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Clearing the Site

When the site is located in a wooded area, the first operation is to clear all timber,standing or fallen. If camouflage is necessary, trees or brush outside the designated

cleared area should not be removed. Construction equipment operations are usually the

most rapid and efficient means of clearing a site. Use of the equipment is limited only byunusually large trees and stumpsterrain which hinders their maneuverability and

maintenance requirements. The construction equipment used include bulldozers,

winches, power saws, rippers, motor graders, and scrapers. In addition, hand toolsare used in certain clearing operations. Brush may be disposed of by burning on the site;

however, check to see if a burn permit is required. Timber of suitable dimensions

should be stockpiled at the perimeter of the site. This timber should be saved for possible use in construction of loading ramps. All stumps, roots, boulders,

vegetation, and rubbish must be excavated and moved clear of the site.

Setting out Processes inConstruction and CivilEngineeringSetting out is the process by which information is taken from the construction designdrawings, and pegs, profiles or other marks are then set to control the constructionworks and ensure that each element of the works is constructed in the right positionand to the correct level.Learners will work with traditional methods to achieve an understanding of theessential mathematical and practical skills required for the Setting out process,including the application of basic principles of techniques to ensure appropriate

levels of accuracy. The use of modern electronic instruments and awareness ofemerging technology will also be addressed.Construction projects are normally designed on a coordinate grid and calculations arecarried out to convert these into a form useful for setting out. Learners must attain areasonable standard of arithmetic and trigonometry in order to successfully completethis unit. Spreadsheets and dedicated software play an important role in reducing thecalculation load for the engineer.This is essentially a practical unit, through which learners will come to understandsetting out as a key part of the construction process, and be able to carry out thestandard tasks and calculations involved.

Foundation

Whenever construction workers begin work on a new building, they must first assess whereand how they will build the foundation. The foundation is a structure, commonly made ofconcrete for homes, that transfers the weight of the building onto the earth below. There aredifferent types of foundation designs and each serves a different specific purpose, butgenerally, every foundation works to transfer the weight load of a structure to the soilbeneath.

Most small and medium homes are built upon a shallow foundation. These are usuallycomprised of concrete strips that are laid about a meter beneath the soil, or of a single largeconcrete slab that is also set about a meter beneath the soil. When applicable, the foundation


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therefore greatly improve the stability of the material behind the wall, assuming that this isnot a retaining wall for water.

Earth Pressure Method For RetainingWalls

Cantilever walls

When designing sheet retaining walls it is normal to assume that theeffective lateral stresses acting on the wall are given by simpleRANKINE active and passive zones. Friction on the wall is usuallyignored as this leads to conservative (safe) designs.

Rankine Active and Passive PressuresThe earth pressures acting on the wall are strongly dependent on thedeformations in the surrounding soil. When the wall moves away fromthe soil the stress on the wall drops reaching a minimum, the ACTIVEpressure, with the soil deforming plastically. When the wall movesinto the soil the stress increases, finally reaching a maximum, thePASSIVE pressure, when again the soil is deforming plastically.
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For most retaining walls the long term, fully drained, situation usuallygoverns the wall stability. For the analysis of fully drained conditionsthe Mohr-Coulomb criterion needs to be expressed in terms of effective

stress using the effective strength parameters c and ?. For design itis also conservative to use the critical state strength parameters, thatis c = 0 and ? = ?v. The effective lateral stresses on the wall arethen
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Ka and Kp are known as the active and passive earth pressurecoefficients. For soil at failure the earth pressure coefficients aresimply related by

.For any vertical wall it is possible to relate the horizontal effectivestress to the vertical effective stress, determined from the verticaloverburden, by an earth pressure coefficient. The coefficient willdepend on the slope of the soil surface and the wall roughness.

Published values are available for many situations.

Stability Limiting EquilibriumWhen assessing the stability it is normal to assume triangular pressuredistributions, and this is in fact quite realistic if the wall is rigid. For acantilever wall the stresses acting at failure will then be as shownbelow, with the wall rotating about a point just above the toe of thewall. The stability of the wall depends mainly on the passive forcedeveloped below the excavation.

For design we need to determine the required depth of penetration forstability and then to size the wall to resist the maximum moment. Todetermine the depth of penetration required for a given height H weneed to consider both moment and force equilibrium:

? F = 0
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? M = 0If the soil is dry the pressures and forces are as shown below

We have 2 equations with 2 unknowns, x and d, and hence we can

determine the required depth of penetration for the wall. Theequations can be solved graphically or by computer. Alternativelysimplifying assumptions about the forces below the pivot can be madeto enable analytical solutions to be obtained as described in many textbooks.


