Geo technical Past paper

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/THE INSTITUTION OF ENGINEERS, SRI LANKAPART III EXAMINATION -DECEMBER 2012305 - GEOTECHNICAL ENGINEERINGTime allowed: 03 hoursAnswer any five questions

Question 1:(a) The concrete retaining wall shown in Figure QI retains a dry sand backfill. The angleof shearing resistance of the sand was found to be 35 degrees. As shown, there is asurcharge loading on the surface of the backfill. The bulk unit weight of the fill is 20.0kN/m3 and the unit weight of concrete is 24 kN/m3. Calculate the lateral active earthpressure. (4 marks)(b) Determine the factor of safety against overturning of the wall about its base (neglectpassive earth pressure effects at the toe). (5 marks)(c) If the foundation soil has a safe bearing capacity of250 kN/m2, determine the factor ofsafety against bearing capacity failure.

(6 marks)(d) Determine the factor of safety against sliding (neglect passive earth pressure effects atthe toe). Assume that the same sandy soil exists below the foundation. (5 marks)30kPaI I I I I l I l l m m 1 1 1 I 1 1 1 1 1 1 1 1 1 1 11 I1 1 1 1 1 1 1 I 1 1 1 I 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 11 1 1 I 1 1 1 1 1 1 1 1 I 1 1 1 1 1 1 1 1 1 1 1 1 1 11 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

~ - : ( > J 1 > ; ; ' ' 'P ' : ' ' ' i i I(_:,~,l.::\~,:; - : ~ ~ ~ ~ - ) ~ ~ ~ : n': ~,~Jt...r.'\.~--.~; . t;:?;~)r ; . .,/ ; i :-~. .I..'(J '.~If - , < ) . . 111~: 'it , > f .~ . ~, ~~1I \ : ;r~'': i.~ ..e :~'1tj~:~~~~:tft .. ' . .t't'.l-:-~' .':- ~t ~ ' r ( - : - t ~ . ,s : . _ .; J ~ f ,-. ..~r:.~:i5'~~

w

a~

2.00 m

Figure Q I

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.:

Question 2:A consolidation test was conducted on an undisturbed sample obtained from a normallyconsolidated clay stratum that is 9 m thick, and it is found that the initial void ratio ofclay was 0.8 and its compression index C, is 0.28. The clay layer is sandwichedbetween two sand layers. The in-situ effective stress at the mid depth of the clay layeris SO kN/m2 and a building causes a stress increase of 60 kN/m2 at the mid depth of theclay layer through its raft foundation. The coefficient of consolidation is found to be 0.8m2/year. The secondary compression index Ca is 0.02.

(a) Calculate the primary consolidation settlement due to the building(8 marks)

(b) Estimate the time taken for 90% of primary consolidation to be completed. T90=0.848 ~mu~(c) What is the secondary consolidation settlement fifty years after the completion of the

building?(4 marks)

(d) Calculate the total consolidation settlement fifty years after the completion of thebuilding.

(2 marks)Question 3:

(a) The expression for the factor of safety of a soil slope using ordinary method of slices isgiven as,

~-p F = -11=1 (c ALn.+Wncas an tan q ; )~~ (wnsin nwith the usual notations.

The unit weight of soil is 18.S kN/m3, c' = 10 kN/m2 and o' = 18.For the slope given in Figure Q 3 find the factor of safety against sliding for the trialslip failure AC using ordinary method of slices.Use the details given in Table Q 3 for the calculations.o 18 m (12 marks)

A

I~~:-- -I I\, ~..........I \,


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Table Q 3Slice v: (kN/m) an (deg) ~Ln m)No.

1 22.4 70 2.9242 294.4 54 6.8033 435.2 38 5.0764 435.2 24 4.3765 390.4 12 4.0906 268.8 0 4.0007 66.58 -8 3.232

(b) Discuss the advantages of using methods of slices over the friction circle method foranalyzing circular failure surfaces in c - soil.

(4 marks)(c) In Sri Lanka, landslides are a common form of natural disasters, especially during

periods of high rainfall.a. Explain the factors which cause landslides.b. Discuss three measures for mitigating those slope failures.

