largest. In order to reduce thismoment, the span length of end span/approach span is designed to be 7.18 of inner spans.'owever, this ratio should not be less than 7.97 because of the effect of uplifting at the endspan/approach span support.

ote? )nd span refers to the last span in a continuous bridge while approach span refers top the first span of abridge.

:/ ,n the design o& a si'pl% supported skew (ridge- which direction o& rein&orce'entshould (e provided)

In the conventional design of steel reinforcement for a simply supported skew bridge, a set ofreinforcement is usually placed parallel to free edge while the other set is designed parallel tothe fi=ed edge. 'owever, this kind of arrangement is not the most efficient way of placing thereinforcement. The reason is that in some parts of the bridge, the moment of resistance isprovided by an obtuse angle formed by the reinforcement bars which is ineffective inresisting fle=ure. In fact, the most efficient way of the arrangement of reinforcement undermost loading conditions is to place one set of bars perpendicular to the fi=ed edge whileplacing the other set parallel to the fi=ed end as recommended by . #. !lark ;0C17


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1ig/ /; The arrangement ofreinforcement in skewed bridge.

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18. What are the &unctions o& grout inside tendon ducts)

Jrout in prestressing works serves the following purposes?

+rotect the tendon against corrosion.Improve the ultimate capacity of tendon.+rovide a bond between the structural member and the tendon.In case of failure, the anchorage is not sub&ect to all strain energy.

19. What is the consideration in selecting the orientation o& wing walls in the design o&(ridge a(ut'ents)

There are three common arrangements of wing walls in bridge abutments based on :r.)dmund ! 'ambly ;0C1C


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alignment with bridge decks which provide supports to parapets. 'owever, they causedisturbances to ad&acent structures and utility services during construction. Doreover, if thebridge is curved, the wing walls may hinder the road curvature.

*ne the other hand, when the wing walls are structurally connected to the abutment, then

structural advantage can be taken by the stability of bo= structure.

1ig/ /< :ifferent orientation of wing walls.

20/ ,n $oints o& precast concrete (ridge seg'ents- what are the &unctions o& appl%ingepo.% adhesive)


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)po=y adhesive is applied in these &oints for the followingInternational Eoad "ederation ;0C11


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In the event of high vertical loads combined with large angle of rotations, rubber bearingsare undesirable when compared with pot bearings. "or instance, elastomeric bearingsrequire large bearing surfaces so that compression can be maintained between the contactsurfaces between the bearings and piers. Doreover, it also leads to uneven distribution ofstress on the piers and some of these highly induced stresses may damage the piers.!onsequently, pot bearings are better alternatives than elastomeric bearings in such anscenario as suggested by :avid 5. ee.

22. What are the shortco'ings o& grillage anal%sis which is co''onl% used instructural anal%sis o& (ridges)

Jrillage analysis suffers from the following shortcomings based on ). !. 'ambly?

(i) "or coarse mesh, torques may not be identical in orthogonal directions. Similarly,twists may differ in orthogonal directions.

(ii) Doment in any beams is mainly proportional to its curvature only. 'owever, moment inan element depends on the curvatures in the beams direction and its orthogonal

direction.

23. Pol%tetra&luoroeth%lene 5PT1E8 is co''onl% used in sliding (earings/ Wh%)

The choice of sliding surface of bearings is of vital importance because the sliding surfacesgenerate frictional forces which are e=erted on the bearings and substructure of the bridge."or instance, +T") and lubricated bron>e are commonly choices of sliding surfaces forbearings. +T") is a flurocarbon polymer which possesses good chemical resistance and canfunction in a wide range of temperature. The most important characteristic of this material isits low coefficient of friction. +T") has the lowest coefficients of static and dynamic frictionof any solid with absence of stick-slip movement ;:avid 5. ee


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the upper stainless steel plate. 'ence, the +T") may be designed with dimples to avoid thelubricant from squee>ing out under repeated translation movements.

24. Should raking piles o& a (ridge a(ut'ent (e placed under an e'(ank'ent)

"or a bridge abutment to be supported on raking piles with different orientations, themovement between the ground and the pile group is difficult to predict. "or instance, i f someof the raking piles of the bridge abutment are e=tended beneath an embankment, then thesettlement of embankment behind the abutment may cause the raking piles to e=periencesevere bending moment and damage the piles as recommended by :r. )dmund ! 'ambly;0C1C


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Chapter 2 ! Concrete Structures

1. What are the &unctions o& di&&erent co'ponents o& a t%pical e.pansion $oint)

In a typical e=pansion &oint, i t normally contains the following components? &oint sealant,&oint filler, dowel bar, +3! dowel sleeve, bond breaker tape and cradle bent.

5oint sealant? it seals the &oint width and prevents water and dirt from entering the &oint andcausing dowel bar corrosion and une=pected &oint stress resulting from restrainedmovement.

5oint filler? it is compressible so that the &oint can e=pand freely without constraint.Someone may doubt that even without its presence, the &oint can still e=pand freely. In fact,its presence is necessary because it serves the purpose of space occupation such that even ifdirt and rubbish are intruded in the &oint, there is no space left for their accommodation.

:owel bar? This is a ma&or component of the &oint. I t serves to guide the direction ofmovement of concrete e=pansion. Therefore, incorrect direction of placement of dowel barwill induce stresses in the &oint during thermal e=pansion. *n the other hand, it links thetwo ad&acent structures by transferring loads across the &oints.

+3! dowel sleeve? It serves to facilitate the movement of dowel bar. *n one side of the&oint, the dowel bar is encased in concrete. *n the other side, however, the +3! dowelsleeve is bonded directly to concrete so that movement of dowel bar can take place. *nemay notice that the detailing of normal e=pansion &oints in 'ighways Standard :rawing is in

such a way that part of +3! dowel sleeve is also e=tended to the other part of the &ointwhere the dowel bar is directly adhered to concrete. In this case, it appears that thisarrangement prevents the movement of &oint. I f this is the case, why should designerspurposely put up such arrangementM In fact, the rationale behind this is to avoid water fromgetting into contact with dowel bar in case the &oint sealant fails. #s +3! is a fle=iblematerial, it only minutely hinders the movement of &oint only under this design.

Bond breaker tape? #s the ma&ority of &oint sealant is applied in liquid form duringconstruction, the bond breaker tape helps to prevent flowing of sealant liquid inside the&oint .

!radle bar? It helps to uphold the dowel bar in position during construction.

2. , & on4site slu'p test &ails- should engineers allow the contractor to continue theconcreting works)

This is a very classical question raised by many graduate engineers. In fact, there are twoschools of thought regarding this issue.

The first school of thought is rather straightforward? the contractor fails to comply withcontractual requirements and therefore as per J. !. !. !lause 89 ;6


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by the contract, even though some engineers argue that slump tests are not as important asother tests like compression test.

