BRACED CUTS.

General Considerations
Shallow excavations can be made without supporting the surrounding material if there is adequate space to establish slopes at which the material can stand. The steepest slopes that can be used in a given locality are best determined by experience. Many building sites extend to the edges of the property lines. Under these circumstances, the sides of the excavation have to be made vertical and must usually be supported by bracings.

Common methods of bracing the sides when the depth of excavation does not exceed about 3 m are shown in Figs 20.26(a) and (b). The practice is to drive vertical timber planks known as sheeting along the sides of the excavation. The sheeting is held in place by means of horizontal beams called wales that in turn are commonly supported by horizontal struts extending from side to side of the excavation. The struts are usually of timber for widths not exceeding about 2 m. For greater widths metal pipes called trench braces are commonly used.

When the excavation depth exceeds about 5 to 6 m, the use of vertical timber sheeting will become uneconomical. According to one procedure, steel sheet piles are used around the boundary of the excavation. As the soil is removed from the enclosure, wales and struts are inserted. The wales are commonly of steel and the struts may be of steel or wood. The process continues until the excavation is complete. In most types of soil, it may be possible to eliminate sheet piles and to replace them with a series of//piles spaced 1.5 to 2.5 m apart. The //piles, known as soldier piles or soldier beams, are driven with their flanges parallel to the sides of the excavation as shown in Fig. 20.26(b). As the soil next to the piles is removed horizontal boards known as lagging are introduced as shown in the figure and are wedged against the soil outside the cut. As the general depth of excavation advances from one level to another, wales and struts are inserted in the same manner as for steel sheeting.

If the width of a deep excavation is too great to permit economical use of struts across the entire excavation, tiebacks are often used as an alternative to cross-bracings as shown in Fig. 20.26(c). Inclined holes are drilled into the soil outside the sheeting or H piles. Tensile reinforcement is then inserted and concreted into the hole. Each tieback is usually prestressed before the depth of excavation is increased.

Figure 20.26 Cross sections, through typical bracing in deep excavation, (a) sides
retained by steel sheet piles, (b) sides retained by H piles and lagging, (c) one of
several tieback systems for supporting vertical sides of open cut. several sets of
anchors may be used, at different elevations (Peck, 1969)

Changes of Soil Properties During Excavation.

The soil at level 1, below ground level – see Fig. 1.9 – is subject to pressure, and thus consolidation, due to the weight of the soil above, and is in equilibrium. If the overlying soil is removed to form a basement then the pressure, and consolidation effects, at level 1 are also removed. The unloaded soil, in this condition, is known as over-consolidated, and is likely to recover from the consolidation and rise in level (heave). This can be likened to the elastic recovery of contraction on a column when its load is removed.

Fig. 1.9 Heave following removal of overburden.

EXCAVATIONS: Electro-osmosis method.

This method is used for fine grained cohesive soils (such as clay), which can be drained or stabilised using electric current. The method was developed by L. Casagrande (1952). If direct current is passed between two electrodes driven into natural soil mass, the soil water will travel from the positive electrode (anode) to the negative electrode (cathode). The cathode is made in the form of well point or metal tube for pumping out the seeping form of well point or a metal tube for pumping out the seeping water. A steel rod, a pipe or steel piling of excavation can serve as the cathode. The arrangement of electrodes is done in such a way that the natural direction of flow of water is reversed away from the excavation, thereby increasing the strength of the soil and stability of the slope (Fig 2.41).

The potentials generally used in the process are from 40 to 180 volts, with electrode spacing of 4 to 5 metres.

FIG. 2.41 ELECTRO-OSMOTIC DRAINAGE.

EXCAVATIONS: Vacuum method: Forced flow.

The above methods are effectíve only in coarse grained soils. For fine grained solis, the welI point system can be extended by the vacuum method. For successful dewatering in the fine, non-cohesive soils, such as silty sands and other fine sands, it is neccesary to apply a suction head to the dewatenng system. Both the well point system and deep well system can be adopted for dewatering such soils by maintaining a vacuum in the well with the use of air tight seals for all points. A hole of about 25 cm diameter is created around the well point and the rise pipe by jetting water under sufficient pressure. While the jetting water is still flowing, medium to coarse sand is rapidly shovelled into the hole to fill it upto about 0.75 or 1 m from the top. The top portion of the hole is then sealed up by tamping bentonite, soil cement or clay (Fig. 2.40). Vacuum pumps are used to create vacuum in yhe sand filling. When the vacuum is drawn on the well point, the ground surface is subjected to unbalanced atmospheric pressure. Although the quantity of water drawn out does not increase much, the unbalanced atmospheric pressure acting on the ground surface consolidates the sub-soil which becomes stiff enough for carring out excavations.

 FIG. 2.40 VACCUM METHOD.

