Deep Excavation Basement

In this section all is about the problems of supporting side slopes or cut faces to the periphery of wide, deep excavations and to smaller sites where the soil batters can be accommodated in the available land area.For large cuts associated with heavy earth moving, the scale and size of these cuts means cross-bracing would be quite impractical, whereas for smaller sites the choice between battered and braced excavations is dependent on available space and cost consideration.

Method to improving the stability of slopes

Regarding the profile of the slope and, for example, weighting the toe of the slope locally with a soil berm to reduce the disturbing movement

Using tensioned ground or rock anchors to increase the effective stress on the potential failure surface,thereby improving soil strength

Intercepting potential failure surfaces with sheet piles or jet grouted columns installed from the face or top of the slope

Increasing the effective vertical stress on the potential failure surfaces by reduction of pore-water pressure by drainage

Improving composite soil strength by regarding the slope and the inclusion of reinforcement to intercept the potential failure surface, using reinforced soil

Driving soil nails through potential failure surfaces

Here is some explanation about reinforced soil and soil nailing:Reinforced soil is the technique whereby fill material (frictional or cohesive) is compacted in successive layers on to horizontal placed sheets or strips of geosynthetic or metallic reinforcement.Soil nailing is the technique whereby in situ ground (virgin soil or existing fill material)is reinforced by the insertion of tension-carrying soil nails. Soil nails may be of metallic or polymeric material ,grouted into a predrilled hole or inserted using a displacement technique.They will normally be installed at a slight downward inclination to the horizontal.In summary, reinforced soil is built-in fill from the base upwards, whereas soil nailing is used in cuts invirgin soils working from the ground surface downwards.

Open excavationOpen excavation does not need any temporary support or timbering to the sides of the excavation. This means that the cost of the excavation is more economic. In economic terms of plants and manpower to cover the extra excavation, backfilling and consolidating must be offset by the savings made by omitting the temporary support works to the site of the excavation. The main advantages of this method is the large amount of free site space required.

The excavation is done to earth Angle of Repose. Angle of Repose is the angle that formed from excavation. The sides of the cut of the excavation will be sloped at the natural angle of degree when the area in which the excavation is being made is large enough to permit it. Normally, the angle would be not less than 1-1/2 feet on the horizontal to each foot on the vertical. When it is not possible to slope to the Angle of Repose, shoring must be erected to prevent a cave-in.

The angle of repose should be measured as accurate as possible by using an inclinometer. A common way of measuring Angle of Repose involves the use of a pole with a weighted string which is lowered into the centre of the trench. The stability of the slope can be determined by using seismitron. It can be placed on the slope surface of excavation. However, the result would be more accurate if it is placed in a drilled hole in the bank. It picks up the tiny sounds of ground movement called microseisms, and amplifies them so they can be heard through earphones.

For open excavation with steel sheet piles, steel sheet piles are used to prevent the soil at the sides of the excavation from sliding down and affect the construction projects. The steel sheet piles function as temporary retaining elements, with a permanent concrete wall.

Perimeter Trench ExcavationsThe perimeter trench method is used where weak soils are encountered and is carried out by excavating a perimeter trench around the proposed basement excavation.

A trench, that is wide enough to enable the retaining walls to be constructed, is excavated around the perimeter of the site, and timbered according to the soil conditions. The trench can be timbered and using sheet piles or precast concrete walls. The arrangement of timbers whether near or distance depends on the types of soils. It may be necessary for runners or steel sheet piling to be driven ahead of the excavation work. This method can be used where weak subsoil are encountered so that the basement walls act as permanent timbering whilst the mound or dumpling is excavated and the base slab cast.

The width and depth of the trench must be sufficient to accommodate the timbering and basement retaining wall, and provide adequate working space. The bottom of the trench should be graded and covered with a 50 to 75mm blinding

layer of weak concrete or coarse sand to protect the base of the excavation from drying and shrinking, and to form a definite level from which to set out and construct

the basement wall. The base of the wall

should be cast first, with a kicker formed for the stem and starter bars left projecting for the stem and the base slab. The stem or wall should be cast in suitable lifts, and as it cures the strut is transferred to the new wall. When the construction of the perimeter wall has been completed the mound of soil, or dumpling, in the centre of the basement area can be excavated and the base slab cast.

The permanent retaining walls are constructed within the trench excavation and the timbering is removed; the dumpling or middle can then be excavated and the base cast and joined to the retaining walls. This method could also be used in firm soils when the mechanical excavators required for bulk excavation are not

Figure 1.2.1 shows the timbering of perimeter trench method.

available. This can be done by excavating the perimeter trench completely before placing the timbering in position.

Perimeter trench is suitable for heights not exceeding 1.00m only. This method is seldom used due to shallow basement requiring support at both sides of the wall.

Raking shores

In this method, inclined members known as rakers are used to give lateral supports to walls.

The following points are to be kept in view for the use of the raking shores:

Rakers are to be inclined in the ground at . However the angle may be

between and .

For tall buildings, the length of the raker can be reduced by introducing rider raker.

Rakers should be properly braced at intervals. The size of the rakers is to be decided on the basis of anticipated thrust

from the wall. The centre line of a raker and the wall should meet at floor level. Shoring may be spaced at 3 to 4.5m spacing to cover longer length of

the bar. The sole plate should be properly embedded into the ground on an

inclination and should be of proper section and size. Wedges should not be used on sole plates since they are likely to give

way under vibrations that are likely to occur.

Figure-1:

Detail of Head of the raker

Figure – 2: Detail of Head of the raker

Figure – 3: Raking shore for multistoried Building where inclination of the rakers has to be limited due to short land width available

Raking shores are generally used to provide lateral support to walls that are unstable or threatened by structural or excavation works. They are also effectively used to retain facades in place whilst internal rebuilding takes place and to brace temporary screens against windloadings. Consequently, whilst classed as a temporary support, they may often remain in place for long periods supporting unstable end walls of derelict vacated buildings and may incorporate advertising hoardings as a disguise.

Wall plates are used to transmit the lateral thrusts to the rakers. Ideally these should be positioned against floors or internal wall lines for best stress distribution. Where this cannot be achieved, internal props or rakers may be introduced or larger plates used to distribute the stresses.

At the base of the rakers, sole plates are used to transmit the loads to the ground. Often these may have housed joints, be inclined and affixed in position by their own foundation pad.

The angle of the raker may be determined by site constraints, but obviously a too steep or too shallow raker may have little effect. Ideally angles between 45 and 60 degrees should be achieved, and groups of tow and three rakers are often effectively.