GB2287730A

GB2287730A - Building foundation construction

Info

Publication number: GB2287730A
Application number: GB9505743A
Authority: GB
Inventors: John James Patrick Brown
Original assignee: Kay Metzeler Ltd
Current assignee: Kay Metzeler Ltd
Priority date: 1994-03-23
Filing date: 1995-03-21
Publication date: 1995-09-27
Anticipated expiration: 2015-03-21
Also published as: GB9405763D0; GB2287730B; GB9505743D0

Abstract

To construct a building foundation, a trench 20 is dug which is to be lined with compressible fill slabs 26 and then filled with poured concrete. To hold the slabs 26 in place prior to pouring of the concrete, weights 10 are placed at the bottom edges of the trench with tapes 18 attached to them and extending up and out of the trench. When the slabs are placed against the sides of the trench, the tapes 18 are pulled taut to secure the slabs, and the free ends of the tapes are fixed at the ground surface 22. Concrete is then poured to fill the trench and once the concrete has stabilised, the free ends of the tapes can be cut off. <IMAGE>

Description

BUILDING FOUNDATION CONSTRUCTION This invention relates to a method of constructing building foundations, and to apparatus for use in constructing building foundations.

It is a requirement in the construction of foundations for buildings that provision be made to absorb ground surge and ground heave which might occur after the foundation has constructed. The term 'surge' refers to movement of the ground in an upward direction, and the term 'heave' refers to movement in a direction along the ground surface. Conventionally this protection is provided by incorporating a compressible fill in the foundation and then surrounding the compressible fill with concrete. The compressible fill is placed on that side of the foundation where ground movement may occur.

The compressible fill used is often in the form of slabs of expanded polystyrene which do not crush when concrete is poured around them, but can be crushed by subsequent ground movement to absorb that movement without disturbing the foundation.

It is necessary for such a slab to be held in the correct position in a foundation trench, whilst concrete is being poured and it is an aim of the invention to assist in providing a temporary fixing for a polystyrene slab.

According to a first aspect of the invention there is provided a method of constructing a building foundation, comprising the steps of digging a trench, placing a weight at the bottom of the trench with a flexible strap secured to the weight and the free end of the strap extending sut of the trench, pouring concrete into the bottom of the trench to embed the weight and thereby to secure the lower end of the strap, placing a compressible fill slab against one side of the trench, passing the strap over the top of the slab, securing the free end of the strap to the ground so as to hold the slab against the trench side, and filling the trench with concrete.

As a final step, the part of the tape protruding above the concrete is preferably cut off.

The term 'strap' as used in this specification includes any flexible tension link, and includes such links as a rope, a cable or a tape. A tape which has significant width is preferred, because this will not cut unduly into the edge of the compressible slab.

The initial fill of concrete preferably submerges the weight.

In order to prevent the slabs being distorted against an uneven trench lip, any space between the top edge of a slab and the adjacent trench lip may be back-filled with sealed bags of polystyrene beads which will conform to the shape of a void to be filled.

According to a second aspect of the invention there is provided apparatus for use in the construction of building foundations, the apparatus comprising, in combination, a plurality of weights, flexible straps attached to the weights and a plurality of compressible fill slabs.

Each weight must be capable of remaining in place at the bottom of a trench whilst concrete is poured around it, and the weight should therefore comprise a body of a density greater than or equal to that of poured concrete.

The weight is preferably a concrete block and preferably has a securing eye on one face to which a flexible strap can be secured. Alternatively, an end of the strap can be embedded in the block.

The strap is preferably of sufficient strength to support the weight so that the strap can be used to lower the weight into a trench.

The strap may have a marking (possibly formed by a knot in the strap) at a predetermined distance above the base of the weight to mark the depth to which the trench should be initially filled with concrete.

If the strap is a plastic tape which can be printed, markings can be placed on the strap at, for example, 500mm intervals so that the progress of concrete filling can be monitored. The tape should be brightly coloured so that it is highly visible to site workers.

The apparatus may include polystyrene beads for use in backfilling between a slab and an adjacent uneven trench edge. The apparatus may also include a plurality of ground anchors which can be inserted into the ground and to which the free ends of the straps can be secured.

According to a third aspect of the invention, there is provided a method of positioning a reinforcement cage in a foundation trench, comprising the steps of placing a compressible fill slab on the bottom of a trench, attaching spacer blocks to the lower face of a reinforcement cage, lowering the cage into the trench so that the spacer blocks rest on the upper surface of the slab, and pouring concrete into the trench on top of the slab, to encapsulate the blocks and the cage.

