US3478656A - Method and apparatus for compacting soil - Google Patents

Description

Nov. 1 8, 1969 c b 3,478,656

METHOD AND APPARATUS FOR COMPACTING SOIL Filed July 24, 1967 4 Sheets-Sheet 1 JOHN K. M DONALD INVE/VTUR BY BUG/(HORN, BLORE, KLAROU/ST 8 SPAR/(MN AT T OR/VE K5 1969 J. K. M DONALD METHOD AND APPARATUS FOR COMPACTING SOIL 4 Sheets-Sheet 2 Filed July 24, 1967 JOHN BUCKHOR/V, BLO/FE, KLAROU/ST 8 SPAR/(MAN ATTORNEYS N 3, 1959 J. K. M DONALD METHOD AND APPARATUS FOR COMPACTING SOIL 4 Sheets-Sheet 5 Filed July 24, 1967 llla Nov. 18, 1969 J. KIM DONALD 3,478,655

METHOD AND APPARATUS FOR COMPACTING SOIL Filed July 24, 1967 4 Sheets-Sheet 4 Ill/[582233521 u "I' -2lZ JOHN K. MDONALD //VVE/V7'0/? fig; J8 5) 2J0 BUCKHOR/V, BLORE, KLAROU/ST a SPAR/(MAN ATTORNEYS United States Patent ABSTRACT OF THE DISCLOSURE I a The present application discloses a method and means for compacting soils and other particulate materials. Ac-

cording to an illustrative embodiment of the method a soil is compacted by applying surface pressure to a zone of the soil to place the same under compression. While the zone is under compression, compressed air or other gas is introduced into the soil under compression to weaken the same. The combinedeffects of the continued surface pressure and increased internal gas pressure within the soil collapses, and thus densifies, the soil to a degree not obtainable by the application of the surface pressure alone. Air or other gas within the soil zone is permitted to bleed therefrom, either through the surrounding soil during the entire operation, or upwardly through the densified soil zone following its collapse and release .of the surface 1 surface for pressing on the material to be compacted such as embodied in an earth tamping foot or earth com- .pacting roller, an external source of a pressurized gas,

and a device for injecting the gas under a controlled pressure into the material. The gas can be injected into the material through small openings in the material contacting surface of the compaction device, through injections arranged'peripherally about the contact surfaceor by means ofa hood placed over a surface-portion of the zone and into which the pressurized gas is pumped.

Applications of the method include the densification of any gas permeable particulate material including, for example, soils, foundry sand, powdered metals, ceramic powders and dental materials.

i :2 BACKGROUND OF THE INVENTION U I i Field of the invention Description of the prior art Commonly known methods of compacting soils and othermaterials include (1) application of surface pressures to the material to press the individual particles of 'material more closely -together, (2) vibration of the. material to effect a more compact rearrangement of the individual particles, (3) application of extraordinary sur- ,face pressuresto the material so as to crush theoriginal 1 particles into smaller, more closely inter-fitting particles, 4) acombination of any of the foregoing three methods,

and (5)-jetting :orinundation of granular soilswith an incompressiblefiuid, usually water.

.. In the manufacture of bricks and similar ..products,wet ,clays,= which are relatively impervious to air, have been worked in :a vacuum, atmosphere prior to extruding or compacting the material to; remove air. therefrom and thereby prevent the formation of air pocketsin the finished product. i

-In the fields of structural and soils engineering, the

effective density ofa soil mass and thus its ability to 31,478,656 Patented Nov. 18, 1969 "ice support superimposed loads, has been increased through the injection of a fluent material such as a grout into the air voids of the mass to create a load-bearing region upon the hardening of the grout. In a related method, sodium silicate has been mixed with soil, and then the mass has been chemically stabilized through the injection of car- 'bon dioxide, which reacts with the sodium silicate to form a stiff glue.

Soils such as quicksand, which are inherently unstable because of high internal water pressures, have been stabilized through the removal of excess Water therefrom by drainage or pumping.

The present invention, however, employs a new technique to the compaction of soils and other materials having some degree of air permeability which gives a greater compactive effect than can be obtained by using the aforementioned surface compaction methods alone, and without the necesstiy of introducing artificial hardening agents or excess water into the material.

