What is bearing capacity of soil?
The pressure which the soil can safely withstand is called allowable bearing pressure or bearing capacity of the soil.
A foundation is used for distributing loads of the superstructure in a large area. The foundation should be designed such that (a) the soil below does not fail in shear and (b) the settlement is within the safe limits.
A foundation is that part of the structure that transmits the weight of the structure to the ground. All the structures which are constructed on land are supported on foundations. Therefore, a foundation is a connecting link between the structure proper and the ground which supports it.
Some Definitions :-
Ultimate bearing capacity of soil (qu)
The ultimate bearing capacity is defined as the gross pressure at the base of the foundation at which the soil fails in shear.
Net ultimate bearing capacity of soil (qnu)
Net ultimate bearing capacity is defined as the net rise in pressure at the base of foundation due to which shear failure of the soil. Net ultimate bearing capacity is equal to the gross pressure minus overburden pressure.
Net ultimate bearing capacity of soil formula,
Safe bearing capacity of soil (qns)
It is defined as the net soil pressure safely applied to the soil considering that only shear failure. It is obtained by dividing the net ultimate bearing capacity by a suitable factor of safely.
Safe bearing capacity of soil formula,
Where, F = Factor of safety , which is equalto 3.0
Gross Safe Bearing Capacity (qs)
Gross safe bearing capacity is defined as the maximum gross pressure that soil can carry safely without shear failure. It is equal to the net bearing capacity plus the original overburden pressure. Thus,
Gross Safe Bearing Capacity formula,
Net Safe Settlement Pressure (qnp)
It is defined as the net pressure which the soil can carry without exceeding the permissible settlement. The maximum allowable settlement varies from 25 mm to 40 mm for individual footing.
Net Allowable Bearing Pressure (qna))
It is the net bearing pressure which can be used for the design of foundations.
As the requirement for the design of foundation are that there should be no shearing failure and moreover the settlements should also be within the limits, the allowable bearing pressure is the smaller of the net safe bearing capacity (qns) and the net safe settlement pressure. Thus,
qna = qns if qnp> qns
qna = qnp if qns> qnp
The net allowable bearing pressure is also called as the allowable soil pressure or allowable bearing pressure or allowable bearing capacity.
Factors affecting bearing capacity of soil
For cohesion less soil,
The factors that affect bearing capacity of soils are as follows;
- Relative density of soil
- Depth of footing
- Unit weight of soil
- Width of footing
- Location of water table
Relative density is more for dense sand and medium sand compared to loose sand.
Hence, if relative density increases the bearing capacity also increases.
Width of footing
From equation (1), width of footing is directly proportional to the bearing capacity i.e. as width of footing increases, bearing capacity also inc
Settlement of foundation on cohesion less soil
The settlement of the foundation on cohesionless soils take place rather quickly after the application of the load. The settlements are generally determined indirectly using the semi-empirical methods.
Static cone penetration method
In this method, the sand layer is divided into small layers such that each small layer has an approximately constant value of the cone resistance of each small layer is determined.
Standard penetration test
A standard penetration test can be used for the determination of the settlement on cohesionless soils.
Plate load test
The settlement of the footing can be estimated from the settlement of the plate in the plate load test. The allowable bearing pressure can be calculated by conducting a plate load test at the site.
The conduct a plate load test, size of the pit is 5Bp x 5Bp, where Bp is the plate size, Which is excavated to a depth equal to the depth of foundation (Df).
The size of the plate is generally 0.3 m square. It is made of steel and is a thickness of 25 mm. Occasionally, circular plates are also used. Sometimes, large size plates of 0.6 sq. m are used.
A central hole of the size Bp x Bp is excavated in the pit. The depth of the central hole (Dp) is found from the following relation:
Dp/Bp = Df/Bf
Dp = (Bp/Bf) x Df
Where, Bf is the width of the pit, and Bp is the size of plate.
How to calculate bearing capacity of soil from plate load test?
For conducting the plate load test, the plate is placed in the central hole and the load is applied by a hydraulic jack. The reaction to the jack is provided by a reaction beam.
Sometimes, trusses are used instead of a reaction beam to take up the reaction. Alternatively, a loaded platform may be used to provide reaction. A seating load of 7 KN/m2 is first applied, which is released after some time.
Then, the load is applied in increments of about 20% of the estimated safe load or one-tenth of the ultimate load. The settlement is recorded after 1, 5, 10, 20, 40, 60 minutes, and further after an interval of one hour.
These hourly observations are continued for clayey soils until the rate of settlement is less than 0.2 mm per hour. The test is conducted until failure or at least until the settlement of above 25 mm has occurred.
The ultimate load for the plate qu(p) is indicated by a break on the log-log plot between the load intensity q and the settlements.
If the break is not well defined, the ultimate load is taken as that corresponding to a settlement on one-fifth of the width (Bp). On the natural plot, the plate load test is obtained from the intersection of the tangents drawn as shown.
Limitation of plate load test
The plate load test has the following limitations:
- Size effect: The results of the plate load test reflect the strength and therefore the settlement characteristics of the soil within the pressure bulbs.
- Scale effect: The ultimate bearing capacity saturated clays are independent of the plate size but it increases with the size of the plate for cohesionless soil.
- Time effect: A plate load test is actually a test of short duration. For clayey soils, is doesn’t give the ultimate settlement. The load- settlement curve isn’t truly representative.
- Interpretation of failure load: The failure load isn’t well-defined, except in the case of general shear failure. An error may be involved in other types of failure.
- Reaction load: It is not practicable to provide a reaction of greater than 250 KN. Hence, the test on a plate of size more than 0.6 m widths is difficult.
- Water table: The water table level affects the bearing capacity of the sandy soils. If the water table is above the footing level, it has to be lowered by pumping before placing the plate.
How to increase bearing capacity of soil?
The bearing capacity of soil can be increased by the following methods;
Increasing the foundation depth
Increasing the depth of the foundation is the general method for increasing the bearing capacity of the soil. Because the soil has a high pressure at more depth. Hence it will be more compact.
Draining the subsoil
Drainage is another well-known method that increases the bearing capacity of the soil. Drains are placed near the footing base, then the subsoil water is collected and drained out by a pipe in this process.
Blending granular materials
In this method, a certain amount of granular materials i.e. crushed stone, gravel, sand, etc is blended into the weak soil by ramming. Hence the poor soil becomes stronger and bearing capacity is increased.
Confining the soil
The soil is confined with the help of sheet piles in a bounded area. This method is suitable for sandy soils.
Driving sand piles
This method is also helpful to increase the bearing capacity of soft soil by reducing the void volume. Here, wooden piles are used to make holes in the weak soils, and then sand is filled into the holes by ramming.
I hope this article remains helpful for you.
Happy Learning – Civil Concept
Civil Engineer – Pradip Thakur