A further aspect of the invention provides spacer blocks in the form of pre-cast concrete blocks which may have a pair of tapes secured in an upper surface which can be tied around reinforcing bars on the lower face of a reinforcement cage to attach the blocks to the cage.

The invention will now be further described, by way of example, with reference to the accompanying drawings in which: Figure 1 is a perspective view of a weight for use in the invention; Figures 2, 3, 4 and 5 are sequential stages in the construction of a building foundation in accordance with the invention; Figure 4a shows a situation which is alternative to that shown in Figure 4; Figure 6 is a view of a spacer block for use in the invention; and Figure 7 is a cross-section through an alternative form of foundation.

The block 10 shown in Figure 1 is a concrete cube of dimensions approximately 50 mm3 with an embedded eye 12 in the top face. The block should have a flat bottom face with the eye 12 in a top face, and it is advantageous if the block has at least one straight edge, with the eye 12 being positioned a fixed distance from this straight edge, for reasons which will become apparent below.

The dimensions suggested of a 50 mm3 block are given as an example only; other block sizes may be equally useful.

To construct a building foundation in accordance with the invention, it is first necessary to dig a trench 20. The dimensions of the trench will depend on building regulations requirements but typically will be between 1800 and 4600mm deep by 600mm wide.

When the trench has been dug, a weight 10 is lowered to the bottom of the trench on a flexible strap 18. The strap 18 is attached to the eye 12 on the top of the block either by looping the strap through the eye or by tying off one end of the strap to the eye. The block 10 is placed against one edge of the trench, as shown. Other blocks will be placed along the length of the trench at suitably spaced intervals (eg 500 mm apart). The strap 18 is then held taut by attaching it to a ground anchor 22 placed in the ground surface on one side of the trench.

The strap should be a plastics tape of width at least 75 mm, strong in tension and resistant to stretching. It should be bright in colour and can have printed markings along its length.

Next concrete is poured into the bottom of the trench to form a concrete toe layer 24 (Figure 3). The depth of this layer of concrete is greater than the height of the block 10 and may for example be 300 mm, so that the block is submerged and the junction between the strap 18 and the block is also embedded in the poured concrete. The concrete is preferably poured into the trench from positions opposite to the blocks to avoid the flow of concrete from disturbing the blocks from their positions.

When the concrete layer 24 has stabilised (this takes 10 15 minutes), slabs 26 of compressible expanded polystyrene are placed against one side of the trench (the side where protection against ground surge is required). Adjacent slabs should be carefully aligned and butted up against one another. The rest of the trench is then to be filled with more concrete, but it is necessary to ensure that the slab 26 is correctly held in place whilst the rest of the concrete is being poured in. To do this, the strap 18 is released temporarily from the ground anchor 22, the slabs 26 are placed against the side trench wall and the strap is then re-secured to the ground anchor 22 so that it holds a slab in place. The length of the strap must be sufficient to reach the anchor 22 after having passed over the slab. Similar securing of the slabs will take place along the length of the trench.

Figure 4a shows an alternative situation, where the edge of the trench 20 is not quite straight. Where there is a gap 36 between the top edge of the slab 26 and the trench wall, a sealed bag 38 of polystyrene beads is positioned so that when the strap 18 is pulled taut over the slab, the slab is not distorted and cracked.

The point at which the lower end of the strap 18 emerges from the concrete layer 24 should be spaced from the trench wall approximately a distance equal to the thickness of the slab 26, and this distance will be determined by the position of the eye 12 relative to the face of the block 10 which is positioned against the trench wall.

Finally, as shown in Figure 5, the trench is completely filled with concrete 30, and at this point the strap 18 extends out of the concrete body. The strap can then be cut off at 28 flush with the surface of the concrete foundation. The ground anchor 22 can then be removed and re-used. The foundation is then complete.

It is normally only necessary to place a compressible slab 26 on one side of a foundation trench 20. However the same method could be used if slabs are to be placed on both sides of the trench.

In order to avoid having to release the strap 18 when the slabs 26 are to be installed, it may be possible to initially fasten the free end of the strap to the opposite side of the trench as indicated at 22a in Figure 3. The strap can then be moved across to the ground anchor 22 after the slab has been put in place.