(4 marks)

Question 4:(a) Propose the most appropriate laboratory shear tests to be conducted in order to obtain thedesign shear strength parameters of the following applications

(3 marks)i. Shear strength parameters of clay-concrete interface for the estimation of long-term

stability of a concrete gravity retaining wall.ii. Estimation of ultimate bearing capacity of a vertically loaded shallow foundation onsaturated clay.

iii. Stability of the downstream slope of an earth dam under steady state seepage,(b) An interface shear test was conducted in the direct shear apparatus to estimate the

interface shear strength parameters of a clay-steel interface for the design of a sheet pileretaining wall. Three tests were conducted at 50 kN/m2, 100 kN/m2 and 200 kN/m2normal stresses without allowing any pore water pressure generation. The results ofinterface shear tests are given below. Estimate the interface shear strength parameters.

(9 marks)Sample Normal stress Shear stress at failure

(kN/m2) (kN/m2)1 50 232 100 413 200 77

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(c) A normally consolidated clay sample was subjected to a series of stress conditions asgiven below. First the sample was allowed to consolidate at an all-round cell pressureof 100 kN/m2 Then a deviator stress was applied under drained conditions. When thedeviator load was increased toI0 kN/m2, there was a technical problem in the cellpressure system and the cell pressure starts to reduce slowly. The sample failed whenthe cell pressure decreased to 60 kN/m2 Deviator stress at the time of failure wasrecorded as 120 kN/m2. Assume that the sample remained saturated, and there was nogeneration of pore water pressure throughout the test.

i. Draw the effective stress path (q versus p' plot) for the test(4 marks)

ii. Determine the effective strength parameters of the soil(4 marks)

Question 5:a) It is proposed to construct a square footing of 2m x 2m at a depth of 1.5 m below ground

level. The subsurface consist of uniform soil up to a considerable depth. Soil strengthparameters are estimated as c l = 5 kN/m2 and / = 30. The above footing will besubjected to a vertical load of 450 kN at the center with a moment of 90 kNm as shown inFigure Q-5 (a). Maximum expected water table at the site would be 2.0 m below theground surface.Determine the ultimate bearing capacity and the factor of safety of the footing againstshear failure using Hansen's equation

(12 marks)450 kN{?kNm 1.5 m

2m, I,2m 2m,

lG.W.T

Figure Q5aHansen's equation is given by,s. = cN cscdJcgcbc + qN qsqdqiqgiq +~ B rejJ N ysydy iygybyWhere,rejJ = (2 H - Dw) ~ ~ r.; + ;~ (H - D ,.)2

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Where,H = 0.5xBtan 45+12)D; = Depth to water table below the footing vet = Wet unit weight of soil up to D;r =Submerged unit weightYe f f = Effective unit weight

(b) The subsurface details of a site where there is a proposal to construct a hotel is given inFigure Q5b. The subsoil in this area consists of a fine sand layer of thickness about 3.5 mfollowed by a soft clay layer of thickness 4 m. Soft clay is underlain by a dense sand layerof considerable thickness. Individual pad footings are proposed to support the superstructure. A typical 2m x 2m square footing at a depth 1.5m will carry a net load of 600kN. Water table is present at 2m below the ground surface.Estimate the expected immediate settlement of the footing using the method proposed byParry (1971).

(8 Marks)BH-l

Depth (m) SPT. . . . - -~ l.5m ,12m ~2m 1m , Ywel = 15.5kN / m

- ~ :l.5m Fine sand

Ysal = 16.5kN / m3

4m Soft clay YSal = 15kN / m

1m -I- 4J. 2m -l-5

3mI54m 35m 36m 47m 36

Course sand Ysal = 18kN / m3 8m >50Figure Q b

Parry's (1971) settlement equation

s = 200qB CDCwCrNWhere,sqBCD

=Settlement in mm2= Bearing pressure in MN/m

= width of the footing in meters= factor for excavation

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Cw = factor for the influence of water tableCr =Factor for thickness of compressible layer under foundation

Question 6:As a junior engineer of ABC Construction Company, you are asked to perform the followingcalculations for a pile design, where a 0.6m diameter driven pile has to be considered for thesite conditions shown in Figure Q 6. Soil properties for respective layers are given in Table Q6.1.(i) Compute the skin friction in loose sand and clay layers considering that the dense sand

layer is providing only the end bearing. You may consider the critical depth Z c ) as 3.0 m.Plot the skin friction distribution.

(8 Marks)(ii) Compute the end bearing capacity in dense sand.

(4 Marks)(iii) Calculate the maximum allowable load this pile can carry with a factor of safety of3.

