TECHNICAL REPORT 2
REVIEW OF FOUNDATION SYSTEM FOR A BUILDING
For:
Introduction
This report outlines different types of foundations and their suitable application areas in construction. In addition, it presents the plan view, design considerations, the elevation and the significant information that are needed from soil investigations and geotechnical design. Soil investigation report outlines the outcome of the geotechnical examination report for the proposed building and complex in Dubai UAE (Peck, Hanson & Thornburn, 1974). The investigations were conducted in accordance with the conformation which is dated 8th April 2016. The aim of the investigation is to determine the conditions of both surface and subsurface at the proposed location. In addition, the conditions of the mechanical, chemical and physical properties of the foundation ground will be determined so as to present the structural engineer with adequate information for the design of the most safe and appropriate foundation.
There are different types of foundations that can be used to erect a building or structure depending on the geological properties of the site. Foundations are generally grouped into two types: shallow foundations and deep foundations. The general categories of foundations are further subdivided based on the type of footing used. For shallow foundation, the following are subdivisions under the category; spread footing, raft or mat foundations, combined footing and cantilever or strap footing. On the other hand, the deep foundation has the following sub-groups under its category; shaft foundations, piles and buoyancy foundations.
It is imperative for structural engineers to have deep knowledge of different types of foundations so as to apply the appropriate foundation to the correct site for the correct structure. Various types of foundations have different types of properties which make them better than their counterparts. Also, these properties make them applicable in different type of situations and site compositions.
Shallow foundation
As previously outlined in the introductory part of this report, shallow foundations are subdivided into the following categories; spread footing, raft or mat foundations, combined footing and cantilever or strap footing (Peck, Hanson & Thornburn, 1974).
Spread footing
Spread footing foundation is shallow type of foundation which is made from either reinforced concrete or normal concrete. The spread footing foundation is applied in making a pad utilized in building wall loads and column over adequately wide spoil area.
Elevation and plan view
Figure 1: Spread footing elevation and plan view. Retrieved from http://www.abuildersengineer.com/
Design considerations
Spread footing type of foundations has several design properties which must be considered while designing the foundation. In most cases, spread footing type of foundation can be designed in either a circular design or a square design so as to support column loading. The footing can be constructed in rectangular footing or long footing to offer support to wall loading.
Soil investigation information
Spread footing type of foundation requires different type of information from the soil investigation. The information from the soil investigation will determine whether spread footing foundation can be employed in a particular site or not. As a result, soil investigation should focus on various properties in case spread footing type of foundation is to be used in the construction of the foundation (Peck, Hanson & Thornburn, 1974).
The type of soil investigation information which is required for this type of foundation includes N value which is greater than 50. In addition, the soil investigation report should present information related to the density of the soil. For this type of foundation, the site of construction should have N value more than 50 and the soil should be very dense. Other soil properties such as bearing capacity are also significant in determining the type of foundation for the site. The soil investigation is conducted using a series of soil testing techniques including static cone penetration test, standard penetration test, unconfined compressive strength test and plate load test.
Geotechnical design
It is imperative for the structural engineers to obtain soil information and outline specific procedure for the capacity of the pile. Some of the information significant for the geotechnical design of spread footing type of design includes unit of weight, load, cohesion area, friction angle and factors of safety. Additionally, it is imperative to outline whether the design is for single pile or a collection of piles based on the need for the amount of piles and the size or load of the foundation. The procedure used in the process is based on the different properties of the soil at the construction site. The stress diagram is imperative given that it determines the amount of Q base and the Q friction (Reese, Isenhower & Wang, 2006).
Raft or Mat foundation
Mat or raft is a type of shallow foundation that us applied in sites where the soil is weak. It is highly used in areas where the soil investigation report indicates that the soil has a low bearing. In addition, it is used in sites where the total area is occupied by specific footing that is not less than half of the loaded area of the structure (Peck, Hanson & Thornburn, 1974).
Elevation and plan view
Figure 2: Mat foundation elevation and plan view. Retrieved from http://www.abuildersengineer.com/
Design considerations
There are certain considerations which must be employed in the design of this type of foundation before it is implemented in a construction site. The raft foundation should be made in such as way that it is s large footing which provides support for various columns and walls under a larger part of the structure or a full structure.
