Retaining Wall (R1)

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Transcript of Retaining Wall (R1)

Sub-Structure Design Retaining Wall

Learning OutcomeStudent 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 Function To

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 Factors1.

Soil Condition Compacted and loose earth, sand, loam, clay etc. Unit weight (kN/m3), repose (friction) angle & cohesion (kPa) Weather Soil Pressure Water, building weight and traffic

2. 3.

Instability in Retaining WallExternalSliding - horizontal displacement Settlement and/or Overturning Failure 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 Materials Bricks Coarse Stones Concrete Blocks Reinforced Concrete

Types of Retaining Walls

Mass / gravity retaining wall These

can be constructed from mass concrete, brickwork or stonework. Massive and heavy Without reinforcement Not to withstand tension

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

Mass and gravity retaining wall ii) Masonry (gravity) wall From bricks, blocks or stones or rock

Brick walls - Simple - For low height ( 3 m)

Mass and gravity retaining wall ii) Masonry (gravity) wall Rubble walls- Medium height

( 6 m) -Normally use limestones

Mass and gravity retaining wall ii) Masonry (gravity) wall

Stone walls - Simple - For low height ( 3 m)

Mass and gravity retaining wall iii) GabionsFree-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 wall iii) Gabions

Mass and gravity retaining wall iv) Crib WallsWood, 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 wall iv) Crib Walls

Reinforced retaining wall i) 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 wall i) Concrete Cantilever

Has intermittent supports either from the soil side (tensile) or from the outside (compression). Thin vertical concrete webs. Spacings = to height. 8-14m high.

Reinforced retaining wall ii) Internal and External Counterfort Wall

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 slope Soldier / King Pile constructed of wide flange steel H sections spaced about 2 3 m apart, driven prior to excavation Bored 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-stressed Economical 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 trmie 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

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

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 WallTypes of Hybrids retaining walls Anchored For 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 on:

Walls depends

Height of wall Surcharge load Soil condition Availability of Space for construction Ground water and rainfall density Availability of raw materials Aesthetic value Design life Consequences of failure

Design of Retaining WallFirst Fundamental Stages Stability Analysis Soil Pressures

- The active pressure (pa) is given by pa = kazwhere = unit weight of soil (kN/m2)

Fill level

ka = coefficient of active pressurez = height of retained fillzFA

FP

A Friction force, FF Wt

Design of Retaining WallFirst Fundamental Stages Stability Analysis Soil Pressures

- The passive pressure (pp) is given by pp = kpzwhere = unit weight of soil (kN/m2)

Fill level

kp = coefficient of passive pressurez = height of retained fillWs Ww

z

A Friction force, (1.0Gk+1.0Vk)

Wb

Design of Retaining WallFirst Fundamental Stages Stability Analysis Sliding

FA = 0.5 pah

Fill level

FP= 0.5 pphFF = W1h1

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

h2

Design of Retaining WallFirst Fundamental Stages Stability Analysis Overturning

Mres 2 Mover Mover = 1/3 FA z Mres = Ww x x1 + Wb x x + Ws x q Ifx1

Fill level

FA Soil Vertical Load, Ws

failed use of heel beam can be considered or resize the structure.y

Ww

q

A

x

Friction force, (1.0Gk+1.0Vk)

Structure self weight, Wb

Design of Retaining WallSecond Fundamental Stages Bearing Pressure Analysis For serviceability, thus all =1.0 Similar to foundation subjected to

Fill level

Eccentric Vertical load & Overturning Moment Vertical Load N = WTResultant Force, Hk

Moment about center of the base M = FA y + Ww(D/2 x) -Ws(q D/2) Ify q

Soil Vertical Load, Vk

M/N D/6, thus eccentricity lies within middle third of the base, hence:

A

P1 = (N/D) +(M/I) X (D/2) Where I = D3/12, thus P1 = (N/D) + (6M/D2) P2 = (N/D) -