The second school of thought is to let the contractor to continue their concreting works andlater on request the contractor to prove that the finished works comply with other contractual

requirements e.g. compression test. This is based upon the belief that workability is mainlyrequired to achieve design concrete compression strength. In case the compression test alsofails, the contractor should demolish and reconstruct the works accordingly. In fact, this is arather passive way of treating construction works and is not recommended because of thefollowing reasons?

(i) Korkability of freshly placed concrete is related not only to strength but also todurability of concrete. )ven i f the future compression test passes, failing in slumptest indicates that it may have adverse impact to durability of completed concretestructures.

(ii) In case the compression test fails, the contractor has to deploy e=tra time andresources to remove the work and reconstruct them once again and this slows downthe progress of works significantly. 'ence, in view of such likely probability ofoccurrence, why shouldnt the )ngineer e=ercise his power to stop the contractor andsave these e=tra time and costM

3. What is the &unction o& shear ke%s in the design o& retaining walls)

In determining the e=ternal stability of retaining walls, failure modes like bearing failure,sliding and overturning are normally considered in design. In considering the criterion ofsliding, the sliding resistance of retaining walls is derived from the base friction between the

wall base and the foundation soils. To increase the sliding resistance of retaining walls, otherthan providing a large self-weight or a large retained soil mass, shear keys are to be installedat the wall base. The principle of shear keys is as follows?

The main purpose of installation of shear keys is to increase the e=tra passive resistancedeveloped by the height of shear keys. 'owever, active pressure developed by shear keysalso increases simultaneously. The success of shear keys lies in the fact that the increase ofpassive pressure e=ceeds the increase in active pressure, resulting in a net improvement ofsliding resistance.

*n the other hand, friction between the wall base and the foundation soils is normally about

a fraction of the angle of internal resistance ;i.e. about 7.F < where is the angle of internalfriction of foundation soil. Khen a shear key is installed at the base of the retaining wall, thefailure surface is changed from the wall base/soil hori>ontal plane to a plane withinfoundation soil. Therefore, the friction angle mobili>ed in this case is instead of 7 . F i n theprevious case and the sliding resistance can be enhanced.

4. ,n designing concrete structures- nor'all% 'a.i'u' aggregate si3es areadopted with ranges &ro' 0'' to 20''/ =oes an increase o& 'a.i'u'aggregate si3e (ene&it the structures)

To answer this question, lets consider an e=ample of a cube. The surface area to volumeratio of a cube is A/b where b is the length of the cube. This implies that the surface area tovolume ratio decreases with an increase in volume. Therefore, when the si>e of ma=imum


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aggregate is increased, the surface area to be wetted by water per unit volume is reduced.!onsequently, the water requirement of the concrete mi=es is reduced accordingly so thatthe water/cement ratio can be lowered, resulting in a rise in concrete strength.

'owever, an increase of aggregate si>e is also accompanied by the effect of reduced contact

areas and discontinuities created by these larger si>ed particles. In general, for ma=imumaggregate si>es below 97mm, the effect of lower water requirement can offset thedisadvantages brought about by discontinuities as suggested by ongman Scientific andTechnical ;0CF1


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high strength concrete to increase the ductility of concrete. In addition, fire resistance of


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high strength concrete is found to be less than normal strength concrete as suggested by*dd ). J&orv ;0CC9ontal% refers to formwork formed vertically and when compared withformwork erected in hori>ontal plane, the amount of falsework required is smaller.

The item "ormwork e=ceeding 77mm wide, hori>ontal or at any inclination up to andincluding 8oto the hori>ontal% refers to formwork to be erected in hori>ontal position and ingeneral it requires much falsework to support this type of formwork. Therefore, the rate forthis item is higher than the one mentioned in the above paragraph.

10. , & concrete co'pression test &ails- should Sch'idt ha''er test (e adopted asan alternative test to prove the concrete strength)

The Scmidt hammer test is based on the elastic rebound of hammer which presses onconcrete surface and it measures the surface hardness of concrete. Since the test is verysensitive to the presence of aggregates and voids at the concrete surface, it is necessary totake more than 07 readings over the area of test. 'owever, it should be noted that Schmidt


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hammer test measures surface hardness only but not the strength of concrete. Therefore, itmay not be considered a good substitute for concrete compression test.

11. What is the indication o& shear slu'p and collapse slu'p in slu'p tests)

There are three types of slump that may occur in slumps test, namely, true slump, shearslump and collapse slump.

True slump refers to general drop of the concrete mass evenly all around withoutdisintegration.

Shear slump implies that the concrete mi= is deficient in cohesion. !onsequently, it mayundergo segregation and bleeding and thus is undesirable for durability of concrete.

!ollapse slump indicates that concrete mi= is too wet ad the mi= is deemed to be harsh andlean.

12. ,n erection o& &alsework- &or a rectangular panel inside a &alsework should it (e(raced along the two diagonals)

Khen a rectangular panel is sub&ect to an eccentric load or a lateral load, it tends to deforminto a parallelogram with one diagonal shortening and the other elongating. Theoretically, itis sufficient to brace along one of the diagonals ;the one in tension


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1ig/ 2/# rebate.

15. , & a contractor proposes to increase concrete cover (e%ond contractualspeci&ication 5i/e/


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Shrinkage of grout is several times that of concrete with the same mass.The effect of creep of grout is far more than that of concrete.'eat of hydration of cement with water is more than normal concrete and this leads tothe problem of severe cracking.

17. Which t%pe o& (ar rein&orce'ent is 'ore corrosion resistant- epo.%4coated(ars- stainless steel (ars or galvani3ed (ars)

Based on the e=periment conducted by the Building Eesearch )stablishment, it was shown


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that the corrosion resistance of galvani>ed steel was the worst among the three types of barreinforcement. "or galvani>ed steel bars, corrosion started to occur when a certain chloridecontent in concrete ;i.e. 7.9O by cement weight< was e=ceeded. 'owever, for epo=y-coatedbars, they e=tended the time taken for cracking to occur when compared with galvani>edsteel bars.

The best corrosion resistant reinforcement among all is stainless steel. In particular,austenitic stainless steel stayed uncorroded even there was chloride contamination inconcrete in the e=periment. Eeference is made to (. K. 5. Treadaway ;0CFF


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from the adhesion and interface friction. Kith such differences in mechanism in bonding,the behaviour of bond between deformed bars and plain round bars in the presence of rustvaries. Eeference is made to !IEI# Eeport 091.

20. eneral Speci&ication &or Civil Engineering Works 5BB2 Edition8 Clause 9/0B

speci&ies that t%ing wires &or rein&orce'ent ad$acent to and a(ove Class 1< and19 &inishes should (e stainless steel wires/ Wh%)

I f plain steel tying wires are used for reinforcement ad&acent to !lass "9 and "8 finishes, itposes the problem of rust staining which may impair the appearance of e=posed concretesurfaces. The rate of corrosion of plain steel tying wires is similar to normal steelreinforcement. 'owever, for tying wires with very small diameter, upon long e=posure itstands a high chance of rusting completely and these rust will stain the formwork andsignificantly affect the concrete finish. Therefore, stainless steel tying wires are specifiedfor locations in the vicinity of high quality of finishes to avoid rust staining by corrodedtyping wires.

ote? Tying wires are wires used for fi=ing and connecting steel reinforcement bars.

