Expansive Base Pile Construction

Abstract

Pile installation with expansive base wherein the soil in a desired bearing strata in the vicinity of the pile tip is displaced by the tip into a region confined by a rim or skirt so that the soil forms a solid plug between and frictionally locked to the pile and rim or skirt.

Description

This invention relates to supporting of structural loads and more particularly, it concerns novel pile arrangements providing increased loading capacity in certain strata at various depths in the earth.

Piles are elongated structural elements which are driven or otherwise placed vertically in the ground; and they serve to transfer a load, such as from the footing of a building, to a stable region in the earth. In general, the extreme upper surface of the earth is too soft and unstable to provide a sufficiently firm and permanent support for a heavy structural load such as a large building. Thus, in most heavy construction, piles are sunk into the earth and are used to transmit the surface load down to subsurface strata or layers which have a great supporting capability.

In many locations, several strata of different densities and load bearing capacity occur at different levels and at different thicknesses between the surface of the earth and bedrock. Often, certain of the more dense strata would suffice to support a given load, except that they are of insufficient thickness; and the pile pierces through them before sufficient load resistance is built up between the bearing stratum and the pile. As a result, the pile must be driven to a deeper supporting stratum, or even to bedrock, before sufficient load resistance is achieved. This, of course, can become quite expensive since piles of extraordinary length must be employed and considerable driving is required.

Attempts have been made in the past to make effective use of intermediate earth strata in the support of pile members. Such attempts have involved the use of so-called "bulb" piles which are elongated members having a bulbous configuration at their lower end. The enlarged lower end of the pile serves to spread out its earth contact area, thereby reducing the stress imposed on the earth through the pile. Bulb piles, however, are expensive to construct. Also, it is difficult to produce a balanced properly-shaped bulbous configuration at the lower end of these piles. Also, such piles are limited to concrete or other castable material; and they must be formed on site.

According to the present invention there are provided novel pile arrangements which make use of the support potential of intermediate earth strata by reduction of the unit stress produced by the pile on the earth. Moreover, these novel pile arrangements do not require on site formation of the piles, nor are they subject to the difficulties of construction associated with conventional bulb piles.

In one aspect, the present invention involves selective densification of the earth in the immediate vicinity of the pile tip at the desired bearing stratum. This densification is such as to make the surrounding earth effectively an integral part of the pile base, thereby providing an enlarged base for distributing the load carried by the pile.

According to the present invention, a soil displacement pile (i.e., a pile which forces aside the earth as it moves through the earth), is driven into a region in which there is located a perimeter defining tension element such as a rim or sleeve. The earth displaced by the pile is forced into the space between the pile and the rim. The rim is thus placed in tension and serves to maintain the displaced soil under pressure. This compacts and densifies the soil and increases its shear or rupture strength. At the same time, the pressures developed serve to increase the frictional contact between the soil, the pile and the rim. By proper choice of the rim and pile diameters and rim height, the soil density and frictional characteristics can be increased to a point where the soil between the pile and the rim actually forms a solid plug locked to the pile, thereby providing an effective enlarged base between the stem and rim.

The present invention also permits selective soil densification at different levels by taking advantage of the different soil characteristics in different strata. For example, the pile system of the present invention can be driven through upper earth layers where the soil is not very dense and would not provide adequate support. Thereafter, when the pile system reaches a more dense strata, the above-described plugging action will take place to form an enlarged pile tip at a location where it can most effectively function.

This selective plugging is achieved with the present invention because the spaces between the rim and pile in the softer less dense strata are such that the soil displacement which takes place as the pile is driven has relatively little effect on plugging or friction characteristics. However, when the pile reaches a stratum of more dense soil, the soil displacement produced by the pile has a much greater effect on plugging and friction characteristics so that a soil base is effected at the level where the soil has greates support potential.

Various further and more specific objects, features and advantages of the invention will appear from the description given below, taken in connection with the accompanying drawings, illustrating by way of example preferred forms of the invention.

