Transcript Slide 1
Kuat Geser Tanah (Shear Strength) (Source : CIL-610 Foundation Engineering) oleh: A. Adhe Noor PSH, ST., MT Staf Pengajar Program Studi Teknik Sipil Jurusan Teknik Fakultas Sains dan Teknik Universitas Jenderal Soedirman Strength of different materials Steel Tensile strength Concrete Soil Compressive strength Shear strength Complex behavior Presence of pore water Shear failure of soils Soils generally fail in shear Embankment Strip footing Failure surface Mobilized shear resistance At failure, shear stress along the failure surface (mobilized shear resistance) reaches the shear strength. Shear failure of soils Soils generally fail in shear Retaining wall Shear failure of soils Soils generally fail in shear Retaining wall Mobilized shear resistance Failure surface At failure, shear stress along the failure surface (mobilized shear resistance) reaches the shear strength. Shear failure mechanism failure surface The soil grains slide over each other along the failure surface. No crushing of individual grains. Shear failure mechanism At failure, shear stress along the failure surface () reaches the shear strength (f). Mohr-Coulomb Failure (in terms of total stresses) Criterion f c tan Friction angle Cohesion f c f is the maximum shear stress the soil can take without failure, under normal stress of . Mohr-Coulomb Failure (in terms of effective stresses) Criterion f c' ' tan ' ' u ’ Effective cohesion f c’ ’ u = pore water pressure Effective friction angle ’ f is the maximum shear stress the soil can take without failure, under normal effective stress of ’. Mohr-Coulomb Failure Criterion Shear strength consists of two components: cohesive and frictional. f c' ' f tan ' f ’ c’ ’f tan ’ frictional component c’ ’f ' c and are measures of shear strength. Higher the values, higher the shear strength. Determination of shear strength parameters of soils (c, or c’, ’) Laboratory tests on specimens taken from representative undisturbed samples Most common laboratory tests to determine the shear strength parameters are, 1.Direct shear test 2.Triaxial shear test Other laboratory tests include, Direct simple shear test, torsional ring shear test, plane strain triaxial test, laboratory vane shear test, laboratory fall cone test Field tests 1. 2. 3. 4. 5. 6. 7. Vane shear test Torvane Pocket penetrometer Fall cone Pressuremeter Static cone penetrometer Standard penetration test Laboratory tests Field conditions A representative soil sample z vc hc hc vc Before construction vc + D hc hc vc + D After and during construction z vc + D Laboratory tests Simulating field conditions in the laboratory vc 0 0 0 hc 0 Representative soil sample taken from the site hc hc vc + D vc hc vc vc Step 1 Set the specimen in the apparatus and apply the initial stress condition Step 2 Apply the corresponding field stress conditions Direct shear test Schematic diagram of the direct shear apparatus Direct shear test Direct shear test is most suitable for consolidated drained tests specially on granular soils (e.g.: sand) or stiff clays Preparation of a sand specimen Porous plates Components of the shear box Preparation of a sand specimen Direct shear test Preparation of a sand specimen Leveling the top surface of specimen Pressure plate Specimen preparation completed Direct shear test Test procedure P Steel ball Pressure plate Porous plates S Proving ring to measure shear force Step 1: Apply a vertical load to the specimen and wait for consolidation Direct shear test Test procedure P Steel ball Pressure plate Porous plates S Proving ring to measure shear force Step 1: Apply a vertical load to the specimen and wait for consolidation Step 2: Lower box is subjected to a horizontal displacement at a constant rate Direct shear test Shear box Dial gauge to measure vertical displacement Proving ring to measure shear force Loading frame to apply vertical load Dial gauge to measure horizontal displacement Direct shear test Analysis of test results Normalforce(P) Normalstress Area of crosssectionof thesample Shear resistancedevelopedat thesliding surface (S) Shear stress Area of crosssectionof thesample Note: Cross-sectional area of the sample changes with the horizontal displacement Direct shear tests on sands Shear stress, Stress-strain relationship Dense sand/ OC clay f f Loose sand/ NC clay Expansion Compression Change in height of the sample Shear displacement Dense sand/OC Clay Shear displacement Loose sand/NC Clay Direct shear tests on sands Shear stress, How to determine strength parameters c and Normal stress = 3 Normal stress = 2 f3 f2 f1 Normal stress = 1 Shear stress at failure, f Shear displacement Mohr – Coulomb failure envelope Normal stress, Direct shear tests on sands Some important facts on strength parameters c and of sand Sand is cohesionless hence c = 0 Direct shear tests are drained and pore water pressures are dissipated, hence u = 0 Therefore, ’ = and c’ = c = 0 Direct shear tests on clays In case of clay, horizontal displacement should be applied at a very slow rate to allow dissipation of pore water pressure (therefore, one test would take several days to finish) Shear stress at failure, f Failure envelopes for clay from drained direct shear tests Overconsolidated clay (c’ ≠ 0) Normally consolidated clay (c’ = 0) ’ Normal force, Interface tests on direct shear apparatus In many foundation design problems and retaining wall problems, it is required to determine the angle of internal friction between soil and the structural material (concrete, steel or wood) P Soil S Foundation material f ca ' tan Where, ca = adhesion, = angle of internal friction THE END