21. 1or long slender structures like (ea's- propping is reDuired a&ter re'oval o&&or'work/ Wh%)

#fter concreting, the time at which striking of formworks should not be too long, otherwise itwould affect the colour of concreted structures. "or long span concrete structures, when theyhave attained sufficient strength to support their self-weight, creep deflection may occur inthese structures i f propping is not provided after the removal of formwork. Therefore, re-propping is carried out after removing formwork and these props should not be allowed to

stand too long because creep loads may overstress them.

ote? +ropping refers to provision of falsework to support slabs and beams during their gain in concretestrength after concreting.

22. What is the di&&erence in application (etween open stirrups and closed stirrupsin concrete (ea's)

*pen stirrups are provided principally to resist shear forces in concrete beams and they areapplied in locations in which the effect of torsion is insignificant. 4-shaped stirrups areplaced in the tension side of concrete beams in which shear cracks would occur. 'owever,

when concrete beams are designed to resist a substantial amount of torsion, closed stirrupsshould be used instead.

23. 1or colu'n rein&orce'ents- wh% is helical rein&orce'ent so'eti'es designedinstead o& nor'al links)

The use of links for column design in Britain is very popular. 'owever, in 4.S.#. engineerstend to use helical reinforcement instead of normal links because helical reinforcement hasthe potential advantage of protecting columns/piles against seismic loads. Doreover, whenthe columns reach the failure state, the concrete outside hoops cracks and falls off firstly,

followed by the eventual failure of the whole columns. The peeling off of concrete outsidehelical reinforcement provides a warning signal before the sudden failure of columns assuggested by J. +. Danning ;0C69


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normal link reinforcement.

"or instance, helical reinforcement is adopted in the design of marine piles in Jovernmentpiers.

ote? 'elical reinforcement refers to shear reinforcement which is spiral in shapes.

24. What is the di&&erence (etween epo.% grout- ce'ent grout and ce'ent 'ortar)

)po=y grout consists of epo=y resin, epo=y hardener and sand/aggregates. In fact, there arevarious types of resin used in construction industry like epo=y, polyester, polyurethane etc.Though epo=y grout appears to imply the presence of cement material by its name, it doesnot contain any cement at all. *n the other hand, epo=y hardener serves to initiate thehardening process of epo=y grout. I t is commonly used for repairing hairline cracks andcavities in concrete structures and can be adopted as primer or bonding agent.

!ement grout is formed by mi=ing cement powder with water in which the ratio of cement ofwater is more or less similar to that of concrete. Setting and hardening are the importantprocesses which affect the performance of cement grout. Doreover, the presence ofe=cessive voids would also affect the strength, stiffness and permeability of grout. It isversatile in application of filling voids and gaps in structures.

!ement mortar is normally a mi=ture of cement, water and sand. They are used as bedding forconcrete kerbs in roadwork.

25. What is the purpose o& skin rein&orce'ent &or deep (ea's)

In BSF0 07, it states that secondary reinforcement should be provided for beams e=ceeding187mm deep at a distance measured 6/ depth from the tension face. )=perimental worksrevealed that at or close to mid-depth of deep beams, the ma=imum width of cracks arisingfrom fle=ure may be about two to three times larger than the width of the same crack at thelevel of surface where the crack originally forms.

The presence of crack is undesirable from aesthetic point of view. Doreover, it posespotential corrosion problems to reinforcement of deep beams. To safeguard against thesecrack formation, skin reinforcement is designed on the sides of deep beams to limit theformation of fle=ural crack widths. Though the principal function of skin reinforcement is to

control crack width, it may be employed for providing bending resistance of the section.


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Chapter ; ! =rainage Works

1. What is the &unction o& waterstops in $oints o& (o. culverts and drainage

channels)

The principal function of waterstops is to prevent liquids ;e.g. water


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1ig/ ;/ Small spatialrequirement of bo= culverthan pipes.

;/ What are the li'itations o& ational @ethod in calculating runo&&)

!omputation of runoff is a complicated matter which depends on many factors like theground permeability, rainfall duration, rainfall pattern, catchment area characteristics etc.Basically, Eational Dethod is a means to find out the ma=imum discharge suitable fordesign purpose. In this method, it is assumed that the rainfall duration is the same as the

time of concentration and the return period of rainfall intensity is the same as the peakrunoff. Time of concentration refers to the time required for the most remote location ofstormwater inside the catchment to flow to the outlet. Khen the time of concentration isequal to the rainfall period, the ma=imum discharge occurs and rainfall collected inside thecatchment comes to the same outlet point.

Eational Dethod provides the peak discharge only and it cannot produce a hydrograph. If amore detailed pattern of runoff is required, unit hydrograph or other methods have to beused. The accuracy of rational method depends very much on our correct selection ofrunoff coefficient and delineation of catchment area.

Eational Dethod is a rather conservative method. *ne of the basic assumptions of therational formula is that the rainfall intensity must be constant for an interval at least equal tothe time of concentration. "or long duration of rainfall, this assumption may not hold true.Doreover, the runoff coefficient in Eational Dethod is difficult to be determined accuratelyand it depends on many factors like moisture condition of soils, rainfall intensity and duration,degree of soil compaction, vegetation etc. In addition, In Eational Dethod the runoffcoefficient is independent of rainfall intensity and this does not reflect the actual situation.

ard to the workers during the lifting-up process of manhole coversbecause cast iron manhole covers are very heavy to normal workers. !onsequently,research has been conducted and ductile iron is considered as a better choice than cast ironbecause it can resist the same traffic loads with lower self-weight. Doreover, as ductile iron isless brittle than cast iron, the traditional cast iron manhole covers are more susceptible todamage and thus requires higher maintenance cost.

'owever, ductile iron manhole covers do suffer from some demerits. "or instance, owing


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to their relative low self-weight, vehicles passing over these manhole covers would lead tothe movement of covers and generate unpleasant noises. To solve this problem, instead ofincreasing the self-weight of ductile iron manhole covers which similarly causes safetyproblems to workers during regular maintenance, the covers can be designed to be attached tothe manhole frames which hold them in firm position.

5. Wh% is it pre&era(le to design stor'water drains to 'atch so&&it)

Stormwater drains collect stormwater in their corresponding catchment areas duringrainstorm and convey the collected water through outlets to the sea. Therefore, inconsidering the hydraulic design of stormwater drains, other than normal drainage pipecapacity to be taken into consideration, one should check the backwater effect due to tidalcondition at outlets i f the drains are located quite close to the downstream end of outlets.