In the drawings:

FIG. 1 is a diagramatic representation of a soil cross-section showing a conventional pile and a pile according to the present invention;

FIG. 2 is an enlarged perspective view, partially cut away, of the base portion of the pile according to the present invention shown in FIG. 1;

FIG. 3 is a section view taken along line 3--3 of FIG. 2;

FIG. 4 is a diagramatic representation of a slug of soil and forces acting thereon for use in explaining the principles of the present invention;

FIG. 5 is a graph used in explaining the operation of one aspect of the present invention;

FIGS. 6 and 7 are cross-section views similar to FIG. 3 showing modifications to the base construction of FIG. 3;

FIG. 8 is a perspective view similar to FIG. 2 showing a modified base construction;

FIG. 9 is a fragmental section view, taken in elevation, of an alternate base configuration;

FIGS. 10-13 are plan views of various configurations of the base portion of the pile according to the present invention shown in FIG. 1;

FIG. 14 is a view similar to FIG. 3 showing a modified spoke configuration for the pile base of the present invention; and

FIG. 15 is an elevational view showing in outline, a multiple base pile according to the present invention.

In FIG. 1, there is diagramatically represented an illustrative earth profile which extends from an upper surface 20 down to bedrock 22. The distance between these regions and the number, the thickness and the characteristics of various intermediate strata will vary depending on the geographical location and the geological history of the site. In the illustrative case, however, the bedrock 22 is indicated as being approximately 160 feet below the upper surface 20. There is shown an upper layer 24 of "soft soils" which extends down from the surface 20 to a depth of about 60 feet. Below this, and extending downwardly about 10 additional feet is an intermediate stratum 26 of "medium dense to dense soils". Finally, a lower layer 28 of "fine sandy silty clay" extends downwardly an additional 90 feet to the bedrock 22.

A conventional 12 inch square cross-section pile 30 is shown on the left side of FIG. 1. This pile is indicated as being designed to support a load of two hundred tons. The upper "soft soils" layer 24 is of insufficient density or continuity to provide any reliable support for the pile 30 and accordingly, no indication of load resistance is shown in this region. Actually, there are surface soils which produce a negative load resistance and tend to pull a pile down. In such case, the extend of such downward pull must be added to the load support requirements for the pile.

The intermediate stratum 26 and the lower layer 28 cooperate to provide a frictional resistance to downward pile movement equal to about 180 tons. It will be appreciated that this frictional resistance will increase as the pile is driven deeper into these layers since then more pile surface area is exposed to the frictional action of the soil. There is also indicated an end bearing restraint of 20 tons which completes the load resistance for the pile. The amount of end bearing restraint depends upon the density of the soil in this region and on the cross-sectional area of the lower end of the pile. In the present case, because the pile cross-section is one square foot and the load is 20 tons, the pressure exerted on the soil is somewhat less than 300 pounds per square inch.

It will be appreciated that in the situation described, the pile makes no particular use of the potential end bearing support characteristics of the relatively high density intermediate stratum 26. In this situation, only the comparitively minor frictional restraint available from this stratum is utilized. The reason for this is that when the pile tip reaches the intermediate stratum, there is, up to this point, no significant frictional resistance available. Thus, the end bearing requirements would equal substantially the entire loading restraint, i.e., about 200 tons. Moreover, since the pile tip has a cross-section of only 1 square foot, the pressure exerted on the soil in this stratum would equal nearly 1 and 1/2 tons per square inch. This is in excess of the loading limits of the stratum and the pile tip will break through and will proceed down into the lower layer 28 toward the bedrock 22.

On the right side of FIG. 1, there is shown a pile 32 constructed according to the present invention and designed to support a similar load of about 200 tons. As shown, the pile 32 comprises a stem portion 34, of 12 inch square cross-section, and a base portion 36 of enlarged configuration. As indicated, the enlarged base portion is 31/2 feet in diameter and thus possess a bottom area in excess of 91/2 square feet. This enlarged base then distributes the 200 ton load so that the pressure exerted on the soil is less than 300 pounds per square inch, or about the same as the pressure produced in the softer lower layer 28 by the bottom of the left hand pile 30. Thus, the denser intermediate stratum 26 is rendered capable of supporting the same two hundred ton load using a far shorter pile than would normally be required.

