Soil as a structural material

All building structures rest on the surface of the Earth and the foundations are the final part of the structure. Loads imposed on the planet Earth by buildings are trivial but locally the behaviour of the surface of the Earth matters. The purpose of the foundations is to ensure that the stress on the local surface is within the safe bearing stress of the soil. The concept of foundations is the same as using snowshoes—Fig. 3.23.

If a hole is dug in the surface of the Earth rock will eventually be found. This rock may be many metres below the ground level so the foundations are usually placed on the soil that lies above the rock. This layer of soil may be very compressible under load so the foundations will move downwards causing them to settle. This means that the building may move downwards as a rigid body or it may settle differentially causing the building to tilt and distort. So not only strength but foundation movement has to be considered by the engineering designer. Unfortunately for engineering designers, the behaviour of soils under load is complex. Due to this complexity a specialist subject has come into existence called soil mechanics.

The first stage in understanding the engineering behaviour of soils is to identify the types of soil that are found. These are broadly classified as rocks, granular soils and clayey soils. These are often found in layers or strata, so immediately under a building site there usually several different types of soil.

Fig. 5.11

Examples of rocks are granite, sandstone and chalk, granular soils are sands and gravels and clayey soils are various types of clay. Foundations on rock are rarely a problem for buildings as they are strong and stiff, but foundations on granular or cohesive soil need careful consideration.

150 Building Structures: From Concepts to Design

There are two basic differences between the behaviour of structural elements and the structural behaviour of soil. The first is that the part of soil loaded by the foundations of a structure cannot act in isolation in same way a column can. The loaded part of the ground is affected by adjacent ‘unloaded’ parts of the ground.

Fig. 5.12

How much of the adjacent soil is affected by the load is hard to determine but it can be signifi-cant. To see why this happens look at two simplified models of soil. One is of unlinked elastic coil springs and the other of elastic spheres. Both are in pits with completely rigid sides.

Fig. 5.13

For the first model the behaviour is quite simple. As the load is applied through the rigid foundation each coil spring deflects vertically under its share of the load. As the load increases so does the deflection. The bottom and sides of the pit do not move.

Fig. 5.14

This model assumes that the soil under the foundation does act as an isolated structure;

the springs. This is the same as assuming there is a finite column of elastic soil under the foundation and that soil outside the ‘rigid box’ is unaffected.

Structural materials 151

Fig. 5.15

The second model using elastic spheres is more complex as there are three phases of behav-iour. The pit of elastic spheres will not be tightly packed so the load causes compaction of the spheres. This compaction can be seen by shaking a jar of rice or sugar and noting the depth before and after shaking.

Fig. 5.16

The second effect is the restriction on the shape into which the spheres can deform. Because the spheres are touching each other and the sides of the pit, each sphere cannot deform freely.

Fig. 5.17

As the confined spheres are compressed, the restriction on their lateral deformation causes horizontal loads on the rigid pit walls. As the spheres deform, the foundation moves down into the pit, this reduces the overall volume of the pit and the size of the voids between the spheres.

Fig. 5.18

152 Building Structures: From Concepts to Design

When all the voids are filled the pit will be completely filled with the elastic material. This is quite a different structure from the pit filled with barely touching spheres. This means that the pit filled with elastic spheres will have three phases of behaviour.

Phase 1 Reduction of voids by compaction.

Phase 2 Deformation of spheres until the voids are filled.

Phase 3 Deformation of pit filled with elastic material.

This gives a three-part load/deflection diagram.

Fig. 5.19

It is possible to find single-sized spherical stones occurring naturally, these can be seen on some shingle beaches. For this ‘soil’, the model of elastic spheres is reasonable but the rigid pit restriction is not. A new model of this soil would be an ‘infinitely wide’ layer of elastic spheres with finite thickness. This layer rests on a rigid base.

Fig. 5.20

As the foundation is loaded, the spheres compact locally. Then the touching spheres begin to deform. With no rigid pit walls the adjacent, ‘unloaded’ spheres have to provide lateral forces.

Structural materials 153

Fig. 5.21

The lower spheres will provide the lateral force if they are heavy enough. Each layer of spheres transfers its load to a lower layer and each lower layer will have more spheres.

Fig. 5.22

In this way the area of loaded spheres increases with depth and the level of load in the spheres reduces.

Fig. 5.23

As soil is not usually made from elastic spheres and there is not a rigid base, the stressed volume under a single foundation becomes bulb shaped.

