Evaluation of Design Methods for Shallow Foundations
This chapter provides the reader with (1) a basic understanding of bearing capacity failure (general, local and punching) of shallow foundations; (2) their onshore and offshore applications and two general design specifications capacity (ULS) and settlement (SLS); (3) the largest database of centrifuge and field load tests on shallow foundations compiled to date, covering two load types (axial compression and uplift) and soil types (clay and sand);
(4) the most comprehensive capacity and settlement model factors statistics based on the database; and (5) calibration of resistance factors with the method in Section 2.5.3 for ULS and SLS. The SLS resistance factors address a significant gap in the literature focused primarily on ULS.
Type and Selection of Shallow Foundations
Shallow Foundation Type
A shallow foundation is the most common foundation type. It will distribute the loads over a wide horizontal area at a shallow depth below the ground surface to lower the intensity of applied loads to levels tolerable for the foundation soils (Kimmerling 2002). The benefits of using a shallow foundation include (1) it requires less excavation and a shorter construction period and thus reduces labour costs, (2) construction procedure is simple and causes less soil disturbance, (3) equipment used in the construction is also simple and less costly and (4) there is less uncertainty in the prediction of shallow foundation behaviour. Common types of shallow foundations are described next (Kimmerling 2002):
1. Continuous strip spread footings: The most commonly used type of foundation for buildings generally have a minimum length (L) to width (B) ratio (L/B) of at least 5 (i.e. L/B > 5). They support a single row of columns or a bearing wall to reduce the pressure on the bearing materials. Plane strain conditions are assumed to exist in the direction parallel to the long axis of the footing.
- 2. Isolated spread footings: These footings are designed to distribute the concentrated loads from a single column to prevent shear failure of the bearing materials beneath the footing and to minimize settlement by reducing the applied bearing stress. The size of an isolated spread footing is a function of the loads distributed by the supported column and the strength and compressibility characteristics of the bearing materials beneath the footing. For bridge columns, isolated spread footings are typically greater than 3 m by 3 m. These dimensions will increase when eccentric loads are applied.
- 3. Combined footings: These footings are similar to isolated spread footings, but they support two or more columns. They are primarily used when the column spacing is non-uniform (Bowles 1997) or when isolated spread footings become so closely spaced that a combined footing is simpler.
- 4. Mat or raft foundation: The foundation consists of a single heavily reinforced concrete slab that underlies the entire structure or major portion of the structure. When spread footings would cover more than about 50% of the footprint of the plan area of structure, mat foundations are often economical (Peck et al. 1974). The main advantage of a mat foundation is its ability to reduce differential settlement. In situations where a raft foundation alone does not satisfy the design requirements, it may be possible to enhance the performance of the raft (increase the capacity and reduce the settlement) by the addition of piles (piled raft foundation) (Poulos 2001), but this is outside the scope of this book. Design and applications of piles will be presented in Chapter 6.
Selection and Application of Shallow Foundations
Shallow foundations are often selected when structural loads will not cause excessive settlement of the underlying soil layers. Granular (or cohesionless) soils and heavily overconsolidated cohesive soils are generally more suitable to support shallow foundations than normally consolidated or lightly overconsolidated cohesive soils, particularly when a foundation is supported by a structural fill (Kimmerling 2002). Cohesionless soils tend to be less prone to settlement under applied loads. Settlement of cohesionless soils usually occurs rapidly as loads are applied. Heavily overconsolidated cohesive soils have relatively high strength and low compressibility characteristics. Normally consolidated or lightly overconsolidated cohesive soils will experience consolidation settlement as a result of changes in water content. Therefore, bearing capacity and settlement of such soils must be evaluated as part of the preliminary design process when considering support of a shallow foundation. When an intermediate geomaterial (IGM) (transition between soil and rock) or rock is at or near the ground surface, a shallow foundation could be the most economical foundation system. There are several scenarios in which shallow foundations cannot be used: (1) the construction site is located near a river or the sea because of a possibility of scour; (2) the water table level is high and pumping out the water from the pit or canal is uneconomical; (3) the soil near the ground surface is too soft to provide sufficient resistance; (4) the weight of the superstructure is high and load is distributed unequally.