The structural configuration of the base portion 36 of the pile 32 is illustrated in FIGS. 2 and 3. As there shown, the base portion comprises an outer rim 38, connected by a plurality of radial spokes 40, to a central hub 42. The hub 42 is configured to accommodate the lower tip of the pile stem portion 34 and to receive downward driving and loading forces from the stem. These forces are transmitted via the spokes 40 to the rim 38. A cylindrical skirt 44 is attached, as by welding, to the rim 38 and extends upwardly therefrom.

The pile 32 is installed by assembling the stem portion 34 on the base portion 36 and driving downwardly on the stem portion to force the entire assembly down through the earth.

During driving of the pile down through the upper layer 24 of soft soils, the soil will readily pass through the spaces between the spokes, hub and rim of the base portion 36. Thus, no substantial restraint to driving is offered by the soil in the upper layer 24. When, however, the base portion 36 reaches the denser soil comprising the intermediate stratum 26, the soil packs more tightly in the spaces between the spokes, hub and rim. This results in a plugging or arching action whereby the soil becomes held so tightly by frictional engagement with the spokes, hub and rim that it effectively becomes an integral part of the pile base itself. The base portion is thus enlarged or completed by the earth plug between the hub 42 and the rim 38. As a result, the base portion serves to distribute the loading force of the pile over an area corresponding to the cross section enclosed by the rim.

FIG. 3 illustrates the manner in which the plugging action occurs. As the pile stem 34 moves downwardly through the earth, it displaces soil laterally along paths indicated by the arrows 46. As can be seen, this displace soil enters the spaces between the pile stem 34 and the rim and skirt 38 and 44. In addition, the soil directly under these spaces also enters them as the assembly moves down. It will be appreciated that the soil passing through the spaces thus becomes compacted. This compaction has relatively little influence on friction where the soil is soft, as in the upper layer 24. HOwever, in the intermediate stratum 26, where the soil is already medium dense to dense, the compacting results in a rapid pressure rise which greatly increases the friction between the soil and the various surfaces of the spokes, rim and skirt and the pile stem. The unit friction is thus increased by this pressurization. At the same time, the total friction is increased as the pile is driven further down since greater amounts of rim, spoke, skirt and stem surfaces are subjected to the action of the pressurized soil. Eventually, the total frictional restraint comes to exceed the total end resistance of the base portion 36. At this condition, if the pile stem and base assembly are forced further down, the soil in the spaces between the stem, rim and spokes will not slide up, but instead, will force down against the underlying soil and cause it to fail. However, at this condition, the total loading is now spread over a substantially greater area and the unit loading on the underlying soil is considerably reduced.

In order for plugging action to occur, it is necessary also that the shear strength of the soil performing the plugging action be sufficiently high to sustain the load to which the pile is subjected. Otherwise, a shearing action will occur within the soil itself between the stem, rim and spokes of the pile base. It is an important feature of the invention that the combination of the soil displacing stem portion 34 and the perimeter defining and pressure retaining rim 38 and skirt 44 serves, by mutual cooperation of these elements, to increase the shear strength of the soil between these elements to a point where the soil will act as a solid structural portion of the pile base and will not fail under pile loading.

The manner in which shear strength of the soil between the stem, spokes and rim of the pile base is increased by the present invention can be seen in FIGS. 4 and 5. FIG. 4 shows diagramatically a cylindrically-shaped slug 50 of granular, non-cohesive soil. All soils can be classified as either cohesive or non-cohesive. In general, non-cohesive soils are those which are made up of solid rock particles which may vary in size from gravel down to fine sand. Such soils are generally pervious enough to permit the water occupying the interstices between the soil particles to escape as the pile penetrates the soil, thereby allowing densification of the soil to occur. When the particles become much smaller than silty sand and become shaped in a special way, however, they do interact with water and retain it. The soil then becomes cohesive and sticks together. Clay is a classic example of a cohesive soil. To a certain extent, the presence in soil of organic materials will contribute to cohesiveness. Most soil is a blend of cohesive and non-cohesive components, the relative amounts varying from location to location and from depth to depth. For structure supporting purposes, a large non-cohesive characteristic is desired since it is not subject to hydrostatic effects and it will not shift in time to the extent that most cohesive soils will. While the present invention can be used in many cohesive soils, its action in increasing soil shear strength to improve plugging is especially effective in the case of non-cohesive soils.

