EP0318172A2

EP0318172A2 - Method for densification of particulate masses

Info

Publication number: EP0318172A2
Application number: EP88310473A
Authority: EP
Inventors: William E. Hodge
Original assignee: Phoenix Engineering Ltd
Current assignee: Phoenix Engineering Ltd
Priority date: 1987-11-23
Filing date: 1988-11-08
Publication date: 1989-05-31
Also published as: EP0318172A3; CA1338305C; JPH01244013A; ZA888485B; AU2500888A

Abstract

A saturated particulate mass is densified by applying a compacting force by means of a vibrating probe (10) within a region of the mass, while actively withdrawing water from the region toward the source of application of the force.

Description

Field of the Invention

This application pertains to a method for densifying or compacting saturated (or nearly saturated) particulate masses by actively withdrawing water from the particulate mass while applying a vibratory force to the mass.

Background of the Invention

Vibrating densification probes are conventionally used to compact particulate masses before structures are placed thereupon. If this were not done then subsequent settling of the particulate mass or failure due to liquefaction beneath the structure could damage the structure and/or endanger the safety of persons or objects within or near the structure. In the prior art, densification probes are relatively simple devices consisting of an extended conduit which is forced into the particulate mass and a motor means for causing the probe to vibrate within the mass, thereby compacting or "densifying" the particulate mass.
The inventor believes that conventional densification probes waste considerable amounts of energy due to the presence of pressurized pore water trapped in the interstices between individual particles which make up the particulate mass. More particularly, the inventor believes that pressurized pore water in the region surrounding a conventional densification probe absorbs a considerable amount of the energy produced by vibration of the probe and that this energy does not contribute to the desired compaction of the particulate mass. By contrast, the vibrational energy produced by a conventional probe tends to increase the pressure of the entrapped pore water, which escapes from the densification region by flowing upwardly therethrough. Such flow tends to loosen, rather than densify the mass.
As explained by the inventor in his United States patent No. 4,664,557 entitled "Method and Apparatus for Constructing an Underwater Fill", the density of a mass of fill located under water may be significantly increased by drawing water into the fill pile as fresh fill material is added to the pile, the reason being that water which flows into the sides of the accumulating fill pile tends to support the side slopes of the pile as they are formed, consequently enabling the formation of a steeper, denser pile. The inventor now believes that a similar principle may be applied to significantly improve the process of densification of particulate masses which may or may not be constructed under water, but which are saturated (or nearly saturated) with pressurized pore water. This is accomplished with the aid of a densification probe capable not only of applying a vibrating force to the mass, but also of withdrawing water from the region surrounding the vibrating probe, thus relieving the pore water pressure, preventing upward flow of pore water through the mass (and consequently preventing loosening of the particulate mass), reducing the amount of energy which is lost due to absorption by pore water, and increasing the amount of energy available for compaction of the particulate mass. The pore water is caused to flow toward the probe, which promotes closer packing (i.e. densification) of the particulate mass since the particles are dragged towards the probe rather than away from it. As the particles are packed more closely together pore water trapped between the particles is liberated and must reach the surface before the particulate mass can settle into a smaller volume. Under normal (i.e. prior art) conditions considerable time is taken for the liberated water to reach the surface. By actively withdrawing the entrapped pore water the invention promotes much faster densification of the mass.
Neither the inventor's patent aforesaid nor the prior art contemplate the improved densification technique presently envisaged by the inventor. That is, although vibratory particle compaction and fill drainage techniques are both separately known, they have not heretofore been used in combination to attain the significant improvements in densification which the inventor has been able to achieve in accordance with the present invention.

Summary of the Invention

The invention provides a method of densifying a saturated particulate mass. A compacting force is applied to the mass and the release of water from within the mass is concurrently controlled to prevent water release in a direction away from the source of application of the force. Preferably, water is actively withdrawn (i.e. pumped) from the mass toward the source of application of the force.

Brief Description of the Drawings

Figure 1 is a cross-sectional plan view of a densification probe capable of densifying a particulate mass in accordance with the preferred embodiment;
Figure 2 is a cross-sectional view of the densification probe of Figure 1, taken with respect to line 2-2 of Figure 1 and completed to show the full cross-section.
Figure 3 is a cross-sectional view of the densification probe of Figure 1, taken with respect to line 3-3 of Figure 1 and completed to show the full cross-section.