Stormwater drains are normally designed to match soffit to avoid surcharging by backwatereffect or when the downstream pipes are running full. ormally pipe si>e increases fromupstream to downstream. "or the case of matching drain invert, when outlet pipes are fullysurcharged by tidal effect of the sea or when the downstream pipes are fully filled withstormwater, pipe sections immediately upstream of the outlet are also surcharged too.'owever, for the case of matching pipe soffit, the immediate upstream sections of outletpipes are not totally surcharged even though downstream pipes are running full. 'owever, itis not always practical to maintain soffit for all pipelines because it requires sufficient drop toachieve this.

Doreover, the flow of stormwater is mainly by gravity in the design of stormwater drains. Incase the drains are designed to match invert, then it stands a high probability that the flow in

the upstream smaller pipes has to be discharged against a head.

ote? Datching soffit means that all pipelines are aligned continuously with respect to the pipelines crownlevel.

1ig/ ;/2 Datch soffit 3S match invert.

6. What is the application o& inverted siphons) What are the disadvantages o&using


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inverted siphons)

Inverted siphons are designed at locations in which a sewer system is blocked byunderground utilities or stormwater drains. They are sometimes called depressed sewersbecause it is claimed that there is no actual siphon action. They connect the upstream and

downstream sewers with 4-shaped vertical alignment such that they are always running full.

The drawbacks of inverted siphons are?

(i) They induce additional head loss to the sewer system which is undesirable in hydraulicperformanceL

(ii) 4-shaped siphons create sediment accumulation problem and previous e=perienceshowed that inverted siphons were easily blocked due to siltationL

(iii) Daintenance of invert siphons is difficult due to its inaccessibility.

7. What is the 'echanis' o& cavitation in pipes and drains)

!avitation refers to the formation of air bubbles in fluid in low-pressure condition which islower than the saturation pressure. It is a potentially damaging condition in which the fluid inpipes or sewers is at high velocities. By Bernoullis )quation, at high flow velocities, thepressure head of fluid is reduced accordingly. #s the fluid pressure is less than saturationpressure, dissolved gases are released from the fluid and these air bubbles will suddenlycollapse when the flow enters into a region of higher pressure. This produces a high dynamicpressure which causes damage to the pipelines due to its high frequency.

8. When a drainage s%ste' 5i/e/ u4channels with catchpits8 is connected to a 'aindrainage channel- a seg'ent o& short pipe is used/ What is the reason o& such

arrange'ent)

There are three scenarios of such connection arrangement? ;a< a new drainage system isconnected to an e=isting drainage channel ;b< an e=isting drainage system is connected to anew drainage channel ;c< a new drainage system is connected to a new drainage channel.

"or all scenarios, what engineers consider is the total amount of differential settlement orlateral movement to be encountered between the drainage system and main drainagechannel. "or scenario ;b< and ;c


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1ig/ ;/; Short pipe.

B/ What are the &unctions o& (edding under stor'water drains)

Bedding, which are normally made of granular materials or concrete, serves four mainfunctions as suggested by *. !. Poung and 5. 5. Trott?

To enhance a uniform support under pipes in order to reduce the bending momentlongitudinallyLTo increase the load-supporting strength of the pipesL"or pipes with spigot and socket &oints, it enables pipes to be supported along pipelengths instead of pipe sockets. *therwise, uneven stress may be induced and it maydamage the pipesL

To provide a platform for achieving correct alignment and level during and afterconstruction.

10. ,n designing sewer pipes- wh% are vitri&ied cla% pipes co''onl% used &or pipesi3e less than ?00'' while precast concrete pipes with PFC lining is used &orpipe si3e e.ceeding ?00'')

The market price of vitrified clay pipes is generally less than that of precast concrete pipeswith +3! lining. Therefore, for small si>e of pipes ;pipe diameter less than A77mm< it ismore economical to use vitrified clay pipes. 'owever, vitrified clay pipes do suffer from theproblem of brittleness and its effect is even severe for larger si>e of pipes. Doreover, it is

rather time consuming to deliver clay pipes products because the ma&ority of them aremanufactured in )urope. 'ence, for larger si>e of sewer pipes ;diameter more than A77mme Q 177mm, water-&etting is normally employed in which water is supplied fromnearby fire hydrants and pressuri>ed water &et is used for clearing blockage.

Kinching method is adopted for all si>es of pipes.

"or instance, for pipe si>e e=ceeding 0787mm, it is stated in Stormwater :rainage Danualthat ma=imum intervals between manholes along straight lengths should be 067m. This isbecause for si>es over 0787m, the main method of pipe maintenance is by winching whosema=imum length of operation is 067m. Similarly, the ma=imum intervals of manholes forother straight pipes are derived from their corresponding maintenance methods.

14. ,n selection o& da's in drainage channels- what are the advantages o& usingru((er da's instead o& steel4gate da's)

The advantages of rubber dams are as follows?

;i< Since rubber is fle=ible in nature it is capable of performing deflation even in thepresence of dirt and sedimentation on the downstream side. 'owever, for rigid


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steel-gate dams, it may not be possible to open when there is e=cessive sediment.

(ii) Since the foundation of rubber dams is comparatively lighter than that of steel-gatedams, it saves both construction cost and time.

(iii) Eubber dams can be designed with longer spans without piers while steel-gate damsrequire intermediate piers for long spans.

15. Should air test or water tests (e selected to test the leakage o& constructedgravit% pipelines)

"or gravity pipes, air tests or water tests are carried out after completion of laying and&ointing of the pipes. These tests are conducted to check the watertightness of &oints and toensure the pipelines are free from damage where leakage may occur.

#ir test has the advantage that the test itself is simple and faster to be carried out. I t doesnot require the disposal of significant quantities of water used in the test which is amandatory requirement for water test. 'owever, in case leakage e=ists in the constructedsegment of gravity pipelines, the position of leakage can hardly be located in air test.Doreover, the rate of water leakage cannot be determined from air tests. In addition, air test isreadily affected by atmospheric condition because air has a relatively high coefficient ofthermal e=pansion. The test is also influenced by the moisture condition of the test pipelinesbecause it affects the passage of air through the pipelines.

"or water test, though it is comparatively slow, it can detect the location of water leakage.'owever, the leakage rate results from water test may not truly reflect its actual leakagebecause pipeline materials like concrete and clay are porous and would absorb water duringthe test.

16. , n designing o& access ra'ps &or drainage channels- wh% should the direction o&access ra'ps (e sloping down towards downstrea')

In the design of access ramps, the direction is normally specified to be sloping down towardsdownstream so as to avoid the occurrence of over-shooting of flowing water for supercriticalflow in case of aligning the ramps in the reverse direction of channel flow.

ote? #ccess ramps refer to ramps used for maintenance vehicles during routine maintenance of channels.