The slug 50 of FIG. 4 being of a granular non-cohesive soil may be considered as being made up of a large number of small solid particles, for example very small marbles. The particles are confined circumferentially by lateral stresses illustrated by lateral arrows .sigma..sub.3. These stresses may be supplied from the surrounding soil, or in the case of the present invention, they are supplied by the rim 38 and the skirt 44. At the same time, the soil is subjected to longitudinal loading stresses illustrated by the vertical arrows .sigma..sub.1. These stresses, in the case of the present invention, are the pile load, applied via the pile stem, spokes and rim acting downwardly, and the underlying soil acting upwardly. Now when the longitudinal loading stresses .sigma..sub.1 exceed the shear strength for the soil, the soil will fail and in the present case, will push through the spaces between the stem, rim and spokes.

The relationship between the lateral restraining stresses .sigma..sub.3, the longitudinal loading stresses .sigma..sub.1, and the shear stress in a non-cohesive granular soil are shown graphically in FIG. 5. The rupture envelope is defined by a straight line which begins at the origin O of the Mohr diagram and rises upwardly at an angle .phi.. The angle .phi. increases with soil density so that, as shown, the rupture line A for a less densely packed soil will have a shallow angle .phi..sub.1, whereas the rupture line B for a more densely packed soil will have a steeper angle .phi..sub.2.

It can be seen in FIG. 5 that the maximum longitudinal stress is a function of the lateral stress and the angle .phi.. For example, for the angle .phi..sub.1 and the lateral stress .sigma..sub.3 the corresponding maximum longitudinal stress is .sigma..sub.1. Note that the maximum longitudinal stress .sigma..sub.1 is related to the lateral stress .sigma..sub.3 and angle .phi..sub.1 by a circle M.sub.1 whose center is located on the abscissa to the right of .sigma..sub.3 which is tangent to the rupture envelope defined by .phi..sub.1. If the angle were to remain equal to .phi..sub.1 but the lateral stress increase to .sigma..sub.3 ', then there is a substantial increase in maximum longitudinal stress as shown by .sigma..sub.1 ' in FIG. 5. M.sub.2 is the circle which determines the location of.sigma.of .sigma.1'. Now if in addition to an increase in lateral stress from .sigma..sub.3 to .sigma..sub.3 ' there was an increase in .phi. from .phi..sub.1 to .phi..sub.2, there would be an even greater increase in maximum longitudinal stress .sigma..sub.1 " as determined by circle M.sub.3.

The pile driving arrangements of the present invention increase the rupture shear stress or longitudinal load bearing capacity of the soil between the pile stem, spokes and rim in both of the two ways described above. Firstly, because the pile stem displaces soil and forces it into the spaces surrounded by the rim 38 and skirt 44, the soil becomes more densely packed. Thus, the characteristic angle .phi. is raised from .phi..sub.1 to .phi..sub.2. Secondly, the rim and skirt each have substantial tensile strength and thus restrain the outward radial movement of the soil displaced by the pile stem. This causes an increase in the lateral restraining force .sigma..sub.3. As a result of these two factors, the shear strength of the soil between the stem, rim and spokes is very substantially increased, and a very positive plugging action can be achieved.

As indicated above, effective plugging which will permit the soil between the pile stem, spokes and rim to act as an integral part of the pile base and contribute to its support, depends upon both an increase in the shear strength of the soil and an increase in the friction between the soil and the surfaces of the pile stem, spokes and rim.

The friction increase can be provided by increasing the surface area of the spokes and rim. In this connection, the skirt 44 forms an extension of the rim 38 and contributes to an increase in frictional surface area. In addition, the number and vertical height of the spokes 40 may be increased to raise the total friction level. The relative sizes of the soil displacing pile stem and of the spaces between the stem, spokes and rim required to introduce plugging and the total friction surface area of the stem, rim and spokes required to secure the plug to the pile depends, of course, on the nature of the soil at the depth plugging is desired. This information can be obtained by means of soil tests made at the location where the pile is to be installed.