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Note that as the factor ofsafety increases the maximum moment alsoincreases.The factor ofsafety can be dramatically reduced by surcharge loadingson the supported ground next to the wall. For a uniform surchargethen the effective active pressure can be increased by Ka ss, while for a

concentrated load from a footing the Coulomb method of trial wedgescan be used to determine the active force on the wall. In the lattersituation allowance must be made for the fact that the point ofapplication of the load will also change.

Consideration must also be given to the water pressures acting on thewall.

For economic reasons cantilever walls are usually limited toexcavations less than 6 m deep.
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Fig. 77.

Excavations To Pier Shafts

When excavating for a shafta hole is excavated as deep as possible without the earth

falling in. Verticalsheeting from 9 by 1 inch to 9 by 2 1/2 inches, according to the nature

of the soil, is temporally strutted against thefacesof the excavation

Walings running right across the excavation are held in position against two oppositesides and strutted by means of walings forced between them against the other two sides.

Cleats are fixed to the endsof the first pair of walings, so as to prevent the other pair of

walings from being forced out of position.

Another layer of earth is now excavated and a second tier of sheeting is placed with theupper end overlapping the lower end of the first tier, and a second row of walings is

inserted to secure both tiers together, and uprights are placed tightly between the walings

at the corners of the shaft.
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When the shaft is of considerable depth the timbering is prevented from sliding down the

shaft by making one pair of walings to project well beyond the shaft at the top and to rest

upon the surface of the ground, as in Fig. 78, a timber being notched over all the walingsto act as a tie. If the shaft be very deep a pair of the bottom walings is buried in the side

of the cutting, being formed in two pieces with a keyed and scarfed joint in the middle

similar to A (Fig. 251 and Plate VI.).

Excavations to shafts should never be less than 4 feetsquare, as a man cannot work in aless space than this.

The above system of timbering may be used for shafts up to about 10 feet square; but if

shafts larger than this have to be sunk, the system shown in Fig. 79 must be used. In this

case the sheeting and waling pieces are inserted as explained with reference to Fig. 78,and two struts are placed at rightanglesto one another across thecentre of the shaft, one

of these struts being in one piece and the other in two pieces, the ends of the latter being

supported at one end by small pieces of timber placed between the side walings and the

uprights, and at the other end by similar small pieces of timber placed between the centralstruts and uprights, as shown by the disjointed details in Fig. 79.

Plan.
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Fig. 78.

Excavating Tunnels

It is sometimes far more economical to cut tunnels for such work as drain laying than toexcavate very deep trenches. These tunnels may be of any size; but they are usually just

large enough to enable one man to work, i.e. not less than 4 feet square. The method of

excavating' a tunnel is as follows: If the soil is good, sections from 2 to 3 feet long are

dug out, poling boards are then temporarily-fixed round theopening, ahead-piece held inposition, and finally strutted by means of upright struts, as shown in Fig. 80, these latter

being kept in position by spiking them to the heador by having cleats fixed to the head.Another length of tunnel is excavated and another series of poling pieces is inserted withthe ends overlapping the first series of poling pieces, and another frame composed of

head and struts is fixed against these overlapping ends.
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Detail of A.

Fig. 79. Detaill of B.

Failure Of Timbering

Serious accidents happen from time to time owing to the failure of timbering in

excavations. This failure is usually due to one of the following two reasons: -


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Fig. 80.

1. The struts fail under the pressure of the earth; or,

2. The struts drop out by the shrinkage of the soil.

1. This is perhaps the most frequent cause of failure, as the pressure from soils is oftenunderestimated, particularly that of clay soils, which sometimes swell with enormous

force when exposed to the atmosphere. When excavations are made in soft soil in the

vicinity of heavy buildings the lateral pressure of the soil is usually very great.

To prevent such failures the timber should be of ample size, and should be examinedbefore being inserted in the excavations, and all faulty pieces rejected; while in very deep

trenches the struts in the lower parts of the trenches should be larger than those near the

surface.

2. To prevent struts dropping out of place they should be examined from time to time andtightened when necessary; but a better plan is to spike them to the walings or to fix cleats

underneath them. The struts supporting stages should always be spiked or fixed in some

way, as they are very likely to become loosened by the weight and motion of theexcavator, who stands upon them to work.

Retaining Walls - Types

By Tim Carter

1993-2010 Tim Carter

Summary: Retaining walls create level surfaces in sloping areas. Build a retaining wall

improperly and nature will regain her curves! Ancient civilizations have passed on


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knowledge of how to build a retaining wall. Four ways of building retaining walls have

been used and improved through time, and these types are discussed here.

Retaining Walls

Many of us live in parts of the country that are not so level. The geography andtopography can range from slightly rolling to mountainous. Those of us who do live in

these regions are quite familiar with retaining walls. These walls allow us to create stepsor level areas on a sloped surface. However, if constructed improperly, the forces of

nature (gravity, water, etc.) will topple a retaining wall in short order.

