GB2183703A - In situ concrete pile construction and under cutting tool - Google Patents

Abstract

In constructing a concrete pile in situ is a shaft having a substantially smooth internal wall is formed in the ground. Regions with diameters (d) greater than that (D) of the remainder of the shaft are then cut into the internal wall, these can be spaced, circular regions but are preferably in the form of a helix of constantor-vanable pitch. By cutting a helix into the internal wall, it has been found that the load bearing and shear resistant capacity of the concrete pile can be considerably increased. A tool for carrying out the cutting is also disclosed. Comprises cutting blades (3) pivotted on a support (1) such that when they are rotated in one direction they are urged into a retracted position (dotted lines Fig 2) and when rotated in the opposite direction they can adopt a cutting position. Springs (5) urge the blades (3) into the cutting position. <IMAGE>

Description

SPECIFICATION Concrete pile construction This invention relates to concrete pile construction.

Conventionally, piles are constructed using an auger to bore a hole into the ground and then filling the resulting straight shafted hole with concrete reinforced with steel. The pile thus formed relies on a combination of end bearing and shaft friction/adhesion (resulting from the shear strength of the soil) to develop its working load.

For some piles bored, especially in clay formations, a reduction coefficient is applied to the soil's strength to take account of the real shear resistance, or adhesion, developed along the pile shaft.

A drilling machine currently available forms screw piles in the ground by the force of lateral soil compaction. The machine comprises a hollow screw which is screwed into the ground with a continuous helical motion. The ground is forced sideways and compacted to form a casing. Once the right depth has been reached, concrete is introduced into the casing under pressure from inside the hollow screw.

Not only is this machine complex and expensive, but it is wholly unsatisfactory for use in clay or similar soil since the lateral compacting force, if it were made sufficient to compact a clay soil, would create shear forces in the casing which would tend to weaken rather than strengthen the resulting pile. In fact, the machine is designed for use in reiatively weak or sandy ground.

The normal manner of auger pile construction, applicable to a wider variety of soil, is to use a mobile crane with a self powered auger mounted on the boom. The auger is driven via a rotary table on to a kelly bar.

According to one aspect of the present invention there is provided a method of constructing a pile of concrete or other cementitious material, the method comprising: forming in a medium in which the pile is to be constructed, an elongate hollow shaft with a substantially smooth internal wall; then cutting into the internal wall of said shaft to form regions with diameters greater than that of the remainder of the shaft; and subsequently filling the shaft with concrete or other cementitious material In one embodiment, said regions form a continuous helix. In another embodiment, said regions form discrete portions of a helix. Whether or not the helix is continuous, its pitch may be constant or variable, as required. In a further embodiment, said regions are in the form of circles spaced apart along the length of the shaft.

According to another aspect of the present invention there is provided an apparatus for constructing a shaft for receiving concrete or other cementitious material to form a pile, comprising: means for forming said shaft in a medium in which the pile is to be constructed; a rotatable support; and a cutting member pivotally mounted to the support for movement between a cutting position and a retracted position and configured so that on rotation of the support in a first direction said medium acts on said cutting member to urge it radially inwardly into said retracted position, and on rotation of the support in a second direction opposite to said first direction, the cutting member can adopt the cutting position to cut into an internal wall of said shaft to form regions having a diameter greater than that of the remainder of the shaft.

Preferably, the cutting member is constructed so as to provide a trailing, convex surface, and a leading concave surface. The concave and convex surfaces may meet at a cutting edge.

The shaft forming means may also be mounted on the support.

The invention also provides a tool for cutting into an internal wall of a shaft and comprising: a rotatable support; and a cutting member pivotally mounted to the support for movement between a cutting position and a retracted position and configured so that on rotation of the support in a first direction said medium acts on said cutting member to urge it radially inwardly into said retracted position, and on rotation of the support in a second direction opposite to said first direction, the cutting member can adopt the cutting position to cut into an internal wall of said shaft to form regions having a diameter greater than that of the remainder of the shaft.

