1 INTRODUCTION

This paper presents a case study of a retaining sys-tem for a deep basement excavation in Kenny HillFormation of Kuala Lumpur. The office buildingconsists of 7 storeys and 4 levels of basement and isfounded on a raft foundation. The depth of thebasement excavation varied from 14 m to 17 m. Twosides of the excavation were adjacent to the existingbuildings while the other sides were facing publicroads. Owing to the variation in subsoil conditionand cost consideration, a combined retaining systemwas adopted. The combined retaining system in-cluded tie back anchors, internal horizontal struttingand inclined-struts-and-berm systems to support theearth retaining walls consisting of contiguous boredpiles of size 600 mm to 900 mm diameters.

Figure 1. Borehore location and instrumentation layout plan.

2 DESCRIPTION OF PROJECT

The site was originally a gently hilly terrain with re-duced levels vary from about RL88m to RL118m.

The site had been flatten into platforms during thedevelopment of the adjacent properties by cut andfill. The level of the site was in the range of RL97mto RL100m after earthworks. Figure 1 shows thesite layout and survey level of the site.

Two rows of 4-storey shop offices are adjacent tothe site at the western and northern sides.These shoplots are supported by pile foundation. The shop lotsand the proposed building are divided by side lanesof about 8 m wide. The vacant land in between theshop lots at the north-western direction was beingdeveloped at the time of detailed design for the pro-posed building. The development consisted of singleto two levels of basements.

3 GEOLOGICAL SETTING AND SUBSOILCONDITIONS

The site is underlain by metasedimentary residualsoils of the Kenny Hill Formation and granitic rocks.The granitic rock boundary is about 0.6 km away,running in a north-west direction to the site. TheKenny Hill Formation belongs to CarboniferousAge, originally formed of interbedded sedimentaryrocks, namely shale, mudstone and sandstone.During the intrusion of magma, in the process offorming granitic rock, these sedimentary rock were

A basement excavation using tie back, internal horizontal strutting andinclined-struts-and-berm system

C.S. Lim, S.M. Tan & L.C. HiewSSP Geotechnics Sdn Bhd, Selangor, Malaysia

ABSTRACT: This paper presents a scheme of a retaining system with a combination of tie back anchors, in-ternal horizontal strutting and inclined-struts-and-berm systems for the basement excavation of an officebuilding located in Kuala Lumpur, Malaysia. The difficulties encountered in design and construction as wellas the performances of the retaining system are presented. The subsoil was mainly residual soil of the KennyHill Formation. The building consists of 7 storeys and 4 levels of basement, supported by a raft foundation.The area of the excavation was about 3240 m2. The depth of the excavation varied from 14 m to 17 m belowthe existing ground level. The performance of the retaining systems has been monitored by instrumentation.

metamorphosed to metasedimentary rocks namelyphyllite and schist. The residual soils formed in-situfrom the parent rock, mainly consist of sandySilt/Clay. In between these materials are layers ofclayey silty Sand and Gravel.

A total of 34 exploratory boreholes were sunkover the adjacent development site in 1983. In linewith the proposed office block, an additional soil in-vestigation consisting of 4 boreholes was conducted.The locations of the additional boreholes are shownin Figure 1. Figure 2 shows the soil profile obtainedfrom the four boreholes.

Stiff and medium dense overburden soil with SPTN values of 15 to 20 was found within the first 5 min boreholes BHA1 and BHA2. Underlying theoverburden soil was hard material having N valuegenerally greater than 100. Hard material was ex-posed at ground surface for borehole BHA4. Pres-ence of backfill material was suspected in boreholeBHA3 as the original ground level at this area wasfound to be around RL98m to RL90m. The exis-tence of some decayed wood at 11 m to 13 m deepin BHA3 substantiated this finding. The total thick-ness of backfill material including residual soil wasabout 13 m.

Figure 2. Subsoil borehole profile.

Quartzite and sandstone were encountered inboreholes BHA1 and BHA4 at 11 m and 15 m depthrespectively. Rocks were not found in boreholesBHA2 and BHA3 within termination depth of 18 m.Based on the groundwater monitoring results, therecommended ground water levels used for designwere minimum of RL90m and maximum ofRL92.5m. The basement excavation level wasRL84.0m.

4 CONTIGUOUS BORED PILE RETAININGWALL

Contiguous bored piles (CBP) were designed as thepermanent wall for the basement excavation. CBP iscost effective compared to other type of wall such asdiaphragm wall and scant pile wall.

The construction sequence was excavation instages with the successively installation of groundanchor or propping system until the lowest level.The basement raft was then cast followed by othersbasement structure components. The layout plan ofthe proposed retaining system for the project isshown in Figure 3.

Figure 3. Layout plan of retaining system.

4.1 Temporary anchor tie back system

Ground anchors was provided for excavation sidesthat faced the roads and the shop offices at the Westside as shown in Figure 3. For the excavation adja-cent to existing buildings, the horizontal length ofground anchor was limited to not more than 11.8 mto prevent hitting the foundation piles of the adjacentbuildings. Four to five rows of anchors were in-stalled for excavation depths of 14 m to 16 m.