SUMMARY OF THE INVENTION The present invention involves the injection of compressed air or other nonreactive gas into an air permeable soil or other particulate material while a compaction device is pressing on the material surface. Somehow the combination of the resulting increased internal gas pressures within the material and the simultaneous surface pressure weakens the soil to such an extent that it collapses, or compacts, to a greater degree than would result from the application of either pressure alone. It has been found in working with soils that the applied gas pressure necessary to achieve the optimum compactive effect varies with the applied surface pressure, the type of soil and the Water content of the soil. In general, the optimum applied gas pressure for a given surface pressure must be determined empirically on a case by case basis.

Principal objects of the invention are to provide:

(1) a new method of compacting soils and other particulate materials utilizing the injection of air or other gas pressure into the material as one of the necessary steps of the method;

(2) a new method as aforesaid which gives a greater compactive effect than can be obtained using prior. ord inary compaction methods relying solely on applied surface pressures or vibration;

-(3) an improved compaction method as. aforesaid which is substantially as simple and economical to perform as prior known methods;

(4) a compaction-method as aforesaid which utilizes (5) apparatus for carrying out the aforesaid method;

- and Y i (6) apparatus as aforesaid which is eflicient, practical, economical to manufacture and potentially adaptable to existing compaction equipment.

Although the method and apparatus as described here- 'inafter will referspecifically to the compaction ofasoil utilizing the'injection of air under pressure, reference to soil and air is illustrative only. It is to; be understood that the description applies to other particulate materials "and' to other compressible'fluids as well as to soil and to h ir. 1 e5;

BRIEF DESCRIPTION. OF THE DRAWINGS I FIG. 1 is a schematic side elevational View, partly in section, showing a laboratory apparatus for performing the method of the present invention;

FIG. 2 is a diagram of a loose mass of soil;

FIG. 4 is a diagram on an enlarged scale of some of the soil particles of FIG. 2, illustrating the theory of the present invention;

FIGS. 5 through 11 are schematic sectional views of various forms of compaction feet in accordance with the apparatus of the present invention;

FIG. 12 is a schematic sectional view of a compaction roller incorporating compaction feet as shown in FIG. 11;

FIG. 13 is a schematic sectional view of a modified form of compaction roller for carrying out the method of the present invention;

FIG. 14 is a schematic sectional view of another modification of compaction roller in accordance with the invention;

FIG. 15 is a schematic end view of a modified form of compaction roller in accordance with the invention;

FIG. 16 is a schematic side view of the compaction roller of FIG. 15;

FIG. 17 is a schematic vertical sectional view of still another modification of compaction roller in accordance with the invention;

FIG. 18 is a partial sectional view taken along the line 18-18 of FIG. 17;

FIG. 19 is a sectional view through a peripheral portion of a roller similar to that of FIG. 17 but modified to exclude tamping feet;

FIG. 20 is a schematic side elevational view of a vibratory tamping device in accordance with the invention; and

FIG. 21 is a schematic side elevational view of a vehicle mounted multiple shoe vibrator in accordance with the invention.

DETAILED DESCRIPTION Description of method With reference to the drawings, FIG. 1 diagrams a laboratory apparatus 10 which has been used to demonstrate the effectiveness of the method of the present invention. The apparatus includes a generally rectangular housing 12 which supports from an overhead portion 14 a suspended hydraulic ram 16 including a piston rod 18 carrying at its lower end a compaction foot 20. The ram is connected by a suitable hose 22 to a hydraulic pump 24. A suitable valve 26 in hose 22 between the pump and the hydraulic ram controls the applied pressure of the compaction foot, such pressure being indicated on a pressure gauge 28.

Compaction foot 20 is shown in section and includes a lower, soil contacting surface 30' which has a recessed central portion 32 which receives a perforate insert 34. Compressed air can be introduced into the perforate insert through an internal foot passage 36 which connects at an upper surface of the foot with an air hose 38 leading to a tank 40 of air under pressure. Air hose 38 includes an air pressure regulating valve 42, and tank 40 is equipped with an air pressure gauge 44.

Soil 46 to be compacted is placed within the lower end of the housing. With the compaction foot in slight contact with the soil, a sensing arm 48 of a penetration gauge 50 is placed in contact with an upper surface portion of the compaction foot. The gauge is mounted on the end of a horizontally extending arm 52 forming part of a penetration gauge support stand 54.

