Hydraulic and earth engineering – Foundation – Columnar structure
Reexamination Certificate
2001-01-25
2002-05-28
Will, Thomas B. (Department: 3673)
Hydraulic and earth engineering
Foundation
Columnar structure
C405S255000, C405S256000, C405S257000
Reexamination Certificate
active
06394703
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to capping beams which are formed across the tops of piles in the construction industry, and in particular, but not exclusively, to an improved method and apparatus for forming a capping beam for piles formed using auger piling methods.
BACKGROUND OF THE INVENTION
It is known to construct subterranean walls by forming a series of adjoining or nearly adjoining concrete piles by using auger piling techniques such as continuous flight auger (CFA) auger piling, described in detail in U.K. patent application no. 9515652.7 (GB 2 303 868) the disclosure of which is incorporated into the present application by reference. Auger piling comprises the steps of rotating an auger into the ground, and then withdrawing the auger, with our without rotation, while pumping concrete to its lower end, thereby forming a concrete pile. A reinforcement member may be inserted into the concrete before it sets so as to provide additional structural strength.
There are three known methods of constructing such a wall. The simplest method is to form a contiguous bored pile wall, in which a series of piles are formed in a line but without touching one another. This is a relatively straightforward operation and the wall will not be watertight owing to the gaps between the piles.
An alternative technique is to form an interlocking bored pile wall. In this technique, a series of ‘female’ piles are formed in the desired line of the wall and concreted with a weak concrete mix. No reinforcements are used. A complementary series of ‘male’ piles is then formed by boring down at the midpoint between two adjacent female piles, thereby cutting into the weak concrete mix. Each male pile is then concreted and reinforced in the usual manner, so as to leave a series of reinforced, hard concrete piles with the gaps therebetween filled by the weak concrete female piles. This is a great improvement over the contiguous bored pile technique, but does require a great deal of vertical piling accuracy and is still not entirely watertight owing to the properties of the weak concrete of the female piles.
The technique which results in the highest structural integrity is known as secant wall piling. This is similar to the interlocking bored pile wall construction method outlined above, but strong concrete is used for both the female and the male piles. This means that when forming the male piles, it is necessary for the piling auger to remove concrete from the hardened female piles. This is a difficult and time-consuming process, resulting in significant wear on the piling auger. However, the result is a wall which has excellent integrity against water penetration.
In all these methods, and also in general piling applications where a group of piles are formed relatively close to one another, it is often desirable to install a capping beam across the tops of the piles at or close to ground level. This capping beam provides a sound, generally level surface upon which construction of a superstructure can take place, and is useful in seeking to equalize differential settlement or movement of the piles. Such beams are often of width larger than the pile diameter and typically have a depth of about 0.5 m.
It is possible to prepare previously-installed piles by forming a trench along the line of the piles, breaking the concrete away from the top of each pile and exposing the steel reinforcing elements so that a beam may be cast across several piles. This is an inefficient process and does not assist tolerance control of pile installation.
Often before piling commences, temporary guide walls are cast at ground level, to a depth corresponding approximately to the depth of the desired capping beam, around commercially-available polystyrene spacers which replicate the expected profile of the finished wall. The polystyrene may then by removed before piling commences, or left in place to be broken up and pushed into the ground by the piling auger. The resulting guide walls are then used to help position the piling auger and to assist with tolerance control. This is particularly important when forming a wall by piling, since the component piles need to be as nearly parallel to one another as possible so as to achieve structural integrity in the composite wall. By providing guide walls, additional vertical stability during piling is achieved, although it is to be remembered that the piling auger may still be subject to uneven lateral forces during penetration due to the prevalent ground conditions. When the piles have been completed, the temporary guide walls are removed, the tops of the piles are broken down and the reinforcing steel is exposed. Blinding (e.g. stone or concrete chippings) is then placed at ground level and shutters erected so as to form a casting mold for the required capping beam, which is then cast in a standard manner. This technique has a number of disadvantages. Firstly, it is time-consuming and costly to break down the hardened concrete from the tops of finished piles. Secondly, the guide walls are discarded, which is wasteful. Thirdly, the polystyrene spacers are generally over-sized, which means that the piling auger will tend to have a high degree of play within the guide walls—often as much as 10 cm or even more, which can lead to considerable positional inaccuracy with little lateral restraint. The reason that the polystyrene spacers are oversized is so as to ensure that once the temporary guide walls have been cast, there remains sufficient room between them to accommodate the piling auger. Finally, the polystyrene used for the spacers is not environmentally friendly and may contaminate the surrounding area upon disposal. Chemical removal of the polystyrene is even more undesirable, since this involves the use of organic solvents which can be toxic and damaging to the environment.
SUMMARY OF THE INVENTION
According to a first aspect of the present invention, there is provided a method of forming a capping beam across two or more piles, the method comprising the steps of:
i) forming an excavation in the ground;
ii) inserting a precast or preformed guide wall structure in the excavation, the guide wall structure including means for receiving and guiding a piling auger along its longitudinal axis at a plurality of locations in the guide wall structure;
iii) forming a plurality of cast-in-situ piles by applying a piling auger to the ground through the means for receiving and guiding the piling auger provided in the guide wall structure; and
iv) removing the guide wall structure and filling the trench with concrete so as to form the capping beam.
Preferably, shuttering elements are provided between the guide wall structure and the sides of the excavation prior to piling so to precisely define a casting mold for the capping beam. The shuttering elements are left in place as the guide wall structure is removed, and concrete is cast into the space between the shuttering elements.
It is also preferable to apply blinding to the bottom of the trench so as to help define a surface layer which will become the bottom of the capping beam.
By employing a reusable, preformed or precast guide wall structure, the time and material waste involved in forming a cast-in-situ guide wall structure is saved, and the need for single-use polystyrene spacers is avoided. Furthermore, because the shuttering elements which determine the configuration of the resulting capping beam are put in place before commencement of piling, the need to break down the tops of hardened concrete piles prior to casting the capping beam is avoided. Excess concrete and spoil which may fall into the guide wall structure and/or between the shuttering elements can be removed before the concrete has hardened and deposited, for example, between the shuttering elements and the sides of the excavation as a filler.
In general, the shuttering elements are placed snugly on each external side of the guide wall structure. If a wider capping beam is desired, the shuttering elements may be placed at any desired location between t
Cementations Foundations Skanska Limited
Dubno Herbert
Pechhold Alexandra K
Will Thomas B.
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