Unit13026Theory; Retaining Walls

BUILDING TECHNOLOGY

CARPENTRY

THEORY UNIT 13026

Retaining Walls

LEVEL 3 CREDIT 2

Purpose

People credited with this unit standard are able to demonstrate knowledge of retaining wall construction.

Unit. 13026 Theory; Retaining walls Page. 2

Department of Building Technology Unitec Revised 2012

CARPENTRY

RETAINING WALL

References :

Construction Technology, Volume 2 and 4. R Chudley. 1987

Cement and Concrete Ass. of NZ Bulletin IB OlO (1983)

“Landscape Retaining Walls”

BRANZ Bulletin No. 394 (2000) “Low Retaining Walls”

Cantilever Masonry Retaining Walls – FIRTH

At the end of this unit you must be able to:

(a) Describe the principles of retaining wall design.

(b) Describe the construction of retaining walls in various materials, including ground

anchors and reinforced earth.

(c) Describe the types and application of tanking materials and the protection of the

same.

(d) Describe the installation of water stops.

(e) Describe the technique of back filling and drainage.

(f) Describe the use of temporary protection to excavated faces.



INTRODUCTION.

The function of a retaining wall is to resist the lateral thrust of mass earth, water pressure

and surcharges on one side of the wall without deflecting.

A. RETAINING WALL DESIGN

Retaining walls must be designed to ensure that:

1. Overturning does not occur.

2. Sliding does not occur.

3. The soil on which the wall rests is not overloaded.

The shape and type of wall used is determined by:

1. Position of the wall in relation to the boundary, (if applicable).

2. Height of the wall.

3. Type of soil to be retained.

4. Material to be used for the wall.

5. Surface finish required e.g. A house basement wall will be quite different to a garden

wall.

6. Type of drainage required.

A retaining wall is designed to support the weight of soil in the wedge between the angle

of repose and the back of the wall. The angle of repose is a natural angle which the soil is

self supported.

If ground water is not a problem, it is necessary to retain the soil from the angle of

rapture, ,– the angle at which the soil will break away if it is not supported.

Foundations – The soil under the retaining wall will need to support both the

retaining wall and the overturning load placed on the wall.

To prevent the retaining wall from sliding, ribs are added underneath the foundation

footing.



Rip

(Alternatively positions)

if required to prevent sliding.

Surcharge

Angle of

rupture

Drive way

Hydrostatic pressure (active water pressure) must be prevented by:

weep holes or

field drain.

Surcharges (active loads)

Sloping soil - sloping away and above the retaining wall.

Building weight - directly supported by a retaining wall or adjacent to.

Road or access way - placed above a retaining wall

Building consent

Retaining walls more than 1.5m high

or on the boundary will require a

building consent.

Retaining walls above 1.5m high

must be designed by a registered

engineer, except low garden walls.

Retaining walls with a surcharge.

Batter

Slope one face of the retaining wall, this will reduce the amount

of materials used.

Batter the entire wall i.e. crib wall will reduce both the materials and footing

widths.

Toe

Weep hole

Stem

Heel

Field drain

Angle of

repose

Ground level



Gravity concrete wall

www.wikipedia.org

B.

TYPES OF RETAINING WALLS

1. Mass concrete, brick or rock.

(Gravity Wall)

This type of retaining wall relies on weight,

(mass) thickness of the stem, and raked at

the front of the wall.

2. Crib walls (Gravity Wall)

A system of headers and stretchers

(timber or pre cast concrete) laid

at a rake of 1 in 4 with a maximum

height of 5m, using 900mm headers.

Note:

It is not necessary to support

the wall on a concrete footing, however,

it is recommended for higher walls.

Crib wall

http://en.wikipedia.org/wiki/File:Retaining_Walll_Type_Function.jpg



3 Reinforced concrete masonry

walls

Masonry units are used as permanent

formwork. Walls are usually thicker than

reinforced concrete, to allow space for

reinforcing and concrete.

4. Reinforced concrete wall

Engineered designed retaining

walls are available in a variety

of shapes and types such as:

(a) Cantilever

(b) Buttress

(c) Counterfort

The correct placement of reinforcing steel

is vital to the strength of the wall.

Reinforced concrete

masonry cantilever wall



5. Ground anchor reinforced concrete

For high retaining walls

pre-stressed ground anchors

are drilled into (a considerable

distance) the retained ground

and grouted to form an anchor.

The stressing of the anchor is

affective when the grouting has

attained sufficient strength.

