GB2261891A - Installing columns e.g. below ground - Google Patents

Abstract

A method of installing a column, comprises: casting a concrete bored pile (3) in a hole in the ground, so that the top end of the pile is below ground level (GL); and securing a column (4) to, or forming a cast column on, the top end of the pile (3), after the concrete of the pile has set. Preferably, a hollow sleeve (40) of a diameter larger than the diameter of the pile (3) is provided between the top of the pile and the surface of the ground, the centreline of the sleeve (40) being coincident with the centreline of the pile (3), to gain access to the top of the pile. The above method may be incorporated into a method of constructing a basement, known as the "top-down" method of construction. In this method, once the columns (4) are installed, simultaneous construction of the basement space and the upper floors (14, 16, 18) of the building may take place. <IMAGE>

Description

INSTALLING PRE-FOUNDED COLUMNS The present invention relates to a method of installing pre-founded columns.

Pre-founded columns are used mainly in the construction of deep basements of high rise buildings following the so-called "top down" method of basement construction. The main advantage of the top-down method is that it enables construction of the superstructure (i.e. the floors above ground level) to proceed at the same time as construction of the basement is in progress. One of the functions of the pre-founded columns is to support the load of the superstructure during basement construction.

Figure 1 illustrates the top-down method of basement construction. In this method, the sequence of construction of the various basement floors starts from the ground level, followed by the first basement level, second basement level and progresses downwards until the lowest basement floor is reached. Since the construction of the basement and the superstructure proceeds simultaneously, it is necessary for the columns within the basement depth to be constructed well in advance of any other parts of the building.

These basement columns are referred to as pre-founded columns and they are constructed from the ground level before basement construction commences.

An important inherent requirement of a pre-founded column is that it must be installed accurately in the required position and also its deviation from the true vertical direction must be as small as possible. Such small construction tolerances are not easy to achieve.

At present a typical pre-founded column consists of a heavy steel member, either an H section or a tubular section, which is supported at its lower end on a large diameter bored pile. The bored pile terminates at the level of the lowest basement floor. The upper end of the column is at ground floor level, see Figure 1.

The construction of a known form of pre-founded column is shown in Figure 2. It is important to note that this illustrated method of installing columns suffers from a number of inherent problems. All operations are by remote control above ground, since there is no access below ground level. Furthermore, the placement of fresh concrete is often carried out in a pile hole filled with water or bentonite slurry.

Another severe constraint is time, since the entire column installation process must be completed before the concrete for the bored pile sets (i.e. hardens).

The setting time of concrete is normally between one to two hours after pouring.

The problems associated with the present method of construction are: 1. It is difficult to achieve the required construction accuracy with respect to (a) verticality and (b) position of the bottom end of the column, since there is no access for workmen below ground level.

2. For the purpose of transferring load to the bored pile, the column usually requires a number of shear connectors and a long bond length in the bored pile (i.e. that length which is embedded in the concrete of the bored pile).

3. Sometime it is difficult to insert the column to the required depth into the fresh concrete of the bored pile or the column may deviate from its intended position during insertion.

4. The entire process of installing a column must be completed within a short time span, usually about one to two hours, since the column must be inserted into the fresh concrete before the concrete hardens. Any delay will render the whole column useless.

Apart from the above factors the steel member is usually fabricated from steel of extraordinary thickness, so that it can withstand the enormous loads which are supported on the column. Such steel members are not readily available and are not easy to transport, so that delay in supply is common.

Furthermore, these members are relatively expensive compared with reinforced concrete columns of the same load carrying capacity.

According to a first aspect of the present invention there is provided a method of installing a pre-founded column, comprising: casting a concrete bored pile in a hole in the ground, so that the top end of the pile is below ground level; and securing the column to the top end of the pile after the concrete of the pile has set. The column may be preformed or may itself be cast in-situ.

A hollow sleeve may be provided between the top of each pile and the surface of the ground, the centreline of the sleeve being coincident with the centreline of the pile. The sleeve is preferably of a significantly larger diameter than the diameter of the pile and may extend below the level of the top of the pile.

Preferably, a pile cap is cast on to the top of the pile, prior to securing the column. The pile cap may comprise a kicker or insertion guide, which is shaped to centre the column onto the top of the pile cap during installation of a preformed column, or to centre the formwork, if the column is cast in-situ.