For a better understanding of the present invention, and to show how the same may be carried into effect, reference will now be made by way of example to the accompanying drawings, in which: Figure 1 is a vertical section through a cutting tool; Figure 2 is a half section along the line l-l, and a half plan of the tool of Figure 1; Figure 3 shows the shape of one form of a blade; Figures 4a to 4e are graphs of a maximum pitch of a helix cut using the tool versus the increase in a pile diameter; Figures 5a and5b are graphs of the increase in pile shaft load versus the increase in pile diameter; and Figure 6 is a diagram illustrating the terminology used in the following description.

Figure 7 is a plan view of another form of cutting tool; Figure 8 is a partial elevation in the direction of arrow VIII in Figure 7; and Figure 9 shows diagrammatically a double bladed cutting member.

Figures 1 to 3 show a tool which is suitable for cutting in a pile shaft which has been bored. The tool may be used to cut a series of discrete underreams or a continuous helix or to form a continuously enlarged diameter pile, say below casing level, according to a method to be described in more detail hereinafter.

The tool has a support bar 1 mounted about a central mounting post 2. Blades 3 are mounted to the support bar 1, one on each side of the post 2. The blades are shown in their operational (full line) and retracted (broken line) positions in Figure 1 . The blades are generally Z shaped. Adjustment bolts 4 are provided to alter the depth to which the blades cut, to permit variation in pile diameter.

One form of cutting blade is shown in Figure 3.

The cutting blades 3 are spring loaded to open by way of springs 5. This arrangement automatically allows cutting to proceed as the tool is turned in the normal drilling direction. The blades 3 can retract against the springs whilst entering the borehole casing, or at the end of the cut. They may retract by vertical pull only, but reverse rotation would assist their retraction especially when entering - in either direction - any casing placed in the pile bore.

A debris bucket 6 is provided below the blades 3 to allow the material cut from the shaft to be collected and, when the operation is completed and the tool removed from the pile bore, the bucket 6 can be emptied via a bottom opening discharge 7.

The blades may be fitted with picks or cutters for use in stronger soils or weak rocks.

The cutting blades could be mounted above the auger used to bore the pile shaft to save changing tools during pile construction. The cuttings may be caught on the auger, although a modified auger with fins on flights to hold the cuttings more positively may be desired.

Figures 7 and 8 illustrate a further embodiment of the cutting tool. In this embodiment, the cutting blades 13 (shown in their cutting (full line) and retracted (broken line) positions) have a modified cutting configuration, and are mounted slightly differently. Each cutting member has a shaft 8 fixedly connected to the blade 13 and pivotally mounted with respect to a support bar 11. The cutting members are coupled to each other by way of springs 15: this has the advantage over the embodiment of Figures 1 to 3 that the urging inwardly of one of the blades by the action of the soil assists, via the spring, the drawing inwards of the other of the blades. Further, if one spring should break, this will not render a blade completely inoperative, since its action is also controlled by the other spring.

The shaft 8 is shown welded to each blade 13, but it will be appreciated that a complete unit (shaft and blade) could be machine-formed if necessary.

The adjustment bolts 14 are arranged to abut the inner ends of the cutting members to allow for more accurate adjustment of the distance to be cut into the shaft wall.

Figure 9 shows a double-bladed cutting member having two blades 16, 17, the cutting points of which are fixedly spaced by an angle 0 (e.g. 0 = 40 ). The blades 16, 17 are arranged so that the leading blade 16 cuts into the shaft wall for a distance slightly less than that of the trailing blade 17. In this way the blade 16 can perform the bully of the cutting, and the trailing blade 17 can carry out fine paring.

The tools described above can be used to construct piles having an enlarged diameter overcut over a percentage of the depth of the pile. One form of this overcut is a continuous helix, another is a series of circular "underreams", and another is discrete enlarged width portions of a helix. It is preferable, when using the tool of Figures 1 to 3 or 7 to 9 to make two diametrically opposed cuts simultaneously so as to balance the action. The conventional auger tool, used for boring the hole, is removed from its kelly bar and is replaced by the cutting tool which is then lowered into the pile.

The term "underream" is used here to refer to a shallow circular portion providing a generally small increase in diameter to the shaft. This is in contrast to a conventional use of the term underream, which implies a portion of a very large increase in diameter, and having a substantial depth.

The helix(es) may be cut either while the tool is descending or ascending the bore, but the latter is preferred since (a) the rate of vertical movement is more controlled by virtue of the crane rope working under direct engine drive (rather than possibly under an uncontrolled braking action) and (b) the cuts are not disturbed by subsequent travel of the tool.