With the foregoing apparatus the effectiveness of the present method is demonstrated as follows: a constant predetermined mechanical contact pressure is applied by the compaction foot to the soil surface through proper adjustment of the regulating valve 26. The compaction foot is permitted to densify the soil 46 under the sole influence of the predetermined surface contact pressure of foot 20 until such foot pressure, of itself, is incapable of effecting any further substantial densification of the soil, as indicated by the penetration gauge. At this point, and while still applying predetermined foot pressure, compressed air is introduced into the soil through foot insert 34 under a pressure which is sufiicient to cause the air to penetrate the soil without blowing away any of the surface soil surrounding the foot. With the introduction of the compressed air, a substantial additional penetration of the foot beyond that obtained with the application of foot contact pressure alone will be observed on the penetration gauge. Thereafter, when the air pressure is relieved and the compaction foot withdrawn, the densified soil remains substantially as dense as its most densified condition prior to withdrawal of the foot.

Similar results are obtained using various sequences of events such as applying air pressure prior to mechanical pressure or simultaneously therewith. Similarly, the air pressure may be released at various stages, or modulated.

Soil density tests have confirmed that under the foregoing conditions the soil has a higher retained density after the application of compressed air than the same soil has with the application of the surface contact pressure alone. For example, in an experiment of particular interest using the illustrated laboratory apparatus and an applied surface contact pressure of 100 p.s.i., the dial gauge reading was .367 just prior to the application of air, whereas upon the application of compressed air at 34 p.s.i., the dial penetration gauge recorded a rapid penetration and a reading of .910, or an additional penetration of over /2 inch out of an original compressed depth of less than 3 inches.

The applied air pressure requried to effect the additional compaction varies with the type of soil, the soil moisture content, and the applied mechanical contact pressures of the compaction device. With some only slightly porous soils it has been found that the air pressure must exceed the soil contact pressure applied by the compaction foot. In other soils, for example sand, the foregoing compactive effect has been observed using air at an applied pressure of less than one pound per square inch. Still other, highly cohesive soils have been found to be unresponsive to air applied at pressures up to pounds per square inch. In general, however, clay soils have been found to be least responsive to the effect of air pressure, sandy soils the most responsive, and silt soils intermediate in response to applied air pressure.

Theory Referring to FIGS. 2 and 3, which diagram a loose soil mass and a dense soil mass, respectively, the compaction of a soil mass or mass of any other particulate material requires a rearrangement of the particles 60 of the mass to reduce the percentage volume of voids V existing between the particles in the mass. These voids are usually filled partly with air and party with moisture. When particles are shifted to reduce the volume of voids, air is forced from the voids and bled from the mass. Water present in the voids remains, however, since it is incompressible and cannot readily be removed therefrom through the application of pressure. Thus the degree of water saturation of the voids is a limiting factor with respect to the potential of the mass for densification.

FIG. 4 diagrams four particles 60A, 60B, 60C and 60D of the loose mass of particles 60 shown in FIG. 3, and illustrates the path of particle 60A as it shifts into dashed line position 60A within a portion of the void V occupied initially by air a to form a more compact arrangement with the surrounding particles. It will also be noted that water w also occupies a small portion of void V.

Particle 60A moves into void V by sliding over the surface of particle 60B. This demonstrates that a soil mass is densified by intergranular movement in shear rather than in compression, although the shear stresses are usually generated by external mechanical compressive forces acting at the surface of the mass. However, before particle 60A will slide into void V, the particles frictional resistance to sliding over particle 60B, that is, its resistance to shear, must be overcome. This frictional resistance is dependent on the normal contact pressure P, (intergranular pressure) acting between particle 60A and particle 60B. The greater such contact pressure, the greater will be the particles resistance to shear. The contact pressure P however, is increased by any applied external pressure P increased by pressure P created by surface tention in water w, but decreased by pore air pressure P acting in the voids between particles 60A and 60B. Pore air or gas pressure is the pressure applied to the particles by air or other gases acting within the voids between particles. I Normally pore air pressure P in a soil would be close to atmospheric pressure. However, in the present method it is theorized that the injection of air into the soil mass temporarily increases the pore air pressure P, to such an extent that it tends to force the particles apart, thereby reducing the contact pressure P between the particles and thus their resistance to shear, so that the particles can slide more readily over one another into more compact arrangements upon the continued application of external mechanical contact pressure. To express the phenomenon somewhat differently, the air appears to act as a sort of lubricant between the particles to temporarily weaken the overall resistance of the entire mass to shear. While the mass is thus temporarily weakened, a given applied mechanical contact pressure is able to more completely shift the particles of the mass into more compact arrangements than would be the case if the same contact pressure were applied with the mass in its normal condition.