In order to improve the alignment between the column and the top of the pile or pile cap, the column and the top of the pile cap may be provided with cooperating surfaces. Preferably, the base of the column is provided with dowels which project in a direction parallel to the centreline of the column, and the pile cap is provided with corresponding holes.

According to a second aspect of the present invention, there is provided a method of constructing a building, comprising the steps of: placing concrete piles such that their upper ends are substantially at or below the lowest floor level of a desired basement space; securing a column on to each pile; constructing the building upwards on the columns; and during the period of the upward construction, excavating the ground around the columns to create the said basement space, the columns being secured to the concrete piles after the concrete piles have set.

For a better understanding of the present invention, and to show how it may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which: Figures la to g show the various steps in the "top-down" method of basement construction; Figures 2a to d show the various stages of construction of known pre-founded columns; and Figures 3a to e show the various steps in the construction of pre-founded columns according to the present invention.

Referring to the drawings, Figure la shows the first step in the known "top-down" method of basement construction. In this method, the sequence of construction of the various basement floors starts from ground level GL. Once the ground has been cleared, a retaining wall 1 is constructed, which defines the boundaries of a basement space. The retaining wall 1 extends below a lowest basement level LBL. Next, using conventional piling techniques, holes 2 of a predetermined depth are bored into the ground. When boring through softer strata, the sides of each hole 2 are prevented from caving in by the insertion of a long hollow sleeve or by the use of bentonite mud pumped in to the bore hole behind the advancing drill bit.

Concrete is then pumped into the hollow sleeve or into the bottom of the holes, displacing the bentonite mud or water in the holes, to create cast in-situ piles 3 extending up to the lowest basement level LBL. Hsection steel columns 4 are then inserted from ground level into the fresh concrete of each pile 3, to a predetermined depth.

Once the concrete piles 3 have set, cross beams are fixed to the H-section columns 4 and floor slabs 6 are fixed to the beams to construct the ground floor.

Large holes 8 are provided in the floor slabs 6, to enable the earth beneath them to be extracted.

The next step in construction, is shown in Figure ic. Once the ground floor slabs 6 have been fixed in place, construction begins both upwards and downwards.

Further columns 9 are erected onto the H-section columns 4, for the construction of the upper floors of the building, while simultaneously earth is excavated from below the floor slabs 6 to create a first level basement 10. Once sufficient earth has been removed, first basement level slabs 12 are secured to the retaining walls 1 and to the H-section columns 4 (see Figure Id). During the same period, construction proceeds above ground with the creation of further upper floors (14, 16, 18). Figures le and if show further stages in the construction of consecutive lower basement floors and upper floors.

The completed structure is shown in Figure lg.

The lower basement floor 24 rests on the top of the piles 3. The structure shown in Figure ig appears only to have three upper floors 14, 16, 18. However, in practice, the building would tend to have many more floors above ground than below ground. The advantage of the top-down method is that the basement floors and upper floors may be constructed simultaneously, so speeding up the overall time of construction.

Figure 2 shows the method of construction of a known pre-founded column 4. As shown in Figure 2a, the first step in construction is the casting well below ground level GL of a large diameter bored pile 3. A temporary casing 30 is provided, which extends from just above ground level GL to just below the pile cut off level C. Once the pile has been cast, a column guide 32 is rapidly installed over the top of the pile hole and is accurately centred over the pile hole by surveyors working above ground with theodolites. The column guide 32 is welded to the top of the temporary casing 30, but is provided with manual adjusting devices which enable the hole in the centre of the column guide, which guides the H-shaped column 2, to be moved in the horizontal plane.

As shown in Figure 2c, an H-shaped column 4 is then installed in the column guide 32. Where shear connectors 34 are provided on the bottom of the Hshaped column 4, great care must be taken in inserting the H-shaped column 4 into the column guide 32. Once the H-shaped column 4 is correctly aligned, it is gradually lowered into the fresh concrete until, as shown in Figure 2d, a predetermined bond length Q is achieved.

Figures 3a to e show the various steps in constructing pre-founded columns, according to the present invention. Step 1, shown in Figure 3a, is the installation of a collar casing 40 of a diameter approximately 150mm larger than the required diameter of the bored pile 3. The collar casing 40 extends from just above ground level to approximately the cut off level C of the pile 4. This cut-off level C:is positioned at or below the floor of the lowest desired basement level LBL.

