FIGURE 1.1 Individual footing
CHAPTER 1 INTRODUCTION
1.1SHALLOW FOUNDATIONS—GENERAL
The lowest part of a structure which transmits its weight to the underlying soil or rock is the foundation. Foundations can be classified into two major cate-gories: that is, shallow foundations and deep foundations. Individual footings (Fig. 1.1) square or rectangular in plan which support columns, and strip footings which support walls and other similar structures are generally referred to as shallow foundations. Mat foundations, also considered shallow founda-tions, are reinforced concrete slabs of considerable structural rigidity which support a number of columns and wall loads. When the soil located immediate-ly below a given structure is weak, the load of the structure may be transmitted to a greater depth by piles and drilled shafts, which are considered deep foundations. This book is a compilation of the theoretical and experimental evaluations prently available in literature as they relate to the load-bearing capacity and ttlement of shallow foundations.
平和
陈昭羽
FIGURE 1.2 General shear failure in soil
香水怎么用The shallow foundation shown in Fig. 1.1 has a width B and a length L .The depth of embedment below the ground surface is equal to D f . Theoreti-cally, when B /L is equal to zero (that is, L = !), a plane strain ca will exist in the soil mass supporting the foundation. For most practical cas when B /L "1/5 to 1/6, the plane strain theories will yield fairly good results. Terzaghi [1]defined a shallow foundation as one in which the depth, D f , is less than or equal to the width B (D f /B " 1). However, rearch studies conducted since then have shown that, for shallow foundations, D f /B can be as large as 3 to 4.
1.2TYPES OF FAILURE IN SOIL AT ULTIMATE LOAD Figure 1.2 shows a shallow foundation of width B located at a depth D f below the ground surface and supported by a den sand (or stiff clayey soil). If this
FIGURE 1.3 Local shear failure in soi l
foundation is subjected to a load Q which is gradually incread, the load per unit area, q = Q /A ( A = area of the foundation), will increa and the foun-dation will undergo incread ttlement. When q becomes equal to q u at foundation ttlement S = S u , the soil supporting the foundation
undergoes sudden shear failure. The failure surface in the soil is shown in Fig. 1.2a , and the q versus S plot is shown in Fig. 1.2b . This type of failure is called general shear failure , and q u is the ultimate bearing capacity . Note that, in this type of failure, a peak value q = q u is clearly defined in the load-ttlement curve.
If the foundation shown in Fig. 1.2a is supported by a medium den sand or clayey soil of medium consistency (Fig. 1.3a ), the plot of q versus S will be as shown in Fig. 1.3b . Note that the magnitude of q increas with ttlement up to q = q´u , which is usually referred to as the first failure load [2]. At
this
铝镁匹林time, the developed failure surface in the soil will be like that shown by the solid lines in Fig. 1.3a. If the load on the foundation is further incread, the load-ttlement curve becomes steeper and erratic with the gradual outward and upward progress of the failure surface in the soil (shown by the broken line in
Fig. 1.3b) under the foundation. When q becomes equal to q
u (ultimate bearing
污蔑的意思
capacity), the failure surface reaches the ground surface. Beyond that, the plot of q versus S takes almost a linear shape, and a peak load is never obrved. This type of bearing capacity failure is called local shear failure.
八字词语有哪些3木Figure 1.4a shows the same foundation located on a loo sand or soft clayey soil. For this ca, the load-ttlement curve will be like that shown in Fig. 1.4b. A peak value of load per unit area, q, is never obrved. The ultimate bearing capacity, q
u
, is defined as the point where ∆S/∆q becomes the largest
FIGURE 1.4Punching shear failure in soi l
FIGURE 1.5 Nature of failure in soil with relative density of sand (D r ) and D f /R and almost constant thereafter. This type of failure in soil is called punching shear failure . In this ca, the failure surface never extends up to the ground surface.
The nature of failure in soil at ultimate load is a function of veral factors such as the strength and the relative compressibility of soil, the depth of the foundation (D f ) in relation to the foundation width (B ), and the width-to-length ratio (B /L ) of the foundation. This was clearly explained by Vesic [2] who conducted extensive laboratory model tests in sand . The summary of Vesic’s findings is shown in a slightly different form in Fig. 1.5. In this figure, D r is the relative density of sand, and the hydraulic radius, R , of the foundation is defined as
R A
雪上加霜造句
P =
(1.1)
