Loadings in buildings consist of the  combined dead loads and imposed loads which exert a downward pressure upon  the soil on which the structure is founded and this in turn promotes a  reactive force in the form of an upward pressure from the soil. The structure  is in effect sandwiched between these opposite pressures and the design of  the building must be able to resist the resultant stresses set up within the  structural members and the general building fabric. The supporting subsoil  must be able to develop sufficient reactive force to give stability to the  structure to prevent failure due to unequal settlement and to prevent failure  of the subsoil due to shear. (1)To enable a designer to select, design and  detail a suitable foundation he must have adequate data regarding the nature  of the soil on which the structure will be founded and this is normally  obtained from a planned soil investigation programme.

Soil investigation. Soil investigation  is specific in its requirements whereas site investigation is all embracing,  taking into account such factors as topography, location of existing  services, means of access and any local restrictions. Soil investigation is a  means of obtaining data regarding the properties and characteristics of  subsoils by providing samples for testing or providing a means of access for  visual inspection. The actual data required and the amount of capital which  can be reasonably expended on any soil investigation programme will depend  upon the type of structure proposed and how much previous knowledge the  designer has of a particular region or site.

The main methods of soil investigation  can be enumerated as follows:

1. Trial pits - small contracts where  foundation depths are not likely to exceed 3m.

2. Boreholes - medium to large contracts  with foundations up to 30m deep.

Classification of soils. Soils may be  classified by any of the following methods:

1. Physical properties; 2. Geological  origin; 3. Chemical composition; 4. Particle size.

It has been established that the physical  properties of soils can be closely associated with their particle size both  of which are of importance to the foundation engineer, architect or designer.  All soils can be defined as being coarse-grained or fine-grained each  resulting in different properties.

Coarse-grained soils: these would  include sands and gravels having a low proportion of voids, negligible  cohesion when dry, high permeability and slight compressibility, which takes  place almost immediately upon the application of load.

Fine-grained soils: these include the  cohesive silts and clays having a high proportion of voids, high cohesion,  very low permeability and high compressibility which takes place slowly over  a long period of time.

There are of course soils which can be  classified in between the two extremes described above. BS1377 deals with the  methods of testing soils and divides particle sizes as follows:

The silt, sand and gravel particles are  also further subdivided into fine, medium and coarse with particle sizes  lying between the extremes quoted above.

Shear strength of soils. The resistance  which can be offered by a soil to the sliding of one portion over another or  its shear strength is of importance to the designer since it can be used to  calculation the bearing capacity of a soil and the pressure it can exert on  such members as timbering in excavations. Resistance to shear in a soil under  load depends mainly upon its particle composition. If a soil is granular in  form, the frictional resistance between the particles increases with the load  applied and consequently its shear strength also increases with the magnitude  of the applied load. Conversely clay particles being small develop no  frictional resistance and therefore its shear strength will remain constant  whatever the magnitude of the applied load. Intermediate soils such as sandy  clays normally give only a slight increase in shear strength as the load is  applied.

Compressibility. Another important  property of soils which must be ascertained  before a final choice of foundation type and design can be made is  compressibility, and two factors must be taken into account:

1. Rate at which compression takes  place.

2. Total amount of compression when full  load is applied.

When dealing with non-cohesive soils  such as sands and gravels the rate of compression will keep pace with the  construction of the building and therefore when the structure is complete  there should be no further settlement if the soil remains in the same state.  A soil is compressed when loaded by the expulsion of air and/or water from  the voids and by the natural rearrangement of the particles. In cohesive  soils the voids are very often completely saturated with water which in  itself is nearly incompressible and therefore compression of the soil can  only take place by the water moving out of the voids thus allowing settlement  of the particles. Expulsion of water from the voids within cohesive soils can  occur but only at a very slow rate due mainly to the resistance offered by  the plate-like particles of the soil through which it must flow. This gradual  compressive movement of a soil is called consolidation. Uniform settlement  will not normally cause undue damage to a structure but uneven settlement can  cause progressive structural damage.

Foundation types. There are many ways in  which foundations can be classified but one of the most common methods is by  form resulting in four basic types thus:

1. Strip foundations –light loadings  particularly in domestic buildings. Heavier loadings can sometimes be founded  on a reinforced concrete strip foundation.

2. Raft foundations - light loadings,  average loadings on soil with low bearing capacities and structures having a  basement storey.

3. Pad or isolated foundations- common  method of providing the foundation for columns of framed structures and for  the supporting members of portal frame.

4. Pile foundations - method for  structures where the loads have to be transmitted to a point at some distance  below the general ground level.

combined  dead loads 组合恒载 

imposed  loads 外加荷载 

downward 向下的    

promote  产生    

reactive  force  反作用，反力 

resultant  stresses 合应力 

member 构件     

fabric 构造，建筑物 

planned  soil investigation programme 土质勘查报告

soil  investigation 土质勘查   specific 专门的  

embrace 包含，包括      

topography 地形 

existing  services 现有的设施 

providing  samples for testing 取样试验  

visual  inspection 外观检查，检视   

capital 资金

borehole钻孔  

coarse-grained  soil 粗粒土

fine-grained  细粒土 

proportion  比例 

void 空隙  

negligible  可以忽略的 

cohesion  粘结力    

permeability  渗透性 

compressibility  压缩性 

cohesive 粘结性的 

silt 淤泥     

clay 粘土 

quote  引用，提到

shear  strength 剪切强度，抗剪强度 

resistance  n.反抗, 抵抗, 抵抗力, 阻力, 电阻, 阻抗 

bearing  capacity  承载能力 

timbering  n.木材, 建筑用材, 木材桁构 

excavation  挖掘 

granular  form 粒状形态

frictional  resistance 摩擦阻力 

magnitude  n.大小, 数量, 巨大, 广大, 量级 

applied  load外加负载, 外施荷载 

conversely  adv.倒地，逆地 

clay  particle 粘土颗粒  intermediate  adj.中间的

ascertain 确定 

non-cohesive  无黏性的  

gravel 砾石 

state 状态 

expulsion 排出 

voids 空隙 

rearrangement  重新排列 

saturate  with 使…饱和 

incompressible  不被压缩的 

gradual 逐渐的 

consolidation  固结 

uniform 均匀的    

uneven  不均匀的