The method set out above provides a simple low cost way of efficiently securing slabs against the side of trenches, so that they stay put whilst the remaining concrete pour 30 takes place. Using this method, it is never necessary to enter the trench for the purposes of fixing the compressible fill.

The block 110 shown in Figure 6 is of similar dimension to that shown in Figure 1, and has two tapes 112 which can be tied together to secure the block to something else.

This block could be used in place of the block/weight of Figure 1, with the strap being secured to the block by tying together the two tapes 112 to form a loop.

Figure 7 shows an alternative foundation construction technique, where compressible fill slabs 32 are laid on the base of a foundation trench, and a reinforcement cage of reinforcing bars 34 is incorporated in the foundations.

Spacer blocks 110 (Figure 6) are tied to the lowermost face of the reinforcement cage using the embedded tapes 112. The cage 34 is then lowered into the trench with the lower limbs of the cage supported on the blocks 110. In this way the cage 34 is held in its desired position (typically 50mm above the slabs 32) so that it does not move when concrete is subsequently poured over and around the framework 34, and the blocks 110 spread the load of the cage so as to prevent the reinforcement bars from penetrating into the material of the slabs.

Claims

1. A method of constructing a building foundation, comprising the steps of digging a trench, placing a weight at the bottom of the trench with a flexible strap secured to the weight and the free end of the strap extending out of the trench, pouring concrete into the bottom of the trench to embed the weight and thereby to secure the lower end of the strap, placing a compressible fill slab against one side of the trench, passing the strap over the top of the slab, securing the free end of the strap to the ground so as to hold the slab against the trench side, and filling the trench with concrete.

2. A method as claimed in Claim 1, wherein the part of the tape protruding above the concrete is cut off after the trench has been filled with concrete.

3. A method as claimed in Claim 1 or Claim 2, wherein the initial fill of concrete submerges the weight.

4. A method as claimed in any preceding claim, wherein any space between the top edge of a slab and the adjacent trench lip is back-filled with sealed bags of polystyrene beads which will conform to the shape of a void to be filled.

5. Apparatus for use in the construction of building foundations, the apparatus comprising, in combination, a plurality of weights, flexible straps attached to the weights and a plurality of compressible fill slabs.

6. Apparatus as claimed in Claim 5, wherein each weight has a density greater than or equal to that of poured concrete so that it is capable of remaining in place at the bottom of a trench whilst concrete is poured around it.

7. Apparatus as claimed in Claim 5 or Claim 6, wherein the weight is a concrete block and has a securing eye on one face to which a flexible strap can be secured.

8. Apparatus as claimed in Claim 5 or Claim 6, wherein the weight is a concrete block and has an end of a flexible strap embedded in the block.

9. Apparatus as claimed in any one of Claims 5 to 8, wherein the strap is of sufficient strength to support the weight so that the strap can be used to lower the weight into a trench.

10. Apparatus as claimed in any one of Claims 5 to 9, wherein the strap has a marking at a predetermined distance above the base of the weight to mark the depth to which the trench should be initially filled with concrete.

11. Apparatus as claimed in Claim 10, wherein the marking is a knot tied in the strap.

12. Apparatus as claimed in Claim 10, wherein the strap is a plastic tape with printed markings placed on the strap at intervals so that the progress of concrete filling can be monitored.

13. Apparatus as claimed in any one of Claims 5 to 12, including polystyrene beads for use in backfilling between a slab and an adjacent uneven trench edge.

14. Apparatus as claimed in any one of Claims 5 to 13, wherein the apparatus includes a plurality of ground anchors which can be inserted into the ground and to which the free ends of the straps can be secured.

15. A method of positioning a reinforcement cage in a foundation trench, comprising the steps of placing a compressible fill slab on the bottom of a trench, attaching spacer blocks to the lower face of a reinforcement cage, lowering the cage into the trench so that the spacer blocks rest on the upper surface of the slab, and pouring concrete into the trench on top of the slab, to encapsulate the blocks and the cage.

16. Spacer blocks for positioning a reinforcement cage in a foundation trench, the blocks being in the form of pre-cast concrete blocks which have a pair of tapes secured in an upper surface which can be tied around reinforcing bars on the lower face of a reinforcement cage to attach the blocks to the cage.

17. A method of constructing a building foundation substantially as herein described with reference to the accompanying drawings.

18. Apparatus for constructing a building foundation substantially as herein described with reference to the accompanying drawings.

19. Spacer blocks for positioning a reinforcement cage in a foundation trench, substantially as herein described with reference to the accompanying drawings.