Pursuant to the foregoing theory and experience, both air pressure and mechanical contact pressure should be applied, and the contact pressure should be maintained throughout the collapse of the mass to achieve the maximum compactive effect. If air injection occurs without the application of contact pressure, the soil mass will be merely puffed up, orloosened, by the .air, and pore air pressures eventually will be lowered to normal or near normal by the bleeding off of air through the mass. It is desirable to maintain the mechanical contact pressure on a given mass until the end of the air injection period to minimize the possibility of the air having a subsequent loosenng effect on the mass following its densification. Air which is not injected under excessive pressures for the type of soil under compaction does not appear to have a loosening effect on the soil after the compaction foot is released. It is thought that this is due, to the natural ability of porous sandy soils to bleed off the compressed air during and following the compaction process fast enough to prevent loosening, and the natural ability of morecohesive soils to withstand loosening while a slow I bleeding off of the excess air takes place.

The foregoing theory appears to explain why a very porous soil such as dry sand is more responsive to the injection of air than a less porous soil such as a silt or clayrAir is able to penetrate the porous soil more readily and more completely to exert its friction-reducing in-- :fluence.

The theory would also appear to explain why, in general, a wet soil is less responsive to the effects of air than the same soil when dry. When the voids of the soil mass are filled or nearly filled with water as is the case with a wet soil, the water prevents or at. least restricts the passage of air into the mass and reduces the effective surface area of the particles against which the pore air pressure can act.

The .theory furthermore explains why a cohesive soil such as a wet clay is less responsive to air injection: than i a soilsuch as dry sand having no cohesion, The cohesive forces tending to hold "soil particles together, are .'independent of the frictionally resistive forces of v the same .2 particles and are also independent of.the normal contact pressure acting between the particles. Thus a lessening of the normal contact pressure by increasing pore air pressure has no effect on the cohesiveness of the particles. .If the shear stresses induced in the massare not sufiicient to overcome the cohesiorrofthe mass, then the mass will Apparatus carrying out the method With reference to FIGS. 5 through 8, several compaction foot configurations are shown, each incorporating a means for injecting air under pressure into the soil at or adjacent to the soil contacting surface of the pressure foot.

More specifically, FIG. 5 discloses a compaction foot having a soil contacting lower surface 72 and a hollow interior defining an air chamber 74 in communication witha source of compressed air (not shown). A series of straight passages 76 and tapered passages 78, 79 extend through the bottom of the foot and into communication with the soil over the area of soil contacting surface 72. Of course, the passages could all be straight or all be tapered in either direction shown, as desired.

FIG. 6 shows another form of compaction foot 80 having a soil contacting bottom surface 82 in contact with the soil 84. Bottom surface 82 has a recess 86 occupying most of the surface area of the foot and defining an air chamber in communication with a source of air pressure (not shown) through a central passage 88. Recess 86 is separated from the soil by an insert member 90 which is pervious to air but not to soil particles and which serves as a means for disbursing air into the soil over a wide area. Insert member 90 may be made of sintered metal or porous ceramic.

FIG. 7 discloses a compaction foot 92 having a recess 94 in the soil contacting surface 96 of the foot, with the recess taking up a relatively small area of such surface. Foot 92, like compaction foot 80, has a pervious insert 98 to disburse air from the recess into the adjacent soil.

FIG. 8 discloses a compaction foot 100 having a soil contacting surface 102 with an annular rim 104 at its periphery. Surface 102, like the corresponding surface of the foot 92 in FIG. 7, has a recess 106 with a pervious insert 108 and a central passage 110 connecting the recess to a source of air pressure. In certain soils, the recess below insert 108 will remain full of pervious compacted soil which is well dried by the passage of air therethrough and which will thus protect the relatively expensive insert 108 against wear.