There are a number of modifications of the present invention which may enhance its effectiveness under different installation conditions. FIG. 6, for example, shows an arrangement for the pile base 36 wherein the spokes 40 between the hub 42 and the rim 38 are inclined downwardly away from the hub at a negative angle (-.alpha. ). This positions the rim 38 and skirt 44 ahead of the stem 34. This serves to enhance the plugging action since it ensures that a greater portion of the soil displaced by driving the stem will be restrained by the rim and skirt.

FIG. 7, on the other hand, shows an alternate arrangement where the spokes 40 are inclined upwardly away from the hub 42 at a positive angle (+.alpha.). This positions the rim 38 and skirt 44 behind the stem 34. While the effectiveness of plugging is reduced with this arrangement, it permits a more stable interconnection between the base and stem for driving the base with the stem.

FIG. 8 shows a modification wherein the spokes 40 are tilted at an angle .beta. relative to the longitudinal axis of the stem-base combination. Also, pins 60 are provided to secure the hub 42 to the stem 34 in a manner permitting them to rotate together. Thus, by turning the stem, the base can be made to rotate and the tilted spokes 40 will bore through the softer upper soils in auger-like fashion. They will also assist more effectively in the soil plug formation and friction lock action when the pile is later driven down into the bearing stratum. The auger action may also be enhanced by the provision of spiral corrugations on the outer surface of the rim and skirt as illustrated at 62 in FIG. 8. Also, either the skirt 44, or the spokes 40 may be roughened or provided with an embossed surface, as indicated at 63, to enhance their frictional characteristics.

The rim and skirt may be formed from a single piece as illustrated in FIG. 9. As there shown, a sheet metal skirt 64 of light gage steel is folded and hemmed along its lower edge to form a stiff rim 66. Similar crimping 68 may be provided at various levels to provide additional stiffness where needed. Also, vertical ribs 70 may be added or formed by crimping where reinforcing is needed in a vertical direction.

FIGS. 11--13 show alternate rim and spoke configurations. In FIG. 10, a pair of concentric rims 72 and 74, interconnected by spokes 76, are provided to increase the number of spaces in which soil plugs may be formed. This is advantageous where a base of large cross-section is to be formed in soil which requires relatively small spaces to form effective plugs. FIGS. 11 and 12 illustrate the use of rim and skirt configurations which form square and triangular perimeters 77 and 78 to provide the soil pressure retention required for plugging action. While square and triangular skirt configurations are illustrated, others may also be employed. It is important, however, that they provide a perimeter defining enclosure to maintain sufficient pressure retention to achieve soil plugging in the vicinity of the pile stem.

FIG. 13 illustrates an arrangement wherein the spokes 40 are of different lengths so that the hub 42 is not centered in the rim 38. This serves to offset the rim with respect to the pile stem; and is useful, for example, in driving close to existing structures and footings.

FIG. 14 shows a pile base 34 with modified or tapered spokes 40a. As can be seen, the spokes 40a are widest where they connect to the hub 42, and are narrowest at the rim 38. This helps to counteract the higher moments produced at the spoke-hub junctions when the pile system is driven downwardly by means of the stem.

FIG. 15 shows a pile system comprising a common stem 80 and a plurality of base portions 82a, 82b at different levels. Each base portions is of a construction similar to those described above and each makes use of a plugging action as above-described. By use of plural base portions, the loading may be distributed among different bearing strata at different levels.

It will be appreciated that the pile construction arrangements of the present invention may be employed in the case of tension or anchor piles which are required to resist upward forces.

Although the various base configurations described herein have been shown as arranged to be driven by the pile stem, it should be understood that the rim and skirt may be independently drive; and in fact, need not have any physical connection to the stem except via the soil plug formed between them. In such case, the spokes may be eliminated entirely. For example, the base may simply consist of a rim and/or a skirt driven in place by some means, such as a vibrator. The stem may thereafter be driven as an ordinary pile; and when it reaches the level of the base, the soil confining and pressure retention characteristics of the base will cause the soil displaced by the stem to form a load supporting soil plug frictionally interlocked with the stem.

The base, including the rim, skirt, spokes and hub, may be made of metal or some other material which is capable of being driven into the soil and which is capable of restraining the soil displaced by the stem sufficiently to maintain the lateral pressure restraint necessary for the plugging action. The base material should also be capable of resisting the corrosive effects of a subterranean environment. Some materials which may be employed are steel masonry, concrete, certain plastics, ceramics and wood or paper products, as well as combinations thereof.

