COMPACTION OF SOIL

Abstract: The invention concerns an impact compaction apparatus for compacting a soil surface. The apparatus has a wheeled frame (15) with one or more impact compactor masses (10) connected rotatably to the frame for delivering periodic impact blows to the soil surface when the frame is moved over the surface. In accordance with the invention, a variable vertical force can be applied to the impact compactor mass(es), thereby to vary the blow energy delivered to the soil surface at each impact blow.

Description: "COMPACTION OF SOIL' BACKGROUND TO THE INVENTION THIS invention relates to the compaction of soil and in particular to the compaction of soil using an impact roller or impact compactor mass. The term "impact roller", as used initially in US patent 2,909,103, refers to a soil compaction machine including a compactor mass of non-round shape which, when towed over a soil surface, produces a series of periodic blows on the soil surface. The compactor mass of an impact roller has a series of spaced apart, salient points on its periphery. Each such salient point is followed by a re-entrant portion of the periphery and each re-entrant portion is followed in turn by a compacting face. As the impact roller is towed over the soil surface, for instance by means of a tractor, it rises up on each salient point and then falls forwardly and downwardly as it passes over that point, with the result that the following compacting face applies an impact blow to the soil surface. The action of the compactor mass is to accumulate potential energy as the compactor mass rises up on each salient point, then to deliver this energy as an impact blow as the compactor mass then falls and the compacting face strikes the soil surface. A typical three sided compactor mass is illustrated in Figures 1(a) and 1(b) of the accompanying drawings. The mass has equi-angularly spaced salient points 1, re-entrant portions 2 adjacent the salient points 1, and radiused compacting faces 3 between each re-entrant portion 2 and the next salient point 1. The mass is mounted on an axle 4 and is towed in the direction of the arrow 5 by a suitable tractor. The towing force causes the mass to rise up on each salient point 1 in turn, as illustrated by Figure 1(b), and then to strike a blow on the surface 6 of the soil as the relevant compacting face 3 falls down on that surface. The coupling between the tractor and the compactor mass is resilient in nature to allow for the necessary forward and downward falling motion undergone by the mass as it passes over each salient point. In Figure 1(a) the letter R refers to the radius from the axle 4 to the extremity of a salient point 1, while the letter r refers to the radius from the axle 4 to the surface of the compacting face 3. Potential energy is created by raising the mass, about the salient point 1, through a distance of R-r, as illustrated by Figure 1(b), and the stored potential energy can be quantified as Mg(R-r) Joules where M is the mass in kilograms, g is the gravitational constant in m/sec 2 , and R and r are expressed in metres. For example an impact roller mass may weigh 10 000kg and have dimensions R = 0,9m and r = 0,75m. The potential energy stored in the mass when it rises on its salient point, the centre of mass then being raised 0,15m above its lowest position, is 0,15m x 10 000kg x 9,81m s 2 = 14 715 Joules, which is nominally stated as 15kJ. In civil engineering construction practice, impact compactor machines with energy values of between 10 and 25U are now commonly used, depending on the nature of the soil and the degree of compaction required. Figure 2 of the accompanying drawings illustrates graphically a relationship between the number of compactor blows and the amount of surface settlement of the soil for a typical soil. It must be noted that the amount of surface settlement is a measure of the degree to which soil density is improved and also the depth below the surface to which the improvement in soil density takes place. Two curves are shown in Figure 2, one for a mass designed to produce 15kJ of impact energy at each blow and another for a mass designed to produce 25kJ of impact energy at each blow. In each, the curve flattens out once a certain surface settlement has been achieved so that, after a certain number of blows the amount of surface settlement hardly varies irrespective of the number of further blows that are applied to the soil. The curve for the 25kJ mass is initially somewhat steeper than that for the 15kJ mass i.e. a greater surface settlement is achieved with fewer blows in the case of the 25kJ mass than in the case of the 15kJ mass during the initial stages of compaction. While fewer blows are required with the higher energy mass to give a required surface settlement, it is not always desirable to continue using the higher energy mass. This is because compaction of the soil is accompanied by shearing and rupture of the soil structure thereby allowing re-arrangement of soil particles and expulsion of air from the soil voids. Once the desired surface settlement has been achieved by high energy blows, for example point A in Figure 2, it is preferable to reduce the energy per blow and then proceed to consolidate the soil structure, closing up the shear planes and ruptures to reach, say, point B in Figure 2. On most soil compaction projects it would not however be economically viable to provide two separate impact compaction machines, one for initial compaction and the other