Bearing Capacity Of Soil | 7-Effective Methods To Improve It.
What Is The Bearing Capacity Of Soil
Bearing Capacity of soil is a frequently heard term in Civil & Geotechnical Engineering. Which deals with the feasibility study of characteristics of soil whether the soil over which the structure is going to be constructed is capable enough to withstand its load. In this article, we are going deep dive into what is Bearing Capacity of soil and how to improve it in order to build a safe structure over the soil.
The foundation should be designed to satisfy two essential conditions:
- It must have some specified safety against ultimate failure.
- The settlements under working loads should not exceed the allowable limits for the super-structure.
The value of bearing capacity of the soil used to design foundations (i.e. for determining the dimensions of the foundations) is determined on the basis of the above two criteria.
In general, the supporting power of a soil or rock is referred to as its bearing capacity. The term bearing capacity is defined after attaching certain qualifying prefixes, as defined below :
1. Gross pressure intensity (q)
The gross pressure intensity q is the total pressure at the base of the footing due to the weight of the super-structure, self-weight of the footing, and the weight of the earth fill if any.
2. Net Pressure intensity (qn)
It is defined as the excess pressure or the difference in intensities of the gross pressure after the construction of the structure and the original overburden pressure. Thus, if D is the depth of the footing
where y is the unit weight of soil above the level of footing.
3. Ultimate bearing capacity (qf)
The ultimate bearing capacity of soil is defined as the minimum gross pressure intensity at the base of the foundation at which the soil fails in shear.
4. Net ultimate bearing capacity (qnf)
It is the minimum net pressure intensity causing shear failure of the soil. The ultimate bearing capacity of soil qf and net bearing capacity qnf are evidently connected by the relation
qf = qnf + γ D
5. Net safe bearing capacity (qns)
The net safe bearing capacity is the net ultimate bearing capacity divided by a factor of safety F :
qns = qns / F
6. Safe bearing capacity (qs)
The maximum pressure which the soil can carry safely without the risk of shear failure is called the safe bearing capacity. It is equal to the net safe bearing capacity plus original overburden pressure
qs = qns + γ D= (qnf/F)+ γ D
Sometimes, the safe bearing capacity is also referred to as the ultimate bearing capacity qf divided by a factor of safety F.
7. Allowable bearing pressure (qa)
It is the net loading intensity at which neither the soil fails in shear nor there is excessive settlement detrimental to the structure in question. The allowable bearing pressure thus depends both on the sub-soil and the type of building concerned, and is generally less than, and never exceeds, the safe bearing capacity of soil.
Methods of Estimating The Safe Bearing Capacity
The bearing capacity of soil can be determined by the following methods :
- Analytical methods involving the use of soil parameters
- Plate load test on the soil
- Penetration test
- Presumptive bearing capacity values from codes
ANALYTICAL METHODS TO DETERMINE BEARING CAPACITY OF SOIL
A number of analytical methods have been developed to determine the ultimate bearing capacity of the soil. These methods use two important shear parameters of soil: (i) Angle of internal friction Φ and (ii) cohesion c.
These parameters are determined in the laboratory, by conducting shear tests on soil samples (preferably, undisturbed samples) collected from the boreholes or test pits. Out of the various theories developed, only two are briefly given here:
(i) Rankine’s analysis and (ii) Terzaghi’s analysis.
Rankine considered the equilibrium of two soil elements, one immediately below the foundation (an element I) and the other just beyond the edge of the footing (element II), but adjacent to element I. When the load on the footing increases and approaches a value qnf. A state of plastic equilibrium is reached under the footing.
Rankine’s Formula of calculating the minimum depth of the foundation is expressed by.
Minimum depth of foundation Dmin=P/W (1-sin Φ /1+sin Φ) 2
where P=gross bearing capacity
W =density of soil
Φ =angle of the response of soil
An analysis of the condition of complete bearing capacity failure, usually termed as general shear failure was made by Terzaghi by assuming that the soil behaves like an ideally plastic material.
