Hydraulic and earth engineering – Foundation – Columnar structure
Reexamination Certificate
2000-04-06
2002-06-11
Shackelford, H. (Department: 3673)
Hydraulic and earth engineering
Foundation
Columnar structure
C405S231000, C405S233000, C405S237000, C175S267000, C052S155000
Reexamination Certificate
active
06402432
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method and apparatus for installing a pile and/or a concrete or grout column in the ground, in particular by way of soil displacement.
BACKGROUND OF THE INVENTION
It is known to install load-bearing piles or columns by various methods. A first method involves hammering a preformed pile into the ground in a series of discrete steps. This method can be effective, but there is a risk of causing damage to the pile or to the ground due to the discontinuous hammering action. Furthermore, much noise and vibration is caused by hammering. An alternative method is to use a jack to install a pile or column made up of a number of discrete sections. A first section is pushed into the ground by the jack, which is then reset, and a second section is then welded or bonded to the top of the first section. The jack is then activated again, and the process repeated until the required depth has been reached. This method is low in efficiency since the jack needs to be retracted after completing each single stroke so that the next element of the pile can be inserted, particularly since a typical stroke length is less than 50 cm.
A second known method is continuous flight auger piling, in which an auger with a continuous flight is caused to enter the ground by way of rotation. Soil is excavated by way of the auger flights before or during the time that the auger is withdrawn from the ground. As the auger is being withdrawn, concrete is pumped through the stem of the auger to the tip, thereby leading to the formation of a load-bearing pile or column. Such a method is described in the present applicant's U.K. patent application no. 9515652.7, the disclosure of which is incorporated into the present application by reference.
Alternatively, as disclosed for example in WO 95/12050, it is possible to use an auger head which does not excavate soil, but instead displaces the soil and compacts it into the surrounding ground. This has the advantage that less spoil is generated, and can lead to better maintenance of ground integrity and greater density in the vicinity of the pile installation.
However, both these methods require that an auger or similar device be screwed into the ground, which takes a relatively long time and generally means that specific combinations of torque and thrust must be applied in accordance with the ground conditions in order to achieve penetration, and these can be difficult to achieve. Another disadvantage with these screw piling methods is that the piling tool is subjected to a large degree of wear. Furthermore, we have found that there appears to be an inverse relationship between the rotational torque required to produce downward thrust by way of the pitch of the auger flights and the “crowd force” (i.e. the total force applied along the longitudinal axis of the auger during penetration) generated at the rig which can be used to achieve penetration into the ground. Indeed, the crowd force in itself is not sufficient to achieve the desired penetration, particularly when premature resistance to penetration, e.g. caused by the presence of a subterranean stratum of granular material such as gravel, is experienced.
SUMMARY OF THE INVENTION
According to a first aspect of the present invention, there is provided a method of installing a load-bearing pile or column in the around, wherein:
i) a hole-forming tool or pile having a longitudinal axis is pushed into the ground, substantially in the direction of its longitudinal axis, in a substantially non-percussive manner to a first depth; and
ii) the hole forming tool or pile is then pushed further into the ground, substantially in the direction of its longitudinal axis, in a substantially non-percussive manner to a second depth while being rotated about its longitudinal axis.
The term “non-percussive” is to be understood as the application of a substantially continuous force over a relatively protracted period of time, e.g. over a number of seconds or even minutes. This is to be distinguished from percussive methods of piling, in which, for example, a weight is repeatedly dropped onto the top of a pile so as, in effect, to hammer the pile into the ground. In this case, most of the force is applied over a relatively short period of time, e.g. fractions of a second. Furthermore, the rate of change of applied force with respect to time will, in practical terms, generally have a discontinuity, as opposed to non-percussively applied force, which will tend to have a rate of change with respect to time which is substantially continuous.
The present invention is of particular use when installing or forming piles in soft ground overlying a granular stratum, such as gravel or the like. The rotational motion of the hole-forming tool or pile helps to overcome premature resistance to penetration which may otherwise prevent the attainment of desired depths. Generally, the hole-forming tool or pile is pushed into the soft overlying soil until it reaches the granular stratum, at which point the hole-forming tool or pile is additionally rotated. Rotation combined with the pushing force is surprisingly effective in penetrating granular strata, especially with certain hole-forming tool or pile tip geometries, and enables the resulting pile to be well-founded.
The rotation may be continuous rotation in either direction; alternatively or in addition, back and forth rotation may be used, which rotation may be less than or more than one revolution. Where back and forth rotation is used, it has been found that an oscillation frequency of around 1 Hz is effective in aiding penetration of granular strata, although frequencies of an order of magnitude higher or lower, e.g. around 10 Hz to 0.1 Hz, are also envisaged. In some applications, even higher or lower frequencies, e.g. around 100 Hz to 0.01 Hz, may be useful.
This method may be used either to install a pile, such as a steel or pre-cast concrete pile, directly into the ground, or may be used to insert a hole-forming tool, such as a hollow cylindrical tube provided with a sacrificial end plate, into the ground so as to allow a load-bearing concrete or grout column to be cast-in-situ prior to or during removal of the tool. The pile or tool dimensions and the force applied to the pile or tool are advantageously determined in accordance with the ground conditions.
Preferably, the hole-forming tool or pile is pushed in a continuous motion to a given depth of at least 1 m into the ground, and in some applications, at least 2 m or even 5 m into the ground. Once the given depth has been reached, the continuous force may be reapplied one or more times so as to push the hole-forming tool or pile to a greater given depth, such as the depth of a granular stratum.
In this way, we have found that it is possible in certain ground conditions to attain depths in excess of 5 m in a time of around 16 seconds, as opposed to 4 minutes using a rotating auger.
According to a second aspect of the present invention, there is provided a method of boring into the ground, wherein:
i) a hole-forming tool having a longitudinal axis is pushed into the ground, substantially in the direction of its longitudinal axis, in a substantially non-percussive manner to a first depth; and
ii.) the hole-forming tool is then pushed further into the ground, substantially in the direction of its longitudinal axis, in a substantially non-percussive manner to a second depth while being rotated about its longitudinal axis.
The hole-forming tool is generally withdrawn from the ground after it has reached the required depth, although in some applications the tool may be sacrificed and left in the ground.
In some embodiments, the hole-forming tool has a generally pointed tip, since this can be effective in penetrating granular strata when pushed into the ground with concomitant rotation. However, a particularly preferred embodiment utilises a hole-forming tool with a generally flat base. We have found that upon pushing such a flat-based tool into the ground, soil under the flat base tends to be co
Darke Roger Martin
England Melvin Gerard
Fleming Wilfred George Kenneth
Dubno Herbert
Kvaerner Cementation Foundations Limited
Lee Jong-Suk
Shackelford H.
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