Trench Fill Foundation Design Decisions.

A typical trench fill foundation is shown in Fig. 11.7 where (a) indicates a typical section, (b) shows the typical design forces, and (c) illustrates the possible externally applied ground strains which can prove critical. Such strains can and do cause serious damage to buildings and their finishes.

The considerations to be made therefore in the design decisions relate to:

(1) The depth and type of suitable ground-bearing strata relative to foundation loading.

(2) The likelihood of large horizontal ground strains due to moisture changes in the sub-strata, mining activity or frost.

(3) The economy of trench fill versus normal strip footings.

Fig. 11.7 Trench fill conditions.

 

Considering (1), strip footings can prove economic for medium loads at shallow-to-medium depths on firm-to-stiff sub-strata, for example, 100–300 kN/m2 at 600–1500 mm deep on firm-to-stiff clay, firm-to-dense sand or firm-to-stiff sandy clay or clayey sand.

Considering (2), provided that the sub-stratum is not  a sensitive clay, i.e. not highly shrinkable, and provided that there is no likelihood of large ground strains from  mining or other activities, then trench fill footings should  be considered. Standard trench strips should generally be avoided where lateral forces may be picked up from below ground on the sides of the footing. A void or cushion can however be adopted to prevent the transfer of such forces where this is appropriate. Alternatively, the foundation and its superstructure may, in certain circumstances, be designed to resist the force transferred.

Considering (3), the economics of any potential solution vary with time and are dependent on:

(a) Material costs and availability,
(b) Excavation technology and machinery,
(c) Weather conditions and likely stability of trenches, and
(d) Manpower availability and other conditions relating to a particular site.

As a general rule providing that the trenches can remain stable for a suitable period to allow excavation and casting of concrete, and that conditions (1) and (2) are appropriate, then trench fill should be considered in any comparison exercises, particularly if the width of a normal strip footing would be dictated by the working space required for brick- laying below ground.

Under such conditions, the solution is likely to prove  competitive both from a cost point of view and speed of construction. The speed of construction can be particularly important where trench stability is time related and where foundations are being constructed through a winter period.

Under such conditions a mass concrete trench fill could  be adopted. However, the use of stone or concrete is an  economic decision based upon the ground conditions,  the long- and short-term stability of the trenches, and the availability of the materials (see Table 10.2).

Table 10.2 Foundation selection to suit varying site conditions