Borehole Plate Test

The present invention relates to the measurement of soil compressibility and more particularly to an improved method and apparatus of insitu soil testing for determining soil compressibility at the base of a borehole.

The testing of soil compressibility at a construction site is important for determining the type of foundation which must be built to support a given building. Current methods available for determining the insitu characteristics of soil compressibility are basically of two types, plate bearing and pressuremeter.

In the plate bearing method, a plate is situated on the smoothed surface of the ground and its deflection is measured under incrementally increasing load. An acknowledged serious limitation of this test is that only near-surface soil contributes to settlement whereas the depth of influence of an actual foundation may be many times deeper. Therefore, if the soil changes with depth, the plate load test is not usable, or a pit must be prepared at great expense to conduct the test deep below the existing ground elevation.

The pressuremeters basically consist of a multi-chamber inflatable bag wherein lateral soil compressibility is monitored from a measured pressure versus volume relationship. The limitation of the pressuremeters is that only the lateral compressibility, not the vertical compressibility, is measured whereas placed on the soil or rock.

To overcome these limitations, two different types of tests have been proposed. The first is to conduct a type of plate test in the base of a borehole and the second involves laboratory testing of so-called "undisturbed" soil samples. But in each instance, the disturbance of the stress environment in the soil at the test site is reflected in the test results making them somewhat less than reliable.

Accordingly, a primary object of the present invention is to provide an improved method and apparatus of insitu soil testing for determining soil compressibility.

Another object is an insitu soil testing method and apparatus which produces a direct measure of soil compressibility in a vertical direction.

Another object is an insitu soil testing method and apparatus wherein tests may be conveniently conducted at the base of a borehole.

A related object is an insitu soil testing method and apparatus wherein tests may be conducted at any desired depth.

Another object is an insitu soil testing method and apparatus wherein the natural lateral stress environment of the soil at the base of a borehole is reconstituted prior to testing.

A specific object is an insitu soil testing method and apparatus wherein the test soil is laterally stabilized for substantially one-dimensional vertical compression testing.

Another object is an insitu soil testing method and apparatus providing for drainage of the test soil to enable measurement of the drained load deformation characteristics.

Finally, an object is an insitu soil testing method and apparatus which are simple, economical and efficient.

The insitu soil testing method of the present invention includes the steps of boring a hole in the soil to be tested, reconstituting the lateral stress environment in the soil adjacent the bottom of the borehole and then performing a vertical compression test directly on the undisturbed material at the base of the borehole. The apparatus for performing the method of the invention may comprise a cylindrical container adapted to be lowered to the bottom of a borehole prior to testing. The container has an inflatable circumferential membrane around the exterior periphery thereof which is inflated to reconstitute the lateral stress environment. An open bottomed compartment at the lower end of the container has an axially slidable load plate therein from which an elongated load rod extends axially up through the borehole to the surface. A fluid cell may be interposed between the load plate and soil at the base of the borehole. For a closed cell, the load may be applied either from the surface with dead load or a screw mechanism, or in the hole with a fluid cylinder unit. For an inflatable cell, the load rod need only be anchored whereupon the load is applied directly upon inflation.

Further alternatives to the basic method further include the use of a flat end plate directly without a fluid cell or a multi-stepped load plate for correcting problems of perimeter shear.

For measurement of substantially one-dimensional vertical compression, a thin-walled cylindrical housing is pushed into the soil at the bottom of a borehole to laterally stabilize the soil prior to vertical loading.

FIG. 1 is a partially sectional side view of the soil testing apparatus installed in a borehole.

FIG. 2 is an enlarged detail sectional view as seen on line 2--2 in FIG. 1;

FIG. 3 is a foreshortened side view of an optional flat load plate;

FIG. 4 is a foreshortened side view of an optional two-stepped load plate;

FIG. 5 is a foreshortened side view of an optional flat load plate including a drainage passage therethrough; and

FIG. 6 is a side view of an alternate shielding structure for the load plate.

An apparatus 10 for insitu testing of soil compressibility is shown in FIGS. 1 and 2 installed within a borehole 12 which is formed in the soil to be tested.

A generally cylindrical container 14 having a continuous sidewall 16, top wall 18 and medial wall 20 is pushed to the bottom of borehole 12 to the extent of engagement with the bottom 22 of the borehole.

The boring of hole 12 tends to relieve the lateral stress in the soil adjacent the hole. To compensate for this, container 14 has an inflatable circumferential membrane 24 around the exterior periphery of sidewall 16. A fluid conduit 26 extends from the membrane into the container through sidewall 16 and upwardly through top wall 18 for communication with a fluid source. By pumping fluid through conduit 26, membrane 24 may be inflated to a volume and pressure corresponding to the insitu lateral stress environment of the soil prior to boring.

A circular load plate 28 is slidably movable within container 14 below medial wall 20 with its stem 30 extending upwardly through a central opening 32 in wall 20.

A fluid cylinder unit 34 includes a cylindrical housing 36 which bears against the stem 30 for applying a vertical load thereto. The piston 38 within housing 36 is formed as the lower end of an elongated rod 40 which extends upwardly through top wall 18 to the surface for use in applying a load to the soil as explained hereinbelow. A fluid conduit 44 extends from the bottom cylindrical housing 36 to the surface for supplying fluid to the fluid cylinder unit 34 or monitoring the pressure therein.

