key terms Compressibility of Soil Compressibility of Soil is the increase in stress caused by foundations and other loads compressing the soil layer Consolidation is the time dependent settlement of soils resulting from the expulsion of water from the soil pores One Dimensional Consolidation is the settlement restricted to vertical direction only Soil Settlement may be divided into three categories 1 Immediate or Elastic Settlement caused by the elastic deformation of dry moist and saturated soils without any change in moisture content 2 Primary Consolidation Settlement caused by a volume change in saturated cohesive soils due to expulsion of water that occupies the void spaces 3 Secondary Consolidation Settlement caused by plastic adjustment of soil fabrics It is an additional form of compression that occurs at constant effective stress Drainage Path Hdr is the longest vertical path that a water particle will take to reach the drainage surface Preconsolidation Pressure or past maximum effective stress is the maximum vertical effective stress a soil was subjected to in the past Normally Consolidated Soil is one that has never experienced vertical effective stresses greater than its current vertical effective stress Over Consolidated Soil is one that has experienced vertical effective stresses greater than its existing vertical effective stress basic settlement formula H H eo e 1 eo primary consolidation settlement Normally Consolidated Soils Sc H C c 1 eo log pf po Where H thickness of stratum Cc compression index eo initial void ratio po initial vertical effective soil stress pf final vertical effective soil stress po p Over Consolidated Soils When pf pc When pf pc Overconsolidation Ratio OCR Sc H C s 1 eo log pf po Sc H C s 1 eo log H pc po C c 1 eo log pf pc OCR pc po Where pc preconsolidation pressure Po overburden effective stress If OCR 1 the soil is normally consolidated Compression Index Cc Skempton For Remolded Clay For Undisturbed Clay Rendon Herreo C c 0 007 7 C c 0 009 10 C c 0 141G1 2 1 eo G 2 38 Nishida Swell Index Cs Rendon Herreo C c 1 15 eo 0 27 C s C c to C s 1 5 1 10 C c C c 0 0463 100 Gs E1 From the given soil profile shown the ground surface is subjected to a uniformly distributed load of 80kPa 1 Compute the compression index Use Skempton s formula for undisturbed clay 2 Compute for po at mid height of the compressible the settlement due to primary clay layer 3 Compute consolidation C c 0 009 10 0 009 42 10 0 288 sat w Gs e 1 e 9 81 2 70 0 52 1 0 52 20 78 KN m3 Po d 2 19 0 9 81 3 2 20 78 9 81 1 5 13 6 73 884 KPa Pf Po P 73 884 80 153 884 Sc H C c 1 eo log pf po 4 0 288 1 0 60 log 153 884 73 884 0 229m E2 From the figure shown a soil profile with the corresponding properties The soil is acted upon by a uniformly distributed load p 60kPa at the ground surface 1 Compute the settlement of the clay layer caused by primary consolidation if the soil is normally consolidated C c 0 009 10 0 009 48 10 0 342 Po d 3 20 9 81 8 18 9 81 16 5 4 162 09 KPa Pf Po P 162 09 60 222 09 KPa Sc H C c 1 eo log pf po 6 0 342 1 0 85 log 222 09 162 09 0 152m 2 Compute the settlement of the clay layer caused by primary consolidation if the preconsolidation pressure of clay is 230kPa Use Cs 1 5Cc C s C c 1 5 Sc H C s 1 eo 1 5 0 342 0 068 6 0 068 pf po log 1 0 85 log 222 09 162 09 0 03m 3 Compute the settlement of the clay layer caused by primary consolidation if the preconsolidation pressure of clay is 200kPa Use Cs 1 5Cc C s C c 1 5 Sc H C s 1 eo 1 5 0 342 0 068 C c 1 eo P c P o H log log P f P c Sc 6 0 068 1 0 85 log 200 162 09 6 0 342 1 0 85 log 222 09 200 0 070m secondary settlement Ss H C 1 e p log T2 T1 Where SS secondary settlement C secondary compression index ep void ratio at the end of primary consolidation eo e eo in situ void ratio Po P e Cc log T1 time for completion of primary settlement T2 time after completion of primary settlement Po E1 Assume a buried stratum of clay 1 83m thick will be subjected to a stress increase of 33 6 kPa at the center of clay The magnitude of the pre construction soil overburden pressure Po 48 kPa at the center of the clay layer A laboratory compression test indicates that the clay has a pre consolidation pressure of 72 kPa Compression index is 0 30 and the value of swell index is 0 05 Void ratio of clay is 1 50 1 Compute the primary settlement Pf Po P 48 33 6 81 6KPa Pf Pc P c P o H C c 1 eo log P f P c C s 1 eo log Sc H Sc 1 83 0 05 1 1 50 log 72 48 1 83 0 30 1 1 50 log 81 6 72 0 018 m 2 If full consolidation settlement primary compression settlement will require approximately 8 years compute the settlement due to secondary compression of clay over a period of 20 year time span Assume secondary compression index 0 008 e Cc log Po P Po 0 30 log 48 33 6 48 0 069 e p eo e 1 50 0 069 1 431 Ss H C 1 e p log T2 T1 1 83 0 008 1 1 431 log 20 8 0 002m the effects of secondary compression ST Sc Ss 0 018 0 002 0 020m 3 Estimate the total settlement to be expected over a 20 year time span considering E2 A soil profile is shown in the figure A uniformly distributed load of 50 kPa is applied at the ground surface The clay is normally consolidated 1 Compute the compression index C c 0 141G1 2 1 eo G 2 38 0 141 2 70 1 2 1 0 80 2 70 2 38 0 177 2 Compute the primary consolidation settlement Po d 3 16 6 18 9 81 3 19 08 9 81 124 95KPa Pf Po P 124 95 50 174 95 KPa Sc H C c 1 eo log pf po 6 0 177 1 0 80 log 174 95 124 95 0 086 m e Cc log Po P Po 0 177 log 124 95 50 124 95 0 026 e p eo e 0 80 0 026 0 774 Ss H C 1 e …