The compaction feet of FIGS. 7 and 8 are believed to provide more effective air penetration of the-soil than the compaction feet of FIGS. 5 and 6 because air entering the soil through the feet of FIGS. 7 and 8 must travel a greater distance, as indicated by the arrows 112 of FIGS. 7 and 114 of FIG. 8, before reaching soil areas of low pressure than does pressurized air entering the soil through the feet of FIGS. 5' and 6, as indicated by the arrow 116 of FIG. 6. Air under high pressure entering the soil through the feet of FIGS. 7 and 8 will gradually travel from regions of the soil under high compaction pressures to regions of gradually decreasing compaction pressures as the air pressure itself decreases. However, in the foot of FIG. 6, air enters the. soil under high pressure at points closely adjacent surrounding regions of the'soil near the soil surface under very low pressure, and therefore the air under high pressure, seeking regions of lower pressure, is more likely to flow to these low pressure soil regions near the soil surface and actually blow away surface soil surrounding the compaction foot than the foot 7 the foot and sealed at 132. As the foot approaches the soil, the shell makes contact first and progressively collapses, trapping air in the cavity defined by the soil surface, the shell and the lower surface 134 of the compaction foot. The descending foot then compresses the air and forces it into the soil directly ahead of the foot itself.

All of the foregoing described foot configurations could be applied to either a simple single or multiple tamping foot device manually operated or mounted on a vehicle, or they could be mounted as so-called sheepsfeet on a footed compaction roller, with or without a vibrations generating means in conjunction with the tamper or roller. However, when used on compaction rollers it is especially important that means he provided for applying the compressed air only when each foot is in proximity to the soil surface to prevent blowing away of surface soil and to save air.

FIGS. 10 and 11 disclose two different valving arrangements operative in response to soil contact pressure to admit air into a tamping foot only when the foot is in contact with the soil. Both arrangements are especially suited for use when the foot is provided on the periphery of a compaction roller. In such an arrangement the roller drum itself may be used as an air pressure accumulator. In FIG. 10 a tamping foot 136 projects from the peripheral surface of a tamping roller 138 and is surrounded in spaced relationship by an outwardly biased but collapsible bellows structure 140. The outer end portion of the bellows mounts one end of a series of valve actuating rods 142 which slide in openings 143 in the roller drum. A series of air passages 145 in the drum surrounding the foot are normally closed by valve members 146, pivoted at 147, when the foot is out of contact with the soil. However, as the foot begins to penetrate the soil, the bellows starts to collapse, causing rods 142 to slide inwardly of the drum to contact valve members 146 and unseat them, thereby enabling air within the drum to enter the bellows and the soil surrounding the foot. As the foot leaves the soil, the bellows will expand, causing rods 142 to retract and valve members 146 to reseat themselves in the air passages.

In FIG. 11, a compaction foot 150 has a hollow interior 151 with a central opening 152 in the soil contacting surface 153 of the foot. The interior is in communication with the pressurized interior 155 of a compaction drum 156 through a valve port 157 in the drum. However, a cone-shaped inner end 158 of a plunger-type valve member 159 normally closes port 157 through the action of a compression spring 160 which surrounds the member between a shoulder portion 161 of an enlarged head 162 of the member and the outer surface of the roller drum. The head 162 extends outwardly beyond the soil contacting surface of the foot until the foot comes into contact with the soil, forcing the valve member inwardly against the force of spring 160 to open valve port 157, thereby injecting air from the drum into the soil.

As shown in FIG. 12, a frame portion 164 of the compaction roller which includes drum 156 and several of the compaction feet 150 of FIG. 11, is equipped with a cam 166 which contacts head 162 of each valve member 159 when the associated foot is well out of contact with the soil, thereby opening the valve port so that air will blow dirt from the interior of each foot.

