Bearing Capacity of Shallow Foundations (Research Paper Sample)

Bearing Capacity of Shallow Foundations
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Introduction
Foundation forms the lowest part of a structure, and performs the function of transferring an aggregate load of a given structure to the type of soil on which it stands and resists. A well-designed foundation exchanges the given load all through the ground without causing much stress on the soil. Imposing more stress on the soil leads to shear failure or excessive settlement of the soil thus causing damages to the structure (Carter and Liu 2005, p.118). Therefore, it is essential for structural and geotechnical engineers, who specialize in the foundation design, to critically evaluate the bearing limit of the soil amid design procedures. The most extreme load, can apply on the soil that is subgrade on a foundation without developing any form of any failure of the shear, and where limiting settlement in a permissible upper bond so that serviceability damages of the superstructure can be prevented (Gourvenec 2008, p.182).
There exist different types of foundations in building construction depending on the nature of the soil for a given site and the structure to be established. Pile and drilled shaft foundations are designed in buildings involving massive structures where significant depth is needed to support the load (Carter and Liu 2005, p.116). Piles consist of structural members that are made of concrete, steel or timber that helps in transmitting the loads to the lower layers of the soil (Carter and Liu 2005, p.112). These piles are usually divided into two main categories, which are end-bearing piles and friction piles. Loads on the end-bearing piles are transferred at its tip to a firm stratum underneath (Jia 2018, p.114). In the event that the friction piles, the force of the shear was that generated along the surface of the pile resist the heavy loads of the superstructure.
Shallow Foundations versus Deep Foundations
Shallow foundations consist of mat foundations and spread foundations. These types of foundations have a ratio of a depth-of-embedment-to-width slightly less than four. Foundations which have the ratio of depth-of-embedment-to-width higher than four are considered deep foundation (Jia 2018, p.118).
Below is a representation of deep foundations, which are different from shallow ones:
Figure SEQ Figure \* ARABIC 1: Pile foundation, an example of deep foundations, different from shallow foundations (Reese, Isenhower, and Wang 2006 p.574)
Figure SEQ Figure \* ARABIC 2: Drilled Shaft foundation (Carter and Liu 2005, p.112).
Types of shallow foundations
Various forms of shallow foundations are utilized in putting up of buildings in existence. According to Carter and Liu (2005, p.123), shallow foundations are applied where soil layer has a shallow depth of maximum 1.5 meters that can support structural loads. Shallow foundations are usually smaller than their width, as illustrated below.
Strip footing
This type of foundation is provided during the construction of structures that have a load-bearing wall as well as for raw columns that are closely spaced (Meyerhof 2006, p.198). Strip footing of the raw lie on each other alternatively or in contact with each other. Other foundations that are shallow are always replaced by the strip footing, such as isolated footing, raft mat foundation, and spread footing (Reese, Isenhower, and Wang 2006, p.574).
Figure SEQ Figure \* ARABIC 3: illustration of strip foundation, a type of shallow foundation (Reese, Isenhower, and Wang 2006 p.574).
Spread Footing
This type of shallow foundation is also referred to as isolated pad or footing that used to support individual column during construction (Wotherspon, Pender, and Ingham 2014, p.414). This type of foundation takes different shapes, such as square, circular, and rectangular among others that are designed to form a slab of uniform thickness from which a column can be laid (Reese, Isenhower, and Wang 2006, p.574). Besides, it can be placed in different ways that include stepping or haunching, spreading the load over a large area.
Figure SEQ Figure \* ARABIC 4: Spread foundation, a type of shallow foundation (Reese, Isenhower, and Wang 2006 p.574)
Strap or cantilever footing
This type of foundation is similar to spread footing except the fact cantilever footing comprises of two distinct footings that are connected to each other using structural lever or strap (Carter and Liu 2005, p.124). The purpose of holding two footings is to make them behave as one unit while transmitting loads to the ground. The design of individual strap is done in such a manner that their line of action that are combined goes through the resultant of the summation of the load. Notably, the strap or cantilever footing is less expensive as compared with a combined footing under conditions of relatively high allowable soil pressure and the large distance between columns.