Ground anchor support

Acknowledgement to R.Chudley & R. Greeno



6. Reinforced earth retaining wall

A retaining wall system consist of pre-cast concrete panels tied to the

ground with galvanised steel straps. The straps are laid out and encased

in and held by the compacted back filling soil.

www.reinforcedearth.com, www.terraforce.ie/design_alternatives_reinfor...

7. Cantilevered timber retaining wall

Acknowledgement to Carter Holt Harvey Timber

http://www.reinforcedearth.com/

http://www.terraforce.ie/design_alternatives_reinforced_retaining_walls.htm



WORK SHEET 1

1. What is meant by:

(i) angle of repose _______________________________________________________

(ii) angle of rapture _______________________________________________________

(iii) hydrostatic pressure _____________________________________________________

(iv) Surcharge ______________________________________________________________

________________________________________________________________________

________________________________________________________________________

2. Briefly explain the function of a retaining wall? _________________________________

______________________________________________________________________________

______________________________________________________________________________

______________________________________________________________________________

3. List three considerations when designing a retaining wall.

(i)____________________________________________________________________________

(ii)___________________________________________________________________________

(iii) _________________________________________________________________________

4. When is it necessary to obtain a building consent for a retaining wall construction?

______________________________________________________________________________

______________________________________________________________________________

______________________________________________________________________________

5. Why is a rib sometimes added to the underside of a retaining wall foundation?

______________________________________________________________________________

______________________________________________________________________________

6. Show a clear cross section sketch of a cantilevered pole retaining wall.

7. Explain the difference between a counterfort wall and a buttress wall.

______________________________________________________________________________

______________________________________________________________________________

______________________________________________________________________________

______________________________________________________________________________



8. Show a clear cross section sketch of a crib wall headers and stretchers.

9. Describe and with the aid of sketches, the construction of a reinforced earth retaining

wall.

_________________________

_________________________

_________________________

_________________________

C. TANKING

Retaining walls constructed as part of a building must be tanked (waterproofed) before

any back filling is done.

Materials used:

Mastic asphalt

A layer or layers of hot trowelled asphalt (bitumen and fillers) applied to the outside

of the retaining wall.

Reinforced bitumen sheets

A sheet of reinforced high quality bitumen approximately 3mm thick which is heat

sealed to the wall and at all joints.

Multi-layered polyethylene and rubberized asphalt

A self adhesive membrane very quickly and easily applied to horizontal or vertical

surfaces.

Bitument/latex emulsion

A brushable emulsion which must be applied in at least two coats at right angles to

each other.

Butyl sheeting Usually used as roofing but can be used for tanking.

Protection

The surface of the tanking must be protected with sheets of hardboard, fibre cement,

etc before back filling.

Penetrations

Follow manufacturer instructions when sealing pipes etc that may penetrate the

tanking.



D. WATER STOPS A rubber or PVC strip used to waterproof joints in concrete walls

or slabs.

Two main types of waterstop:

1. those located in the wall or slab.

2. those which are surface mounted.

Both types require accurate placing and supporting while the concrete is poured.

The sketches below show some problems that my occur in the placing of waterstops.

E. BACKFILLING AND DRAINAGE

Backfilling (Granular fill)

Scoria 25/7 (scoria size 25 mm to 7 mm) or 50/20 (50mm to 20mm must be laid in

150 - 200mm layers and compacted carefully with a compactor.

An alternative to granular fill is Formflow 2000 x 1000 x 90 mm sheets.

Drainage

65mm or 110mm drains pipes or field drains must be installed at the base of any

tanked walls to prevent hydrostatic pressure against the wall.

F. TEMPORARY PROTECTION TO EXCAVATED FACES

Excavated soil face must be protected against eroding, caused by surface water

running over the excavated face.

To prevent the face from eroding, divert all surface drainage away from the excavation

or cover the soil face with polythene or bitumen sprayed hessian to keep the rain water

off as possible.



WORKSHEET 2

1. Describe how you will prevent an excavated face from eroding.

______________________________________________________________________________

______________________________________________________________________________

______________________________________________________________________________

2. Show a clear cross section sketch of a dumb bell type water stop.

3. Clearly sketch a cross section of a reinforced concrete masonry retaining wall

1.2m high and show the position of the following:

main steel

rib

top soil

back filling (include granular size)

filter cloth

geotextile filter

drain pipe

tanking



Safety to the MAX’ !!?? Best option

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http://www.gme-shields.com/products/steel/4m-series

Documents

Unit13026Theory; Retaining Walls