Various driving techniques may be employed with the present invention. For example, conventional hammers or vibrators, or a combination of both may be used. In the event that vibrators are employed, the stem and/or base may be vibrated at one frequency down to the desired bearing stratum and at another frequency at the bearing stratum. The first vibratory frequency would be that which induces fluidization of the soil through which the stem and/or base are being driven. This allows free flow of soil through the spaces between the stem, rim and spokes. Thereafter, at the desired bearing stratum, the elements are vibrated at a different frequency known to induce compaction and of the soils thereby to effect plugging of the spaces between the stem, spokes and rim.

It will be appreciated that there has been described novel pile installation arrangements whereby a pile of an enlarged base may easily be formed by causing the soil adjacent the base to become effectively an integral part of the base.

Claims

What is claimed is:

1. In combination, a displacement pile positioned in the earth with the lower end of the pile in a region characterized by substantially non-cohesive granular soil, a perimeter defining rim of substantial tensile strength also positioned in the earth and surrounding a mass of soil around the lower end of said pile, the soil surrounded by said rim being tightly packed by the displacement of said pile and thereby having a higher density than the surrounding soil, said soil surrounded by said rim further being subjected to lateral compressive forces imposed thereon by said rim, said lateral compressive forces being sufficient to maintain a shear strength in the soil surrounded by the rim which is substantially greater than that of the surrounding soil whereby the soil within the rim is enabled to sustain column loading imposed thereon by said pile.

2. A combination according to claim 1 wherein said interconnecting means comprise spaced apart spokes extending between said pile member and said rim.

3. A combination according to claim 2 wherein said spokes are inclined with respect to the plane of the lower edge of said rim.

4. A combination according to claim 2 wherein said spokes are tilted with respect to the longitudinal axis of said pile.

5. A combination according to claim 2 wherein said spokes are tapered from a maximum width at said pile to a minimum width at said rim.

6. A combination according to claim 2 wherein said spokes are embossed to increase their frictional interlock with a plug of soil formed therebetween.

7. A combination according to claim 1 wherein said rim includes a perimeter defining skirt integral therewith and extending upwardly therefrom.

8. A combination according to claim 7 wherein said skirt is embossed to increase its frictional interlock with a plug of soil formed therein.

9. A combination according to claim 7 wherein said skirt is formed with spiral grooves to permit movement thereof down through loose soil by rotation therein.

10. A combination according to claim 7 wherein said rim is formed by folding and hemming a lower edge of said skirt.

11. A combination according to claim 7 wherein said skirt is strengthened by folds and hems about its circumference at different distances from its lower edge.

12. A method of installing a displacement pile having a soil displacing pile member and having at the lower end thereof a perimeter defining rim of substantial tensile strength comprising the steps of driving said pile down into the ground through an upper strata of soft soil while said soft soil passes through the space between the soil displacing pile member and the rim, and thence driving said pile further into the ground into a different bearing stratum characterized by a substantially non-cohesive soil, while displacing the soil surrounded by the rim laterally in a direction toward the rim by an amount sufficient both to compact the ground between the pile member and the rim and to subject said rim to substantial tension so as to cause said rim to impose lateral compressive forces on said soil of sufficient magnitude to produce a plug of said soil having a density and a shear strength substantially greater than that of the surrounding soil for frictionally interlocking said plug with said pile and effectively forming an expanded base for said pile capable of sustaining column loading imposed by said pile.

13. A method of installing a pile according to claim 12 wherein said pile and rim are interconnected and are driven together.

14. A method of installing a pile according to claim 12 wherein the rim is placed in the ground in advance of said pile.

15. A method of installing a pile according to claim 12 wherein said rim is placed in the ground at the level of a bearing strata which is primarily of non-cohesive granular soil of greater density than the soil thereabove.

16. A method of installing a pile according to claim 12 wherein said pile and rim are placed in the ground by vibrating said pile at a frequency which tends to fluidize the soil between the pile and rim in the regions above a desired bearing stratum and then vibrating the pile at a frequency which tends to compact the soil between the pile and rim at said desired bearing stratum.