Detailed Description of the Preferred Embodiment

The drawings illustrate a densification probe generally designated 10. Probe 10 has an extended pipe-like configuration with a pointed end 12 which assists in forcing probe 10 into the particulate mass which is to be compacted or "densified". Pointed end 12 could be replaced by a drill bit, or auger to allow the probe to be screwed into the ground through soil layers which are uncommonly dense or hard. It is worth noting that conventional probes could not be manipulated in this way because their power supply cables and general deployment arrangement would interfere. By contrast, probe 10 could be deployed with the aid of a drilling rig which could rotate the probe at will. Normally, the weight of probe 10 will be sufficient to cause probe 10 to enter the ground (probe 10 is vibrated as hereinafter explained to generate the required entry force) but the drilling rig could be used to push or screw probe 10 into the ground. Pipe casing 14 surrounds a cylindrical cavity 16 within which eccentric weight 18 is journaled for rotation on bearings 20, 22 which are mounted at the opposed ends of weight 18. A second pipe casing 24 defines another cylindrical cavity containing air motor 26 which is drivingly coupled to weight 18 such that operation of motor 26 causes weight 18 to rotate about the longitudinal axis of probe 10, thereby generating vibratory forces.
A third pipe casing 28 contains another cylindrical cavity which contains first conduit means 30, second conduit means 32 and third conduit means 34. A well screen 36 is mounted within a cylindrical segment of the outer surface of pipe casing 28 so that water may pass from the region surrounding probe 10 into first conduit means 30 for subsequent withdrawal from probe 10. The relative location of well screen 36 is not crucial. For example, probe 10 could be redesigned to position well screen 36 around, or beneath motor 26 rather than above it as illustrated in the drawings.
Motor 26 is preferably air powered with the aid of an external compressed air source (not shown). Compressed air passes to motor 26 through second conduit means 32 and through the passages within pipe casing 24 indicated with the aid of arrows 38. Air is in turn expelled from motor 26 through the passages in pipe casings 24, 28 indicated with the aid of arrows 40 and then passes into third conduit means 34 for ultimate explusion from probe 10. First conduit means 30 merges into third conduit means 34 as indicated at 42. Air expelled from motor 26 through third conduit means 34 rushes past point 42, thereby creating a low pressure zone within first conduit means 30 adjacent the interior surface of well screen 36. Pore water in the region surrounding probe 10 thus tends to flow through well screen 36, through first conduit means 30 and past point 42 into third conduit means 34, such that the pore water is ultimately extracted from probe 10 through third conduit means 34, together with the air expelled from motor 26.
In operation, probe 10 is positioned on the surface of the particulate mass which is to be compacted or "densified", with pointed end 12 on that surface. The mass in question will be saturated or nearly saturated with pressurized pore water. For example the mass may be partly or completely submerged or it may be below the water table. Probe 10 would normally be positioned perpendicular to the surface of the mass but could be placed at an inclination to the vertical in some cases (for example to densify the side slopes of an underwater fill pile). Compressed air is fed to air motor 26 in the manner aforesaid to rotate eccentric weight 18, thereby causing probe 10 to vibrate and work its way into the particulate mass to a desired depth. By blocking the air/water discharge outlets at the upper end of probe 10 during initial entry of probe 10 into the mass one causes the compressed air to be discharged through well screen 36, thus loosening the particles which immediately surround probe 10 and easing its penetration into the ground. Initial penetration could also be eased by mounting an air or water jet at probe end 12 for activation during penetration of probe 10 into the mass. Once probe 10 has reached the desired depth the air/water discharge outlets at the upper end of probe 10 are unblocked and motor 26 continues to operate, thus withdrawing air and water from probe 10 through third conduit means 34 until the particulate mass has been sufficiently densified.
The invention enjoys at least two significant advantages over the prior art. First, the prior art is incapable of densifying saturated masses which lie on a slope (i.e. tailings dams) because prior art vibrators would tend to liquefy the mass, resulting in failure of the slope. The invention, by contrast, would stabilize the slope while densifying the mass. Second, prior art probes are large, bulky devices which normally require heavy cranes and large power supplies to operate. Probe 10 could however be made in short (i.e. about five foot) lengths and could therefore easily be manhandled and operated in areas of restricted headroom. Moreover, present indications are that a probe constructed in accordance with the invention and driven by a ten horsepower motor will attain the same preformance as a prior art probe driven by a one hundred horsepower motor.
As will be apparent to those skilled in the art in the light of the foregoing disclosure, many alterations and modifications are possible in the practice of this invention without departing from the spirit or scope thereof. For example, although the first, second and third conduit means of the preferred embodiment are hereinbefore described and illustrated as surrounding one another (i.e. first conduit means 30 surrounds second and third conduit means 32, 34; and, third conduit means 34 surrounds second conduit means 32) one need only position first conduit means 30 outwardly of third conduit means 34 so that the low pressure zone aforesaid is created in the interior probe region surrounded by well screen 36 as air rushes past point 42. Accordingly, the scope of the invention is to be construed in accordance with the substance defined by the following claims.

Claims

1. A method of densifying a saturated particulate mass, comprising the steps of :

(a) applying a compacting force within a region of said mass; and,

(b) during application of said force, actively withdrawing water from said region toward the source of application of said force.

2. A method of densifying a saturated particulate mass, comprising the steps of :

(a) applying a compacting force within said mass; and,

(b) during appliction of said force, establishing a low pressure zone within said mass to prevent release of said water in a direction away from the source of application of said force.

3. A method of densifying a saturated particulate mass, comprising the steps of:

(a) applying a compacting force within a region of said mass; and,

(b) during application of said force, pumping water from said region toward the source of application of said force.

4. A method as defined in claim 2, further comprising actively withdrawing water from said region toward the source of application of said force.