Design Example: Trench Fill Strip Footing.

The internal load-bearing wall for a four-storey office block is to be supported on a strip foundation. Borehole investigations produced the consistent soil profiles shown in Fig. 11.13.

Soil analysis shows that the sand fill is an unreliable bearing strata. The weathered sandstone has net allowable bearing pressures of na = 400 kN/m2 for strip footings and na = 550 kN/m2 for pads, both with a maximum of 20 mm settlement. The sandstone bedrock has a net allowable pressure of na = 2000 kN/m2
for pad foundations.

By inspection of the soil profile and analysis in Fig. 11.13, the strip will be founded in the compact weathered sandstone. The relatively even distribution of the loading will not lead to unacceptable differential settlements and, as the sides of the excavations do not collapse in the short-term, mass concrete trench fill footings have been selected as the most appropriate foundation type.

Fig. 11.13 Borehole log for Design Examples 1, 2 and 4.

Loadings
The loadings from the four-storey structure have been  calculated (as working loads) as follows.

Size of base (normal method)
The foundation surcharge is considered small enough to be neglected. The minimum foundation width is given by

In many instances this approximate method is satisfactory.

Where the new foundation surcharge is large, or the allowable bearing pressure is low, the following method should be used.

Size of base (allowing for foundation surcharge)
Dead load from new surcharge

Imposed load from new surcharge

The weight of the new foundation is taken as approximately equal to the weight of soil displaced, and thus is
excluded from the above loads.

The net bearing pressure is

In this case the existing surcharge sS = 0.

As may be seen, the normal method value of B = 0.71 m in this example is sufficiently accurate for all practical purposes.

Final selection of foundation width must take into account the width of the wall, together with an allowance for tolerance. It should also try to suit standard widths of excavator buckets which are in multiples of 150 mm, e.g. 450 mm,  600 mm, 750 mm, etc. In this case a width of B = 750 mm would be appropriate, as shown in Fig. 11.14.

Actual net bearing pressure (ignoring foundation surcharge)

The actual net bearing pressure beneath the strip footing may now be calculated, if required.

Fig. 11.14 Trench fill strip footing design example.