GEOTECHNICAL ENGINEERING ECG 503 LECTURE NOTE 07 TOPIC : 3 GEOTECHNICAL ENGINEERING ECG 503 LECTURE NOTE 07 TOPIC : 3.0 ANALYSIS AND DESIGN OF RETAINING STRUCTURES

LEARNING OUTCOMES Learning outcomes: At the end of this lecture/week the students would be able to: Understand natural slope and made engineered soil slope assessment which include rainfall induced failure and role of suction.

Types of Retaining Structures TOPIC TO BE COVERED Types of Retaining Structures Sheet Pile Wall – Cantilever and Anchored Sheet Pile

2.1 Introduction and overview LATERAL EARTH PRESSURE Introduction & Overview 2.1 Introduction and overview Retaining structures such as retaining walls, basement walls, and bulkheads are commonly encountered in foundation engineering, and they may support slopes of earth mass. Proper design and construction of these structures require a thorough knowledge of the lateral forces that act between the retaining structures and the soil mass being retained.

Retaining walls are used to prevent the retained material from assuming its natural slope. Wall structures are commonly use to support earth are piles. Retaining walls may be classified according to how they produce stability as reinforced earth, gravity wall, cantilever wall and anchored wall. At present, the reinforced earth structure is the most used particularly for roadwork

3 basic components of retaining structure Facing unit – not necessary but usually used to maintain appearance and avoid soil erosion between the reinforces. Reinforcement – strips or rods of metal, strips or sheets of geotextiles, wire grids, or chain link fence or geogrids fastened to the facing unit and extending into the backfill some distance. The earth fill – usually select granular material with than 15% passing the no. 200 sieve.

Component of E.R. Wall

Retaining Wall EARTH RETAINING STRUCTURES Types of Retaining Wall The various types of earth-retaining structures fall into three broad groups. Gravity Walls Embedded walls Reinforced and anchored earth

Masonry walls EARTH RETAINING STRUCTURES Gravity Walls Gravity Walls Gabion walls Crib walls RC walls Counterfort walls Buttressed walls

EARTH RETAINING STRUCTURES Gravity Walls Unreinforced masonry wall

EARTH RETAINING STRUCTURES Gravity Walls Gabion wall

EARTH RETAINING STRUCTURES Gravity Walls Crib wall

Types of RC Gravity Walls EARTH RETAINING STRUCTURES Gravity Walls Types of RC Gravity Walls

EARTH RETAINING STRUCTURES Embedded Walls Embedded walls Driven sheet-pile walls Braced or propped walls Contiguous bored-pile walls Secant bored-pile walls Diaphram walls

Types of embedded walls EARTH RETAINING STRUCTURES Embedded Walls Types of embedded walls

EARTH RETAINING STRUCTURES Reinforced and Anchored Earth Reinforced and anchored earth Reinforced earth wall Soil nailing Ground anchors

Reinforced earth and soil nailing EARTH RETAINING STRUCTURES Reinforced and anchored earth Reinforced earth and soil nailing

Stability of Rigid Walls EARTH RETAINING STRUCTURES Stability Criteria Stability of Rigid Walls Failures of the rigid gravity wall may occur due to any of the followings: Overturning failure Sliding failure Bearing capacity failure Tension failure in joints Rotational slip failure In designing the structures at least the first three of the design criteria must be analysed and satisfied.

States of Equilibrium Hydrostatic Pressure and Lateral Thrust LATERAL EARTH PRESSURE Types of Lateral Pressure States of Equilibrium Hydrostatic Pressure and Lateral Thrust Earth Pressure at Rest Active Earth Pressure Passive Earth pressure

Horizontal pressure due to a liquid LATERAL EARTH PRESSURE Types of Lateral Pressure Hydrostatic pressure and lateral thrust Horizontal pressure due to a liquid

Earth pressure at rest LATERAL EARTH PRESSURE Earth Pressure at Rest A z σv σh = Ko σv A B If wall AB remains static – soil mass will be in a state of elastic equilibrium – horizontal strain is zero. Ratio of horizontal stress to vertical stress is called coefficient of earth pressure at rest, Ko, or Unit weight of soil = γ

LATERAL EARTH PRESSURE Earth Pressure at Rest Earth pressure at rest .. cont.

