shear strength-2021-2022.pdf

Shear Strength of the Soil Definition: the ability of soil deposit to support an imposed loading or of a soil mass to support itself is governed by shear strength of the soil. shear strength of cohesive and cohesionless soils shear strength and related deflection (or σ – Є) of...

Shear Strength of the Soil Definition: the ability of soil deposit to support an imposed loading or of a soil mass to support itself is governed by shear strength of the soil. shear strength of cohesive and cohesionless soils shear strength and related deflection (or σ – Є) of a foundation Is conventionally studied in laboratory by testing samples obtained from site as well as filed tests. Shear strength refers to the resistance along a plane that passes between or a long particles surfaces (but not through the particles). It is one of most important characteristics. The problems related shear strength: a) Stability of slopes (eg., hills slides, cuts, embankments, earth dams). b) Ultimate bearing capacity of a soil for foundation design. c) Lateral pressure against retaining walls, sheeting or bracing. d) Friction developed by piles. The shear strength developed either due to compression or tension forces. In many cases compression. When the stress applied on soil exceeded the shear strength of soil, the sliding of soil particles upon each other which cause change in volume, part of soil may move or displaced w.r.t. to surrounded media and this is called the shear failure of soil. Factors affecting shear strength of soils: a) Water content b) Type and amount of loads (static or dynamic) c) Soil structures and disturbance effects. d) Type and conditions of test is very important factor on the shear strength Type of soils w.r.t shear strength: e) Cohesive soils: as clayey soil which posses a bonding among particles. f) Cohesionless soils: as sand which have only friction factors among particles Components of shear resistance of soils : (1) Friction resistance: this developed due to friction that creates among particles under the applied stress this consist from: - Friction due to sliding of particles. - Friction due to rolling of particles. - Friction due to interlocking (2) Cohesion force: this is force developed in clayey soils only among particles which come from bonding. (c) They called shear strength components (c & Ø) Mohr-Coulomb Theory The relation between shear strength and friction and cohesion as Mohr- Coulomb law S  τ  c  σ n.tanυ where: S,τ  shear strength of soil c  cohesion of soil τ shear stress υ  angle internal friction σ n  normal stress act on soil As normal stress (σ ) increase as shear n strength increase due to more dense obtained. Ø C normal stress σn Both normal and shear stresses act on planes passing through a point in an element of mass that has been subjected to external loading. Small element containing point “A” which located in soil mass σ  stresses normal to a plane (normal stress) τ  stresses parallel to a given plane (shear stress) Nine components of stresses define the state of stress at a point (fig.a) There are 3 planes at right angles where only normal stresses acting (fig.b). The stresses (σ , σ , σ ) are called principal stresses and the 1 2 3 planes act on are called principal planes. σ major, σ intermediate , σ minor principal stresses act on major, 1 2 3 intermediate, & minor principal planes, where shear stresses=0 In soil study, it can be considered a biaxial stresses act as shown below Stress on soil Element in Two-Dimensional  1 3 S m a ‫)‪ ( Mohr’s Envelope‬يمثل الحد الفاصل بين حالة الفشل وحالة االستقرار في التربة‪.‬حيث جميع‬ ‫النقاط تحت الـ)‪ (failure envelope‬يمثل حالة االجهاد اقل من تحمل التربة‪.‬اما النقاط على الخط‬ ‫فهي تمثل حالة االجهاد االقصى التي تتحمل التربة وان التربة على وشك الفشل‪.‬وال توجد حالة للتربة‬ ‫فوق خط الفشل‪ ،‬ألن التربة قد انهارت لزيادة االجهاد عن قوة تحملها‪.‬‬ If we omitted the pore water pressure then the design, then the design and analysis based on the total stress this cause the failure because of over-estimated the bearing capacity. Advantages: 1- simple & easy 2- the results can be analyzed easy 3- small time Disadvantages: 1. Only suitable for cohesive soil 2. It is not simulate the field conditions because there is no confined pressure which gives result less than the actual 3. It cannot be control the drainage condition or measured the pore water pressure 4. It is measured of apparent cohesion but not the angle of internal friction Factors affecting the shear strength of soils: It is included the factors which affect the cohesion and angle of internal friction. 1-Cohesion: It is the true strength of the soil that produced from the attraction forces (chemical or electrical) among particles. The forces depend on the contact points in unit area in the plane of shear and also depends on distance among particles, the water content affect cohesion force. As the w/c increases, the number of contact points reduces in unit volume, so cohesion decreases. A- Cohesion cohesionless (granular soils) ≈0 B- As the clay percent increases in soil the cohesion increases C- As the w/c increases the cohesion reduce 2- angle of internal friction: A- B- shape of particles: Φ Angularity > Φ rounded C- method of test affect the Φ value of clayey soils. D- % of clay: E- plasticity: