STRIP FOUNDATIONS FOR HOUSES G. Barnbrook BSc, CEng, MICE, ACIArb

This document

LtaS 8 pages J

The construction of strip foundations - whether strip footings or trench fifi - is generally a straightforward operation. However, in some circumstances and particularly in hazardous soils, special care must be taken. This Guide points out where this special care is needed and offers some practical hints on construction. The section on the correct mix to specify has been updated. For more detailed information, including design guidance, refer to BCA publication 48.048: House foundations for the builder and building designer.

STRIP FOOTINGS TRENCH FILL

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TRENCH SIZE Excavate at least to the depths and widths shown in the specification or on the drawing for the job.

TRENCH EXCAVATION AND FILL ON SLOPING SITES Trenches must always be excavated vertically. This means that on sloping sites, excavators must be level before trenches are dug across the slope, to avoid trench fill being out-of-vertical. Non-vertical foundations could start to rotate when loaded, causing movement of walls.

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SAFETY Remember the safety code for excavation work and follow the Construction (General Provisions) Regulations 1961. Provide support to the excavation sides to prevent danger to personnel in the trench from faffing earth or other materials. This usually means that shoring must be provided in trenches over 1.2 m deep where strip footings are used. The type of shoring must be chosen to suit the ground conditions. Where trench ifil is used, and where no one wifi normally enter the trench, shoring is not usually required. (The bottom of most trenches can be prodded with a steel bar from ground level to check the unifor- mity of the soil for bearing capacity.)

Where an inspection of the trench base is necessary at particular points in trenches deeper than about 1.2 m, quickly installed hand-operated hydraulic speedy' shoring is available to allow safe access. The equipment is also useful if some soil falls into deeper trenches and must be removed by hand.

if a soft spot is found in the trench run, remove the soft soil and reffil, preferably with a lean concrete (1 part cement: 12 parts aggregate).

Ground level

Finished ground level V

SOFT SPOTS

CLAY SOILS Firm clay over soft. Do not over-dig where a finn clay crust (possibly 1 or 2 m deep) overlies generally softer clay. The deeper the strip into the crust, the higher the stress will be in the softer soil, leading to greater settlement.

Effects of weather. As soon as a trench is excavated in clay soil, place the strip footing or trench fill concrete straight away (preferably on the same day) to avoid the clay base either drying out or becoming wet.

If delay does occur in concreting, and the clay base dries out or becomes wet, the trench must be 'bottomed out', i.e. dug back 75 mm at least to clay of original moisture content - then concreted immediately.

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Keeping the sides straight. The sides of trench fill concrete in clay soil should be kept vertical at the top section.

If the top edges of the trench sides collapse during excavation, the trench fill concrete level must be kept down to maintain vertical sides. Alternatively, formwork can be used to produce higher vertical sides ii this is essential at certain points.

Remember that in some clay soils with a high shrinkage potential, concrete in the wider top section of a trench would increase the risk of clay heave problems while the foundations remain without the weight of the house on them - this is because swelling clay would be able to exert upward thrust against the underside of any sideways-projecting concrete. Bucket marks. When a trench is excavated in clay soils, the base often has some tooth marks made in it by the excavator bucket. Provided that the clay soil between the tooth marks is not loose in the trench base, there is no need to 'bottom-out' the trench by hand. However, if there is any loose soil it must be removed.

Keep this dimension as large as possible

Firm clay crust

Soft clay

Moisture driven out a foundation placed on a dried clay base may suffer heave movement later when the clay takes up moisture and swells

Over-wetting: a foundation placed on a swollen clay base will settle later as the clay compresses under the house load

DEPTH IN WET SANDY OR GRAVEL SOILS In these types of soil, do not excavate unnecessarily deep. You must dig down to a level where bearing capacity is satisfactory, but a depth of 350 mm is often sufficient to allow removal of surface vegetable soil. However, keep drain trenches well away from shallow foundations (see page 7). Remember that the bearing capacity of sand and gravel soils near or below ground water level can be as little as half that of the soil well above ground water level. Keep foundations in wet granular soils as high as possible.

WATER PROBLEMS Do not pour concrete into standing or flowing water in a trench. Deal with a water problem during excavation - for example, by using pipes to connect up temporarily interrupted field drains. However, site drainage should finally be permanently routed around and away from a house.

SETTING-OUT Where strip footings or trench fill are narrow - e.g. only 450 mm wide - set out the foundation accurately to avoid the house wall being more than about 25 mm off centre line.

maximum

With narrower trench ifil, even greater care must be taken to produce accurate setting-out. Always aim to have trench ifil at least 150 mm wider than the wall to be built on it to allow for some small margin of error in setting-out and stifi have the wall approximately central on the foundation.

Strip footings. For strip footings, drive short steel pins into the trench bottom to establish the top of the concrete.

350 mm mm.

Try to keep this dimension larger than the foundation width

tj foundation

Wall width W

W + 150 mm mm. (but total width depends on wall load, soil conditions and structure type)

MARKING THE TOP LEVEL

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Trench fill. For trench fill, there are usually only about four courses of brickwork, or one or two courses of blockwork, above the trench ff1 concrete up to damp-proof course, so it is important to level the concrete top accurately. Establish the concrete top level using long steel pins in the trench bottom or by using short horizontal pins in the trench sides.

READY-MIXED CONCRETE The concrete used in strip foundations is often supplied from ready-mixed concrete companies. A designated mix, GEN 3, to British Standard 5328, should be ordered. This will comply with the requirements of most speci1ring authorities, and of the Building Regulations, for normal non-aggressive soils.

Concrete of this type is reasonably self-compacting as poured and levelled and would not require additional compaction in normal circumstances. Do not add extra unauthorized water to the concrete on site. This will make the concrete weaker.

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ACCESS Temporary access. If easier placing of concrete is required, make a good access to both sides of the house to allow several points of discharge into the trenches.

Superplasticized concrete. Where access is very difficult, superplasticized or 'flowing' concrete (containing a special admixture to make the concrete flow) can be supplied to allow placing from only one point in the trench with little or no labour.

Pumped concrete. Alternatively, a lorry- mounted pump can deliver concrete easily across inaccessible parts of a site and place it where required.

Both methods are particularly useful for trench fill, where greater quantities of concrete are used than for strip footings.

Ready-mixed concrete for strip foundations

For normal ground conditions: Order Designated mix GEN 3 20 mm aggregate 75 mm slump for strip footings, or

125 mm slump for trench fill

For aggressive ground conditions, e.g. sulfate. or acidic soil.: Order A higher-grade designed mix with possibly a different cement type Ask the Engineer for guidance. (See BCA publication 48.048)

DAYWORK JOINTS The strip foundations of one house or terrace block should be completed in one pour, wherever possible, to avoid daywork joints. If a daywork joint becomes unavoidable, e.g. because of concrete supply break-down, fmish the first pour using one of the methods described below. (An Engineer can advise you which is the best method to use in any particular case.) Method A. Finish with an edge as near vertical as possible, using formwork if necessasy and keeping the vertical face of the concrete at the joint either rough or indented.

Method B. A preferable alternative with trench fill in clay soils, or in soils where higher than normal differential settlement may occur, is to form a wedge-shaped joint using corrugated sheet.

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Method C. The best protection against differential setfiement at a joint is provided by installing some horizontal steel dowel bars.

Always keep a few suitable dowel bars and some expanded metal or corrugated sheet available on site for emergency daywork joints in foundations.

BRICKWORK REINFORCEMENT Brickwork reinforcement may sometimes be specified for walls above daywork joints. This should be installed as shown.

..s... 500 mm 500 mm

TL U TiETJI ii I[ Till l[ilL IL lET

Pegs at ground level

Timber form drilled for bars (optional: depends on area of expanded metal to be supported)

Dowel bars

Pegs at ground level

ps

. Pegs in trench base

Expanded metal

Timbor form (oponal)

Expanded __________ metal f(n

- -, 17- '. " Method A for strip footings

Four bars 16 or 20 mm diameter and 0.9 m long, with half their length left protruding. For concrete depth over 1 m, increase the number of bars proportionally.

Pegs at ground level

DPC

_____________________________________

Stainless steel _____________________

brick reinforcement __________

in both wall leaves

Daywork lox1t Concrete foundation H

STEPS IN LEVEL ON SLOPING SITES On sloping sites, it is necessary to follow the rules for steps in strip footings given in the Building Regulations (e.g. Approved Document A for England and Wales). Strip footings. The step height. h, should not be greater than the concrete thickness, t. The length of lap at the step should be at least equal to 2 x h, or t or 300 mm. whichever is greatest.

Trench fill. The Building Regulations do not specifically deal with steps in trench fill foundations. The following guidelines are given in BS 8103: - For trench fill less than 500 mm deep, use the rules given above for strip footings. - For trench fill 500 mm deep or more, the step should not be greater than the concrete thickness, t. The length of lap at the step should be at least 1 m or twice the step height. h, whichever is greater.

DRAINS AND OTHER SERVICES Services through the house sub-structure. In the case of a strip footing (below left), some of the bricks or blocks in the wall have to be cut out. With trench fill (below right), a pipe, duct or box is fixed into the trench before the concrete is placed. The duct should be sensibly larger than the drain or service to be installed, and positioned with reasonable accuracy.

Service trenches. Beware of weakening the soil support to a foundation by digging trenches adjacent to it. The depth. d, should be large enough for L to exceed h.

For further reading see back cover. 7

h (not greater than t)

Lap at least 2 x h or 300 mm. whichever is greatest

Ship footing Trench fill

h (not greater than t)

Lap at least 1 m or 2 x h, whichever is greater