Excavation beyond depth of 1.5 m is generally categorised as deep excavation. The problems generally encountered in deep excavation are:
- The collapsing of the sides of the trench.
- The prevention of water entering the trench from the sides or from the bottom of trench.
Only the first aspect is treated in the sections treated below. The secured aspect is dealt separately elsewhere. Following methods of bracing are commonly employed.
1. Stay Bracing
This arrangement is similar to that followed for shallow excavations. This type of bracing is used in moderately firm ground and when the depth of exaction does not exceed 2 m. Here vertical sheets or poling boards are placed on opposite sides of the trench and they are held in position by one or two rows of struts. The sheets are placed at the spacing of 3–4 m and generally extend to the depth of trench. The thicknesses of poling boards are about 40–50 mm and of width 200 mm. The struts may be of 100 mm × 100 mm size for trench up to 2 m width and of 200 × 200 mm width for trench width exceeding 2 m (Fig. 15.16).
Figure 15.16 Stay bracing
2. Box Sheeting
This arrangement is made for loose soil and when the depth of excavation does not exceed 4 m. Sheeting planks, wales and struts are used to form box like structure as shown in Fig. 15.17. In this arrangement the planks are placed closer or sometimes touching each other. Tow longitudinal rows of wales keep the sheets in position. Struts hold the wales in position.
Figure 15.17 Box sheeting
In very loose soils additional bracings are provided. In this arrangement the planks are placed horizontally (in plan) and are supported by wales and struts as shown in Fig. 15.18.
Figure 15.18 Box sheeting for very loose soils
3. Vertical Sheeting
In soft ground up to 10 m depth of trenches, the work is carried out in stages. This method is similar to box sheeting. Here at each stage of excavation one offset is provided for each stage separate vertical sheets, horizontal wales, struts and braces are provided. The offset is provided at 3–4 m depth and of 30–60 cm wide at each stage. Suitable working platform is provided (Fig. 15.19).
Figure 15.19 Vertical sheeting
4. Runners
In situations where immediate support is needed, in case of very loose and soft ground, as the excavation progresses the special arrangement as shown in Fig. 15.20 is made. Here the runners are long thick wooden sheets with iron shoe at one of its ends is used to drive the runners. The wales and struts are provided as usual.
Figure 15.20 Arrangement of runners
5. Sheet Piling
When the depth of excavation exceeds 10 m the use of vertical timber sheeting becomes generally uneconomical. In such situations other methods of sheeting and bracing are commonly employed. One such procedure is driving of steel sheet piling around the boundary of the excavation. As the soil is removed from the enclosure wales and struts are inserted.
The types of sheet piles commonly used are shown in Fig. 15.21.
Figure 15.21 Cross-section of sheet pilling
The strength and stiffness of piling is in the increasing order as flat arch and z-piling. Flat and arch web types are used for shallow to deep excavation whereas z-type is used for deep to very deep excavations where the heaviest pressure is expected.
As the excavation proceeds wales and struts are inserted. The wales are commonly of steel, and the struts may be of steel or wood. Excavation is then proceeded to a lower level, and another set of wales and struts is installed. This process is continued until the excavation is completed. In order to prevent local heaves in most of the soils it is necessary to drive the sheet piles several cms below the bottom of excavation (Fig. 15.22).
Figure 15.22 Sheet piling arrangements
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