By cutting a helix in the pile bore after boring a pile shaft to its required depth, an oversize pile can be achieved with the result of a higher adhesion factor, which may approach unity. The depth of cut is significant in increasing the adhesion factor since full undrained shear strength can theoretically be mobilised between a series of pile shaft enlargements. The spacing of the enlargements, i.e. pitch of the helix, is also important since it is desired to increase pile shaft load on the basis of full soil shear strength arising from the oversized pile cylinder dimension rather than on the basis of a series of individually aggregating end bearing components.

These requirements can be developed in order to determine the optimum helix pitch (P). (Refer to Figure 6).

In the equations below: D = Conventional (Augered) Pile Diameter d = Increase in Pile Diameter P = Helix Pitch os = Conventional pile adhesion factor (typically 0.45 for LONDON CLAY) = = Increased Pile adhesion factor for oversize helix pile (Max 1.0) L = Pile Shaft length Cu = Clay Undrained Shear Strength Nc = Bearing Capacity factor (Typically 9) Shear resistance over the portion of enlarged diameter = surface area x shear strength = n(D + d) LOX'CM and End bearing on enlarged diameter = cross sectional area of increase (i.e. portion of increased diameter) = x vertical strength (bearing capacity factor x shear strength) = [(D + d)2 - D2]NcCu 4 for mobilisation of shaft resistance rather than end bearing over a length P p = L < Nc [(D + d)2 - D2] 4oc (D + d) Typical results are presented in Table 1 and Graphs 5a-e.

The increase in pile working load per given diametric increase is significant, as demonstrated below: Shaft load on conventional pile = nDLaC, Shaft load on helical cut pile = n(D + d)La'Cu Thus increase - (D + d)o*' Dcc for the typical value 7r = 0.45 and assuming n' = 0.85 (as min value) Increase in unit pile shaft load 1.89 (D + d) D As an alternative to increasing pile load, the pile length can be reduced for a similar pile shaft load by multiplying the conventional pile shaft length by the approximate reciprocal of the above term.

These factors are illustrated in Table II and Graphs 6a & b.

The use of an increased adhesion factor ' as 0.85 is invalid for small values of d since the increase in factor must be progressive, leading to a more probable gradual increase in pile shaft load or decrease in shaft length as shown dotted on graphs 5a & b.

As can be seen from graphs 5a & b the increase in pile diameter (d) (i.e. depth of helix cut) should increase with the pile diameter. That is, the increase in cc would appear to develop earlier in the increase in d for smaller diameter piles compared to larger diameter piles.

Hence, for piles having a diameter in the nominal range 450 - 600mm, the increase in pile diameter would appear to be a minimum of 1 50mm for achievement of significant increase in oc. Similarly for larger piles (of 1000/1500 nominal diameter) minimum pile diameter increase would appear to be of the order of 250mm for the same effect.

As seen from Table 1 and Graph 5, the optimum minimum pitch for piles in the 450-600mm pile diameter range would appear to be of the order of 750mm at the assumed minimum diametric increase of order 1 50-200mm and nearer 1000mm as the increase approaches 250mm. Similarly for the larger diameter piles in the range 1000-1500mm the optimum maximum pitch would appear to be of the order 11 00mm at the assumed minium diametric increase of order 250mm.

The normal manner of auger pile construction is to use a mobile crane with a self powered auger mounted on the boom. The auger is driven via a rotary table on to a kelly bar. For the smaller diameter piles (nominal pile diameter 450-600mm) the crane is typically a 30RB or equivalent with a single line rope operating the kelly bar.

Single line rope speeds of the order of 47.5 metre/min. are usual for this type of crane, whilst the rotary table driving the kelly is of variable rotation - nominal range 0 - 195 rpm.

In order to determine the desired rotation speed whilst pulling the auger out of the pile hole, the following formula is used.

Rope rpm = Rope Speed (m/min) Minimum rpm Maximum Helix Pitch Maxlmum Hellx Pltch It is preferable to produce a double cut in the helix as the cutting is being carried out so as to balance the movement on the cutting tool described earlier.