Geotechnical design of shallow foundations can vary from very simple footings for small lightly loaded domestic buildings to the complex requirements of a raft for a nuclear power station (ICE 2012). When the subsurface conditions at shallow depths are reasonable for their use, shallow foundations can be a viable alternative to deep foundations, as they are relatively inexpensive; however, they have not been widely used to support highway bridges yet (Sargand and Masada 2006). FHWA believes that shallow foundations on soils are underutilized because designers encounter one or more of the following obstacles (Samtani et al. 2010): (1) limited knowledge of guidelines from the AASHTO and FHWA that pertain to the application of shallow foundations on soils to support bridges, (2) limited knowledge of performance data for shallow foundations, (3) unrealistic tolerable settlement criteria, (4) overestimation of loads used to calculate settlement and
(5) use of conservative settlement prediction methods. To address these issues and promote the use of shallow foundations as a routine alternative to deep foundations for support of highway bridges, FHWA and several state DOTs have published a set of technical reports since the 1980s (e.g. DiMillio 1982; Moulton 1986; Gifford et al. 1987; Baus 1992; Lutenegger and DeGroot 1995; Briaud and Gibbens 1997; Sargand and Hazen 1997; Kimmerling 2002; Sargand and Masada 2006; Samtani et al. 2010; Abu- Hejleh et al. 2014; Agaiby and Mayne 2016; Xiao et al. 2016; Allen 2018; Moon et al. 2018; Samtani and Allen 2018).
In offshore geotechnical engineering, shallow foundations also become an economic and sometimes the only practical solution as an alternative to pile foundations (Randolph and Gourvenec 2011). Historically, offshore shallow foundations either comprised large concrete gravity bases supporting large, fixed substructures or steel mudmats used as temporary support for piled jackets. Recently, offshore shallow foundations have become more diverse, including concrete or steel bucket foundations, as shown in Figure 4.1. Spudcans are a type of temporary shallow foundation used for mobile drilling rigs (commonly called jack-ups) that are presented separately in Chapter 5. Because of the increasing demand for oil and gas worldwide, design practice in offshore geotechnical engineering grew out of onshore practice, but two application areas tended to diverge over the last thirty years, driven partly by the scale of offshore foundation elements (that are typically much larger than those used onshore) and partly by fundamental differences in construction (or installation) techniques (e.g. Randolph et al. 2005; Houlsby 2016). Compared to onshore shallow foundations, offshore shallow foundations are usually required to withstand much larger
Figure 4.1 Applications of offshore shallow foundations ("Reprinted from Proceedings of the 16th International Conference on Soil Mechanics and Geotechnical Engineering: Geotechnology in Harmony with the Global Environment, Vol I, Mark Randolph, Mark Cassidy, Susan Gourvenec, and Carl Erbrich, Challenges of offshore geotechnical engineering, I 23— 176. Copyright (2005) with permission from IOS Press.”.)
horizontal loads and overturning moments. In the design of offshore shallow foundations, more attention is placed on the capacity in which the cyclic loading effect is critical (Andersen 2009, 2015).
General Considerations in Shallow Foundation Design
From a geotechnical perspective, the decision to use a shallow foundation to support a structure is made based on two fundamental requirements (e.g. Kimmerling 2002; AASHTO 2017): (1) checking that an adequate margin of safety is provided against bearing capacity failure, overturning or excessive loss of contact and sliding along the foundation base - ULS and (2) checking that the level of deformation or settlement under working load conditions is tolerable or acceptable - SLS. Because of this point, the key design issues include the following (Poulos et al. 2001):
- 1. Calculation of the bearing capacity of shallow foundations with appropriate allowance for the combined effects of vertical, horizontal and moment loading - Section 4.3 (the focus of this chapter).
- 2. Calculation of the total and differential settlements under vertical and combined loading, including any time dependence of these foundation movements - Section 4.4.
- 3. Calculation of the foundation movements because of moisture changes in the underlying soil.
- 4. Structural design of the foundation elements.
According ro Eurocode 7 (CEN 2004), shallow foundations can be designed in one of three ways (Bond and Harris 2008): (1) use conventional and conservative design rules and specify control of construction (e.g. presumed bearing resistance); (2) carry out separate analyses for each limit state, both ULS and SLS; and (3) use comparable experience with the results of field and laboratory measurement and observations that usually represent a high degree of site understanding and low risks associated with potential failure or excessive deformation of the structure. The second approach, which is more general, will be discussed in some detail in Sections 4.3 and 4.4. In accordance with the classification in Table 2.8 from Poulos et al. (2001), most approaches are Category 2 methods, which are based on the understanding of foundation behaviour and soil mechanics principles, such as bearing capacity theory and elasticity theory for settlement calculation.