Terzaghi’s gave the following equation for local shear failure:
Terzaghi’s Bearing capacity equations:
Qu = c Nc + γ D Nq + 0.5 γ B Nγ
Qu = 1.3 c Nc + γ D Nq + 0.4 γ B Nγ
Qu = 1.3 c Nc + γ D Nq + 0.3 γ B Nγ
C: Cohesion of soil, γ: unit weight of soil, D: depth of footing, B: width of the footing.
Nc, Nq, Nr: Terzaghi’s bearing capacity factors depend on soil friction angle, φ.
Nq=e2 (3π/4-φ/2)tanφ / [2 cos2(45+φ/2)]
Nγ=(1/2) tanφ( Kpr /cos2φ -1)
Kpr=passive pressure coefficient
What Are The Test For Soil Bearing Capacity?
PLATE LOAD TEST
Plate Load Test is a field test to determine the ultimate bearing capacity of the soil and the probable settlement under a given loading. The test essentially consists of loading a rigid plate (usually of steel) at the foundation level and determining the settlements corresponding to each load increment. The ultimate bearing capacity is then taken as the load at which the plate starts sinking at a rapid rate.
These tests involve the measurements of the resistance to penetration of a sampling spoon, a cone, or another shaped tool under dynamic or static loadings. The resistances empirically correlated with some of the engineering properties of soil, such as density index, bearing capacity, etc. Two commonly used penetration tests are
- Standard penetration test
- Dutch cone test.
Standard Penetration Test
The test (IS: 2131-1981) is performed in a clean hole, 55 to 150 mm in diameter. A casing or drilling mud shall be used to support the sides of the hole. A thick wall with split tube sampler, 50.8 mm outer dia. and 35 mm internal dia. is driven into the undisrupted soil at the very bottom of the hole under the blows of 65 kg drive weight with 75 cm free fall.
The minimum open length of the sampler should be 60 cm. The sampler is first driven through 15 cm as a seating drive. It is further driven through 30 cm and the number of blows required for this is counted. This number of blows is termed penetration resistance N.
Dutch Cone Test
This test is used for getting a continuous record of the resistance of soil by penetrating steadily under static pressure a cone with a base of 10cm2 (3.6 cm in dia.) and an angle of 60° at the vertex. The cone is carried at the lower end of a steel-driving rod which passes through a steel tube (mantle) with an external diameter equal to the base of the cone. Either the cone or the tube, or both together can be forced into the soil by means of a jack.
To know the cone resistance, the cone alone is first forced down for a distance of 8 cm and the maximum value of resistance is recorded. The steel tube is then pushed down up to the cone, and both together are further penetrated through a depth of 20 cm to give the total cone resistance and the frictional resistance along the tube.
The cone test is considered very useful in determining the bearing capacity of soil pits in cohesionless soils, particularly in fine sands of varying density. The cone resistance q. (kg/em’) is approximately equal to 5 to 10 times the penetration resistance N.
PRESUMPTIVE BEARING CAPACITY FROM BUILDING CODES
For the design of foundations of lightly loaded structures and for a preliminary design of any structure the presumptive safe bearing capacity may be used. The presumptive safe bearing capacities of various types of soils are given in the Table given by the National Building Code.
Types of Soil
Soft, wet clay or muddy clay
Fine, loose, and dry sand
Black cotton soil
Moist clay and sand-clay Mixture
Medium, compact and dry sand
Laminated rock such as sandstone & Limestone
Hard rocks such as granite, diorite, trap
What Are The Methods To Improve Safer Bearing Capacity Of Soil?
The safe bearing capacity of soil can be improved in many ways
Sometimes, the safe bearing pressure of soil is so low that the dimensions of the footings work out to be very large and uneconomical. In such a circumstance, it becomes essential to improve the safe bearing pressure, which can be done by the following methods
- Increasing depth of the foundation
- Compacting the soil
- Draining the soil
- Confining the soil
- Replacing the poor soil
- Chemical treatment.
#1. Increasing depth of the foundation
It has been found that in granular soil, the bearing capacity increases with the depth due to the confining weight of overlying material. However, this is not economical since the cost of construction increases with depth. Also, the load on the foundation also increases with an increase in depth. The method is useful only when a better bearing stratum is encountered at greater depth.