17. When (ranch pipelines are connected to 'ain pipelines- so'eti'es H4$unctions

or &itting (ranched pipelines to 'ain pipelines (% &or'ation o& holes in 'ainpipelines are used/ Which one is a (etter choice)

By using standard precast units of P-&unction branch pipelines, it is beyond doubt that &ointsbetween branched pipelines and main pipelines are properly formed and the quality of &ointsis relatively less dependent on workmanship. 'owever, it suffers from the problem that withfi=ed precast units of P-&unctions, sometimes it may be difficult for contractors to determinethe precise orientation of specific angles of P-&unctions with respect to gullies. ;e.g. gulliesare connected through side branches to carrier drainse the pile bent. Khen the reaction forces of these piles, hori>ontal forces ;e.g. dueto berthing and deberthing of vessels< and vertical forces ;e.g. superimposed deck loadsed by drawing a force polygon, it is noted that lateral resistance of the pile bent isdependent on the vertical load, i.e. lateral resistance is small when vertical loads are high.


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1ig/ 9/2 "orce polygon of pile bent.


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13. 1or underwater concreting- tre'ie pipes are nor'all% used with the aid o&hoppers/ So'eti'es tu(es are inserted inside the hoppers/ Wh%)

In placing concrete by tremie pipes, hoppers are connected to their top for receiving freshly

placed concrete. 'owever, air may be trapped inside the tremie pipes i f there is rapidfeeding of fresh concrete. To release the trapped air inside the tremie pipes, hoses ;calledventilation tubes< are inserted and lowered down through the hoppers. Eeference is made to!arl #. Thoresen ;0CFFontal load due to berthing of largevessels. Since the widths of open berth piers are relatively small so that they provides ashort lever arm to counteract the moment induced by berthing loads. Doreover, the deadload of open berth piers are normally quite light and therefore the resisting moment

provided by the dead load of pier structures may not be sufficient to counteract the momentgenerated by berthing loads.

To aid in providing adequate resistance to the overturning moment by the berthing load, thesoil resistance above bedrock contributes to stabili>ing moment. "or commonly adoptedmarine piling type, i.e. driven steel tubular piles with reinforced concrete infill, driven pilescan at most be founded on top of rockhead surface. In case the rockhead level is shallow,then the little soil cover may result in insufficient lateral resistance to the berthing load.

15. Wh% are high and narrow (ea's not desira(le in concrete piers)

Based on past e=perience in other countries ;!arl #. Thoresen ;0CFF


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The elasticity of fenders is related to the ability to release the stored energy during berthingof vessels. 'owever, it has no effect on the reaction force and the deflection of fendersystem. The amount of energy that a fender can absorb is dependent on the reaction-deflection curve and is represented by the area under the curve. The higher is the reactionforce, the higher amount of energy would be absorbed by the fender provided that the

resistance of ships hull is sufficient to withstand the force without permanent deformations.#lthough stiff and soft fender may have the same deflection under the same ma=imumreaction force acting on the berthing vessel, the amount of energy absorbed by stiff fendersis much higher than that of soft fenders. !onsequently, stiff fenders should be employed forberthing purpose.

*n the other hand, in mooring operations where vessels are constantly sub&ect to waveaction, it is desirable to keep the tension force on the rope to a low value. In this connection, itis recommended to use soft fenders.

/ What is the signi&icance o& direction o& approaching velocities o& ships during(erthing operation)

*ne of the ma&or effects of angle of approaching velocities of ships is its influence of theenergy to be absorbed by the fender system. !onsider several ships berth on the same pier atthe same speed but with different angle of approach, though their kinetic energies are thesame, the amount of energy absorbed by fender differs. The amount of energy absorbed byfender is?

K R 7.8mv6;k6@r6cos6


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Chapter ? ! oadworks

1. ow can unrein&orced concrete pave'ent &unction without 'esh rein&orce'ent)

"or concrete carriageway, it is normally classified into two types? reinforced andunreinforced concrete pavement. The reinforcement in reinforced carriageway ;in the form ofmesh< is used for controlling cracking. Then one may query how unreinforced pavement cancontrol cracking without the use of mesh reinforcement. To answer this question, one shouldpay attention to the features of unreinforced concrete pavement. In accordance with'ighways Standard :rawing o. '007C, an appro=imately mm wide groove with a depth ofabout one-third to one-fourth of slab thickness is designed with a regular spacing ;normally8mards. "or normal

fencing design, when vehicles crash into safety fencings, it will give way so as to absorb asmuch energy as possible, thus reducing the impact forces on the vehicles. Doreover, itserves to realign the vehicles along the carriageway when vehicles hit on them. 'owever, forconcrete profile barriers they are not designed to absorb energy during vehicle crashing, butto hold the vehicles hitting on them. In this connection, concrete profile barriers aredesigned with curved profiles so that vehicles can mount and go up partly on them, and yetthey will not cause overturning of vehicles. Eeference is made to #rthur Kignall, +eter S.(endrick and Eoy #ncil.

"or shallow-angle crashing of cars, they would climb on the lower slope face of concrete

profile barriers. *n the other hand, when a car hits at a large angle to the barrier, the


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bumper collides with the upper sloping face of concrete profile barrier and the car ridesupwards. This provides the uplift of the car whose wheels move up the lower sloping faceof the barrier. It is not intended to lift the car too high, otherwise it may result in rolling.Since the friction between the wheels and barriers provide e=tra lifting forces, it isundesirable to design rough finish on these faces. In essence, the kinetic energy of the car

during collision is transformed to potential energy during its lifting up on profile barrier andfinally converted back to kinetic energy when the car returns to the road.

ote? "or details of concrete profile barriers, reference is made to 'y: Standard :rawing o. '6070#.

3. Should $oints o& concrete ker(s (e in line with the $oints in concrete

carriagewa%)

In normal practice, &oints are provided in road kerbs to cater for concrete e=pansion andcontraction. 'owever, the location of &oints in kerbs is not arbitrary and they should match

with &oints in concrete carriageway. *therwise, it is very likely that cracks may form inconcrete kerbs at location of pavement &oints ;Dinistry of Transport ;0C88


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"or ground penetrating radar, it sends radio waves into the ground and receives signalsfrom reflections from utilities. It has the advantage of locating both the depth and alignmentof utilities. Dore importantly, i t can detect both metallic and non-metallic utilities. 'owever,it suffers from the disadvantage that it is quite e=pensive and interpretation of data is notsimple. Eeference is made to :, :S: ;6777


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9. What is the &unction o& a sand la%er underl%ing paving sla(interlocking

(locks)

ormally after the laying of sub-base layer of the paving slab/interlocking blocks, a 7mm

thick sand bedding is screeded and tamped over the pavement area. Then paving slabs arelaid hori>ontally with &oints of 6-mm wide and are laid in uphill direction. #fter completingthe laying of paving slabs, sand used for filling &oints is spread over the surface of the unitsand brushed into the &oints such that all &oints are completely filled. The paving slabs arethen bedded into final position by using plate vibrators.