EXCAVATIONS: Deep well system.

When the depth of excavation is more than 16 m below the water table, deep well drainage system may be used with advantage. The system is also useful where artesian water is present. A 15 to 60 cm diameter hole is bored and a casing with a long screen (5 to 25 cm) is provided. A submeisible pump with a capacity to

push the water upto a height of 30 m or more is installed near the bottom to the well. Each well has its own pump. Along with the deep wells arranged on the outer side of the area under excavation, a row of well points is frequently installed at the toe of the side slopes of the deep excavation.

 FIG  2.39  DEEP WELL SYSTEM.

EXCAVATIONS: Shallow well system.

In this system, a hole of 30 cm in diameter or more is bored into the ground to a depth not excceding 10 m below the axis of the pump. A strainer tube of 15 cm diameter is lowered in the bore hole having a casing tube. A gravel filter is formed around the strainer tube by gradually removing the casing tube and simultaneousty pouring filter material, such as gravel etc. in the annular space. A suction pipe is lowered into the filter well so formed. The suction pipes from a number of such wells may be connected to one common header leading to the pumping unit.

EXCAVATIONS: Well point system.

A more complicated dewatering system based on gravity flow is the installation of well points. A well point is a perforated pipe, about ½ to 1 m long and 5 to 8 cm in diameter, covered by cylindrical wire gauge screen. In an expensive type well point, the steel tube is covered with two brass screens, the inner of fine mesh and the outer perforated (Fig. 2.36). A conical steel drive point is attached to the lower end of the pipe, with a neoprine ball valve fitted in the point to allow jetting of water to pass through it for driving it. When operating on suction, the ball is in the position shown and the soil water enters the outer screen, through the mesh, and down the flutes of the inner tube. Holes near the bottom of the latter and just above the shoe admit water to the inside where it is drawn up the riser, along the header to the pump for discharge through the pipes to a drain.

FIG. 2.36  DETAILS OF WELL POINT.

FIG. 2.37 LOWERING OF WATER TABLE BY WELL POINT SYSTEM.

The well points are placed in a row or ring, and the riser pipes are attached through a common manifold or header pipe to a special well point pump (Fig. 2.37). For inserting yhe well point into ground by jetting, water is pumped down the well point under pressure from where it emerges with a great velocity through the tip of the drive point. The emerging jet-stream dislodges the surrounding soil and the well point can be lowered to the desired depth. A further advantage of jetting is that water under pressure washes away soil fines from around the well point leaving a relatively coarser material lo settle and form a natural filter around the well point. The hole formed around the riser pipe and the welI point by jetting water is filled with coarse sand. The sand also helps in directing drainage to the well point.

The suction pump used in the well point system has a capacity of bringing water to the surface from a maximum depth of about 6 m. The well points are generally spaced between 1 to 2 m. For dewatering excavations which are more than 6 m below the water table, a multi-sage well point system (Fig. 2.38) is used. Excavations exceeding 16 m depth are preferably drained by deep well system. In the multi-stage well point sysem, the ground is first stripped to the natural water level where the first stage of well points is installed. After excavating about 5 m, second stage is installed to further lower the water table for advancing excavations. The other stages are put successively, upto a maximum depth of 16 m is reached. In the well point system, a round the dock pumping schedule is essential, as the interruption in pumping can have catastrophic consequences. Hence one auxiliary pumps for each two pumps is use should always be available.

 FIG. 2.38 MULTI-STAGE WELL POINT SYSTEM.

EXCAVATIONS: Ditches and Sumps.

This is the simplest form of dewatering used in shallow excavations in coarse grained soils. Shallow pits, called sumps are dug along the periphery of the area-drainage ditches. The water from the slopes or sides flows under gravity and is collected in sumps from which it is pumped out [Fig. 2.35(a)]. If the seepage (ie. flow of water) is significant, it may cause softening and revelling or sloughing of the lower part of the slope. There is also possibility of piping in the sump bottom, because of upward flow. In such circumstances, the sump can be weighted down with an inverted filter consisting of layers of successively coarser material from the bottom of the sump-pit upwards [Fig. 235(b)].

FIG. 2.35 EXCAVATION DRAINAGE WITH SUMP.

EXCAVATIONS IN GROUND WITH SUB-SOIL WATER.

Excavations of foundation trenches in ground having high water table, or in water-logged area pose great problems because of water oozing in the trench from sides, bringing with it the soil from the sides. The timbering, if provided, would become loose and collapse. Excavatíons can be carried out by dewatering the sub-soil water. Foundation dewatering can be done by the following methods:

(i) Ditches and sumps 
(ii) Well point system
(iii) Shallow well system
(iv) Deep well system
(v) Vacuum method.
and (vi) Electro-osmosis method.