SHEAR STRENGTH I YULVI ZAIKA

MATERI Keruntuhan mohr coulomb, stress paths, kuat geser tanah non kohesif dan kohesif, evaluasi kuat geser di lapangan, tegangan normal dan tegangan geser pada sebuah bidang

Strength of different materials Steel Concrete Soil Tensile strength Compressive strength Shear strength Complex behavior Presence of pore water

Shear strength of soil Soil cannot resist tension Soil can resist compression. For excessive compression failure occurs in the form of shearing along the internal surface within the soil Structural Strength of soil = f ( soil s shear strength) Shear Strength Soil s ability to resist sliding Important for foundation design Lateral earth pressure calculations Slope stability analysis

Shear Strength due to sliding friction Normal Force Sliding Force

What is Shear Strength? Shear strength in soils is the resistance to movement between particles due to physical bonds from: A.Particle interlocking b.. Atoms sharing electrons at surface contact points C.. Chemical bonds (cementation) such as crystallized calcium carbonate

Influencing Factors on Shear Strength The shearing strength, is affected by: soil composition,: mineralogy, grain size and grain size distribution, shape of particles, pore fluid type and content, ions on grain and in pore fluid. Initial state: State can be describe by terms such as: loose, dense, over--consolidated, normally consolidated, normally consolidated, stiff, soft, etc. Structure : Refers to the arrangement of particles within the soil mass; the manner in which the particles are packed or distributed. Features such as layers, voids, pockets, cementation, etc, are part of the structure.

Shear failure of soils Soils generally fail in shear Strip footing Embankment 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 Criterion (in terms of total stresses) f c tan Cohesion c f Friction angle f is the maximum shear stress the soil can take without failure, under normal stress of.

Mohr-Coulomb Failure Criterion (in terms of effective stresses) c f tan Effective cohesion c f Effective friction angle u u = pore water pressure 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 f c f tan f tan frictional component c c f

NOTE c and are measures of shear strength. Higher the values, higher the shear strength.

Mohr Circle of stress Soil element 1 1 3 3 q q q cos sin 3 1 3 1 3 1 Resolving forces in and directions, 3 1 3 1

Mohr Circle of stress 3 1 3 1 3 1 3 1 3 1

Mohr Circle of stress 3 1 3 1 3 1 3 1 3 1 P D = Pole w.r.t. plane q (, )

Mohr Circles & Failure Envelope Failure surface c tan f X Y X Y Soil elements at different locations Y ~ stable X ~ failure

Mohr Circles & Failure Envelope The soil element does not fail if the Mohr circle is contained within the envelope GL Y c c Initially, Mohr circle is a point c c +

Mohr Circles & Failure Envelope As loading progresses, Mohr circle becomes larger GL Y c c c.. and finally failure occurs when Mohr circle touches the envelope

Orientation of Failure Plane 1 Failure envelope 3 q 3 (90 q) (, f ) 1 3 q 1 3 1 P D = Pole w.r.t. plane Therefore, 90 q = q q 45 + /

Mohr circles in terms of total & effective stresses v v u X h = X h + X u effective stresses total stresses h v h u v or

Failure envelopes in terms of total & effective stresses v v u X h = X h + X u If X is on failure Failure envelope in terms of effective stresses effective stresses Failure envelope in terms of total stresses total stresses c c h v h u v or

Mohr Coulomb failure criterion with Mohr circle of stress X v = 1 h = 3 ( c 1 3 )/ X is on failure 3 1 Therefore, 3 c cot sin c cot ( 1 3 )/ 1 Failure envelope in terms of effective stresses 1 3 effective stresses

Mohr Coulomb failure criterion with Mohr circle of stress 3 c cot sin 1 1 3 ( ) ( ) sin c cos 1 3 1 3 ( ) 1 sin ( 1 sin ) c cos 1 3 1 3 ( 1 sin ) cos c ( 1sin ) ( 1sin ) 1 3 tan 45 c tan 45

If c=0 then sinφ = σ 1f σ 3f σ 1f +σ 3f Or σ 1 = 1+sinφ = σ 3 1 sin φ tan 45 + φ Failure envelope in terms of effective stresses 3 ( 1 3 )/ 1 effective stresses

exercises A cylindrical soil (sand) sample was subjected to axial principal stress (1) and radial principal stress (3). The soil can not support additional stress when 1 = 300kPa and 3 =100 kpa. Determine the friction angle and the inclination of the slip plane to the horizontal.

Stress Point v stress point t stress point h X ( v - h )/ h v s ( v + h )/ v h s t v h 31

Stress Path During loading t Stress path adalah tempat titik tegangan Stress path s Stress path adalah cara yang lebih mudah digunakan untuk melihat Proses pembebanan dari awal sampai mencapai selubung keruntuhan 3

SELUBUNG KERUNTUHAN t failure tan -1 (sin ) c c cos stress path s During loading (shearing). 33

FIELD TEST VANE SHEAR Measure undrained shear strength of soil Suitable for very soft clay to medium stiff clay Principle of equipment operation : vane pushed and rotated The vane shear equation : s u = πd 3 T h d +1 3 Where: T : Maximum torque H : height of vane d : diameter of vane

Standard Penetration Test (SPT)