4.1.1 Removable Ground AnchorsSince all ground anchors encroached into adjacentland, a non-conventional removable ground anchorsystem was used to facilitate the removal of anchorsafter completion of excavation. The details of theanchor system, namely non-explosive loop system,are shown in Figure 4. The difference between theloop system anchors and conventional anchors isthat the entire length of the prestressed strand is notin contact with the grout column. The strands areseparated with the grout column by poly-ethlene(PE) tube fill with grease for the purpose of removal.

The load transfer of the anchor is not through thebonding between strands and the grout, but througha loading transfer body to the surrounding ground.The load is transferred from the tendon to the load-ing body, from which the load is transmitted to thesurrounding by means of compressive action. Withthis system, high anchor retrieval rate was reported.

Detailed load transfer mechanism and descriptionof the non-explosive loop system ground anchorswere reported elsewhere (Chua & Prasanthee 1997).

Figure 4. Composition of temporary loop system anchor.

4.2 Internal diagonal strutting

Taking advantage of the shape of the excavation,internal strutting was installed at the northern cornerof the excavation as shown in Figure 3. Internalstrutting was proposed due to the limited space be-tween the excavation and the adjacent building. Inaddition, the backfill material, as revealed in BHA3,could not provide sufficient anchorage for groundanchors with length limited by the existing building.The strutting consisted of four levels for the excava-tion depth of 14 m as shown in Figure 5a and 5b.

4.3 Inclined-struts-and-berm-system

Inclined-struts-and-berm system was introducedwhere the space between the excavation and the ex-isting building was insufficient for ground anchorsinstallation. Furthermore, the use of internal strut-ting was not suitable due to long distance to the op-posite wall.

Three rows of inclined strutting were proposed.The site was first excavated with a temporary bermto foundation level for the construction of raft foun-dation. After completion of the raft foundation, thefirst row of inclined prop was installed followed bypartial excavation at the earth berm for the installa-tion of the second level strutting. The last part of theearth was removed upon the installation of the low-est row props. Plate 1 shows the construction of in-clined-struts-and-bem-system.

5 THE PERFORMANCE OF THE RETAININGWALL SYSTEM

An instrumentation scheme was proposed around theexcavation to monitor movements and settlements ofground and adjacent buildings, deflection of the re-taining wall and variation of ground water table.

The layout of the instrumentation is shown inFigure 1. Initial readings were taken before the ex-cavation work. Subsequently, readings were takenonce a week during basement excavation.

The excavation started in June 1997. The totalvolume of excavation was about 50,000m3. Raft slabwas cast in October 1998. Figures 5a and 5b showthe deflection of the wall as revealed from the incli-nometers I1 & I2 respectively.

I1 and I3 represent the deflection of wall asbraced by internal strutting system. Only I1 is pre-sented. At completion of basement structure, themaximum deflection behind the wall as revealedfrom I1 is about 103 mm at ground level and gradu-ally reduced to 4.5 mm at final excavation level.However, the excessive wall deflection in the top 2mwas due to extra loading contributed by stock pilingof construction material and traffic of constructiontrucks in the vicinity of inclinometer and thus notrepresentative. The maximum deflection of the CBPwall should be about 60 mm at the ground surface.

On the other hand, I2, represented the deflectionof wall retained by anchor tie back system. It indi-cated deflection behind the wall of 13 mm at 2 mbelow ground level and 5 mm at final excavationlevel on completion of basement construction. Thedeflection of wall was remarkably reduced by theanchor tie back system as compared to the internaldiagonal strutting system. This finding suggests thatanchor tie back system, with the use of preloading, ismore effective in preventing wall lateral movementcompared to unpreloaded internal strutting system.The lateral movement of wall braced by inclined-struts-and-berm system is not available.

Figure 5a. Lateral ground movement behind wall (I1).

It is worth to note that, the movement behind wallnotably increased from the date the excavationreached final level up to the day prior to the castingof raft slab although the construction activity wasvirtually stagnant at adjacent area. This suggeststhat the wall movement is time dependent.

Figure 5b. Lateral ground movement behind wall (I2).

The settlement profiles of the two sets of settle-ment markers are shown in Figures 6a to 6b. Thecomparison of ground settlement at all settlementmarkers revealed that the maximum settlement wasabout 2.8 mm at Marker S10. The settlement is neg-ligible as compared to deflection of walls. The set-tlement also suggested that the ground settlementdue to lowering of ground water through the perme-able CBP wall was insignificant.

Figure 6a. Settlement marker S4 to S6.

