TIMBERING OF TRENCHES - SOILS.
When the depth of trench is large, or when the sub-soil is loose, the sides of the trench may cave
in. The problem can be solved by adopting a suitable method of timbering. Timbering of trenches,
sometimes also known as shoring consists of providing timber planks or boards and struts to give
temporary support to the sides of the trench. Timbering of deep trenches can be done with the help
ofthefollowingmethods:
1. Staybracing.
2. Boxsheeting
3. Verticalsheeting
4. Runnersystem
5. Sheetpiling.
1. Stay bracing. This method (Fig. 2.31) is used for supporting the sides or a bench excavated in
fairly firm soil, when the depth of excavation does not exceed about 2 metres. The method consists
of placing vertical sheets (called sheathing) or polling boards opposite each other against the two
walls of the trench and holding them in position by one or two rows of struts. The sheets are placed
at an interval of 2 to 4 metres and generally, they extend to the full height of the trench. The polling
boards may have width of about 200 mm and thickness of 44 to 50 mm. The struts may have size
100 x 100 mm for trench up to 2 m width and 200 x 200 mm for trench up to 4 m width.
FIG. 2.31 STAY BRACING.
2. Box sheeting. This method is adopted in loose soils, when the depth of excavation does not
exceed 4 metres. Fig. 2.32 (a) shows the box like structure, consisting of vertical sheets placed
very near to each other (sometimes touching each other) and keeping them in position by
longitudinal rows (usually two) of wales. Struts are then provided across the wales.
Another system of box sheeting, shown in Fig. 2.32(b), is adopted for very loose soils. In this
system, the sheeting is provided longitudinally, and they are supported by vertical wales and
horizontal struts [Fig. 2.32 (b)]. If the height is more, braces are also provided along with struts.
FIG. 2.32 BOX SHEETING.
3. Vertical sheeting. This system is adopted for deep trenches (upto 10 m depth) in soft ground.
The method is similar to the box sheeting [Fig. 2.32 (a)] except that the excavation is carried out
in stages and at the end of each stage, an offset iS provided, so that the width of the trench goes on
decreasing as the depth increases. Each stage is limited to about 3 m in height and the offset may
vary from 25 to 50 cm per stage. For each stage, separate vertical sheeting, supported by horizontal
wailings and struts are provided (Fig. 2.33).
4. Runner system. This system is used in extremely loose and soft ground, which needs immediate
support as excavation progresses. The system is similar to vertical sheeting of box system, except
that in the place of vertical sheeting, runners, made of long thick wooden sheets or planks with
iron shoe at the ends, are provided. Wales and struts are provided as usual (Fig. 2.34). These
runners are driven about 30 cm in advance of the progress of the work, by hammering
FIG. 2.33 VERTICAL SHEETING. FIG. 2.34 RUNNER SYSTEM.
5. Sheet piling. This method is adopted when (i) soil to be excavated is soft or loose (ii) depth of
excavation is large (iii) width of trench is also large and (iv) there is sub-soil water. Sheet piles are
designed to resist lateral earth pressure. These are driven in the ground by mechanical means (pile
driving equipment). They can be used for excavating to a very large depth.
