Generally, a shallow foundation refers to a foundation whose depth equals the width. It is important to understand the assumptions behind various design procedures rather than getting their definition. Shallow foundation involves transferring building loads to the earth near the surface, rather than to a subsurface layer. Types of shallow foundations include spread footing, mat-slab, slab-on-grade foundations, combined footings and continuous strip spread footing
In spread footing, foundations pads of concrete are used to transfer loads from walls and columns to the bedrock. Embedment of spread footing is affected by penetration of soft near surface layers, development of lateral capacity among others (George, 2003, p.42). This type of foundation is common in commercial structures and in residential construction. Mat-slab foundations are commonly used to lower contact pressure by distributing heavy column and wall loads across the whole building area thus considered superior to the conventional spread footing foundation. Various factors are considered while designing mat slab foundation (Houlsby, 2008). They include determining stress distribution of foundation, determining differential settlement and determining the bearing capacity of the foundation. Gross ultimate bearing capacity is calculated using the support method as.
Slab on grade foundations is the one where concrete slab is formed from a mold set into the ground. The concrete slab is then placed on the mold leaving no space between the structure and the ground. This type of foundation is commonly used in warmer climates where there is no need of heat ducting underneath the floor. It is advantageous in that it is cheap and less vulnerable to termite infestation. On the other hand, it is disadvantageous in that it is not accessible from below and has the potential of losing a lot of temperature.
In combined footing, a minimum of two columns are supported on a rein forced concrete footing. In most cases they are used where one of the columns is along a property line and it is impossible to extend the spread footing beyond the edge of the building. Continuous strip spread footing is commonly used in foundations. Strip footings have a minimum length to width ratio of at least 5. The strip footings support a single a bearing wall to reduce pressure on bearing materials (French, 2001, p.10) . Shallow foundation is the most common structural foundation in the construction industry today owing to the fact that it is affordable, construction procedure is simple and labor does not involve a lot of expertise. Shallow foundation is also subject to some disadvantages which include irregularity in the ground surface and the foundation is subject to torsion, pullout and moment.
The design of shallow foundation should be reliable, cost effective and the design should meet its function. Design should be reliable in terms of safety and serviceability. In shallow foundation the engineer should cover issues relating to rock structure, nature of soil and seismic forces. The dynamic characteristics of the subsoil should be determined as well as choosing a suitable form of substructure. It is of importance to do some analysis of the soil when making the foundation due to the fact that soil have been blended, restratified chemically modified, eroded and later deposited in a different location (French, 2001, p.17) Also the foundation should be designed in a manner that it will cater for seismic forces which emanate from deformation of adjacent soil and earthquakes acting in the super structure. In earthquake prone areas, factors such as provision for earthquake overturning moment, differential settlements and nature of the top soil should be considered. Buildings are less affected by overturning moments unless they are slender. The force of an earthquake is determinable and the only task is to determine the magnitude of the seismic forces. The current design practice considers the vertical and the horizontal seismic stresses and necessary measures to counter the stresses when designing.
Modeling of shallow structures will be discussed in the context of offshore structures. Offshore structures are sensitive and their performance mainly depends on their foundation. In modeling, structural analysis is normally incorporated because it creates a numerical model for finite element analysis. This is achieved by making some assumptions that the foundation is rigid and built on elastic soil and the spudcans of a jack-up are attached to the seabed. Then, standard solutions which will address the stiffness factors on the rigid circular foundation should be employed in finite element analyses in either the frequency or the time domain (Houlsby, 2008). However, because soil is not a linear elastic material modeling will be considered unrealistic especially under extreme load conditions. Due to this, some models within the plasticity theory will be used to address the non-linearity of soil. Plasticity model involve the non-linear behavior of soil thus their analysis can only be done in the frequency domain rather than the time domain (George, 2003, p.42).
Of influence in the shallow foundation is the groundwater. The rise and fall of the water table as well as the chemistry do affect the soil load capacity and the type of foundation. Rain water is of much importance than the water from below because it contributes to the rigidity of a foundation (French, 2001, p.33).
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