Soil investigation information
It imperative to conduct several soil tests to determine the different properties of the soil at the construction site so as to determine whether this type of foundation can be applied at the site. The soil investigation report attain its soil information through performing various tests using various techniques such as static cone penetration testing, standard penetration testing and unconfined compressive strength test (Reese, Isenhower & Wang, 2006).
The type of soil information necessary for this type of foundation is the bearing capacity. The bearing capacity of the soil is obtained from the shear strength considerations of the soil under investigation.
Geotechnical design
Raft foundations employ the use of various soil properties which are highly cohesive clays. It should be noted that raft foundations does not employ the use of soil investigation information like sensitive or soft clays (Hambly, 1979). In other words, raft foundation can only be employed in soils which exhibit highly cohesive clay properties unlike sensitive or soft clays.
Combine footing
Combined footing is used to relay the load from the building to the underlying ground. This type of foundation is applied under specific depth under which the bearing strata lies, the condition of the soil and the type of structure to be constructed. In most cases, combined footing is employed when more than two columns in a straight line are conducted on one spread footing.
Elevation plan and view
The elation plan can be either rectangular or trapezium in shape depending on the soil properties and the type of structure to be constructed.
Figure 3: Combined footing elevation and plan view. Retrieved from http://www.abuildersengineer.com/
Design considerations
The elation plan can be either rectangular or trapezium in shape depending on the soil properties and the type of structure to be constructed. The construction of this type of foundation depends of the soil properties of the construction site and the type of structure to be erected at the site. Various types of combined footing plans have different properties and advantages which makes them suitable for applications in different situations (McCarthy & McCarthy, 1977).
Soil investigation information
It is imperative for the structural engineers to determine the safety factor of the acceptable bearing capacity under the footing which is suitable to the loads (Reese & Welch, 1975). The determination of the safe bearing capacity of the foundation is attained through the use of a series of techniques such as static cone penetration test, standard penetration test and unconfined compressive strength test (Reese & Welch, 1975). The allowable safety capacity bearing is determined based on the shear strength consideration.
Geotechnical design
The information gathered from the soil investigation performed at the construction site is used to determine whether this type of foundation can be applied or not. In most cases, combined footing type of foundation is applied in clay and soft soil (McCarthy & McCarthy, 1977).
Cantilever or strap footing
The application of cantilever footing or strap footing is done in constructions that required a combination of columns that are situated along the lot line or the building’s property.
Elevation and plan view
Figure 4: Strap footing elevation and plan view. Retrieved from http://www.abuildersengineer.com/
Design considerations
The strap or cantilever footing type of foundation can be designed using either a rigid beam or as a rectangular shape (Meyerhof,1951). The type of design used in this type of foundation depends on the properties of the soil and the type of the structure to be erected at the identified site.
Soil investigation information
It is imperative for structural engineers to obtain certain type of soil information that is pertinent to this type of foundation before the construction is started. The soil information that should be used from the soil investigation reported is the N value. It is imperative that for the N value to be more than 50 for this type of foundation to be applied at a given construction site. Also, it is important that the type of soil be very dense (Yamamoto et al., 2009).
In addition, the application of this type of foundation relies heavily on the information on the bearing capacity of the soil. The bearing capacity of the soil is determined using different methods such as the unconfined compressive strength test, standard penetration test, plate load test and static cone penetration test.
Geotechnical design
The geotechnical design for this type of foundation mandates the structural engineers to perform a procedure and gather information pertinent to capacity of piles. The geotechnical design of this type of foundation requires; unit of weight, load, cohesion area, friction angle and factor of safety. It is also imperative for the engineers to determine whether the foundation is constructed for multiple or single pile (Yamamoto et al., 2009). The information can be obtained based on the size and load of the foundation. On the other hand, the procedure for determining the geotechnical design and the capacity of the pile is based on the properties of the soil at the selected construction site. The Q base and the Q friction are determined using the stress diagram.
Deep foundation
As opposed to shallow foundation, deep foundations are made deep inside the soil surface so as to provide sufficient support for the overlying structure above the foundation. In most cases, deep foundations are made in soils which are known to be weak such as sand and soft clay (Reese & Welch, 1975). Deep foundations are constructed deep into the soil so as to achieve the soil level and property with a high bearing capacity as opposed to the soil found on the surface or shallow levels of the construction site. Similar to shallow foundation, deep foundation is further categorized into various categories. Deep foundation has the following sub-groups under its category; shaft foundations, piles and buoyancy foundations.