Next, an inner temporary casing 42 of a diameter smaller than the diameter of the collar casing 40 is centred within the collar casing 40 and installed in a hole 44 which is bored to the desired level of the bottom of the pile. Pile reinforcement 46 is then positioned within the inner casing 42 and concrete is poured into the hole 44. Pouring continues until the bore hole 44 is filled to a level slightly greater than 0.6 metres above the desired cut-off level C of the bored pile 3, at which point the inner casing 42 is withdrawn from the bore hole 44.

After the concrete hardens, the water or bentonite mud inside the collar casing 40 is pumped out, and workmen descend to the level of the top of the pile 3.

Concrete is then hacked and stripped back to the cut off level C by the workmen. The collar casing 40 is pushed down below the cut off level C to a depth sufficient to seal off the ingress of water. Once the top of the pile 3 has been prepared in this way, a pile cap 50 is cast in-situ on to the pile reinforcement 46, protruding from the top of the pile 3. The pile cap 50 comprises a concrete kicker 52 comprising an annular element with a central tapered hole 54. The kicker 52 ensures accurate positioning of a preformed column 4 on the pile cap 50, or alternatively ensures accurate centring of formwork for the construction of a cast insitu column (not shown). A suitable de-bonding agent 56 is placed between the pile cap 50 and the collar casing 40 to ensure that the collar casing 40 may be withdrawn once the column 4 has been installed and the concrete of the pile cap 50 has set.

In the preferred embodiment described above, dowel holes (not shown) are formed in the top of the pile cap 50, in the region of the central tapered hole 54 of the concrete kicker 52. Cast into the base 62 of the precast concrete column 4, are a plurality of dowel bars (not shown) which project at right angles from the base 62. The number, size and spacing of the dowel bars corresponds to the number, size and spacing of the dowel holes in the top of the pile cap 50. After all the dowel holes have been filled with liquid cement grout, the pre-cast reinforced concrete column 4 is lowered onto the pile cap 50. The tapered hole 54 in the kicker 52 guides the base of the column 62 so that the protruding dowel bars are able to align accurately with the corresponding dowel holes formed in the bottom of the reinforced concrete column 4.

In an alternative arrangement (not illustrated), the pile cap 50 is provided with dowel bars which protrude from its top surface. These dowel bars engage corresponding holes provided in the base 62 of the column 4 when the column 4 is lowered into place.

The column 4 may also be cast in-situ, in which case the pile cap 50 and kicker 52 are installed as in the previous method. However, the next step in the process is to construct a reinforcement tower surrounded by suitable formwork (not shown). The reinforcement tower and formwork is constructed from the level of the pile cap 50 upwards, to the desired height above ground level GL. Concrete is then poured into the top of the formwork to construct the column.

To ensure structural continuity between the column and the top of the pile cap 50, reinforcement bars 60 are cast into the top of the pile cap 50. The bars 60 form the starter bars of the reinforcement tower of the cast in-situ column

Claims (11)

1. A method of installing a pre-founded column, comprising: casting a concrete bored pile in a hole in the ground so that the top end of the pile is below ground level; and securing a column to the top end of the pile, after the concrete of the pile has set.

2. A method as claimed in any one of the preceding claims, in which a hollow sleeve is provided between the top of the pile and the surface of the ground, the centreline of the sleeve being coincident with the centreline of the pile.

3. A method as claimed in claim 2, in which the sleeve is of a larger diameter than the diameter of the pile.

4. A method as claimed in claim 3, in which the sleeve extends below the level of the top of the pile.

5. A method as claimed in any one of the preceding claims, in which after the pile has set a pile cap is cast on to the top of the pile.

6. A method as claimed in claim 5, in which the pile cap comprises a kicker, which is shaped to centre the column onto the top of the pile cap.

7. A method as claimed in any one of the preceding claims, in which the bottom of the column and the top of the pile cap are provided with cooperating surface elements.

8. A method as claimed in claim 7, in which the cooperating surface elements comprise dowels and corresponding holes.

9. A method as claimed in any one of the preceding claims, in which the column is cast-in-situ.

10. A method of constructing a building, comprising the steps of: placing concrete piles such that their upper ends are substantially at or below the lowest basement level of a desired basement space; securing a column on to each pile after it has set; constructing the building upwards on the columns; and during the period of the upward construction, excavating the ground around the columns to create the said basement space.

11. A method of installing a pre-founded column, substantially as described herein, with reference to and as shown in Figures 3a to 3e of the accompanying drawings.