Sub-Structure DesignRetaining Wall

Learning Outcome

Student will be able :To sketch and describe clearly the function of

retaining walls.To perform stability check for retaining wall.To describe design procedure for retaining

wall

Main FunctionTo resist that force without excessive movement

The selection of the most appropriate type of retaining wall based on the height of soil to be retained

Active and Passive Earth Pressure

Active Earth Pressure

Passive Earth Pressure

Soil Stability Factors

Instability in Retaining WallExternal

Sliding - horizontal displacementSettlement and/or OverturningFailure of wall base (allowable soil pressures exceeded)Curved surface behind and under the wall (soil shear failure)Internal Deformability of the wall material (wall section failure) Failure of an individual anchor of tiedback walls

Sliding failureDue to sliding along the bottom of the wall.

The frictional passive earth resistance is inadequate.

By shear surface immediately adjacent to the bottom of the wall.

Shallow shear failure

Due to excessive shearing stresses – curved.May have same conditions as sliding failures.

Deep -seated shear or base failure

Due to excessive shearing stresses – curved – deep.May occur when there is a deposit soil, such as clay, underlying a firm deposit.

Settlement FailureExcessive wall movement - compression of the soil on which a wall is founded.

When toe pressure is significantly greater than heel pressure, results in excessive forward/outward tilting. Underlying soft deposit – compression behind the wall due to the weight of an approach cause to tilt backward/ inward

Building MaterialsBricksCoarse StonesConcrete BlocksReinforced Concrete

Types of Retaining Walls

Mass / gravity retaining wallThese can be constructed from mass concrete, brickwork or stonework.

Massive and heavyWithout reinforcementNot to withstand tension

Mass and gravity retaining walli) Concrete (gravity) wall Up to 3 m

Mass and gravity retaining wallii) Masonry (gravity) wall From bricks, blocks or stones or rock Brick walls- Simple- For low height ( 3 m)

Mass and gravity retaining wallii) Masonry (gravity) wall Rubble walls

- Medium height ( 6 m)-Normally use limestones

Mass and gravity retaining wallii) Masonry (gravity) wall Stone walls

- Simple- For low height ( 3 m)

Mass and gravity retaining walliii) Gabions

Free-draining walls - filling large baskets with broken stone.Baskets are made of galvanised steel mesh, woven strips, plastic mesh, bamboo slats, nylon or polypropylene. A typical basket is rectangular about 50 cm by 15 cm.

Mass and gravity retaining walliii) Gabions

Mass and gravity retaining walliv) Crib Walls

Wood, steel or pre-cast concrete constructed in the form of a series of interlocking units.

Filled with loose earth and crushed rocks to allow water to flow.

Good for static earth up to 6.3m.

Mass and gravity retaining walliv) Crib Walls

Reinforced retaining walli) Concrete CantileverReinforced concrete is used to

withstand tension and structural size.

Wall footing was used as cantilever to counter soil pressure on wall.

Up to 8 m high.Above 8 m uneconomical

Reinforced retaining walli) Concrete Cantilever

Reinforced retaining wallii) Internal and External Counterfort Wall

Has intermittent supports either from the soil side (tensile) or from the outside (compression).

Thin vertical concrete webs.

Spacings = ¼ to ½ height.

8-14m high.

Embedded Wall Contiguous or interlocking individual

piles or diaphragm wall-panels to form a continuous structure.

May be cantilever, anchored or propped.

Embedded WallTypes:Sheet Pile - driving steel sheets into a slopeSoldier / King Pile –constructed of wide flange steel H sections spaced about 2 - 3 m apart, driven prior to excavationBored Pile -a soil replacement rather than a soil displacement method (to minimise vibration)Diaphragm - a water tight barrier

Embedded Wall

Sheet Pile

Embedded Wall

Soldier Pile

Embedded Wall

Bored Pile

Embedded WallPre-stressedEconomical for over 4 m high.Pre-compression technique in the masonry cross section - flexural tensile capacity and enhanced resistance to lateral loading.Diaphragm wall – 50 to 100 cm thick and up to 7m, extending to the excavation bottom.Construction of shallow concrete or steel guide walls – excavate using thin-grab clamshell – pump in Bentonite slurry to provide temporary support – lower prefabricated reinforcing cage - replaced slurry by trémie concrete –proceed to the next panel.

Embedded Wall

Reinforced Soil Walls• To provide a stable earth

retaining system- Reinforced Soil- Soil Nailing

Reinforced Soil WallsReinforced soil walls are constructed of compact backfill.Strips or ties, made from galvanised steel, are embedded to absorb the tensile forces within the fill.The strips are attached to a thin outer skin to retain the face.The face is composed of precast concrete panels for durability and aesthetic reasons.Normal height 15m; length 0.8-1.2 height

Reinforced Soil Walls

This kind of reinforced soil walls, including the facing, reinforcements, reinforced fill and the back of wall.

Reinforced Soil Walls

Environmental wall: This method of reinforced wall is used to retain a great quantity of soil from collapse.

Reinforced Soil Walls

The general elements of a reinforced soil wall.