Figure 7 presents the ground water variation asrevealed from the water standpipe, namely P1, dur-ing the course of basement excavation. The waterlevel was initially about 9.1 m below ground leveland gradually reduced to 13.5 m when excavationdepth reached 14.1 m. The level was 10.6 m atcompletion of basement construction. The compari-son between the excavation level and the ground

water level suggested that the CBP wall was not ef-fective to maintain ground water level for the soilencountered at the site. However, since the soil wasrather impermeable, the problem of ground waterseepage was not serious and manageable withoutcarrying out any grouting treatment to the CBPwalls.

The water level at water standpipe was some 0.5m above the final excavation level. The water levelstarted to raise after the raft slab had been cast. Theraise in ground water level was assisted by casting ofskin wall on the CBP wall which made the wall wa-ter tight. The water level after casting of the skinwall of the lowest level of the basement was about9.7 m below the ground level which was about thesame level as the top of skin wall. However, groundwater table again reduced to a steady water table ofabout 11.0 m below ground level few weeks aftercasting of skin wall. This phenomenon may due tothe flow of ground water into the adjacent wall atwhich raft slab or skin wall had not been con-structed.

Figure 6b. Settlement marker S7 to S10.

Figure 7. Ground water variation from P1.

-16.000

-14.000

-12.000

-10.000

-8.000

-6.000

-4.000

-2.000

0.000

5-Ju

n-97

12-J

un-9

719

-Jun

-97

26-J

un-9

73-

Jul-9

710

-Jul

-97

17-J

ul-9

724

-Jul

-97

31-J

ul-9

77-

Aug

-97

14-A

ug-9

721

-Aug

-97

28-A

ug-9

74-

Sep

-97

11-S

ep-9

718

-Sep

-97

25-S

ep-9

72-

Oct

-97

9-O

ct-9

716

-Oct

-97

23-O

ct-9

730

-Oct

-97

6-N

ov-9

713

-Nov

-97

20-N

ov-9

727

-Nov

-97

4-D

ec-9

711

-Dec

-97

18-D

ec-9

725

-Dec

-97

1-Ja

n-98

8-Ja

n-98

15-J

an-9

822

-Jan

-98

29-J

an-9

85-

Feb-

9812

-Feb

-98

19-F

eb-9

826

-Feb

-98

5-M

ar-9

812

-Mar

-98

19-M

ar-9

826

-Mar

-98

2-A

pr-9

89-

Apr

-98

16-A

pr-9

8

Date

Dep

th B

elow

Gro

und

Leve

l (m

)

P1 Excavation Level

Casting of Skin Wall at 4th basement To 9.3 mbgl

Casting Of Raft Slab

-3.00

-2.50

-2.00

-1.50

-1.00

-0.50

0.000 5 10 15 20 25 30 35 40 45 50

Distance From Wall (m)

Set

tlem

ent (

mm

)

09/04/1997

11/05/1997

26/11/1997

12/09/1997

30/12/1997

27/1 /1998

17/03/1998(Comple tion ofBasement)

S4 S5

S6

-3.00

-2.50

-2.00

-1.50

-1.00

-0.50

0.00

0 5 10 15 20 25 30 35 40 45 50

Distance From Wall (m)

Set

tlem

ent (

m)

09/04/97

11/05/97

26/11/1997

12/09/97

30/12/1997

27/1/1998

17/03/1998(Completion ofBasement)

S7

S9

S8

S10

6 CONCLUSIONS

In view of the site constraints, a scheme of a com-bined retaining system, consisting of contiguousbored pile walls braced by a combination of tie backanchors, internal diagonal strutting and inclined-struts-and-berm system, was presented for the 14 mto 16 m deep basement excavation in Kenny Hillformation of Kuala Lumpur.

The performance of the retaining system was sat-isfactory as confirmed by the ground settlement be-hind the wall during basement excavation. Onlynegligible settlement of a few mm was observed.

The deflection of the wall as revealed from incli-nometers suggests that tie back system by means ofpre-stressed anchors has been effective in reducinglateral wall movement.

During basement excavation, the ground waterlevel behind the wall was drawn down to about 0.5mabove the corresponding excavation level for thatstage. This is probably due to the flow of groundwater into basement during excavation. This showsthat CBP wall is not water tight.

As the adjacent buildings are supported by pilesand the subsoil is relatively strong in most part ofthe site, ground settlement was negligible eventhough the ground water table was lowered duringexcavation.

Plate1. The construction of the inclined strutting

REFERENCES

Chua T.S. & Prasanthee R. 1997. Performance of temporaryremovable U-Turn ground anchors in Singapore, Proceed-ings of 3YGEC, Singapore.

CIRIA report 104. 1984. Design of retaining walls embeddedin stiff clay. London, United Kingdom.

Geotechnical control office publication No. 1/90. 1991. Reviewof design methods for excavation. Hong Kong.

L&M Geotechnic Sdn Bhd. 1997. Technical report on designand analysis of retaining wall system for development of 7-storey building of Damansara Height, Kuala Lumpur.

SSP Geotechnics Sdn Bhd 1996. Geotechnical design reportfor development of 7-storey building at Damansara Height,Kuala Lumpur.