(2 Marks)(iv) If the design-working load for this pile is 150 kN, comment on the performance and the

capacity.(2 Marks)

(v) After the construction of the piles, it is proposed to make a fill of2.0m thickness to raisethe ground level. What would be the anticipated negative skin friction acting on the piledue to the above fill?

(4 Marks). - - - -

4m t1 \

6m

1m ~ L-

Loose sandGWT

Clay

Dense sand

Figure Q-6

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Bearlng-capadty factors for the Meyerhof, Hansen, and Veslc bearing ..capacity equationsNote that N, and Nf are the same for all three methods; subscripts identify author for Ny~ N~ Nf NyOO NY 1 t Ny V} Nt/Ne 1an ~(l- si n ~)l0 5.14 1.0 0 .0 0 .0 0.0 0.195 0 .0 0 05 6.49 1.6 0.1 0 .1 0.4 0.242 0.14610 8.34 2.5 0.4 0.4 1.2 0.296 0.24115 10.97 3.9 1.2 1.1 2 6 0.359 0.294

20 14.83 6.4 2.9 2 9 5.4 0.431 0.31525 20.71 10.7 6 .8 6.8 10.9 . 0.514 0.31126 22..25 11.8 7.9 8.0 12.5 0..533 0.30828 25.79 14.7 10.9 lJ.2 16.7 0.570 0.29930 30.13 18.4 15.1 15.7 22.4 0.610 0.28932 35.47 23.2 20.8 22.0 302 0.653 02.7634 42.14 29.4 28.7 31.1 41.0 0.698 0.26236 50.55 37.7 40.0 44.4 56.2 0.746 0.24738 61.31 48.9 56.1 64.0 77.9 0.797 0 .23140 75.25 64.1 79.4 93.6 109.3 0.852 0,21445 133.73 134 ,7 200.5 262.3 271.3 1.007 0.17250 266.50 318.5 5{)7.4 871.7 761.3 1.195 0.131 - IT + 2 as limit when -- + (J'.

Table 1.2(a) Sbape and depth factors for Hansen's expression-- ~-~~.~.-,Shape factors Depth factors. 8 for '=O d~ = OAk for '::::OC =0.2lf

de = 1.0+OAkN 8 k = DI B for DI B S 1e = 1.0+-q .-,Ne .

k(radians):::: tan-I DIB) for DI8 >1

B dq =1+2 tan'(l- sin')2 kq ::::1.0+-, sin,L

B ~ 0.6 dr ::::1.00r = 1.0- O.4lf

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c f 4< ; 3.)lco 3e< ;oco.21 0>c oo 1w

0 0 2 4 6 8 10DISFi g 5.2 7 Excavation correct ion ia cio t, CD , a fte r Pa rry (1971)

-Ow I 8 ) 1 : '~ . Y . . . . . . . . . . . . ' II. JIll .'III '. ' I- IJI I IIII. JI I III '

If 0 < Dw < D DwCw =1+ D + O.75B

If D < Dw < D + 28 C =1+Dw(2B+D-Dw)w 2B (D + O.75B )

~o10~~~~1i~:~ () .s i I I~ 0 7 1c UE ~c () .S 10 )5 2 '()

T IB

F ig . 5 .2 8 C o r re c t i o n f a c to r f o r t h i c k n e s s o f c o m p re s s ib le m a t e r i a l , a f t e rP O ll Y 1 9 7 1

I l f D W> D +28 Cw = 1.00 ICorrectionfactor Cwfor the influence of water table after Parry, 1971)

-- ....... ~ .._......,.............1-- ......-- -.... 'w ---.----Loose sand Clav Dense sandUnit weight (kN/llf') 17.0 18 19.0Undrained cohesion (kPa) 40Friction angle,cj)' 28 38

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C O r l i


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I Interface friction angle 8 2 U 3 UNote: Lateral earth pressure coefficient,K can be assumed as l-SinTable Q 6,2: Adhesion factors-Undrained strength a(kPa)

1 5 9 5

1 81 5 6 52 62 5 5 53 5

~ \O,.....CDO'l~45'

Vn tuc s 01 N c~ g ~~ 2~gg~ sN

10

,

N~~ ~ 1-~ I ./k- I ~ ./V

N : ; V :/ V/ v ...../ Ny . . . . .. . ~V/Vq/. ,. VI Y I

J/1l11V I

-& 40'Ie 35'e. . :; 30'.

C 7 I,S 25':;(>s: 20''0

OJ~ is-

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