Unit-2.pptx Foundation engineering unit - 2

Foundation engineering unit - 2

1. FOUNDATION ENGINEERING SVECW Shri Vishnu Engineering College for Women Dr. B. Sudhir Kumar (Assistant Professor) Civil Engineering

2. UNIT – 02 Earth Retaining Structures Stability of slopes : Types of failures, factor of safety of slopes- Infinite and finite earth slopes in sand and clay Swedish arc method, Standard method of slices Taylor’s Stability Number Stability of slopes of dams and embankments - different conditions. Earth pressure theories : Introduction- Rankine’s & Coulomb’s theory of earth pressure Culmann’s graphical method - earth pressures in layered soils. Retaining walls :– Different types - Type of Failures of Retaining Walls Stability requirements Drainage behind Retaining walls.

3. Reasons for failure of earth slopes Gravitational forces and forces due to seepage of water in the soil, progressive disintegration of the structure of the soil mass and excavation near the base. SLOPES OF EARTH ARE OF TWO TYPES 1. Natural slopes slopes exist in hilly areas 2. Man made slopes The slopes of embankments constructed for roads railway lines, canals etc. The slopes of earth dams constructed for storing water.

4. 1. Infinite slopes The term infinite slope is used to designate a constant slope of infinite extent. Slopes extending to infinity do not exist in nature. However in practice if the height of the slope is very large, it may be considered as infinite slope. 2. Finite slopes A slope of limited extent, bounded by a base and a top surface is called a finite slope. The sl o pes o f e m b a n k m ents and e a rth d am s a r e ex a m p les o f fi n ite slopes.

5. The Geotechnical Engineer should have a thorough knowledge of the various methods for checking the stability of slopes and their limitations.

6. Slope can fail due to one of the following methods Rotational failures Translational failures Compound failures Wedge failures Miscellaneous failures

7. Occurs by rotation along a slip surface by downward and outward movement of soil mass . Slip circle formed is circular for homogeneous soil and non-circular for non homogeneous soils Toe failure Occurs along surface that passes through the toe Most common failure occurs when the slope is steep and homogeneous.

8. b) Slope failure Failure surface intersects the slope above the toe This type of failure occurs when the slope angle is large and when the soil at the toe portion is strong. Base failure Failure surface passes below the toe Occurs when weak stratum lies beneath the toe Base failure

9. Occurs in infinite slopes along a long failure surface parallel to the slope . Shape of failure surface influenced by presence of hard stratum at a shallow depth below slope surface . Common in slopes of layered materials .

10. Plane failure, wedge failure or block failure Failure along an inclined plane Occurs when distinct blocks and wedges of the soil mass become separated Similar to translational failure in many aspects Wedge failure can occur in finite slopes Having two different materials Homogeneous slopes with cracks, joints or any other specific plane of weakness

11. Combination of rotational and translational failures Failure surface is curved at both ends and is plane in the middle portion Occurs normally when a hard stratum is exists at a considerable depth below the toe

12. Stability Analysis of Slopes Infinite slopes Consider an infinite slope in a cohesionless soil inclined at an angle to the horizontal as shown. Consider an element ‘abcd’ of the soil mass.

13. A long natural slope in a c-φ soil is inclined at 12° to the horizontal. The water table is at the surface and the seepage is parallel to the slope. If a plane slip has developed at a depth of 4m.determine the factor of safety. Take c = 8 KN/m 2 and γ sat = 19 KN/m 2

14. (b) An embankment 9 m high is inclined at an angle of 40 ° to the horizontal. A stability analysis by the method of slices gives the following forces per running meter: Σ Shearing forces (T) = 550 kN; Σ Normal forces (N) = 800 kN; Σ Neutral forces (U) = 316 kN. The length of the failure arc is 17 m. Laboratory tests on the soil indicate the effective values c′ and φ′ as 18 kN/m 2 and 20 ° respectively. Determine the factor of safety of the slope with respect to (a) shearing strength and (b) cohesion.

15. If the slope angle β , height of embankment H, the effective unit weight of material ɣ, angle of internal friction φ and unit cohesion c are known, the factor of safety may be determined. Taylor (1937) conceived the idea of analyzing the stability of a large number of slopes through a wide range of slope angles and angles of internal friction, and then representing the results by an abstract number which he called the "stability number". This number is designated as “S n ".