The sand layer serves the same purpose of normal blinding layer under concrete structure?

(i) +rovide a level and flat surface for the paving slab/interlocking blocks to lay onL

(ii) +rotect the foundation ;i.e. underground subgrade and sub-base layer< againstadverse

outside conditions ;e.g. bad weather< during construction of the laying work of pavingslab/interlocking blocks.

10. What is the &unction o& longitudinal $oints in concrete road pave'ents)

# longitudinal &oint consists of a tie bar placed at the mid-depth of a concrete pavementand it is not intended for &oint lateral movement. Then one may doubt the reasons ofplacing longitudinal &oints in concrete pavements. In fact, longitudinal &oints are normallydesigned at a regular spacing e.g. 9.8m to accommodate the effect of differential settlementof pavement foundation. Khen uneven settlement occurs, the tie bars in longitudinal &ointsperform as hinges ;Dinistry of Transport ;0C88


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12. ,n eneral Speci&ication &or Civil Engineering Works 5BB2 Edition8- the designo& road(ase 'aterial is (ased on recipe approach/ Wh%)


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The design of roadbase material is based on recipe approach ;:avid !roney and +aul!roney ;0CC6


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to harden the surface.

15. What is di&&erence in purpose &or conducting sur&ace regularit% test and sandpatch test 5test on te.ture depth8)

The purpose of surface regularity test is to measure the riding quality of pavements and thesame requirement and standard is applied to both concrete and bituminous carriageway. *nthe other hand, sand patch test is used for checking the skid resistance of road pavements.Doreover, it is related to traffic noise because the intensity of noise generated from roadtraffic is related to te=ture depth of carriageway.

16. ,n eneral Speci&ication &or Civil Engineering Works 5BB2 Edition8- it speci&iesthe te'perature reDuire'ents &or (itu'inous 'aterial during and a&ter 'i.ing/What is the reason (ehind this)

Temperature is one of the factors that govern the compaction of bituminous material and theair void content is found to decrease with an increase in compaction temperature. Thisphenomenon is e=plained by the viscosity-temperature relations? the higher is the viscosity ofbinders, the greater is the resistance to compaction. Therefore, in normal contract forbituminous laying, the temperature requirements for bituminous material during and aftermi=ing are specified.

17. Can a su(4(ase la%er serve as a drainage la%er to re'ove water &ro'

pave'ent)

Besides providing load distribution in post-construction stage and working platform in

construction stage, sub-base can also serve as a drainage layer to remove water comingfrom the pavement. "or thick layers of road pavement, it is likely that the water leakage frompavement is insignificant and therefore the sub-base layer mainly serves to support the trafficstresses. 'owever, for thin pavement layers, water penetration is quite substantial andtherefore sub-base may also act as a drainage layer to remove these water.

18. 1or unrein&orced concrete carriagewa%- what is the seDuence o& closing andopening o& e.pansion $oints and contraction $oints)

ets take an e=ample to illustrate the sequence of closing and opening of &oints ;Dinistry of

Transport ;0C88


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1ig/ ?/2 Dovement of &oints in summer and winter.

B/ Are ker(s necessar% in road pave'ents)

In general, kerbs are essential in road pavements due to the following reasons ;based on#rthur Kignall, +eter S. (endrick and Eoy #ncil


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In general, the spacing of dowel bars should not be too close which may pose problemsduring construction. 'owever, it should be not too wide to allow the occurrence of bendingbetween the dowel bars. *n the other hand, regarding the length of dowel bars, it should notbe too long because the induced stress at the end of long dowel bars is insignificant and is noteffective in transferring loads between ad&acent concrete panels. 'owever, i f the dowel bars

are too short, the stress at the face of &oint is increased resulting in concrete crushing.Eeference is made to Dinistry of Transport ;0C88


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Chapter : ! Pu'ping Station

1. What is the di&&erence a'ong ce'ent plaster- ce'ent render and ce'entscreed) #nder what situations should each o& the a(ove (e used)

The purpose of plastering, rendering and screeding is to create a smooth, flat surface toreceive finishes like paint, wallpaper etc.

+lastering is the intermediately coating of building materials to be applied on the internalfacade of concrete walls or blockwalls.

Eendering is the intermediate coating for e=ternal walls only.Screeding is the coating laid on floors to receive finishes like tiles, carpet, and marble...

'ence, these terms differ basically from the locations at which they are applied. :ue todifferent locations of application of plasterwork, the proportion of material component for

plaster and render is different. "or e=ample?

(i) !ement plaster

4ndercoat- cement?lime?sand ;by volume< R 0?9?0A"inishing coat - cement?lime?sand R 0?06?7

(ii) !ement render4ndercoat- cement?lime?sand ;by volume< R 0?6?A"inishing coat - cement?lime?sand R 0??A

2. ,n the design o& cor(el (ea's in a pu'ping station- wh% are shear links designed

in the top 2; o& the section)

What is the general advice on the design)

!orbel beams are defined as >/dQ7.A where > is the distance of bearing load to the beamsfi=ed end ;or called shear span< and d is depth of beams. The design philosophy is based onstrut and tie system. To establish the design model, it is firstly assumed the failure surface,i.e. shear cracks e=tending to 6/ of depth of beam. )=periment results verified that thefailure cracks e=tended only to 6/ of beam while the remaining 0/ depth of concretecontributed as concrete strut to provide compressive strut force to the bearing loading.

'ori>ontal links are normally provided to corbel beams because e=perimental resultsindicated that hori>ontal links were more effective than vertical links when shear span/depthis less than 7.A. "or shear span/depth7.A, it should be not considered as corbel beams butas cantilevers.

In designing corbel beams, care should be taken to avoid bearing load to e=tend beyond thestraight portion of tie bars, otherwise the corners of corbel beams are likely to shear off.Eeference is made to . #. !lark ;0CF


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3. iven a ' high staircase resting on solid concrete- would it (e adeDuate todesign no'inal rein&orce'ent &or the staircase)

"or the design of staircase, there are three main scenarios?

(i) Stairs spans longitudinally

This kind of stairs refers to stairs spanning between landings only without any side supports.In this case, the staircase should be designed as a beam between two end supports ;i.e.landing< and the main reinforcement is provided at the bottom of staircase slabs.

(ii) Stairs spanning transversely

This kind of staircase is supported by sidewalls only and it may also be supported by stringerbeams. "or the case of sidewalls, it acts as a cantilever beam and the main reinforcementare provided the top surface of slab. "or the case of staircase supported sideways by bothsidewall and stringer beam, it should be designed transversely with end supports as sidewalland stringer beam and reinforcement is provided at the bottom of the staircase.

(iii) Stairs resting on solid support"or stairs resting on solid supports, only nominal steel reinforcement is provided to controlthermal and shrinkage cracking.

4. ,n selecting screw pu'ps in polder sche'e pro$ects- what are the &actors that

a&&ect the design capacit% o& screw pu'ps)

The commonly used angles of inclination for screw pumps are 7 o, 8oand Fo. "or screwpumps of relatively high lifting head, like over A.8m, angle of inclination of F o is normallyused. 'owever, for relatively lower head and high discharge requirement, angle of inclinationof 7oshall be selected. In general, for a given capacity and lifting head, the screw pumpdiameter is smaller and its length is longer for a screw pump of 7 o inclination whencompared with a screw pump of Foinclination.

To increase the discharge capacity of screw pumps, a larger number of flights should be

selected. In fact, screw pumps with 6 flights are more economical that that with flights interms of efficiency and manufacturing cost. Doreover, the discharge capacity is also


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determined by the screw pump diameter and si>es of 77mm to 8777mm are available incurrent market.

9/ What are the co'ponents o& a waterproo&ing s%ste' in the roo& o& a t%picalpu'ping station)

In the design of a waterproofing system at the roof of a pumping station, normally thefollowing components are?

(i) #bove the structural finish level of the concrete roof, a screed of uniform thickness isapplied to provide a smooth surface for the application of waterproofing membrane.;Screed of varying thickness can also be designed on the roof to create a slope fordrainage.< The screed used for providing a surface for membrane should be thin andpossess good adhesion to the substrate. Doreover, the screed aids in the thermalinsulation of the roof.

(ii) #bove the screed, waterproofing membrane is provided to ensure watertightness of theroof.

(iii) #n insulation board may be placed on top of waterproof membrane for thermalinsulation. In cold weather condition where the loss of heat at the roof is significant, theinsulation board helps to reduce these losses. *n the contrary, in summer the roof isheated up by direct sunlight and the insulation layer reduces the temperature rise insidethe pumping station.

?/ ,n pu'ping stations one o& the choices &or the 'aterial o& water tanks is &i(re4rein&orced plastic 51P8/ What are the advantages associated with this kind o&'aterial)

There are two main advantages for "E+ water tanks?

(i) I t possesses high strength to weight ratio and this leads to the ease of site handling.

(ii) I t is highly resistant to corrosion and thus it is more durable than steel water tank.

:/ ,n the construction o& pu'p troughs &or acco''odation o& screw pu'ps- what isthe construction 'ethod to ensure close contact (etween the screw pu'ps and thepu'p trough)

In the construction of screw pump troughs, trape>oidal-shaped troughs are usually formed by

using normal formwork. In order to enhance close contact between screw pumps andtroughs, upon lifting the screw pumps into the troughs screeding works is carried out. Screwpumps are set to rotate and screeds are placed between the gap of screw pumps blade andtrape>oidal-shaped troughs during the rotating action of screw pumps. #fter the screed sets,it serves to prevent leakage of water during the pumping operation of screw pumps.

/ Wh% are voids &illed with lightweight in&illing 'aterial in ra&t &oundation o& pu'pingstations)

To reduce the dead load and hence to reduce the settlement of pumping stations, the voids

inside the raft foundations are filled with light material. I f instead concrete is placed insidethese voids, it poses severe thermal cracking problem and drastically increases loads to


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pumping stations. The use of general f il l is also not desirable because its self-weight iscomparable to that of concrete. *n the contrary, if these voids are left vacant, water maypenetrate into these voids during future operation and increases the dead load of pumpingstations during its normal operation. Therefore, lightweight in f il l material, which is non-water-absorbing and non-biodegradable, is designed inside these voids to avoid ingress of

water and to reduce the dead load of the structure.

9. What is the di&&erence in arranging pu'ps in series and in parallel)

"or identical pumps with similar functions, i f the pumps arranged in series, the total head isincreased without a change to ma=imum discharge. *n the other hand, for pumps arrangedin parallel to one another, the discharge is increased without any changes to ma=imumhead.

10. , n ter's o& pu'ping per&or'ance- how should engineers deter'ine the use o&

radial &low pu'ps and a.ial &low pu'ps)

Specific speed is usually defined for a pump operating at its ma=imum efficiency. In order tominimi>e the cost of future operation, it is desirable to operate the pumps as close to thema=imum efficiency point as possible. The specific speed for radial flow pumps is relativelysmall when compared with that of a=ial flow pumps. This implies that radial flow pumpstend to give higher head with lower discharge while a=ial flow pumps tend to give higherdischarge with lower head.


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Chapter ! ecla'ation

1. What are di&&erent approaches &or recla'ation in deep water region and shallowwater region)

To illustrate the different approaches adopted for reclamation in deep water and shallowwater region, the following e=ample is used?

In deepwater region, consider the seabed level is NF.8m+:. #fter laying of geote=tiles and0 .8m thick sand blanket, the top level of sand blanket is about N1m+:. Split barges aredeployed for dumping public f il l to N6.8m+:. #fterwards, end dipping of public f il l bytrucks will be carried out up to @6.8m+: which is the designed reclamation level. Betweenlevel N6.8m+: and @6.8m+:, it is too shallow for split barges to enter the water, thus themethod of end dipping is used instead.

"or shallow water region, the seabed level is taken as N8.8m+: in this e=ample. Kith thelaying of geote=tiles and 0 .8m sand blanket into position, the top level of sand blanket isabout N9m+:. In this case, split barges are also used for reclamation work between thelevel N9m+: and N6.8m+:. #fter that, i f end dipping is used for reclamation work above N6.8m+:, then in considering the relative thin layer of fill above seabed ;0.8m sand blanket @0 .8m sand blanket


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"or geote=tiles used in reclamation, they serve mainly the following two purposes?

(i) they separate reclamation f i l l from marine mudL

(ii) they may act as reinforcement to enhance the stability of reclamation. 'owever, thereinforcement function is still under heated debate because its performance as

reinforcement depends on several factors like the directional strength of wovengeote=tiles and damage effect by installation of vertical band drains.

"or sand?

(i) i t spreads the load of future public dump on top of itL

(ii) i t acts as drainage path for dissipation of e=cess pore water pressure for band draininstallation.

4. eotechnical ,nstru'entation is &reDuentl% e'plo%ed &or 'onitoring thecondition o& recla'ation/ So'eti'es two pie3o'eters are installed inside the

sa'e (orehole/ What is the reason (ehind this)

"or standpipes, they normally contain one plastic tube between its intention is to measurewater level only. 'owever, for pie>ometers, they are used for measuring pore water pressurein a certain depth below ground. "or instance, i f there are two clayey layers below ground atdifferent depths, a multiple pie>ometer including two separate pie>ometers may be sunk atthe same borehole to determine the pore water pressure at these layers respectively. Thisarrangement has the advantage that it saves the cost of installation of separate boreholes forseveral pie>ometers. 'owever, the installation of multiple pie>ometers within the sameborehole is affected by occurrence of leakage along the pipes as suggested by DariusTremblay ;0CFCometer is a type of pie>ometer which measures pore water pressure at acertain level. It consists of plastic pipes without holes. The tip of the standpipe pie>ometer is

perforated and the annular space between the tip of the pie>ometer and soil is filled withsand while the annular space between other parts of plastic tube and soil is filled withcement/bentonite grout to seal off water from entering the region of pie>ometer tip. Thisenables the pore water pressure in the region of pie>ometer tip to be measured. In essence,standpipe pie>ometers are installed to study the pore water pressure of a specified depthbelow ground. 'owever, it suffers from the disadvantage that the response time is relativelyslow in clayey soils. Eeference is made to Darius Tremblay ;0CFCones are created in which a >one of soilsurrounding the band drains are disturbed. The compressibility of surrounding soils is


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increased and it results in the reduction of their permeability. In fact, the surrounding soils areremoulded during the installation process and the effectiveness of band drains is


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reduced. In essence, for the reduced permeability of soils around band drains, it takeslonger time to complete the consolidation process.

