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

LEARNING OUTCOMESLearning 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.

TOPIC TO BE COVEREDTypes of Retaining StructuresSheet Pile Wall Cantilever and Anchored Sheet Pile

LATERAL EARTH PRESSUREIntroduction & Overview2.1 Introduction and overviewRetaining 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

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

Gravity Walls Gravity WallsMasonry walls

Gabion walls

Crib walls

RC walls

Counterfort walls

Buttressed wallsEARTH RETAINING STRUCTURES

Gravity WallsUnreinforced masonry wallEARTH RETAINING STRUCTURES

Gravity WallsGabion wallEARTH RETAINING STRUCTURES

Gravity WallsCrib wallEARTH RETAINING STRUCTURES

Gravity WallsTypes of RC Gravity WallsEARTH RETAINING STRUCTURES

Embedded Walls Embedded wallsDriven sheet-pile walls

Braced or propped walls

Contiguous bored-pile walls

Secant bored-pile walls

Diaphram wallsEARTH RETAINING STRUCTURES

Embedded WallsTypes of embedded wallsEARTH RETAINING STRUCTURES

Reinforced and Anchored Earth Reinforced and anchored earthReinforced earth wall

Soil nailing

Ground anchors EARTH RETAINING STRUCTURES

Reinforced and anchored earthReinforced earth and soil nailingEARTH RETAINING STRUCTURES

Stability Criteria Stability of Rigid WallsFailures 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 failureIn designing the structures at least the first three of the design criteria must be analysed and satisfied.EARTH RETAINING STRUCTURES

LATERAL EARTH PRESSURETypes of Lateral Pressure Hydrostatic Pressure and Lateral Thrust

Earth Pressure at Rest

Active Earth Pressure

Passive Earth pressure

States of Equilibrium

LATERAL EARTH PRESSURETypes of Lateral Pressure Hydrostatic pressure and lateral thrustHorizontal pressure due to a liquid

LATERAL EARTH PRESSUREEarth Pressure at Rest Earth pressure at restIf 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, orUnit weight of soil =

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

LATERAL EARTH PRESSUREActive Earth Pressure Active earth pressureEarth pressure at restzvhABPlastic 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 Mohrs circle in the subsequent slide.Unit weight of soil =

ACTIVE EARTH PRESSURE (RANKINES)(in simple stress field for c=0 soil) Fig. 1X = Ko zzzKo zxA

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

LATERAL EARTH PRESSUREActive pressure distributionActive Earth Pressure

LATERAL EARTH PRESSUREActive pressure distributionActive Earth PressureBased on the previous slide, using similar triangles show that : where zo is depth of tension crackFor pure cohesive soil, i.e. when = 0 :

LATERAL EARTH PRESSUREFor cohesionless soil, c = 0Active pressure distributionActive Earth Pressure

LATERAL EARTH PRESSUREPassive Earth Pressure2.2.4 Passive earth pressureEarth pressure at restzvhABIf the wall is pushed into the soil mass, the principal stress h will increase. On the verge of failure the stress condition on the soil element can be expressed by Mohrs circle b.The lateral earth pressure, p, which is the major principal stress, is called Rankines passive earth pressure Unit weight of soil =

PASSIVE EARTH PRESSURE (RANKINES)(in simple stress field for c=0 soil) Fig. 2X = Ko zzzKo zxP

LATERAL EARTH PRESSUREPassive Earth Pressure

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

LATERAL EARTH PRESSUREFor cohesionless soil,Passive pressure distributionPassive Earth Pressure

LATERAL EARTH PRESSUREIn conclusionEarth PressureWall tilt

LATERAL EARTH PRESSURETypes of Lateral Pressure Rankines TheoryAssumptions : Vertical frictionless wall Dry homogeneous soil Horizontal surface 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

LATERAL EARTH PRESSURETypes of Lateral Pressure Active pressure for cohesionless soil

LATERAL EARTH PRESSURETypes of Lateral PressureEffect of a stratified soil Effect of surcharge

LATERAL EARTH PRESSURETypes of Lateral Pressure Effect of sloping surface

LATERAL EARTH PRESSURETypes of Lateral PressureActive pressure,Passive pressure,whereand

LATERAL EARTH PRESSURETypes of Lateral Pressure Tension cracks in cohesive soils

LATERAL EARTH PRESSURETypes of Lateral Pressure Effect of surcharge (undrained)

LATERAL EARTH PRESSURETypes of Lateral Pressure Passive resistance in undrained clay

LATERAL EARTH PRESSUREThe stability of the retaining wall should be checked against :(ii) FOS against sliding (recommended FOS = 2.0)

FOS against overturning (recommended FOS = 2.0)Stability Criteria

LATERAL EARTH PRESSURE Stability AnalysisPpThe stability of the retaining wall should be checked against :

2.3.1 FOS against overturning (recommended FOS = 2.0).. overturning about A

LATERAL EARTH PRESSURE2.3.2 FOS against sliding (recommended FOS = 2.0)Stability CriteriaFriction & wall base adhesion

LATERAL EARTH PRESSURE2.3.3 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 Stability Criteria

LATERAL EARTH PRESSURE Stability AnalysisBase pressure on the founding soil

Stability AnalysisLATERAL EARTH PRESSUREFigure 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 > 2.5Factor of safety against sliding > 1.5Maximum base pressure should not exceed 150 kPaWorked example :

Stability AnalysisLATERAL EARTH PRESSURETHE PROBLEM

LATERAL EARTH PRESSURE Stability AnalysisTHE SOLUTIONP6

LATERAL EARTH PRESSURE Stability AnalysisDetermination of the Earth Pressure Coefficients

LATERAL EARTH PRESSURE Stability Analysis

Sheet1

ELEM.FORCE (kN/m)TOTALL. ARM (m)MOMENT (kNm/m)

HORIZONTAL

Active

P10.271 x 30 x 216.264.573.17

P20.333 x 30 x 3.534.971.7561.20

P30.5 x 0.271 x 17 x 2 x 29.214.1738.41

P40.333 x 17 x 2 x 3.539.631.7569.35

P50.5 x .333 x (20-9.81) x 3.5 x 3.520.781.16724.25

P60.5 x 9.81 x 3.5 x 3.560.091.16770.13

SUM180.94336.50

Passive

Pp0.5 x 3 x 18 x 1.5 x 1.560.750.530.38

VERTICAL

W10.5 x 4.9 x 2458.81.75102.90

W20.6 x 4.5 x 2464.82.25145.80

W32 x 2.5 x 17 + 2.9 x 2.5 x 20 + 30 x 2.53053.25991.25

W40.9 x 1.5 x 1824.30.7518.23

SUM452.91288.55

LATERAL EARTH PRESSURE Stability AnalysisTo check for stability of the retaining wall

FOS against overturning > 2.5 (ii) FOS against sliding > 1.5Thus it is not OK

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

Stability AnalysisLATERAL EARTH PRESSURE 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 kPa120.8 kPaIn 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 :

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