Trench Fill Strips.

A brief description of trench fill strips is given previously. The design of such strips is relatively simple, and it is true to say that there is more design involved in making the decision to adopt such a foundation than in analysing and sizing the appropriate trench fill.

Trench fill is often used in an attempt to:

(1) Reduce the foundation width where brickwork below ground would need a wider footing to suit working space,

(2) Reduce the labour content of construction, and

(3) Speed up the construction of the footing, for example, in conditions where trench supports are not necessary for short periods but would be required if the trench were left open for a significant time.

The saving in excavation, labour, time and/or temporary works can in some situations be quite considerable. However, in loose ground the quantity of concrete used can become both difficult to predict and/or considerable in quantity particularly if trenches meet or cross at right angles.

Strips excavated through poor ground to reach suitable bearing strata can prove troublesome due to instability of the trench sides, particularly at changes in direction of the strip (see Fig. 11.1). This can be overcome by using suitable trench supports. However, the problem can often be more economically assisted by good design.

Fig. 11.1 Trench instability at change in direction.

For example, Fig. 11.2 shows two alternative designs for the same house foundations: in (A) the trenches would fail under much less critical conditions than the trenches in (B) since this scheme avoids trench direction changes and hence avoids the corner failure conditions of the trench sides.


Fig. 11.2 Trench fill alternatives.

A disadvantage in some situations is the tendency of the trench strips to pick up, via passive resistance, any longitudinal or lateral ground strains which may occur in the strata around the foundation. This can prove to be a major problem in active mining areas and in sub-strata sensitive to moisture changes such as shrinkable clays. In some situations this problem can be overcome by the insertion of a compressible batt against the trench faces (see Fig. 11.3),  but this must be considered for all directions and for conflicting requirements since passive resistance is often exploited in the superstructure and foundation design.


Fig. 11.3 Trench fill with compressible side formers.

In addition the high level of the concrete can create problems for drainage and services entering the building if these are not pre-planned and catered for. The top surface should be low enough so as not to interfere with landscaping and planting. In some situations concrete trench fill can create undesirable hard spots, and stone trench fill should be  considered.

Stone trench fill used under the strip loads to transfer the loads to the lower sub-strata is more yielding than concrete trench fill which may produce excessive differential movement between the main strip load area and the general slab (see Fig. 11.4).


Fig. 11.4 Stone versus concrete trench fill.

In soft wet conditions, the soft materials at the surface of the trench bottom can be absorbed into the voids of a first layer of no fines stones blinded by a second layer of well graded stone. The second layer prevents the soft materials from oozing up through the hardcore. This can prove to be a clear advantage for difficult sites where the material is  sensitive and wet and where good clean trench bottoms are difficult or impractical to achieve. By this method a stable trench fill can quickly and easily be achieved in relatively poor ground (see Fig. 11.5).

Fig. 11.5 Trench fill in poor ground.

Compaction difficulties can be experienced in narrow trenches cut in dry or relatively stiff sub-strata where
compaction of the fill at the edges is partly restricted by the frictional resistance of the trench sides. This tends to show itself in the concave surface of the compacted layer (see  Fig. 11.6). However, this can be overcome by using suitably graded stone in relatively thin layers and by extra com- paction at the edges of the trench.


Fig. 11.6 Concave compacted surface.

Selection of suitably graded and shaped stone is particu- larly important, for example, single sized rounded stone will tend to compact automatically during filling in a sim- ilar way to say filling a trench with marbles. The marbles immediately fall into contact on more or less the maximum compaction due to the standard radius involved. However, in some locations it is important to avoid forming a field drain within the fill which may attract moving water, therefore well graded material is essential in these situations.

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