Figure SEQ Figure \* ARABIC 5: Shows the elevation of strap foundation (Reese, Isenhower, and Wang 2006 p.574)
Figure SEQ Figure \* ARABIC 6: plan for strap foundation (Reese, Isenhower, and Wang 2006 p.574)
Raft or Mat foundations
The foundation is composed of a big slab that support numerous columns and walls under a sizeable portion of the structure or the entire structure. This type of foundation is most applicable in situations where walls and columns are so close or allowable soil pressure is low (Wotherspon, Pender, and Ingham 2014, p.417). Under these conditions, individual footings are likely to move too close to each other or overlap. The foundation is essential in minimizing the differential settlements that can occur in non-homogeneous soils or in constructions that may experience large variations in the loads on particular columns (Liyanapathirana, Carter, and Liu 2006, p.43).
Figure 2: Mat foundation (Liyanapathirana, Carter, and Liu 2006, p.43)
Bearing capacity for undrained shallow foundations
Most buildings are set up on shallow foundations, bearing directly on either human-made or natural soils. The soils on which the buildings are set up are situ, and they behave in a different way compared to similar materials that have been interfered with or those taken to the laboratory for testing (Eslami and Fellenius 2007, p.886). Various studies show significant progress in devising advanced models that incorporate the impact of the structure of the soil on the capacity of the bearing (Carter and Liu 2005, p.124). Destructuring is a critical phase during which the entire structure of soil can be partially or wholly lost with only infinitesimal alterations in stress state causing excessive strains (Eslami and Gholami 2002, p.94). As a result, substantial errors in the determination of foundation behaviour may develop if the influence of the structure of the soil is neglected while evaluating predictions.
Various researchers establish equations for the assessment of the capacity of the bearing of undrained foundations which are shallow. The equations developed by Terzaghi consider the foundation size, shape, the soil properties variations, and depth embedment of the soil (Eslami and Fellenius 2007, p.883). However, these equations fail to consider the impact of the structure of soil on the resistance of the bearing of the foundation. The engineers usually apply the factor of safety in design, which has enabled them to develop shallow foundations on man-made and natural soils by ignoring the impact of the structure of the soil on the behaviour of the bearing. Zdravković, Potts, and Jackson (2003, p. 9) note that it is essential to incorporate the complex behaviour of structured soil while making predictions of bearing capacity, especially during the design of offshore structures.
Researchers have carried out various numerical simulations to establish the impact of the structure of the soil based on the response of the displacement of the load of relatively foundations that are shallow. The simulations incorporate Structured Cam Clay (SCC) model that is derived from the Buland’s Modified Cam Clay model (Zdravković, Potts, and Jackson (2003, p. 9).
Structured Cam-clay model
This model incorporates six primary parameters defining the soil structure besides the ordinary parameters for deformed soil behaviour outlined in the Modified Clay model (Eslami and Gholami 2002, p.94). The extra parameters are represented with symbols b, Pco, y, w, a and c, where;
b refers to destructuring index that indicates the rate of deformation
Pco denotes the size of the first yield surface
y indicates the shearing effect of destructuring
w denotes the soil influence of the structure of the soil on the potential plasticity of the soil., Its value is corresponding with the value of y.
a and c represent the additional ratio of voids of the soil , given by the equation;
lefttopIn the above equation p’ shows the current yield surface size.
Finite Model
Analysis of soil sample is done using finite element mesh that consist of 15 nodded cubic strain triangles (Eslami and Fellenius 2007, p.894). This element can perform accurate computations of within the plastic range in problems involving axial symmetry during the undrained loading (Liyanapathirana et al. 2006, p.39).
The capacity of embedded foundations that are shallow under pure moment (M), vertical (V) or horizontal (H) load can be established using the factors of depth wh...