A fluid cell or load cell 46 is positioned below load plate 28 within the open-bottomed compartment 48 at the bottom of container 14. Whereas fluid cell 46 is shown as an inflatable cell having a fluid conduit 50 extending upwardly through load plate 28 and walls 20 and 18, a closed fluid reservoir could also be utilized as explained hereinbelow.

In operation, after borehole 12 is formed and container 14 is pushed to the bottom 22 of the borehole, fluid is pumped into the circumferential membrane 24 through conduit 26. The fluid pressure within the membrane 24 is simultaneously monitored to cut off fluid flow when the insitu lateral stress is achieved. Insitu lateral stress may be calculated by multiplying the product of the depth of testing and soil unit weight by the coefficient of earth pressure at rest. This coefficient may be estimated by empirical means or the lateral stress may be measured by self-boring pressuremeters.

Compressibility tests are then performed by applying a load to the bottom 22 of borehole 12 and measuring the vertical deflection of the soil. There are several possible ways of applying a load. One method is simply to direct fluid downwardly through conduit 50 for inflating fluid cell 46. Measurements of both the volume and pressure of fluid within the cell are determinative of both the vertical load and resultant soil deformation.

To securely fix the container 14 within the borehole for this test, the upper end of rod 40 is secured to a reaction beam 52 as shown in FIG. 1. Beam 52 is securely anchored to the ground surface 54 by a plurality of helical screw anchors indicated at 55.

To fix the lower end of axial rod 40 relative to container 14, a collar 56 is secured to top wall 18 and a pin 57 is insertable through registered bores through the collar 56 and rod 40. Thus, the reaction from load applied to the lower membrane 22 (or the load plates shown in FIGS. 3-5) can be taken up by the pressure applied by the circumferential membrane 24. Vertical movement is then measured by a linear variable differential transducer 58 installed inside container 14 between brackets 59 and 60 on the rod 40 and side wall 16 respectively. This arrangement eliminates bending which could take place in rod 40 in deep holes. Rod 40 is then merely a placement rod. The cylindrical container 14 will thus have to be long enough so that sufficient reaction is provided to allow testing to the desired vertical load range.

In another method of applying a load to the bottom 22 of borehole 12, the fluid conduit 50 to fluid cell 46 is closed, whereupon the volume of the fluid cell 46 is fixed due to the incompressible character of the fluid therein. Hydraulic oil is a typical fluid suitable for load cell 46. Downward pressure is applied to the load plate 28 by directing fluid into fluid cylinder unit 34 through conduit 44. Rod 40 is again anchored as in the previous test. The pressure within fluid cylinder unit 34 and fluid cell 46 are indicative of the vertical load and the volume of flow to the fluid cylinder unit is indicative of the vertical deformation of the soil.

Finally, as a further alternative, both fluid conduits 44 and 50 may be closed and pin 57 may be removed from collar 56, whereupon a downward vertical force may be applied directly through rod 40 from the surface with a dead load such as weight or a screw mechanism, not shown. In this instance, the actual weight or the pressure within fluid cell 46 are indicative of the load and the axial movement of rod 40 corresponds to the vertical soil deformation.

Referring to FIG. 3, an optional flat load plate 61 may be substituted for the load plate 28 and fluid cell 46. The advantage of a flat end plate is to simulate a rigid foundation. An alternative to end plate 61 is to use a different sized end plate such as a multi-stepped plunger or end plate 62 as shown in FIG. 4, to provide multiplicity of results.

A further alternative load plate 63 is shown in FIG. 5 with a disc 64 of porous stone, brass or the like countersunk into the bottom surface so as to be flush. An exhaust opening 66 opens through the top of load plate 63 to provide drainage through the load plate to measure the drained characteristics of load-deformation or to provide means to measure pore water pressure, such as with electrical transducers.

Load-unload cycles may be easily performed by monitoring the deformation and rebound during repeated cyclic loading. Such information may be pertinent to aid in defining the influence of sampling or excavation procedures.

Use of either a dead-load procedure or the fluid cylinder unit 34 will supply a constant load to the load plate. For saturated soils, by taking deformation readings at timed intervals from the initiation of each load, the coefficient of consolidation, or time rate of deformation may be determined. In the case of unsaturated soils, the creep rate is determined.

The apparatus described above and shown in FIGS. 1 and 2 provides loading such that deformations in the soil at the point of loading occur in three-dimensional space. This is usually desirable, since most realistic conditions would have this characteristic. In the special case where only one-dimensional compression is possible, an alternative test may be performed with the apparatus 70 shown in FIG. 6. The apparatus 70 includes an elongated thin-walled cylindrical housing 72 having a lower end 74 adapted for penetration into the soil below the bottom 76 of bore 78. Housing 72 may be a standard Shelby thin-walled tube attached with set screws 80 to an annular top wall 82 having a central bore 84 for slidably receiving the stem 86 of load plate 88. The load plate may be used in conjunction with a fluid cell 90 and the vertical load may be applied either directly through the corresponding load rod 92 or through the fluid cylinder unit 94 interposed between load rod 92 and load plate stem 86. Once the housing 72 is in place, the load may be directly applied since the lateral stress of the soil within the lower end 74 of the cylindrical housing 72 has not been relieved.

Whereas the preferred methods and apparatus for insitu testing of soil compressibility has been shown and described herein, many modifications, alterations and substitutions may be made which are within the intended broad scope of the appended claims. For example, the hole may be bored to incremental depths with tests conducted at each step thereby allowing layered systems to be analyzed individually so that a more precise estimate of settlement may be made.

Thus there has been shown and described a method and apparatus for insitu testing of soil compressibility which accomplishes at least all of the stated objects.