Question

A group of piles is required to support a central bridge pier carrying a load of 4500 kN. The depth of water above the river bed is 3 m, and the soil is a normally consolidated clay (a 0.4, unit weight 18 kN/m3 and Cu/av 0.5) which is at least 30m thick. The client informs you that 300 x 300 mm concrete piles are being considered, although the maximum length that can be shipped in (due to space restrictions around the bridge) is 25 m In addition, each of the piles needs to have 6 m of length protruding above the river bed. Assuming the load includes the weight of the piles above the river bed, and that the factor of safety on the ultimate load for a single pile is 2.0 the client has asked you to: [10] Determine the minimum amount of piles required to carry this load, assuming single pile failure occurs. (a) [10] Calculate the capacity of the pile group determined in part (a), assuming a centre-to-centre spacing of 2m and the occurrence of block failure (state any assumptions you have make in terms of spacing). (b)

Answer 1

Solution 2ater Lenh Layor 1 Rivor bed Lay er 2 Pile 19M Nomally consdlidated day xIe.4. Y=1ekNm Som Lood P 4500 KN FRatu sed o ad Pu 2x4500z 9000KN Fogx P Cros-seation ob Pile A-3oox 300mm Length d25m Capa &ty de ile. QU= Op ca Qus Ap 2put As 2f9 base where Ab sseat onal area o Pil at 300x300 qxtotmm2 Ab resistanu 2pu-unit boint bearug con suli doted ely = 2pu Cup Ne FOrr nurm 2, Ne- q (As por skompton for decp foun ECRct stess base t A (3Yw) 9XY)-(22xYw Bxq.812+09x10-(22x q.01 V 155.GI KNM2 5x155.61 = F0 aH.2 Cup=0.5x ' qxt7.801 00.245 KNM2 2pu= - 6 Pot be oingQup= 700.245x 9xt0tx1o 62.03 KN

Surface anea As = bid a cntadt uth sail (4x300)12 00 mmx 19m 12xt9=22-0 m2 As 2s unit skn fiction eststanre Con salid ated cloy For Cu= undrauned cuheston n the embeddd length o Bile lay er 2, FOr Oat bottom sh uret bed- Avelage effectve stess orer the length 155 G 3.805 KHme .. 5 56 KHm 2 .4 x 34.g02 2s s ZAs 15.5G1X 22. 354.79 KN capadty Pfle Q Qup 3 = G3.02 354.79 417.01 KN /P No biles 21-5= 22 4/4-91 Mmmom amovt ot pile to ay tArs ood 22 battere ot bile. 24 pile by To piraduce a are considened (6x4) 2m 2m 2m These are souare pile +. - 00

Width ot tile grop P 3xSt 300mm 3x2+ 9.3 TV 6.3 m Length of le graup, L a base vt pile -section d Ab=Cmss- 64.89 m2 10.3x 6.G a Mile Centacith sail A9= Swrfate NLea 2CL+B)x= 2 C10.3+6-3 )x 19 = Capacty ob bile goup ogGpyt (Lup) Ab +(2s As p.Nc) Ab+G)AS (1x 38.90x630 8 .Gox9 x 64-39)+ 1, forr pile gravp 69,974.09 KN