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In geotechnical engineering, 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; allowable bearing capacity is the ultimate bearing capacity divided by a factor of safety. Sometimes, on soft soil sites, large settlements may occur under loaded foundations without actual shear failure occurring; in such cases, the allowable bearing capacity is based on the maximum allowable settlement.

There are three modes of failure that limit bearing capacity: general shear failure, local shear failure, and punching shear failure.

Introduction[edit]

A foundation is the part of a structure which transmits the weight of the structure to the ground. All structures constructed on land are supported on foundations. A foundation is, therefore, a connecting link between the structure proper and the ground which supports it.

General shear failure[edit]

The general shear failure case is the one normally analyzed. Prevention against other failure modes is accounted for implicitly in settlement calculations.[1] There are many different methods for computing when this failure will occur.

Terzaghi's Bearing Capacity Theory[edit]

Karl von Terzaghi was the first to present a comprehensive theory for the evaluation of the ultimate bearing capacity of rough shallow foundations. This theory states that a foundation is shallow if its depth is less than or equal to its width.[2] Later investigations, however, have suggested that foundations with a depth, measured from the ground surface, equal to 3 to 4 times their width may be defined as shallow foundations(Das, 2007).

Terzaghi developed a method for determining bearing capacity for the general shear failure case in 1943. The equations are given below.

For square foundations:

 q_{ult} = 1.3 c' N_c + \sigma '_{zD} N_q + 0.4 \gamma ' B N_\gamma \

For continuous foundations:

 q_{ult} = c' N_c + \sigma '_{zD} N_q + 0.5 \gamma ' B N_\gamma \

For circular foundations:

 q_{ult} = 1.3 c' N_c + \sigma '_{zD} N_q + 0.3 \gamma ' B N_\gamma \

where

 N_q = \frac{ e ^{ 2 \pi \left( 0.75 - \phi '/360 \right) \tan \phi ' } }{2 \cos ^2 \left( 45 + \phi '/2 \right) }
 N_c = 5.7 \ for φ' = 0
 N_c = \frac{ N_q - 1 }{ \tan \phi '} for φ' > 0
 N_\gamma = \frac{ \tan \phi ' }{2} \left( \frac{ K_{p \gamma} }{ \cos ^2 \phi ' } - 1 \right)
c′ is the effective cohesion.
σzD′ is the vertical effective stress at the depth the foundation is laid.
γ′ is the effective unit weight when saturated or the total unit weight when not fully saturated.
B is the width or the diameter of the foundation.
φ′ is the effective internal angle of friction.
K is obtained graphically. Simplifications have been made to eliminate the need for K. One such was done by Coduto, given below, and it is accurate to within 10%.[1]
 N_\gamma = \frac{ 2 \left( N_q + 1 \right) \tan \phi ' }{1 + 0.4 \sin 4 \phi ' }

For foundations that exhibit the local shear failure mode in soils, Terzaghi suggested the following modifications to the previous equations. The equations are given below.

For square foundations:

 q_{ult} = 0.867 c' N '_c + \sigma '_{zD} N '_q + 0.4 \gamma ' B N '_\gamma \

For continuous foundations:

 q_{ult} = \frac{2}{3} c' N '_c + \sigma '_{zD} N '_q + 0.5 \gamma ' B N '_\gamma \

For circular foundations:

 q_{ult} = 0.867 c' N '_c + \sigma '_{zD} N '_q + 0.3 \gamma ' B N '_\gamma \

 N '_c,  N '_q and  N '_y , the modified bearing capacity factors, can be calculated by using the bearing capacity factors equations(for  N_c, N_q, and N_y, respectively) by replacing the effective internal angle of friction(\phi ') by a value equal to  : tan^{-1}\, (\frac{2}{3} tan \phi ') [2]

Factor of Safety[edit]

Calculating the gross allowable-load bearing capacity of shallow foundations requires the application of a factor of safety(FS) to the gross ultimate bearing capacity, or:

 q_{all} = \frac{q_{ult}}{FS} [2]

Meyerhof's Bearing Capacity theory[edit]

In 1951, Meyerhof published a bearing capacity theory which could be applied to rough shallow and deep foundations.[3] Meyerhof (1951, 1963) proposed a bearing-capacity equation similar to that of Terzaghi's but included a shape factor s-q with the depth term Nq. He also included depth factors and inclination factors. For more information click here.

See also[edit]

References[edit]

  1. ^ a b Coduto, Donald (2001), Foundation Design, Prentice-Hall, ISBN 0-13-589706-8 
  2. ^ a b c Das, Braja (2007), Principles of Foundation Engineering (6th ed.), Stamford, CT: Cengage Publisher 
  3. ^ {{|publisher=CRC Press LLC|id=|}}

External links[edit]


Original courtesy of Wikipedia: http://en.wikipedia.org/wiki/Bearing_capacity — Please support Wikipedia.
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86 news items

 
DailyPost Nigeria
Fri, 18 Apr 2014 08:11:15 -0700

The Commissioner added that the council also noted that the present nature and increased volume of traffic using the bridge is beyond its bearing capacity. Continuing he stated that the sum of one hundred and six million, seven hundred and six naira ...

Northamptonshire Telegraph

Northamptonshire Telegraph
Wed, 16 Apr 2014 23:56:15 -0700

The move was made by Northamptonshire County Council after a structural survey by bridge owner Tata Steel raised concerns about its load-bearing capacity. Concrete chicanes were installed to prevent HGVs going over the bridge. The county council is ...
 
The Star
Wed, 16 Apr 2014 07:02:08 -0700

Erin Redwood was interested in checking out how moisture can have an impact on bearing capacity on a given object – that is, how much weight it can hold. The North Augusta Middle School eighth-grader tried it, and judges liked her efforts at the ...
 
PR Web (press release)
Wed, 16 Apr 2014 02:11:15 -0700

URETEK Holdings, Inc. specializes in improving the weight-bearing capacity of subsurface soils through the injection of patented, lightweight, expanding structural polymers. The patented URETEK Deep Injection Method provides the industry's most ...
 
Business Mirror
Tue, 15 Apr 2014 04:26:15 -0700

The land undergoes a comprehensive soil investigation, a process of determining the soil-bearing capacity to properly design the roads and houses and ensure its structural integrity. Aside from soil investigation, a flood study is conducted to ...

Dredging Today

Dredging Today
Sun, 13 Apr 2014 23:56:15 -0700

... drainage board driving and site leveling of sand-gravel piles. After its completion, the project will improve the bearing capacity of the foundation of Dachanwan port zone in Shenzhen and play a significant role in promoting regional economic ...
 
Broomfield Enterprise
Fri, 11 Apr 2014 08:57:29 -0700

The construction will increase runway weight bearing capacity from 75,000 pounds to 100,000 pounds, which will make it easier to accommodate different types of planes, Miller said. The airport sees about 120,000 takeoffs and landings each year. The ...
 
Assam Tribune
Sat, 12 Apr 2014 11:00:00 -0700

The load-bearing capacity of the bridge was 10 tonnes, said youth leader Apang Welly of Sebba village. While appreciating the company for deploying equipments to construct the 174-km Nechipu-Sagalee road, part of the Trans-Arunachal Highway, Welly ...
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