Hollow box foundations or buoyancy rafts
Buoyancy type of foundation is applied in areas where appropriate bearing strata is extremely large at correct depths more than the traditional foundation alternatives. Buoyancy foundations are mainly constructed in places with open excavations.
Elevation and plan view
Figure 5: Buoyancy raft elevation and plan view. Retrieved from http://www.abuildersengineer.com/
Design Considerations
In most cases, buoyancy rafts are constructed from hollow structures which are designed to grant either semi-buoyant or full buoyant substructure under which the overall loading on the soil surface is minimized to the required low intensity. This type of foundation can be designed to either caissons or drowned (Paikowsky, 2004). Buoyancy foundations are mainly constructed in places with open excavations.
Soil investigation information
There is specific information needed from the soil investigation report before this type of foundation is applied in the construction of a structure or building. The main soil investigation information is the type of soil present at the selected construction site where the building is to be erected. In addition to the type of soil at the selected site, the soil investigation information should also provide other properties of the soil such as the bearing capacity and friction levels. In most cases, buoyancy raft is applied in weak clayey soil (Paikowsky, 2004). It is applied when the soil at the selected site has a very low bearing capacity. Additionally, this type of foundation can be used in soils with very low bearing capacity which cannot support the building. There are different methods through which the soil investigation is performed; static cone penetration test, standard penetration test and un-drained triaxial shear strength and vane shear strength. The foundation can be made through cobbles using numerous feet into tough and hard rock or frozen grounds.
Shaft foundations
Shaft foundation is a type of deep foundation designed and applied within deep excavations which are supported by lining put in place and filled using either pre-fabricated load bearing material or concrete. This type of deep foundation can be employed in place of a driven pile group. Its application in place of a driven pile group eliminates the need for a pile cap.
Elevation and plan view
Figure 6: Shaft foundation elevation and plan view. Retrieved from http://www.abuildersengineer.com/
Soil investigation information
The main soil investigation information is the type of soil present at the selected construction site where the building is to be erected. In addition to the type of soil at the selected site, the soil investigation information should also provide other properties of the soil such as the bearing capacity and friction levels (Paikowsky, 2004). In most cases, buoyancy raft is applied in weak clayey soil. It is applied when the soil at the selected site has a very low bearing capacity.
Piles
Pile is a type of deep foundation which is applied in different situations. It can be used when the surface has a layer of weak soil. The layer of weak soil at the surface is incapable of supporting the weight of the structure. As a consequence, the load of the structure must bypass this layer into the deeper and stronger layers using piles. Also, pile at a type of deep foundation is applied in situations where the structure has very heavy and concentrated loads like a high rise construction.
Elevation and plan view
Figure 7: Pile elevation and plan view. Retrieved from http://www.abuildersengineer.com/
Design considerations
The pile is made up of strong and long concrete pipes which are submerged deep into the ground past the weak soil layer to the strong soil layer. This type of deep foundation can make use of either Friction piles or End bearing piles. The end bearing pile foundation is applied in strong soils while the friction piles are applied in soft and weak soil. As a consequence, the bearing capacity of the soil is significant soil investigation information required before pile foundation is applied in any construction (Nawari, Liang & Nusairat, 1999).
For the calculation of the pile foundation, it is assumed that the diameter of the pile is 1 M. Also, it is assumed that the N value: >50. As a consequence, ø > 45
Df = 8m
Dia = 1m
γ = 19.5 kN/m3
ø = 44 º
Nq* = 180
K = 0.4
Tan∂ = 0.45
FS = 3
Water table ∆ @ depth 3.4 m
Pv2 = (19.5 – 9.81) x 4.6 = 44.6 KN/m
Pv1 = 19.5 x 3.4 = 66.3 KN/m
Therefore
∑Pv = 66.3 + 44.6 = 111 KN/m
As a consequence, it can be concluded that
A-I = ½ x 66.3 x 3.4 = 112.7 m2
A-II = 66.3 x 4.6 = 305 m2
A-III = ½ x 44.6 x 4.6 = 102.6 m2
The total value of A is determined be summing all the three values of A, thus
∑A = 112.7 + 305 + 102.6 = 520.3 m2
Conclusion
There are different type of foundations that can be applied in different buildings and structures. The various types of foundations exhibit various properties which makes them suitable for different applications. It is imperative to have a comprehensive understanding of different types of foundations so as to apply the appropriate type of foundation for the correct construction.
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