Retaining walls have been in use for thousands of years. The Romans used retaining

walls to aid in the construction of their famous roads. Many of the castles in Europeincorporated retaining walls into their design. Hillside rice paddies in Asia have

depended on retaining walls for hundreds of years. Trial and error construction methods

of the past and advancements in engineering knowledge have indicated that four basic

types of retaining walls seem to perform quite well. Each design has its limitationshowever. The four basic types of retaining walls are: gravity wall; cantilever wall;

counterfort wall; and buttressed wall.

Gravity Walls

A gravity retaining wall is usually a low height (less than 3 to 4 feet) wall which dependson its own weight or mass to hold back the earth behind it. This goal is achieved by

constructing the wall with a volume of material so that when stacked together, the weight

and friction of the interlocking material exceeds the forces of the earth behind it. The wallis thicker at the base than at the top. Also, note that as the front of the wall gets taller it

slants backwards. This is often referred to as 'battering'.

This battering effect creates a visual message of strength, as over time, the wall will

probably succumb to the forces of gravity and begin to tilt outward. By battering the wallbackwards, you extend the visual life of the wall. Retaining walls that appear to be

tipping over, tend to indicate faulty construction and impending failure.

Walls that are battered send a visual message that the wall is 'working' and that it is

continuing to beat the force of gravity. Gravity walls become very cumbersome toconstruct as they get higher, because they require vast quantities of materials. The

thickness of a gravity wall at its base should be one half to three fourths its height. So, if

you intend to build a wall 4 feet high, the base should be 2 to 3 feet wide. As the wallgets higher, it begins to get thinner.

Cantilever Walls

A cantilever retaining wall is one that consists of a uniform thickness wall which is tied

to a footing. A cantilever wall usually is asked to hold back a significant amount of earth,


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so it is a good practice to have these walls engineered. A simple example of a cantilever

retaining wall is a typical basement wall of a house.

The width of the footing for a cantilever wall is very important. The footing is designedto resist tipping or sliding forces which the earth exerts upon the wall. Also, the wide area

of the footer allows the weight of the earth to actually keep the wall from tipping in someinstances.

These walls require significant steel reinforcing in both the footer and the wall structures.The steel also has to extend from within the footer up into the wall so that the two pieces

actually become one integral unit. As you can see, this is why these walls need to be

designed by structural engineers. If you try to guess yourself at the amount, size and

placement of structural steel in this type of wall, you are gambling.

Also, the thickness of both the footer and the wall is extremely critical. Don't be a fool

and try to become a weekend engineer. Spend several hundred dollars and get it right the

first time!

Counterfort Retaining Walls

A counterfort retaining wall is very similar to a cantilever wall, except that it has oneadditional feature. This wall has a triangular shaped wall which connects the top of the

wall to the back of the footer. This added support wall is hidden within the earthen or

gravel backfill of the wall. The footer, retaining wall and support wall must be tied to oneanother with reinforcing steel.

If the structure is poured concrete, often the retaining wall section and the support walls

are poured as one unit at the same time. The support walls add a great deal of strength tothe retaining wall. The supports make it virtually impossible for the wall to becomedetached from the footer. As with cantilever walls, a counterfort wall should be designed

by a competent structural engineer.

If you decide to attempt to construct this type of wall without approved plans, you are

making a huge mistake. If the wall fails, the cost to remove the failed wall, construct thenew one, etc. could be ten times or one hundred or more times the cost of engineering

services. Remember, engineers have to eat just like you and me!

Buttressed Retaining Walls

A buttressed retaining wall is basically identical to a counterfort wall except for onething. The support wall is on the outside of the retaining wall. They are visible. The

buttresses add incredible strength to the wall system. For the retaining wall to fail or tip

over, the buttresses would have to be crushed. The buttress concept was widely used inthe construction of many cathedrals in Europe.


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Because of the height of the cathedral walls, the buttresses helped to stabilize them. They

do the exact same thing in a retaining wall. Once again, if you intend to build one of these

walls, you must give serious consideration to hiring an engineer. Situations whichdemand this type of wall usually have tremendous loads which bear against the walls.

The buttresses can often be designed to be decorative in nature and covered with stone or

some other material. Depending upon the overall length of the wall, you may haveseveral buttresses. They can be spaced to create rooms, parking spaces, handball courts or

any other functional space.

Definition:

A half-timberedbuilding has exposed wood framing. The spaces between the woodentimbers are filled with plaster, brick, or stone.

In Medieval times, many European houses were half-timbered. The structural timberswere exposed. In the United States, harsh winters made half-timbered construction

impractical. The plaster and masonry filling between the timbers could not keep out colddrafts. Builders began to cover exterior walls with wood or masonry.

During the 1800s, it became fashionable to imitate Medieval building techniques.

Timbers were applied to exterior wall surfaces as decoration. False half-timbering

became a popular type of ornamentation in many nineteenth and twentieth century housestyles, including:

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