Active earth pressure LATERAL EARTH PRESSURE Active Earth Pressure A Plastic equilibrium in soil refers to the condition where every point in a soil mass is on the verge of failure. If wall AB is allowed to move away from the soil mass gradually, horizontal stress will decrease. This is represented by Mohr’s circle in the subsequent slide. Unit weight of soil = γ z σv σh B Earth pressure at rest

σz σX = Ko σz σz σx’A ACTIVE EARTH PRESSURE (RANKINE’S) (in simple stress field for c=0 soil) – Fig. 1 σz σX = Ko σz ø Ko σz σz σx’A

LATERAL EARTH PRESSURE Active Earth Pressure Based on the diagram : (Ka is the ratio of the effective stresses) Therefore : It can be shown that :

Active pressure distribution LATERAL EARTH PRESSURE Active Earth Pressure Active pressure distribution z zo

LATERAL EARTH PRESSURE Active Earth Pressure Active pressure distribution Based on the previous slide, using similar triangles show that : where zo is depth of tension crack For pure cohesive soil, i.e. when  = 0 :

Active pressure distribution LATERAL EARTH PRESSURE Active Earth Pressure Active pressure distribution z For cohesionless soil, c = 0

2.2.4 Passive earth pressure LATERAL EARTH PRESSURE Passive Earth Pressure 2.2.4 Passive earth pressure A If the wall is pushed into the soil mass, the principal stress σh will increase. On the verge of failure the stress condition on the soil element can be expressed by Mohr’s circle b. The lateral earth pressure, σp, which is the major principal stress, is called Rankine’s passive earth pressure Unit weight of soil = γ z σv σh B Earth pressure at rest

σz σX = Ko σz σz σx’P PASSIVE EARTH PRESSURE (RANKINE’S) (in simple stress field for c=0 soil) – Fig. 2 σz σX = Ko σz ø Ko σz σz σx’P

Mohr’s circle representing Rankine’s passive state. LATERAL EARTH PRESSURE Passive Earth Pressure Shear stress Normal stress C D D’ O A σp Koσv b a σv c Mohr’s circle representing Rankine’s passive state.

LATERAL EARTH PRESSURE Passive Earth Pressure Referring to previous slide, it can be shown that : For cohesionless soil :

Passive pressure distribution LATERAL EARTH PRESSURE Passive Earth Pressure Passive pressure distribution z For cohesionless soil,

LATERAL EARTH PRESSURE Passive pressure At-rest pressure Active pressure Earth Pressure Wall tilt In conclusion Wall tilt

Rankine’s Theory LATERAL EARTH PRESSURE Types of Lateral Pressure Initial work done in 1857 Develop based on semi infinite “loose granular” soil mass for which the soil movement is uniform. Used stress states of soil mass to determine lateral pressures on a frictionless wall Assumptions : Vertical frictionless wall Dry homogeneous soil Horizontal surface

LATERAL EARTH PRESSURE Types of Lateral Pressure Active pressure for cohesionless soil

Effect of a stratified soil LATERAL EARTH PRESSURE Types of Lateral Pressure Effect of surcharge Effect of a stratified soil

LATERAL EARTH PRESSURE Types of Lateral Pressure Effect of sloping surface

LATERAL EARTH PRESSURE Types of Lateral Pressure Active pressure, Passive pressure, where and

LATERAL EARTH PRESSURE Types of Lateral Pressure Tension cracks in cohesive soils

LATERAL EARTH PRESSURE Types of Lateral Pressure Effect of surcharge (undrained)