As mentioned with respect to FIGS. 10 and 11, where the tamping feet form part of a compaction roller, the entire interior of the roller drum may be used as an air pressure accumulator to supply pressure to the various feet when valves are actuated. However, FIG. 13 diagrams an alternative arrangement whereby only a lower segment 168 of the interior of a roller drum 170 is used as an accumulator to eliminate the need for groundactuated valving for each foot 172. Feet 172 approximate the form of the foot shown in FIG. 7. Accumulator segment 168 does not rotate with the drum, but instead contacts the inner surface of the drum at sliding air seals 174 attached to the ends of segment 168. Air is supplied to the accumulator from an air compressor 176 mounted on compactor frame 177 through a hose 178 that directs air into a hollow portion of a drum axle 179 at a swivel connection 180. Air injected into the axle enters the segment through openings in the axle. With the accumulator extending through the are shown and with the roller moving from right to left as indicated by the arrow, air enters each compaction foot from a time just after the foot enters the soil to a short time after the foot leaves the soil, thereby blowing dirt from each foot to keep the air passages clear. However, the roller could be operated in the reverse direction, in which event air would be injected into each foot and directed toward the soil beginning from a time just before e'ach foot enters the soil and continuing until just before each foot leaves the soil. In either case, a compactive effect greater than that obtainable through use of the roller without air injection would be obtained in air permeable soils.

FIG. 14 discloses an arrangement similar to that of FIG. 13, but incorporated in a plain roller drum 182 having no tamping feet, but having air passages 184 through the periphery of the drum. An air chamber segment 186 within the drum remains stationary as the drum rotates so that only the downwardly directed air passages will receive air. The accumulator may be supplied with air from a compressor on the frame in the manner shown in FIG. 13. Normally roller 182 would roll from right to left as shown, but as discussed with respect to FIG. 13, the desired compactive effect would also be obtained when operating the roller in reverse.

It is important to point out that both the roller of FIG. 13 and that of FIG. 14 may be of either the static or vibratory type. If of the latter type, such a roller could incorporate any of the common types of vibrations generating means, such as, for example, an eccentric, driven shaft extending through the center of the roller drum as shown in United States Patent 3,203,201, to Harbke, or one having a driven rotary eccentric weight as shown in United States Patent 2,025,703, to Baily et al.

FIGS. 15 and 16 diagram an alternate arrangement for supplying air from an external air supply 188 on the frame 189 of a compaction roller having a drum 190, to headers 192 within the drum extending parallel to the cylindrical surface of the drum, along each longitudinal row of tamping feet 194. Each header has a series of laterals 196 which extend into the feet 194 to supply the latter with air. Air is supplied to each header when the header rotates with the drum to a downward position, through a pair of air hoses 197 from the compressor 188. The hoses connect to injection heads 198, 199 mounted by struts 200 from frame members 201, 202 in stationary positions at the lower opposite end walls of the drum. End openings of the headers come successively into register with the injection heads as the drum rotates. The heads are maintained under constant internal air pressure at the drum surfaces by sliding seals 204 so that the headers are successively injected with air as they register with the heads.

FIGS. 17 and 18 disclose an alternative arrangement for supplying pressurized air to tamping feet 210 on a compaction roller drum 212 when the feet roll into predeterimned positions with respect to the ground. Referring first to FIG. 18, compressed air from a compressor (not shown) enters a drilled passage 213 within one end of an axial drum shaft 214 at an end connection 215. Shaft 214 is fixed against rotation relative to a compactor frame 216 by a key 217 in a bearing member 218 carried by the frame.

Air passes through drilled passage 213 and enters the inside of drum 212 at an opening 220. A rotary seal 222 seals the drum against leakage of air at the shaft. Air within the drum enters a valve housing 224 through an orifice 226. If desired, a valve (not shown) operable from outside the drum could be placed at orifice 226 to retain accumulated air pressure within the drum during periods of shutdown. However, so long as the compressor is operating, high pressure air is maintained within chamber 228 of the valve housing.

A sleeve 230 slidably surrounds valve housing 224 and is sealed against air leakage by O-rings 225. The sleeve is connected to a system of radial tubes 232 which lead into the interiors of hollow tamping feet 210 of one peripheral row of such feet. Longitudinal feeder pipes 234 connected to radial tubes 232 direct air into the remaining peripheral rows of feet 210 through short radial pipe sections 236.

Ports 238, 240, as shown in FIG. 17, are placed at predetermined locations in sleeve 230 to provide clear air passage between valve chamber 228 and only those tamping feet 210 which are connected directly or indirectly with radial tubes in register with such ports. With the illustrated arrangement, air is supplied only to the three tamping feet 210 of each peripheral row which are in contact with the soil to inject air into the soil and to the single tamping foot of each peripheral row which is in its uppermost position so as to blow accumulated soil from the perforate insert 242 at the soil contacting surface of each foot.

