Digging Foundations

The depth below ground level to which the foundation trenches should be dug depends both on the load to be imposed and on the type of ground to be found below the site. On large building sites, soil investigation by test boring is undertaken and from the results the required depth is calculated.

Loads are not excessive in small domestic buildings and the foundation depth is usually indicated on the plans and then the condition of the subsoil is checked by the Building Control Officer when the trenches have been dug. In all cases, the foundations must be taken to a depth where the earth is not affected by changes in the weather. They must be deeper than the level penetrated by surface water which saturates the ground and then disperses fairly quickly, partly by natural drainage and partly by evaporation. This level of earth is affected by the drying effect of hot sunshine and drought.

Deeper down, below the level to which frost generally penetrates, the subsoil is much more stable and is subject to less movement, it can therefore be relied on to support calculated loads. In general the depth is about 1.5 m in clay and decreases as the proportion of gravel increases. In firm gravel the foundation depth would be a minimum of 600 mm. Soil is in fact graded and the classifications are: (1) Rock: not inferior to sandstone, limestone or firm chalk; this would need a mechanical pick for excavation. (2) Compact gravel or sand: this would need at least a hand pickaxe for excavation. (3) Stiff clay or sandy clay: this also needs a pickaxe for excavation. It is too hard to be moulded with the hands. (4) Firm clay and sandy clay: this can just be moulded by hand and can be excavated with a spade. (5) Loose sand, loose silty sand and loose clayed sand: can be excavated with a spade. (6) Soft silt, soft clay and soft silty clay: fairly easily moulded by hand and is easily excavated. (7) Very soft silt, very soft clay, very soft sandy clay and very soft silty clay: in winter conditions this soil exudes between the fingers when a sample is squeezed in the fist.

Compact, well graded, sands and gravel-sand mixtures can be expected to carry from four to six tons per square foot. Loose soils of this type could be expected to carry only two to four tons per square foot. Stiff clays could also carry two to four tons per square foot. Firm and sandy clays could be expected to carry one to two tons per square foot.

A typical two-storey house of about 1000 ft2 floor area with brick cavity wall construction, timber floors and tiled roof is considered to weigh about 100 tons. The load per lineal foot on the front and back walls at ground level is about 0.8 tons. For the party wall it is about 1.5 tons and for the gable walls, it is about 1.2 tons.

It can be seen therefore that provided that the foundations are dug to a depth where they will not be affected by the swelling, shrinking or freezing of the subsoil, most soils, can support the average dwelling house without resorting to special constructions.

For a grade 4 soil the required width of concrete would be 600 mm to support ½ tons per lineal foot, or 750 mm to support 1% tons. As this is about the maximum load for house foundations it can be seen that the generally accepted width of 600 mm for the concrete foundation is adequate all around the building on most subsoils.

One rule-of-thumb method of determining the width of the foundation concrete is to make it twice the width of the wall it supports, which again works out about right at not less than 559 mm for a 279 mm cavity wall. The thickness of the concrete is determined by making a scale drawing of a section through the wall and then drawing a line at 45 degrees from the face of the brickwork down through the concrete. Where this line meets the front edge of the concrete denotes the depth or thickness required. However, the concrete must never be less than 150 mm thick; also the depth of the concrete should be at least equal to the amount that it projects beyond the face of the brickwork.

Levelling the bottom of the trenches can be carried out with the boning rods, and pegs are driven into the bottom of the trenches to mark the thickness of the concrete. These pegs are also levelled with the boning rods.

The top of the pegs in the trenches represents the top of the concrete foundation. So if the base of the foundation is to be 1224 mm below datum level and the concrete is to be 150 mm thick, then the top of the peg will have to be 1067 mm below the datum level. A straight edge board is packed up level across the trench from the datum peg and beneath it a peg is driven into the bottom of the trench until the top of this peg is exactly 1067 mm below the bottom edge of the straight edge.

Another peg is then driven into the bottom of the trench as far from the first one as the length of the straight edge will allow and the top of this peg is brought to the same level as the first one. This procedure is carried out from one end of the foundation trenches to the other and right around the building.

The bottom of the trench is then carefully dug out level to 150 mm below the tops of the pegs. The sides of the trenches must be vertical and when all the loose material has been cleaned from the bottom of the trenches, the concrete is poured. No hardcore is ever put into the foundations for walls. The concrete is placed direcly on to the prepared ground.

The concrete mix to use for the foundations is l:2l/2:4. That is one part cement, two and a half parts sand and four parts coarse aggregate. The sand should be ‘sharp’ not the soft building sand used for mortar. It is most important that the materials for concrete making are kept clean, they should not just be dumped on the soft earth where they will pick up dirt. Sand and gravel are washed at the pits and should be kept on a prepared base. Wooden boards will do for small quantities, but large quantities need a base made of weak concrete. Corrugated iron or wooden boards can be used for the sides of the enclosure.

The coarse aggregate can be either gravel or crushed stone which varies in size from 4.5 mm to 19 mm. In some parts of the country, especially where there are quarries providing crushed stone, it is possible to get ‘all-in’ ballast. This is a combined aggregate which can be used where the uniformity of the concrete is not of any importance, such as garden wall foundations, paths and bases for garden sheds. For house foundations and other important work separate aggregates which can be accurately measured to exact proportions must be used.

Use only enough water to make the mix workable so that it can be placed in the trenches and compacted without leaving large air-holes at the sides of the trench. The concrete is not just dumped into the trench and left. It is laid carefully in layers about 100 mm thick and each layer is tamped or prodded with a length of timber to compact it. On large building sites this is done with a poker vibrator which shakes the concrete down into a solid mass.

The top of the concrete is tamped level with the top of the pegs, using a straight edge. The pegs are pulled out and the holes filled in. When the concrete has cured sufficiently, bricklaying can start. Below ground brickwork is solid, or if cavity walls are being built the cavity must be filled up to ground level. Ideally, the top of this filling is sloped to the outside face of the wall and one or two open joints are left in the brickwork as weep-holes to let out any water which may collect at the bottom of the cavity. Foundation brickwork is carried up to not less than 150 mm above ground level and at that point a damp proof course is laid.

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