The bearing capacity of soil is influenced by many factors for instance soil
strength, foundation width and depth, soil weight and surcharge, and spacing
between foundations. These factors are related to the loads exerted on the soil
and considerably affect the bearing capacity.
Apart
from the above factors, there are number of non-load related factors that
influence bearing capacity and need to be considered during the design of foundation
and computing the soil bearing capacity. For instance, earthquake and dynamic
motion, frost action, subsurface void, expansive and collapsible soil,
potential heave, soil erosion and seepage, and soil reinforcement.
Bearing capacity is the ability of
soil to safely carry the pressure placed on the soil from any engineered
structure without undergoing a shear failure with accompanying large
settlements.
1. Soil Strength
Bearing capacity of cohesionless
soil and mixed soil increases unproportionally with the increase of in the
effective friction angle. However, bearing capacity of cohesive soil varies linearly
with the soil cohesion provided that the effective friction angle is zero.
2. Foundation Width
Foundation width affects bearing capacity
of cohesionless soil. The bearing capacity of a footing placed at the surface
of cohesionless soil, where the soil shear strength is considerably dependent
on internal friction, is proportional to the width of the foundation. Bearing
capacity of cohesive soil of constant shear strength and infinite depth is independent
of foundation width.
3. Foundation Depth
The greater the bearing capacity the
deeper the foundation. This is specifically obvious in a uniform cohesionless
soil. In contrary, if the foundation is carried down to a weak soil layer, then
bearing capacity is declined.
Foundations placed at depths where
the structural weight equals the weight of displaced soil usually assures
adequate bearing capacity apart from the case where the structure supported by under-consolidated
soil and collapsible soil subject to wetting.
4. Soil Weight and Surcharge
The contribution of subsurface and
surcharge soil, which are influenced by water table, to the bearing capacity
cannot be ignored. The water table should not be above the base of the
foundation to avoid construction, seepage, and uplift problems. If the water
table is below the depth of the failure surface, then it has no influence on
the bearing capacity.
5. Spacing Between Foundations
It is recommended to consider
minimum spacing between footings, which 1.5 times foundation width, during the
design of foundation in order to avoid reduction in bearing capacity.
6. Earthquake and Dynamic Motion
Repeated movements could increase
pore pressure in foundation soil and consequently bearing capacity is decreased.
Sources of cyclic movements are earthquakes, vibrating machinery, and other
sources like vehicular traffic, blasting, and pile driving.
The foundation soil can liquify when
pore pressures equal or exceed the soil confining stress. Liquefaction reduces
effective stress to zero and causes gross differential settlement of structures
and loss of bearing capacity.
7. Frost Action
Frost heave in certain soils in
contact with water and subject to freezing temperatures or loss of strength of
frozen soil upon thawing can alter bearing capacity over time. Low cohesion
materials containing a high percentage of silt-sized particle are mostly
susceptible to frost action.
8. Subsurface Voids
Bearing capacity of soil decreases
due to subsurface voids which are within a critical depth beneath the
foundation. The critical depth is that depth below which the influence of
pressure in the soil from the foundation is negligible.
9. Expansive and Collapsible Soils
Collapsible and expansive soil can
have large strength and bearing capacity when they are fairly dry. However, the
volume of these soils changes due to changes in water content. This leads to
total and differential foundation movements. Seasonal wetting and drying cycles
may cause soil movements that often lead to excessive long-term deterioration
of structures with substantial accumulative damage.
10. Potential Heave
The potential heave can be determined
from results of consolidometer test which can be performed in accordance with ASTM
D 4546. The results of this test is considered in determining preparation of
foundation soils to reduce destructive differential movements and to provide a
foundation of sufficient capacity to withstand or isolate the expected soil
heave.
11. Soil Reinforcement
Bearing capacity of soft or weak
soil can be increased greatly by installing various forms of reinforcement in
the soil like metal ties, strips, or grids, geotextile fabrics, or granular
materials.
12. Soil Erosion and Seepage
Erosion of soil around and under
foundations and seepage can reduce bearing capacity and can cause foundation
failure.