Bearing Capacity of Soil
Don’t Believe they are settlement…
MIT strata Centre, USA Dancing Couple Building, Prague
When the settlement goes unequal !!!
Methods of Improving Bearing Capacity of Soil
Increasing depth of foundation
Draining the soil
Compacting the soil
Confining the soil
Replacing the poor soil
Using grouting material
Stabilizing the soil with chemicals
Methods of Improving Bearing Capacity of Soil
Bearing capacity is the capacity of soil to support the loads applied to the
ground. The bearing capacity of soil is the maximum average contact pressure
between the foundation and the soil which should not produce shear failure
in the soil. Ultimate bearing capacity is the theoretical maximum pressure
which can be supported without failure; Safe bearing Capacity is the ultimate
bearing capacity divided by a factor of safety.
SBC=UBC/FS
Chapter Three:
Excavation and Timbering of foundation trenches
What is it?
EXCAVATION
Removal of earth to form a cavity in the ground for laying out the foundation of the
structure and enable the site for desired construction.
Isn’t it possible to construct any infrastructure without having the excavation?
TYPES OF EXCAVATION
1. Over site
The removal of top soil
Depth varies from site to site (usually in a 150 to
300 mm range)
required since top soil often contains plant life,
animal life and decaying matters making soil
compressible and thus unstable for supporting
building
2. Reduce Level ( R. L. )
Required in irregular sites to form a level
surface
Consists of both cutting and filling
operations
Level to which the ground is reduced is
called the formation level
3. Trench Excavation
o Excavation of trench of required depth and width before laying out foundation
o Done both manually (with use of spade, pick axe, rammer, etc. and for small
buildings) and mechanically (with use of bulldozers, trench diggers, etc. and
for large buildings)
TYPICAL EXAMPLES OF TRENCH EXCAVATION
i. Battered / Sloped face excavation
Advantage: no temporary support required to
the sides of excavation
Disadvantage: extra cost and time required for
over excavation and back filling
ii. Vertical / Straight face excavation
Advantage: only required amount of soil is
removed and thus min. amount of back filling
Disadvantage: side of excavation require some
degree of temporary support
PROBLEMS IN DEEP EXCAVATION
o Excavation beyond 1.5m deep is deep excavation
o Problems: - collapsing of the sides of trenches
- water coming out of the sides or bottom of the excavation
Precautions to be taken
o “Timbering of trenches ’’ for soil collapsing
o “Dewatering ’’ for water problems
Timbering is defined as providing temporary timber supports to stop the sides of
trenches from falling
DEWATERING
Ground water can cause problem by its
natural tendency to flow into the voids
created by excavation
Water in excavation should be removed since
it can :
o undermine sides of excavation
o make it impossible to adequately
compact the bottom of excavation to
receive foundation
o bearing capacity of the soil is reduced
with water stored in voids of the bottom
of excavation
METHODS OF TIMBERING
1.Stay Bracing
o open timbering
o for firm / stiff / rocky hard soil
o for excavation not exceeding about 2m in depth
o consists of placing vertical sheets called polling
boards, opposite to each other against the walls and
holding them in position by one or two rows of
struts
o polling boards are placed at an interval of 2 – 4m
and extend to full height of trench
o polling board : 200 x 40 – 50 mm., struts : 100 x
100 mm for up to 2m wide excavation and 200 x
200 mm for up to 4m wide excavation
2. Box Sheeting
This method is adopted in loose soils, when the depth of excavation does not
exceed 4 meters. Fig. 2.32 (a) shows 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, 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.
2. Box Sheeting
This method is adopted in loose soils, when the depth of excavation does not
exceed 4 meters.
o closed timbering, box like structure
o two types of box sheeting : (i) Vertical Sheeting and (ii) Horizontal Sheeting
Vertical Sheeting
o for loose / loamy / dry sandy soil
o for depth of excavation not
exceeding 4m
o consists of vertical sheets placed
very near to each other / touching
each other and keeping them in
position by longitudinal rows
(usually two) of wales
o struts are then provided across
the wales
Vertical sheeting for deep trenches
o for up to 10m deep trenches
o for soft ground
o excavation is carried out in stages and at the end
of each stage offset is provided so that the width
of the trench goes on decreasing as the depth
increases
o each stage is 3m in depth
o offset : 25 – 30 cm per stage
o separate vertical sheeting for each stage
supported by horizontal wales and struts
Horizontal Sheeting
o for loose soil
o horizontal sheets are provided longitudinally and supported by vertical
waling and horizontal struts
o if height is more, braces are also provided along with struts
3. Runner System
o closed timbering
o for extremely loose, soft and wet soil
o for soil needing immediate support after excavation
o similar to vertical system except for, runners are provided in place of
vertical sheets
o runners with iron shoes at the end
o runners are driven 30 cm in advance by hammering
o wales and struts are provided as in vertical sheeting
4. 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.
