L2 Soil Stress and Pore Water Pressure

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<ul><li><p>GEO-MECHANICS(CE2204)Soil Stress and Pore Water PressureLecture Week No 2Mdm Nur Syazwani Noor RodiLINTON UNIVERSITY COLLEGESCHOOL OF CIVIL ENGINEERING</p><p>*</p></li><li><p>TOTAL VERTICAL STRESS The total vertical stress (v) acting at a point below the ground surface is due to the weight of everything lying above i.e. soil, water, and surface loadingTotal vertical stresses are calculated from the unit weight of the soilAny change in total vertical stress (v) may also result in a change in the horizontal total stress ( h) at the same pointThe relationships between vertical and horizontal stress are complex (v h)</p></li><li><p> TOTAL VERTICAL STRESS in homogeneous soil</p><p>Depth, z</p></li><li><p> TOTAL VERTICAL STRESS below a river or lake</p><p>zzw</p></li><li><p>z1Soil1z2Soil2Soil3z3 TOTAL VERTICAL STRESS in multi-layered soil</p></li><li><p> TOTAL VERTICAL STRESS with a surface surcharge load</p><p>zVery wide surcharge, q (kN/m2)</p></li><li><p>PORE WATER PRESSUREThe water in the pores of a soil is called pore water. The pressure within this porewater is called pore water pressure (u)The magnitude of pore water pressure depends on: </p><p>the depth below the water table the conditions of seepage flow </p></li><li><p>zPORE WATER PRESSURE under hydrostatic conditions (no water flow)</p></li><li><p>EFFECTIVE STRESS CONCEPT(Terzaghi, 1923) where = Total Vertical Stress= Effective Stress= Pore Water Pressure</p><p>*</p></li><li><p> VERTICAL EFFECTIVE STRESSES</p><p>z</p></li><li><p> EXAMPLE 1</p><p>Plot the variation of total and effective vertical stresses, and pore water pressure with depth for the soil profile shown below 4mGRAVELY SANDSANDSAND GRAVEL4m5mkN/m3kN/m3kN/m3kN/m3</p><p>Water Table2m</p></li><li><p> EXAMPLE 2</p><p>The soil layers on a site consists of:0 4 m Gravel-sand (sat= 2038 kg/m3; B 1957 kg/m3)4 9 m Clay (sat= 1835 kg/m3) </p><p>Draw an effective stress and total stress profile between 0 9m, when the water table is 1m above the top of the clay </p></li><li><p> EXAMPLE 3</p><p>On a certain site a surface layer of silty sand is 4m thick and overlies a layer of peaty clay 7m thick, which in turn is underlain by impermeable rock. Draw effective and total stress profiles for the following condition:Water table at the surfaceWater table at a depth of 5m, with the silty sand above the water table saturated with capillary water</p><p>Unit weight:Silty Sand = 18.5 kN/m3Clay = 17.7 kN/m3</p></li><li><p> EXAMPLE 4A confined aquifer comprises a 5m thick of sand overlain by a 4m thick layer of clay and underlain by impermeable rock. The unit weight of the sand and clay respectively are 19.6 kN/m3 and 18.4 kN/m3. Determine effective overburden stress at the top and bottom of the sand layer, when the levels of the water in a standpipe driven through the clay into the sand layer are:at ground surface1.5m below the ground surface3.0m below the ground surface1.5m above the ground surface3.0m above the ground surface</p><p>and hence comment on the effect of changing water table</p></li><li><p> EXAMPLE 5</p><p>A sediment settling lagoon has a depth of water of 4m above the clay base. The clay layer is 3m thick and this overlies 4m of a medium sand, which in turn overlies impermeable rock. Calculate the effective stresses at the top of the clay and at the top and bottom of the second layer under the following condition:Initially, before any sediment is depositedAfter a 3m layer of sediment of silty fine sand has been depositedAfter draining the lagoon down to base level, with same thickness (3m) of sediment still in place</p><p>Unit weight: Sand = 20 kN/m3; Clay = 18 kN/m3; Sediment = 16 kN/m3</p></li><li><p> EXAMPLE 6</p><p>Plot the variation of total and effective vertical stresses, and pore water pressure with depth for the soil profile shown below for the following condition:initially before constructionimmediately after constructionfew days after constructionmany years after construction. </p><p>4mSANDCLAYkN/m3</p><p>Water Table2mSurface surcharge, q (100 kN/m2)</p></li><li><p>SHORT TERM &amp; LONG TERM STRESSESInitially before construction Stress distribution profile at its original stageImmediately after constructionThe immediately effect after the construction is an increasing in the pore water pressure loading is too rapid and not allow any significant out flow of pore water and the soils are in an UNDRAINED stageFew days after constructionFew days after the construction, the out flow of pore water takes place at the Sand layer due to its high permeability and the sand is in DRAINED stage. i.e. excess PWP is dissipated at the Sand layer whereas Clay Layer is in contrastMany years after construction After many years, excess PWP will dissipated in clay layer despite its low permeability and the soils are in drained stage</p><p>*</p><p>*</p></li></ul>