Question

Question 1 At the site of a proposed building, 12 m of sand (k = 3.5 x 10-5 m/s, e = 0.55, Gs = 2.65) overlies impermeable bedrock. A sheet pile wall is installed to a depth of 9 m to facilitate an excavation which is 6 m deep. The water table outside the excavation is at a depth of 2 m, while the excavation is pumped to keep it dry. (a) How might the value of k quoted above have been obtained? [2] (b) Construct a flow net and use it to estimate the flow rate into the excavation. [8] (c) Calculate the pore water pressure and vertical effective stress at a point either side of the wall, 1.5m above the base of the wall. (d) Investigate the likelihood of both piping and heave failures. 15)

Answer 1

Cal value of k will be obtained by using flow net. The discharge in pumping will be equal to the seepage flow of soil into the excavation. det, No. of equipotantial drop = No No of flow channle - Nt q=k. Nt.H 1 - k = q. No I 1 (b). -h.L E- – – – I -H.W.T. M No of flow channe=3 No of equipotantial duop=9 Total head for flow (H)= 6-2 = you.

od loss per equipotential drop- 9 = 0.44m. nead loss at outside wall on 1.5 m above the base of wall = 0.44x3 =1.32m. · pore water pressure outside the wale = 19-2-1-5-1.31110 - Uklant

pore water pressure inside the wall above 15m base of the wall - Pressure head at point thead loss yet to occure at that point = [3-15+2x0.447x9-8) = 83.35 km? u. Effective stress outside the wall Viru = usrw = 2.6589.81 = 16.77 kr/m² He 1.55 J = Vary x 2 + (sat)& 55-u = 16.77X2 + 90.25x5.5-41 - 103.9219/m² Effective sovess inside the wall T = VsectX 1.5-U - 20.25 1.5-23-25 - 7.125 10/m2

(y). Chitical hydraulic gradient (ie) = Oh! He - 2.657 H0.55 -0.419 The distance be the last last two equipotantials is im. .: hydraulic gradient = 0.44 = 0.44 م ده . The structure is mot safe againest piping

fos for hearing = Effective wt above the base of wall pore pressure at base of the au Pop оf bаcе од огц — 3+D-чухч ХҮo = 46.70 kN/m² Effective stress out base of wall = 72025-9.81783 = 31.32 kunt fos againest heaving= 31.32 = 0.67 <1 46.70 . Structure is not safe againest hearing