Timbering of Foundation
Trenches (With Diagram)
After reading this article you will learn about:- 1. Methods
Adopted for Timbering of Foundation Trenches
2. Measures taken for Foundation of Timbering Trenches.
Methods Adopted for Timbering of Foundation Trenches:
The methods adopted for timbering in consideration of
depth of the trench and nature of soil areas are:
i. Stay bracing:
This method is used in case of supporting sides of trenches
excavated in fairly firm soil and when the depth of excavation does
not exceed 2.0 m.
ii. Box sheeting:
This method is used in loose soil and when the depth of excavation
does not exceed 4.0 m.
iii. Vertical sheeting:
This method is adopted for deep trenches up to 10 m in depth in
soft soil. Vertical sheeting is similar to box sheeting, but done in
stages.
iv. Runner system:
In cases of extremely loose and soft soil, vertical sheeting may
require horizontal runners for keeping the sheets in position. The
vertical sheeting in combination with runner is termed ‘Runner
system’.
v. Sheet piling:
In case where (a) soil to be excavated is loose or soft, (b) depth of
excavation is large, (c) width of trench is also large, and (d) the sub-
soil water exists. The exigencies of the situation require further
improved method for protection of the trench sides — sheet piling is
adopted.
Sheet piles may be of timber, steel or concrete. These are driven in
the ground by mechanical means. The sheet piles are costly,
specially the steel piles. After completion of the foundation works,
piles are extracted for reuse.
vi. Diaphragm walls:
For difficult sites, specially for trenches having superimposed load
of great magnitude very close, diaphragm wall of R.C.C. may have to
be adopted.
The diaphragm walls are not structural members to be integrated
with the main structure. Their function is to protect the sides of the
excavation. When the depth of excavation is large, horizontal struts
from wall to wall are required at one or more places.
:
After completion of the main structure, the trenches are filled and
the struts are removed gradually but the diaphragm wall is left in
position.
Measures taken for Foundation of Timbering Trenches:
Precautionary Measures in Construction of Deep Open
Foundation:
Sometimes, it becomes necessary to take the foundation deep in the
soil. Excavation of deep trenches creates problems, specially when
the soil is loose or granular or soft or of mixed variety or the water
table is high. The walls of the foundation trench which are kept
vertical cave in and crumble. Precautionary measures for protection
of the trench walls are necessary.
i. To protect the trenches, timbering is done. However, in cases
when the space at site is available, the walls of the trenches may be
made sloped instead of keeping them vertical.
The angle of slope should not be more than the angle of repose of
the soil. This will increase the cost of earthwork in excavation and
back-filling of the trenches; but may still be economical in view of
the high cost of timbering materials and construction difficulties.
ii. When the trench is very deep, and sufficient space for making the
walls slope at desired angle is not available, timbering of the
trenches will have to he resorted to; however, in cases, a
combination of timbering and sloping of the walls may be done
according to the suitability of the site.
iii. In case of foundations of depth beyond 4.0 m, timbering of
trenches in any of the methods may not be considered suitable. In
such cases, the trench may be protected by piling. Sal bullah or
timber or precast R.C.C. piles are driven close enough to touch each
other. The length of the piles need be minimum 1.5 times, if possible
2 times, the proposed depth of excavation.
The piles are driven first in rows and then excavation is done and
simultaneously the wall is braced with horizontal bracings and
struts. In cases of excavation of basement, horizontal strutting may
not be possible, inclined strutting may have to be resorted to with
proper anchorage.
iv. According to the situation a combination of piling and sloping of
the sides may be adopted.
The closed pile walls always have gaps, providing passage for water
to come out. If water only comes out, it would not be harmful; but if
mud comes out which means flow of soil it is a sign of danger.
Pumping of water from the trench must be stopped immediately
and the temporary closed pile walls must be checked for stopping
the leakage.
However, it is very difficult to achieve complete sealing of the
leakage and it is not desired also. The soil flow should be stopped
allowing seepage of water. Release of water will reduce lateral thrust
on the closed pile wall from the earth outside the wall.
Sheet piling would have been the best answer; but it is very costly.
The length of the piles need be 2 times the proposed depth of
excavation. After construction, it becomes very difficult to extract
the sheet piles and, in cases, they are left in situ which increases the
cost.
The diaphragm walls which are considered a substitute to sheet
piles and are left in position are also very costly. The height of the
diaphragm wall need be 2 times the’ proposed depth of excavation.
Both sheet piles and diaphragm walls are required to be provided
with horizontal supports which make them costlier.