154 Building Structures: From Concepts to Design

Fig. 5.24

For an elastic sphere to deform into an elastic cube, thus filling all the voids, the material has to be very elastic. A stone sphere could not deform into a cube as it would split first. If the foundation load is increased, the highly stressed spheres will fail or the lateral forces required will become too high for the ‘unloaded’ spheres and these will heave upwards.

Fig. 5.25

The load/deformation curve can be drawn for this model and again this has three phases.

Phase 1 Reduction of voids by compaction.

Phase 2 Deformation of spheres.

Phase 3 Failure by crushing or heaving.

Fig. 5.26

Structural materials 155 So far the models have been used to understand the behaviour of the soil skeleton. If, as is often the case, there is water in the soil, the behaviour of the soil skeleton plus water has to be modelled. The behaviour of this composite structure can be quite different from the behaviour of the soil skeleton. Fill the pits of the first two models with water and assume that the foundations fit into the pits in a watertight manner.

Fig. 5.27

Water is almost incompressible so the load from the foundations is carried by water pres-sure in both of the models with almost no deformation.

Fig. 5.28

The behaviour will be altered if holes are made in the foundation which relieve the water pressure and allow some water to escape.

Fig. 5.29

Now the water that is filling the voids in the soil skeleton has a drainage path. As the water is expelled from the voids, the soil skeleton carries the load as before. If the holes are large then the water will be expelled immediately, but if the holes are very small the water will only seep through the foundation slowly.

156 Building Structures: From Concepts to Design

For the coil spring model with no water, the deflection will double if the load is doubled.

This is because the model is linear.

Fig. 5.30

If the pit is filled with water and only very small holes are made in the foundation, the water will take time to seep through. Initially the water will carry the entire load but as the water pressure is gradually reduced by seepage, the load is transferred to the coil springs. Now the deformation is time dependent, and under a constant load will deform in a non-linear way until the springs are carrying the entire load.

Fig. 5.31

If further load is applied the process will be repeated.

Fig. 5.32

Structural materials 157 In the water-filled soil skeleton, the pressure of the water in the voids is called the pore water pressure. Before loading, the pore water pressure is just the hydrostatic head, but the loads cause an increase in the pore water pressure. This increase is relieved by the drainage until the pressure returns to the hydrostatic pressure. How long this takes is called the seepage rate. The smaller the drainage holes, the longer it takes. In a real soil there is no rigid pit, so the water drains away laterally as well as vertically.

Fig. 5.33

Even with the first model, the presence of water and a low seepage rate dramatically alters the soil structure behaviour. With all the variations of underlying strata, particle size and shape and rate of loading, if water is present the soil structure behaviour of a real soil can be complex.

Just to complicate matters further, if the particle size is smaller than 0.002 mm and the particles are made from certain chemically complex minerals, then the presence of water is always a consideration. If water is poured over a heap of large stones, apart from a small part that wets the surfaces of the stones, all the water will drain away.

Fig. 5.34

However, with a heap of very small particles this will not happen as the wetted area of the particle is enormous compared with its volume.

Fig. 5.35

158 Building Structures: From Concepts to Design

The water does not drain away but is adsorbed on to the surfaces of the particles. Complex electro-chemical actions between the wetted particles now cause them to cohere together to form a clayey soil. In soil mechanics, soils are divided into freely draining soils—non-cohesive or granular soils and non-draining soils—clayey soils. Examples of granular soils are sands and gravels and examples of clayey soils are clays and marls. The difference can be physically experienced by squeezing handfuls of wet sand and clay. The water is readily squeezed from the sand but cannot be squeezed from a clayey soil. This is because the proportion of ‘free’ water in clayey soil is small and the diameters of the drainage paths are also small which causes very low permeability.

For non-cohesive soils, water may or may not be present but in clayey soils water is always present. If a clayey soil is loaded by a foundation, the load is initially carried by an increase in pore water pressure. As with granular soils the pore water pressure in cohesive soils reduces by draining laterally to unstressed areas. But due to the extremely low seep-age rate in cohesive soils this may take years.

In summary three broad statements can be made about soil as a structural material and these are:

• The engineering behaviour of many real soils is difficult to formulate analytically.

Many aspects are not fully understood and these are subjects for research by

Structural materials also have ‘non-structural’ characteristics which influence their use in structures. These include behaviour due to temperature change, exposure to fire, expo-sure to climatic changes and dimensional changes due to moisture variation. The common structural materials behave quite differently under these influences. Because these material characteristics are non-structural they do not directly affect the structural performance of the materials, but they strongly influence their use for structures.

Temperature change and moisture variation both cause a change of size of a structure.

This would not matter if the structure could just grow or shrink but usually there is a dif-ferential change in size between different members of the structure.

Fig. 5.36