hi, i need help for the calculation of the flow rate for question 1

What should be the correct workings/answers? The above figure shows an excavation where sheet -pile cutoff walls are installed on both sides of the excavation to keep water out from the excavation floor. The vertical and horizontal permeabilities of the sand are equal at 5 x10 -4 m/s . The vertical and horizontal permeabilities of the silt are 5 x10 -6 m/s and 2 x10 -5 m/s respectively. The vertical and horizontal permeabilities of the clay are equal at 5 x10 -8 m/s . The dry unit weight of the sand is 16 kN/m 3. The saturated unit weight of the silt is 18 kN/m 3. The saturated unit weight of the clay is 23 kN/m 3. Water is taken as 10kN/m 3. (a) Draw a suitable flownet for the problem. [8 marks] (b) Calculate the flow rate of water infiltrated to the excavation floor per day (and per metre into the plane of paper). [3 marks] (c) What are the total stress, pore pressure and effective stress at Point X in the above figure, under seepage conditio n? [6 marks] Student’s Working: (a) (b) Vertical and horizontal permeabilities of sand and clay are the same. Hence, α = 1.0 Sand permeability is the highest, therefore water flows faster and governs flow, k = 5x10 -4 m/s. No. of flowtubes, Nf = 3 No. of equipotentials, Nd = 8 Head difference = 4 m q = ( 5x10 -4 ) x (3/8) x 4 = 7.5 x10 -4 m3/s/m (= 64.8 m3/day/m ) (c) Taking silt -clay interface as datum. Total head = Pressure head + Elevation head = 11 + 3 = 1 4m (no seepage). Total head = 1 4 – 2.5 = 11 .5m (seepage) Pressure head = Total head – Elevation head = 11 .5 – 3 = 8 .5m Pore pressure = 8 .5 x 10 = 8 5 kPa Total stress = 2 x 16 + 9 x 18 = 1 94 kPa Effective stress = 1 94 – 85 = 109 kPa Qu estion 2 [8 marks] Scenario: You now have to solve the below question from scratch. (d) Further excavation commenced and the soil level within the excavation wall was lowered by another 2 metres, as shown in the below figure. Draw a new flownet for the new excavation condition. Without calculations, discuss the impact of the further excavation on stability of the sheet -pile walls based on your new flownet. [8 marks]