7. , n case a road passes through a reclai'ed land and an e.isting land- what isthe 'ain concern regarding the design o& pave'ents)

"or an e=isting land, it is anticipated that there wi ll be no ma&or settlement within thedesign life of pavement structures. 'owever, for a recently reclaimed land, even withsurcharging and installation of vertical drains, some settlement will still occur after theconstruction. If a road pavement has to be constructed connecting these two areas, specialdesign has to be made in this transition region. In order to avoid the occurrence ofdifferential settlement which may damage the pavement structure, a transition slab may bedesigned to accommodate such movement ;5. S. D. (wong ;0CCA


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touch the sea(ed)

Silt curtains are impermeable vertical barriers e=tending from the seawater surface to itsdesigned depth. The curtains are held in a vertical position by the carrier float on their topand a curtain weight at their bottom. # tension cable is designed at the carrier float to resist

stresses incurred by currents. Doreover, the silt curtains are anchored to the seabed to holdthem in the designed configuration.

In essence, the depth of silt curtains should not be so long and touch the seabed because thebottom segment of the silt curtains would be trapped inside the newly accumulated sediment,thus resulting in sinking of the curtain. !onsequently, it is difficult to remove these sunkencurtains. Doreover, reversal tidal and current actions may cause the movement of bottomregion of curtains which stir up the settled suspensions and induce additional turbidity.

/ What are the &unctions o& slip $oints in (lockwork seawalls)

Slip &oints are &oints which are formed through a complete vertical plane from the cope levelto the toe level of seawalls. These &oints are designed in blockwork seawalls to cater forpossible differential settlements between ad&acent panels of seawalls. The aggregates insidethe half-round channels in slip &oints allow for the vertical movements induced by differentialsettlement and at the same time providing aggregate interlocking forces among ad&acentpanels of seawalls to link the panels in one unit against the lateral earth pressure e=erted onseawall.

Besides, slip &oints provide a path for the relief of water pressure developed and allow thelateral movement ;e.g. contraction< due to seasonal variations.

ote? "or details of slip &oints, reference is made to !):: Standard :rawing o. !77F!.


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Chapter B ! Water etaining Structure and Waterworks

1. When designing a water storage tank- should 'ove'ent $oints (e installed)

In designing water storage tanks, movement &oints can be installed in parallel with steelreinforcement. To control the movement of concrete due to seasonal variation oftemperature, hydration temperature drop and shrinkage etc. two principal methods in designare used? to design closely spaced steel reinforcement to shorten the spacing of cracks,thereby reducing the crack width of cracksL or to introduce movement &oints to allow a portionof movement to occur in the &oints.

ets take an e=ample to illustrate this. "or 7m long tanks wall, for a seasonal variation of 8degree plus the hydration temperature of 7o!, the amount of cracking is about F.Fmm. Itcan either be reduced to 7.mm with close spacing or can be absorbed by movement &oints.#nyway, the thermal movement associated with the seasonal variation of 8o! is commonlyaccounted for by movement &oints.

"or water-retaining structure like pumping stations, the crack width requirement is even morestringent in which 7.6mm for severe and very severe e=posure is specified in BSF771. It turnsout to a difficult problem to designers who may choose to design a heavy reinforcedstructure. *bviously, a better choice other than provision of bulky reinforcement is to allowcontraction movement by using the method of movement &oints together with sufficientamount of reinforcement. "or instance, service reservoirs in Kater Supplies :epartmentcomprise grids of movement &oints like e=pansion &oints and contraction &oints.

2. What is the crack pattern induced (% h%dration due to internal restraint)

ets take a circular column as an e=ample to illustrate this.

Khen the temperature is rising, the inner concretes temperature is higher than outerconcretes temperature and the inner concrete is e=panding. This induces pressure to theoutside and the induced compressive stress will result in formation of radial cracks near thesurface of concrete.

Khen the temperature drops, the concrete at the outside drops to surrounding temperaturewhile the concrete at the central region continues to cool down. The contraction associatedwith inner concrete induces tensile strains and forms cracks tangential to the circular radius.

;/What is the purpose o& adding cooling pipes or even using cold water &or concrete inconcreting operation)

#l l these measures aim at reducing the placing temperature and reducing thermal cracksinduced during concreting of massive pours. Since the final concreting temperature shouldbe the ambient temperature, reducing the initial placing temperature will also lower the peakhydration temperature. Therefore, the temperature difference between the hydration peakand the ambient temperature is reduced accordingly and subsequently the thermal effect toconcrete structure can be reduced by controlling the placing temperature.


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concreting operation)

To answer this question, one must fully understand the effect of formwork on the temperatureof concreting structure. Kithout doubt, with better insulation of structure by timber formwork,the overall rise of temperature and hence the peak hydration temperature is also increased.

'owever, for a well-insulated structure, the temperature gradient across concrete element isreduced. Therefore, the use of well-insulated formwork ;like timber< increases the ma=imumtemperature and reduces the temperature gradient across the structure at the same time.'ence, whether steel or timber formwork should be used to control thermal cracking isdependent on the restraints and the si>e of section.

I f the section under consideration is thick and internal restraint is the likely cause to thermalcracking, then timber formwork should be used. *n the other hand, i f e=ternal restraint is themain concern for thermal cracking, then steel formwork should be used instead.

5. What is the i'portance o& critical steel ratio in calculating ther'alrein&orce'ent)

The fulfillment of critical steel ratio means that in construction &oints or planes of weakness ofconcrete structure, steel reinforcement will not yield and concrete fails in tension first. This isimportant in ensuring formation of more cracks by failure of concrete in tension, otherwisefailure in steel reinforcement would produce a few wide cracks which is undesirable.

6. ,n selection o& waterstop- shall engineers use plain du'(4(ell t%pe or center4(ul( t%pe)

The plain dumb-bell type is used for &oint locati

200 Questions and Answers on Practical Civil Engineering Works _ICE_July 2007

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Transcript of 200 Questions and Answers on Practical Civil Engineering Works _ICE_July 2007