FIG. 19 shows a peripheral shell portion 244 of a plain roller similar to the roller of FIGS. 17 and 18 except for the absence of tamping feet on the former. The valving and air supply arrangements of the two rollers are identical. The major difference is that the drum shell 244 is provided with air injection orifices 246, and porous inserts 248 are set within the orifices. A housing 250- attached to the inner face of shell 244 receives radial air supply tube 252 corresponding to tubes 232 of FIG. 18 leading from the primary valve housing and distributes air into orifice 246.

FIG. 20 illustrates a hand operated vibratory tamper 260 having a tamping plate 262 with a porous air injection insert 264 in the soil contacting surface 265 thereof. Air is supplied to the plate through a hose 266 extending from the plate upwardly along a handle 267 and to an air compressor (not shown). A manually operated valve 268- on the handle controls the admission of air to the plate. A motor driven vibrator 269 is mounted on the plate. If desired, an air driven vibrator could be substitued for the motor driven vibrator shown, in which case air would first be directed through the hose to the vibrator and then exhausted from the vibrator into the tamping plate.

FIG. 21 illustrates diagrammatically the application of the present method to a compaction vehicle 270 mounting at its forward end a plurality of air driven vibrators 272 from which depend a series of compaction feet 274. An air line 276 extends from a compressor on the vehicle to the vibrators to drive the same, and air is exhausted from the vibrators into the tamping feet.

From the foregoing apparatus, it will be apparent that an air pressure can be applied to the soil which is independent of the mechanical contact pressure applied by the foot, roller or tamper, and that the air pressure can be varied to meet the requirements of varying soil conditions. The applied air pressure, being supplied from a source external to the soil, is also independent of the porosity of the soil, and thus again the applied air pressure can be varied to meet changing needs.

Having illustrated several means for carrying out the method, it should be obvious to those skilled in the art that the method and apparatus of the present invention are capable of modification in arrangement and detail. I claim as my invention all such modifications as come within the true spirit and scope of the following claims.

I claim:

1. A method of compacting a permeable mass of particulate material having initial pore gas pressures acting between the particles of said mass,

said method comprising:

injecting a gas into said mass under a pressure sufficient to increase said pore gas pressures to a level greater than said initial pore gas pressures,

while said pore gas pressures are increased, applying a mechanical contact pressure to a surface portion of said mass,

controlling the injection pressure of said gas at a level sufliciently high to densify said mass and sufficiently low to avoid the quick state of said mass,

continuing the application of said mechanical contact pressure during the densification of said mass.

2, A method according to claim -1 including inducing the bleeding of the injected gas from said mass by continuing the application of said mechanical contact pressure until the mass is densified and at least until the applied gas pressure is relieved.

3. A method according to claim 1 including applying the mechanical contact pressure before injecting the gas into said pores and including injecting said gas at a pressure producing positive intergranular pressures within said mass at levels above a predetermined minimum positive intergranular pressure so as to avoid inducing a quick state within said mass.

4. A method according to claim 1 including continuing said injection of gas into said mass during the densification of said mass whereby said pore gas pressures are maintained at a level above said initial pore gas pressures throughout substantially the entire period during which densification of said mass occurs.

5. A method according to claim 1 wherein the injected gas is chemically nonreactive with said particulate material.

6. A method according to claim 1 wherein the pressure of the injected gas is independent of said mechanical contact pressure.

7. A method according to claim 1 wherein said gas is injected into said mass from a position at said surface portion.

8. A method according to claim 7 wherein said gas is injected into said mass at points spaced inwardly of the outer peripheral limits of said surface portion.

9. A method of compacting a plot of ground comprising: I

applying a mechanical contact pressure to the surface of a selected zone of said plot,

while said mechanical surface contact pressure is being applied, injecting gas into the ground within said zone at a pressure sufficiently high to effect a collapse thereof but sufliciently low to maintain said ground surface in a load-supporting condition during said collapse,

and while said ground is collapsing, maintaining said mechanical contact pressure against said surface.

10. A method according to claim 9 wherein the application of said contact pressure and injection of gas simultaneously as aforesaid is repeated at different zones of said plot until the entire said plot is compacted.