#2. Compaction of soil
It has found that compaction of natural soil deposits (loose) or man-made fills results in the improvement of bearing capacity and reduction in the resulting settlements. Compaction of soil can be effectively achieved by the following means:
- Ramming moist soil
. The foundation soil is moistened and then compacted with the help of hand rammers or mechanically operated frog rammers or vibratory rollers. The voids of the soil are very much reduced, resulting in a reduction in settlements.
- Rubble Compaction into the soil
A layer of 30 to 45 cm thick well-graded rubble is spread over the foundation level and well-rammed. If this layer of rubble gets buried in the soil (especially when it is very loose) another layer of 15 cm thick rubble is spread and well rammed manually. This results in an increase in the bearing value of the soil.
- Flooding the soil
The bearing pressure of very loose sands can be increased by flooding the soil. The method is very effective in improving the safe bearing pressure of dune sands, which cannot otherwise be effectively compacted. The Authors have experience improving the bearing power of desert soils by this method at many locations where it was required to support heavy loads.
Heavy vibratory rollers and compactors may compact a layer of granular soils to a depth of 1 to 3 m. If the method of flooding and then vibration is used, sandy soil can be very effectively compacted, resulting in increased safe bearing power and decreased settlements when super-structure loads come on the soil. After flooding the soil, so that moisture penetration is at least 1 to 2 m, form vibrators or platform vibrators (about 1 m X 1.5 m base area, with a pair of eccentrically loaded motors) can be slid on the sand surface with the help of two laborers. A large area can be covered by this process, without the help of sophisticated vibrating equipment.
It is a commercial method that combines the effect of vibration and jetting. A heavy cylinder, known as a vibroflot is inserted in the ground (soil) while the cylinder vibrates due to a rotary eccentric weight. A water jet on the tip of the vibroflot supplies a large amount of water under pressure. As the vibroflot sinks, the clean sand is added into a crater that develops on the surface. The method is very useful when the foundation is required to support heavy loads spread over a greater area.
- Compaction by pre-loading
This method is useful when the footing is founded on clayey soils which result in long-term settlements. Pre-loading results in accelerated consolidation, so that settlements are achieved well before the actual footing is laid. The load used for this process is removed before the construction of the footing.
- Using sandpiles
This method is very useful in sandy soils or soft soils. Hollow pipes are driven in the ground, at close intervals. This results in the compaction of soil enclosed between the adjacent pipes. These pipes are then gradually removed, filling and ramming sand in the hole, resulting in the formation of sandpiles.
#3. Drainage of soil
It is a well-known fact that the presence of water decreases the bearing power of the soil, especially when it is saturated. This is because of the low shearing strength of soil in presence of excess water. Drainage results in a decrease in the voids ratio, and an improvement in bearing power.
#4. Confining the soil.
Sometimes the safe bearing pressure of the soil is low because of settlements resulting due to the lateral movement of loose granular soil. Such a tendency of lateral movement can be checked by confining the soil, outside the perimeter of the foundation area, by driving sheet piles, thus forming an enclosure and confining the soil.
#5. Relacing the poor soil
Replacing the poor quality of soil is one of the remedial measures to improve the bearing capacity. the loose or marshy soil can be removed and replaced by good compacting materials like filling sand or murrums soils.
This method is useful in loose gravels and fissured rocky strata. Bores holes in sufficient numbers are driven in the ground and cement grout is forced through these under pressure. The cracks, voids, and fissures of the strata are thus filled with the grout, resulting in an increase in the bearing value. 6. Chemical treatment. In this method, certain chemicals are grouted in the place of cement grout. The chemical should be such that it can solidify and gain early strength.
#7. Chemical Treatment
Chemical treatment of soil is a process of grouting certain chemicals to the soil in place of cement grout. The chemical should have properties to make the soil solid and help to gain early strength.
The Conclusion, in this above post we have discussed detailed about the bearing capacity of soil and different test to determine the value of soil bearing capacity and various methods to improve it.