LATERAL EARTH PRESSURE Types of Lateral Pressure Passive resistance in undrained clay

LATERAL EARTH PRESSURE Stability Criteria The stability of the retaining wall should be checked against : FOS against overturning (recommended FOS = 2.0) (ii) FOS against sliding (recommended FOS = 2.0)

LATERAL EARTH PRESSURE Stability Analysis The stability of the retaining wall should be checked against : 2.3.1 FOS against overturning (recommended FOS = 2.0) Ph ∑ V A Pp .. overturning about A

Friction & wall base adhesion LATERAL EARTH PRESSURE Stability Criteria 2.3.2 FOS against sliding (recommended FOS = 2.0) Ph ∑ V Pp Friction & wall base adhesion

LATERAL EARTH PRESSURE Stability Criteria 2.3.3 For base pressure (to be compared against the bearing capacity of the founding soil. Recommended FOS = 3.0) Now, Lever arm of base resultant Thus eccentricity

Base pressure on the founding soil LATERAL EARTH PRESSURE Stability Analysis Pp Ph ∑ V Base pressure on the founding soil

LATERAL EARTH PRESSURE Stability Analysis Worked example : Figure below shows the cross-section of a reinforced concrete retaining structure. The retained soil behind the structure and the soil in front of it are cohesionless and has the following properties: SOIL 1 : u = 35o, d = 17 kN/m3, SOIL 2 : u = 30o,  = 25o , d = 18 kN/m3, sat = 20 kN/m3 The unit weight of concrete is 24 kN/m3. Taking into account the passive resistance in front of the wall, determine a minimum value for the width of the wall to satisfy the following design criteria: Factor of safety against overturning > 2.5 Factor of safety against sliding > 1.5 Maximum base pressure should not exceed 150 kPa

LATERAL EARTH PRESSURE Stability Analysis THE PROBLEM SOIL 2 2.0 m 0.5 m 0.6 m 2.9 m GWT 4.5 m SOIL 1 30 kN/m2 4.0 m

LATERAL EARTH PRESSURE Stability Analysis P1 P3 SOIL 2 2.0 m 0.5 m 0.6 m 2.9 m GWT 4.5 m SOIL 1 30 kN/m2 4.0 m P2 P4 PP W41 W3 W2 W1 P5 THE SOLUTION P6

LATERAL EARTH PRESSURE Stability Analysis Determination of the Earth Pressure Coefficients

LATERAL EARTH PRESSURE Stability Analysis

LATERAL EARTH PRESSURE Stability Analysis To check for stability of the retaining wall FOS against overturning > 2.5 (ii) FOS against sliding > 1.5 Thus it is not OK

LATERAL EARTH PRESSURE Stability Analysis (iii) For base pressure Now, Lever arm of base resultant Thus eccentricity Therefore

LATERAL EARTH PRESSURE Stability Analysis qb = 120.8 and 80.5 kPa Since maximum base pressure is less than the bearing pressure of the soil, the foundation is stable against base pressure failure. DISTRIBUTION OF BASE PRESSURE 80.5 kPa 120.8 kPa In conclusion the retaining wall is not safe against sliding. To overcome this the width of the base may be increased or a key constructed at the toe.

Group assignment NO. 1: Form a group of 6 members in each group. Your task is to write up a case study which involve a dam case failure in Malaysia and a slope failure in Malaysia. Your report shall consists of the history of each case, as examples; amount of dam in Malaysia, their purpose, operation, etc. Make sure your case study are not the same as others groups. Penalties will be given accordingly for those who ignore the warnings. Date of submission :

Group assignment NO. 2: Form a group of 6 members in each group. Your task is to write up a case study which involve a ground improvement technique. Your shall selected a real project which will consists of real soil problems and technique to overcome the problems. Make sure your case study are not the same as others groups. Penalties will be given accordingly for those who ignore the warnings. Date of submission :