Question

(b) What do you mean by Timbering of Trenches ? Explain different techniques of Timbering of Trenches in different types of soil conditions.

Answer 1

(b) 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 of the following methods:
1. Stay bracing.
2. Box sheeting
3. Vertical sheeting
4. Runner system
5. Sheet piling.

1. Stay bracing. This method 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 1OO x 100 mm for trench up to 2 m width and 200 x 200 mm for trench up to 4 m width.
2. Box sheeting. This method is adopted in loose soils, when the depth of excavation does not exceed 4 metres. The box like structure, consisting of vertical sheets placed very near to each other (some times 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, is adopted for very loose soils. In this system, the sheeting is provided longitudinally, and they are supported by vertical wales and horizontal struts. If the height is more, braces are also provided along with struts.

3. Vertical sheeting. This system is adopted for deep trenches (up to 10 m depth) in soft ground. The method is similar to the box sheeting 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.

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. These runners are driven about 30 cm in advance of the progress of the work, by hammering.

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.