Reinforced Soil Walls

Discrete Panels: The facing of reinforced soils are constructed sequentially in order to build the walls consistently

Reinforced Soil Walls

This types of facing enables the building of walls that can be easily curved in plan, and well adapted to retaining structures.

Soil NailingClosely spaced steel bars, called "nails," driven into a slope and grouted.

Significantly increase the apparent cohesion capability to carry tensile loads.

Appears similar to reinforced fill but: - nails are inserted directly into an existing earth not installed with the fill.

- commences at the top level and proceeds downwards; for reinforced fill the lower reinforcements are loaded first (by layers).

Soil Nailing

Soil Nailing

Soil Nailing

Soil Nailed Retaining Wall

Hybrids Types Retaining Wall

Types of Hybrids retaining wallsAnchoredFor stabilizing and controlling erosion of steeply sloped areas of the lot.

Hybrids Types Retaining Wall

Hybrids Types Retaining WallTailed GabionGabion elements fitted to geogrid 'tails‘ extending into supported soil.Wire mesh placed in the fill behind the wall can increase the ability of the wall to resist overturning and sliding force.A vertical skin of gabions was anchored to the backfill using metal strips.Concrete block GabionConcrete block facing units fitted with geogrid 'tails' extending into supported soil.

Hybrids Types Retaining Wall

Drain holes Water (rain) can add

weight to the soil and more pressure to the wall.

The condition is aggravated when there is soil movement.

Water must be allowed to flow freely through the wall by using installed drainage

Parallel to the wall Through the wall Concrete apron

Backfill Material:Should be granular and free draining. i.e. sand and stone Avoid clay or clayey silt!

Selection of Walls depends on:

Height of wallSurcharge loadSoil conditionAvailability of Space for constructionGround water and rainfall density Availability of raw materials Aesthetic value Design life Consequences of failure

Design of Retaining Wall

Fill level

Friction force, FF Wt

FP A

z

FA

Design of Retaining Wall

Fill level

Wb

Ww

Friction force, (1.0Gk+1.0Vk)

Ws

A

z

Design of Retaining Wall

Fill level

A

h1

h2

FA = 0.5 pah

FP= 0.5 pph

FF = µW1

The factor of safety against this type of failure occurring is normally taken to be at least 1.5 FF + FP ≥ 1.5 FA

Design of Retaining Wall

Fill level

y q

x

Structure self weight, Wb

Friction force, (1.0Gk+1.0Vk)

Soil Vertical Load, Ws

FA

A

Ww

x1

Design of Retaining Wall

Fill level

y

q

x

Gk(1.0Gk+1.0Vk)

Soil Vertical Load, Vk

Resultant Force, Hk

A

D/2 D/2

P1P2

Soil Bearing Pressure

Design of Retaining Wall

Fill level

y

q

x

Gk(1.0Gk+1.0Vk)

Soil Vertical Load, Vk

Resultant Force, Hk

A

D/2 D/2

P1P2

Soil Bearing Pressure

Drainage pipe

Retaining Wall - Example

Fill level

4.5 m

0.8 m

0.4 m

2.2 m

Given:Unit Weight of Concrete = 24 kN/m3

Ka = 0.33 = 0.45

0.4 m

SolutionKa = 0.33 = 1700 kg/m3 g = 10h = 4.9 mpa =kagh = 0.33 x 1700 x 10 x 4.9 = 27489N/m2

= 27.49 kN/m2

FA = 0.5 pah = 0.5 x 27.49 x 4.9 = 67.35 kNWw(wall) = 0.4 x 4.5 x 24 = 43.2 kNWb(base) = 0.4 x 3.4 x 24 = 32.64 kN

Solution …..Continue….Ws = 2.2 x 4.5x 17

= 168.3 kNWT = Ww + Wb+ Ws

= 43.2 + 32.64 + 168.3 = 244.14 kN

FF = µ WT

= 0.45 x 244.14= 109.86 kN

i) Sliding CheckAssume passive pressure force (Fp) = 0. Hence factor of

safety against sliding isFF/FA = 109.86 /67.35

= 1.63 > 1.5 Satisfactory

Solution …..Continue….

ii) Checking Overturning ∑M res = Ww x (0.8 + 0.4/2) + Wb x (3.4/2) +Ws x (0.8 +0.4

+2.2/2) = (43.2 x 1.0) + (32.64 x 1.7) + (168.3 x 2.3) = 485.78 kNm

∑M over = 1/3x FA x 4.9 = 1/3 x 67.35 x 4.9 = 110 kNm ∑M res = 485.78/ 110 = 4.42 ≥ 2 OK ∑M over

Solution …..Continue….Bearing Pressure AnalysisMoment about center of the base

M = FA 4.9/3 + Ww(x – x1) -Ws(q – D/2)

= N = WT

M/N =

D/6 = 3.4/6 = 0.58M/N D/6, therefore eccentricity lies within middle third of the base P1 = (N/D) + (6M/D2) < allowable bearing pressure

= P2 = (N/D) - (6M/D2)

=