Question

QUESTION 3 (30 marks) A soil profile at a site is shown in Figure 1. A soil specimen is taken from the middle of the clay layer and subjected to a consolidation test, and following properties are reported. • Moisture content of clay = 15% • Specific gravity of soil grains = 2.7 OCR=3.0 • Compression Index (Cc)=0.5 • Recompression Index (Cr)=0.04 a) Calculate the effective vertical stress at the middle of the clay layer (10 marks) b) Calculate the pre-consolidation pressure (6 marks) c) A compacted fill placed on the ground surface will increase the stress at the middle of clay layer by 150kPa. What would be the final consolidation settlement? Dividing the clay layer into sublayers is NOT required. (10 marks) d) If the OCR of clay is X, would the expected settlement of clay due to the fill layer be higher or lower than that obtained from part c above? (Give reasons. No need to show calculations) (4 marks) Water table Ground Level Thompson ******** We Sanat Ysat=20.4kN/m 2.Om Clay Bedrock Figure 1

X=22

Answer 1

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  Solution:- 22n sand Y5ut = 20.9 kr/m3 2m Data given :- - thickness of sand layer x=22m - thickness of clay layer = 2 m moisture content of clay /w) = 15% f Ysut = 21.0 kN/m 3 specific gravity of soil graim = 2.7 - OCR = 3.0 e compression index ce=0.5 1. clay Recompression indere (r=0.04 (a) effective verticle stress at mid layer of clay (5) 6 - Ysub sand X Hsand + Ysubclay Xa Clay Ysubsand - Submerged unit weight of sand = Ysat - Yo Ysubelay - Submerged unit weight clay - Yser - Yw yw = 9.81 kW/my - (21-981)x2 + (a + -34%) x = 5.72an/ - [b] Dicies Past maximum stress (Pre consolidation prayure) Present existing stress 3. = Pre consolidation pressure - 2564 77 kN/m2 =) Preconsolidation presure = 1770.31 kW/m2/
  
  (c) . void ratio of day - eos were = o.se » Teo= 0.405) 1 → Due to compacted fill increase in ventiche strees B A5 = 150 kPa tre. Nowe total final pressure acting at mid layer of clay = 150 + 256 77 = (406.77 kr/m 2 + final pressure ü less than fre consolidatien pressure e hence soil will remain overconsolidated. → Formula for settlement of overcomeolidoted soil » An= G H o logiolo ) Ho = 2m, lo=0:405, 6250.00, x51 = 906.77 kr/m2 Itlo 6 =256.77 kN/m2 ' 406.77 0.04 xam 256.77 - An= 1400405 → AH = 0.011376 m An= 11. 376mm 1
  (d).- if the O.C.R OF THE clay layer is x( i.e. 22) which is higher than previous value of O.C.R hence clay will still remain over consolidated and there is no change in the formula and the value which is used  for settlement (i.e. Cr, final effective stress, present existing effective stress and the void ratio and also thickness of clay layer) therefore there is no change in settlement. it will remain same as above.