11. A method according to claim 9 wherein the gas is injected from a source external to said ground and at a pressure independent of the ambient gas pressure and mechanical contact pressure.

12. Earth compacting apparatus comprising in coma bination:

pressure applying means for applying a mechanical contact pressure to the surface of a localized zone of earth,

gas injection means for injecting gas under pressure into the earth within said zone during the application of mechanical contact pressure by said pressure applying means,

said gas injection means including means for controlling the applied pressure of said injected gas at a level sufficiently high to promote densification of said 1 1 zone and sufiiciently low to maintain the surface of said zone in a load supporting condition.

13. Apparatus according to claim 12 wherein said pressure applying means includes means for maintaining a contact pressure on the surface of said zone during the settlement of earth within said zone and said gas injection means includes means for continuing the injection of said gas into said zone during said settlement.

14. Apparatus according to claim 12 including means for controlling the application of said gas pressure and said contact pressure in accordance with a predetermined sequence of time.

15. Apparatus according to claim 12 wherein said pressure applying means includes an earth compaction roller and said gas injection means includes orifice means embodied in a peripheral portion of said roller.

16. Apparatus according to claim 12 wherein said pressure applying means includes a compaction foot means having an earth contacting surface and said gas injection means includes orifice means in said foot means and spaced inwardly of the periphery of said surface.

17. Apparatus according to claim 16 wherein said orifice means includes plural orifices distributed over said earth contacting surface.

18. Apparatus according to claim 16 wherein said orifice means includes an air permeable portion of said earth contacting surface.

19. Apparatus according to claim 12 including vibration generating means for vibrating said zone while applying said mechanical contact pressure and while injecting said gas.

20. Apparatus according to claim 12 wherein:

said pressure applying means includes an earth contacting surface, said gas injection means includes an opening adjacent said earth contacting surface in communication with a source of gas under pressure,

and means for cleaning soil particles from said opening by blowing gas from said source outwardly through said opening when said opening is out of engagement with said earth.

21. Apparatus for compacting a porous mass of particulate material comprising:

means for intruding a gas under pressure into said mass,

means for applying a mechanical contact pressure to a surface portion of the same said mass to compress said mass and for maintaining said contact pressure during the injection of said gas,

and means for controlling the pressure at which said gas is applied at a level sufficiently high to increase the pore gas pressures Within said mass while said mass is under compression but sufficiently low to maintain positive intergranular pressures within said mass.

22. Apparatus according to claim 21 wherein said means for injecting gas under pressure includes control means for varying said pressure.

23. A method of compacting soil in place comprising:

applying a downward mechanical contact pressure to a surface area of a selected zone of said soil to be compacted, to place the soil Within said zone under compression, said applied pressure being at a level below that required to penetrate said surface area,

and while said zone is under compression, intruding a pressurized gas into the soil within said zone under a pressure sufiiciently high to effect a settling of the soil within said zone and below that pressure required to place said soil in its quick state,

and maintaining said zone under compression during the settling of said soil.

24. A method of compacting a porous unsaturated mass of particulate material having an initial volume and initial pore gas pressures and initial positive intergranular pressures,

said method comprising:

applying a mechanical surface contact pressure to a surface area of said mass while restraining the remaining peripheral areas of said mass against any substantial bodily displacement so as to place said mass in compression,

during the application of said mechanical contact pressure to said surface area, intruding gas into said mass in compression under a pressure sufiicient to increase the pore gas pressures of said mass and below that pressure required to reduce the intergranular pressures to zero,

continuing the simultaneous application of contact pressure to said surface area and the intrusion of said gas to shrink the volume of said mass,

and continuing the application of said contact pressure and the intrusion of said gas during said shrinkage.

References Cited UNITED STATES PATENTS 1,117,333 11/1914 Cooper 94-44 2,384,469 9/1945 Kalix 9448 1,952,162 3/1934 Gee 6136 2,719,029 9/1955 Steuerman 61-36 XR 2,866,422 12/ 1958 Colson 1116 2,975,735 3/1961 Purvance 111-6 3,029,756 4/ 1962 Krivda 1l1--6 3,269,039 8/1966 Bodine 17240 XR JACOB L. NACKENOFF, Primary Examiner US. Cl. X.R.

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