As an iilustration of the above calculations, assume for the smaller pile ranges a minimum helix pitch of the order of 0.75mm is required, the minimum rpm = 47.5 = 32 rpm 2 x 0.75 For the larger pile range minimum rpm = 47.5 = 24 rpm.

2 x 1.0 i.e. slow rotation speeds are generally compatible with the crane rope speeds utilised.

The height of the helix is determined by the pile load per helix pitch and concrete shear strength. It can be deduced from the approximate formula: = = (D + d) P.cc'.C height D.a.(FofS)U D.s.(F of s)u where o = permissible concrete shear strength F of S = pile factor of safety Typical calculations indicate root heights of order 1 00-200mm for the range of pile diameters/loads envisaged.

Using the method and apparatus described above, pile load performance can be improved not only by virtue of the increase in diameter, but also by an increase in the reduction coefficient normally applied to the soil strength.

This latter increase is a result of the ability of the pile underreamed shaft to mobilise more adequately the in situ strength of the soil. The reduction factor may approach unity under ideal "underream" geometry.

The increase in pile working load may be two to three or more times that of a conventionally constructed pile.

Alternatively a shorter pile length may be constructed for a similar pile load although, since generally soil strength increases with depth, the reduction is not a simple inverse to the above increase in working load.

Further, a smaller diameter pile shaft may be bored, possibly to a shorter depth, and cut so as to achieve similar load characteristics to a larger and possibly longer conventionally constructed pile.

The underreamed pile may be constructed as a series of discreet circular or helical "underreams", or by a continuous helix or pair of helix.

The technique is mainly applicable to clay soils, but could also be used with soft rocks, i.e. chalk, provided the bores remain stable.

The shape of profile may be altered to produce a more stable undercut. For example, a shape more like a conventional large diameter underream may be appropriate.

In cases where the tool is used to form a continuously enlarged diameter pile, say below casing level, so as to increase pile shaft load in clays or soft rocks, pile load increase would only be attributable to increase in pile diameter i.e. shaft surface area. Naturally, it will be necessary to alter the configuration of the blades of the tool in these cases.

Claims (13)

1. A method of constructing a pile ef concrete or other cementitious material, the method comprising: forming in a medium in which the pile is to be constructed an elongate hollow shaft with a substantially smooth internal wall; then cutting into the internal wall of said shaft to form regions with diameters greater than that of the remainder of the shaft; and subsequently filling the shaft with concrete or other cementitious material.

2. A method as claimed in claim 1, in which said regions form a continuous helix.

3. A method as claimed in claim 1, in which said regions form discrete portions of a helix.

4. A method as claimed in claim 1, in which said regions are substantially circular and are spaced lengthwise of the shaft.

5. A method as claimed in claim 2 or 3, in which the pitch of the helix varies along the shaft.

6. A method as claimed in claim 2 or 3, in which the pitch of the helix is constant along the shaft.

7. A method as claimed in claims 1,2,3,4 or 5, in which reinforcement is added to said shaft.

8. An apparatus for constructing a shaft for receiving concrete or other cementitious material to form a pile, comprising: means for forming said shaft in a medium in which the pile is to be constructed; a rotatable support; and a cutting member pivotally mounted to the support for movement between a cutting position and a retracted position and configured so that on rotation of the support in a first direction said medium acts on said cutting member to urge it radially inwardly into said retracted position, and on rotation of the support in a second direction opposite to said first direction, the cutting member can adopt the cutting position to cut into an internal wall of said shaft to form regions having a diameter greater than that of the remainder of the shaft.

9. An apparatus as claimed in claim 8, in which the cutting member is biased into the cutting position by biasing means and is movable against the action of the biasing means into the retracted position.

10. An apparatus as claimed in claim 8 or 9, in which the shaft forming means is also mounted on the support.

11. An apparatus as claimed in claim 8, 9 or 10, in which the cutting member is constructed to provide a convex surface and a concave surface, the concave surface and convex surfaces meeting at a cutting edge.

12. A method as claimed in claim 1 substantially as hereinbefore described with reference to the accompanying drawings.

13. An apparatus, for constructing a shaft, substantially as hereinbefore described with reference to, and as shown in, Figures 1 to 3 or 7 to 9 of the accompanying drawings.