16. Stability Number is defined as S n = c / ( F c γH ) =c m / ( γH ) factor of safety = c / c m Stability number- dimensionless quantity Charts prepared indicating Stability Number and slope angle β for different values of Φ T A YLOR S T ABILITY NU M BER AND CHA R T

17. An embankment is inclined at an angle of 35° and its height is 15 m. The angle of shearing resistance is 15° and the cohesion intercept is 200 kN/m 2 . The unit weight of soil is 18.0 kN/m 3 . If Taylor’s stability number is 0.06, find the factor of safety with respect to cohesion. 2. A cutting is to be made in clay for which the cohesion is 35 kN/m 2 and φ = 0°. The density of the soil is 20 kN/m 3 . Find the maximum depth for a cutting of side slope 1 12 to 1 if the factor of safety is to be 1.5. Take the stability number for a 1 12 to 1 slope and φ = 0° as 0.17.

18. LATERAL EARTH PRESSURE Soil is neither a solid nor a liquid, but it exhibits some of the characteristics of both. One of the characteristics similar to that of a liquid is its tendency to exert a lateral pressure against any object in contact. This important property influences the design of retaining walls, abutments highways, railways, tunnels, mining and military engineering. Lateral earth pressure is the pressure that soil exerts in the horizontal direction .

19. TYPES OF EARTH-RETAINING STRUCTURES Earth-retaining structures may be broadly classified as retaining walls and sheet pile walls . Retaining walls may be further classified as: Gravity retaining walls — usually of masonry or mass concrete. Cantilever walls Counterfort walls usually of reinforced concrete. Buttress walls Sheet pile walls may be further classified as cantilever sheet pile walls and anchored sheet pile walls, also called ‘bulkheads’.

20. Gravity Retaining Wall Relies on its own weight to resist lateral earth pressure. Typically made of concrete or stone masonry. The base is wider to enhance stability. Cantilever Retaining Wall Consists of a vertical stem and a base slab with two parts: toe and heel. Uses reinforced concrete and resists earth pressure through bending moments.

21. Counterfort Retaining Wall Similar to a cantilever retaining wall but with counterforts (triangular vertical slabs) attached to the stem and base. Counterforts reduce bending moments and provide additional stability. Buttress Retaining Wall Similar to a counterfort wall but with buttresses on the outside face. Provides additional support against lateral earth pressure. Sheet Pile Wall (Cantilever Type) Thin, vertical walls driven into the ground. Typically made of steel, wood, or vinyl. Used for retaining soil in loose or soft ground conditions. Anchored Bulkhead A sheet pile wall supported by anchors or deadmen connected by rods. Provides additional stability for retaining soil, especially in deep excavations.

22. AT REST-PRESSURE ACTIVE EARTH PRESSURE PASSIVE EARTH PRESSURE Not subjected to any lateral yielding or movements Occurs when soil tends to stretch horizontally Occurs when soil tends to compress horizontally Firmly fixed at its top Not fixed at top Not fixed at top Not allowed to move laterally or rotate freely Allowed to rotate freely or move laterally Allowed to rotate freely or move laterally In elastic equilibrium In plastic equilibrium In plastic equilibrium Retaining walls with basement slab at top Bridge abutment 1. Retaining wall 1. Retaining wall Lateral earth pressures

23. RANKINE’S EARTH PRESSURE THEORY FOR COHESIONLESS SOIL 1. Active Rankine State 2. Passive Rankine State    

24. 1. A retaining wall, 6 m high, retains dry sand with an angle of friction of 30° and unit weight of 16.2 kN/m 3 . Determine the earth pressure at rest. If the water table rises to the top of the wall, determine the increase in the thrust on the wall. Assume the submerged unit weight of sand as 10 kN/m 3 .

25. Effect of Inclined Surcharge—Sloping Backfill

26. A vertical wall with a smooth face is 7.2 m high and retains soil with a uniform surcharge angle of 9°. If the angle of internal friction of soil is 27°, compute the active earth pressure and passive earth resistance assuming γ = 20 kN/m 3 .

27. COULOMB’S WEDGE THEORY Charles Augustine Coulomb (1776), a famous French scientist and military engineer. Coulomb’s theory considers the soil behind the wall as a whole instead of as an element in the soil.

28. A retaining wall is battered away from the fill from bottom to top at an angle of 15° with the vertical. Height of the wall is 6 m. The fill slopes upwards at an angle 15° away from the rest of the wall. The friction angle is 30° and wall friction angle is 15°. Using Coulomb’s wedge theory, determined the total active and passive thrusts on the wall, per lineal meter assuming γ = 20 kN/m 3 . Wall friction angle = (δ) Batter angle of the wall = (𝛼) Slope of the fill = (β) Slope of the fill = (β) =90 - 15 °