Shallow and Deep Foundation Construction - 2750 Words

Shallow and Deep Foundation Construction (Research Paper Sample) Content: Shallow and Deep Foundation Construction Name Institution Affiliation Shallow and Deep Foundation Construction In structural construction, it is mandatory for contractors to design and construct foundations. Foundations are the lowest parts of the building whose purpose is to transmit the weight of the building to the underlying soil. The design used to construct a foundation is dependent on the type of soil, the load on the structure, purpose of the structure and materials used to construct the structure. Two categories of foundations are used in modern construction. These are deep and shallow foundations. A shallow foundation is rectangular or square footing that supports columns, walls, and other structures. Shallow foundations are normally less than 6ft in depth, or their depths are equal to their widths. On the other hand, deep foundations are those foundations that stretch deep beyond the surface of the topsoil in the excavated site. These are depths beyond 6ft from the finished ground. Shallow Foundations Shallow foundations are those foundations that are constructed near the ground surface on the site of construction. In shallow foundations, as a rule of thumb, the founding depth is normally less than the width of the footing. The optimal depth is usually 3-4m. If the bearing capacity is affected by the surface conditions or surface loading, then this foundation constitutes a shallow foundation. Two chief factors are considered when designing shallow foundations. Firstly, the pressure applied on the foundation must not exceed the bearing capacity of the supporting soil on which the foundation rests and secondly, the foundation settlement should not be exceeded as a result of the impact of the applied pressure (Le Pape Sieffert, 2001). Shallow foundations are also referred to as spread footings, and they are classified into several categories, which include among them isolated footings also known as pads, rafts, and strip footings. These foundations find their applications where the surface soils strata are strong and firm enough to hold the imposed structural loads. They are not appropriate or use in weak surface soils or where the soils are highly compressible for example, recent lacustrine, peat, fill or alluvial deposits. There are several advantages of constructing shallow foundations. One is that they are very cheap and easy to construct thus making them affordable. These foundations are mostly made of concrete, which is very competent for firmly holding structural loads and also easily available. Finally, they do not call for advanced expertise in construction. On the contrary, they have a few demerits including limiting the capacity of soil structure, prone to settlements, prone to other surface conditions such as torsion, moment, or pullout and also their stability is dependent on the ground surface topology (Le Pape Sieffert, 2001). Footing Foundations The portion of a structure that serves to transmit its weight to the subsoil is termed as a footing. When the footing supports a single column, then it is referred to as an isolated footing. When it supports a cluster of columns, then it is called a combined footing. When it supports a wall, it is referred to as a strip footing. When the depth of the footing which is the vertical distance between the ground surface and the base of the footing is less that its width, then this footing is known as a shallow footing (Rao, 2010). Sometimes it is necessary to have more columns as opposed to having one column. These columns can be closely or no uniformly spaced. This calls for a combined footing construction. Such footings are advantageous as they help combat rotational moments and large overturning that occur in the longitudinal direction of the column row. Combined footings include trapezoidal footings, mat or raft foundations, cantilever footings and rectangular footings. Types of Shallow Foundations a) Spread/ Isolated Footing These types of foundations are constructed with the purpose of supporting an individual column. They come in various shapes and sizes. Some may be circular, rectangular, square slabs of uniform thickness. In some cases, it is convenient to design them as haunched or stepped to facilitate distribution of the load over a large area. These footings are used in the construction of basements and commercial structures. They normally consist of strips of concrete that facilitate the transfer of column or wall loads to the bedrock. There are some factors such as the penetration that control spread footings. Penetration may result into changed volume due to swell, shrink or frost heave that may occur near the surface layers. b) Mat/ Raft Foundations Usually, these large foundations support many columns and walls. They can be constructed in a large part of the structure or can support the entire structure itself. They are mostly useful when the permissible soil pressure is unfavorably small or where the walls and columns are in proximity such that individual footings overlap or nearly touch each other. Non-homogeneous soils are prone to differential settlements; however, mat foundations can resist this irregularity. They are also efficient where a large variation in loads occurs on the individual columns by alleviating the pressure exerted by the construction materials. Mat foundations are commonly used in the construction of high-rise buildings where thick foundations and extensive reinforcement is required to ensure homogeneous distribution of the load occurs. c) Slab on Grade Foundation This type of foundation is mainly used for supporting structures that have been formed from mold set ground. It is normally elevated through a concrete slab that is inserted into the mold. This ensures that there is no space created between the bedrock and the structure. This type of shallow foundation is popular with contractors who work in warmer climates where heat ducting is not a necessity, and there is no ground freezing or thawing. Slab on grade is cheap to construct and is very sturdy. It is also resistant to attacks from marauder such as termites. d) Strip Footings Strip footings or continuous footings as they are also referred to are basically a row of columns held very closely and are spaced such that their spread foundation overlap or tend almost to touch each other. They are employed to provide support to a load-bearing wall. In such a case, they are preferred as they are more economical to build as compared to a spread footing. e) Combined Footing This is a foundation designed to offer support to two parallel columns, and it is favored where the two columns are in proximity such that their individual columns would overlie. When the property line is too close to a column, it is possible for a spread footing to get eccentrically loaded if maintained within the property line. Thus, a combination with an interior column allows the load to be distributed uniformly. f) Strap/ Cantilever Footing This is formed from two isolated footings that have been connected with a structural lever/strap. This ensures that these two footings work as a single unit although the connecting strap does not oppose any soil reaction. The design of the footings is such that their combined line of action passes through the result of the total structural load. Where the permissible soil structure and the distance between the columns is comparatively greater, this type of footing becomes the most favorable to use. Bearing Capacity This refers to the maximum pressure that a foundation can hold (Pacheco, Danziger, Pinto, 2008). The type of soil over which the foundation stands determines the depth the foundation can penetrate. An effective foundation should have the capacity to transfer the structural load evenly below the ground surface. This ground surface, however, is also influenced by several factors among them being the groundwater level. The water table may be high or low depending on the depth of the bedrock and seasonally shifts depending on the rains. Modeling shallow foundations, therefore, is reliant on the depth of the water table as it is imperative to consider the desired depth to sink the foundation. In places where the water table is very high, certain measures such as dewatering of the grounds have to be undertaken in order to improve the safety of the resultant foundations. Otherwise, these foundations would be catastrophic if unanticipated deformations in the surrounding soils were to occur. To avert such problems, Zeng Steedma (1998) propose constructing walls in the ground that stands over the entire breadth of the water table level. This acts as a reservoir where the water can safely accumulate and can be pumped out through other available mechanisms. For most shallow foundations, water is pumped out using open-end discharge systems. The water table level can influence the soil granular characteristics, for example; it can deteriorate their strength and consequently lead to progressive failure of the structure. As a cautionary maneuver, shallow foundations are constructed in such as way that they are very broad in order to distribute the resultant forces evenly on the ground. This, in turn, lessens the influence of the water table level on the bearing capacity of the structure. Other factors that influence the bearing capacity of shallow foundations are ground shaking, foundation orientation, and type of footing. In the case of ground shaking which may result from earthquakes, the strength of the soil can be affected by cyclic degradation, which ultimately may lead to bearing capacity failure. Liquefaction is also an important variable to consider that can be instigated by the same process. It leads to shear stiffness, which can have adverse effects on the overall foundations (...