for subsequently closing up the shear planes and ruptures. Furthermore, in soil compaction practice it often happens that a crust of hard, dry soil covers a weaker soil material. This condition provides a further illustration of the need to have a choice of energy per blow with a single machine. To break the crust so as to permit subsequent compaction of the underlying soil, it is necessary to use high energy blows. After the crust has been broken compaction may continue with lower energy blows. To continue using the higher energy compactor once the crust is broken, could, as explained above, lead to a weaker soil structure for a economic number of compaction passes. Besides this, the use of a high energy compactor for the underlying soil could lead to a compacted soil density greater than that specified and compaction to a depth greater than required, thereby detracting from the economic advantage of using impact rollers in preference to conventional roller types. In the operation of impact rollers, it is usually necessary for the machine to turn at the end of each pass. This causes the surface of the soil at either end of the work area to be greatly disturbed, with the consequence that the machine and operator are subjected to severe stress when turning, with the added possibility of the machine becoming, bogged down in the case of either cohesionless soil or soil with a high moisture content. The ability on the part of the operator to vary the amount of energy per blow at will during operation would assist in avoiding the problems presently associated with the turning of the machine, as the energy per blow could then be attenuated at the end of each straight pass, thereby substantially reducing the disturbance of the soil surface in the turning area, and reducing the traction required from the tractor wheels while laming, thereby allowing for greatly improved efficiency of operation. A further circumstance in which it would be an advantage for the operator to be able to vary the value of energy per blow during operation is where the soil is too weak to sustain a high energy impact blow. In this circumstance it has been found possible to compact first with a low energy value, say 5kJ, then place a new layer of soil compacted with say lOkJ per blow, then a second and subsequent layers of soil compacted with say 15kJ per blow. It will be evident from the foregoing description of the operational characteristics required from impact rollers that varying the energy per blow needs to be accomplished while the machine is in motion. The operation requirements are not met by stopping the machine and changing to heavier or lighter compacting masses. It is an object of the present invention to provide a means for varying the energy per blow while an impact roller is in motion. SUMMARY OF THE INVENTION This invention provides an impact compaction apparatus for compacting a soil surface, the apparatus comprising a wheeled frame, at least one impact compactor mass connected rotatably to the frame for delivering periodic impact blows to the soil surface when the frame is moved over the surface, and means acting between the frame and the compactor mass for applying a variable vertical force to the impact compactor mass, thereby to vary the blow energy delivered to the soil surface at each impact blow. The force applying means can apply a variable upward force to the compactor mass, a variable downward force to the mass, or it may be capable of applying both variable upward and downward forces to the mass. In view of the fact that the variable upward or downward force needs to be applied with the impact compaction machine in motion and with the axle on which the compactor mass is mounted undergoing cyclical movement in both vertical and horizontal directions, it is preferred that the force applying means have resilience. In one example, the force applying means comprises an air spring and means for supplying air at different pressures to the spring. Alternatively, the force applying means may comprise an hydraulic spring in the form of a cylinder or ram, possibly of double-acting type. To give the pneumatic or hydraulic spring the required resilience, it is proposed to incorporate an accumulator storing gas under pressure, in the apparatus. The force applying means does not necessarily act directly on the compactor mass. In the preferred versions of the invention, the force applying means acts between the frame and an axle on which the compactor mass is mounted. Also it is preferred that the force applying means acts with a vertical component of force between the frame and the axle. Thus the force applying means need not necessarily be vertically acting, as long as its line of application is such as to produce a vertical. component of force. The force applying means may be sufficient for the or each compactor mass to be lifted clear of the soil surface so that transportation thereof can take place without the application of impact blows to the soil surface. The force applying means may be responsive to an automatic sensor operatively associated with the steering mechanism of a traction unit used to tow the compactor mass, or the steering mechanism of the vehicle itself in the case of a self-powered apparatus, the sensor being arranged to cause the force applying means to raise the compactor mass or masses clear of the soil surface in response to a predetermined change in steering direction. The frame may be in the form of a drawn or self-powered carriage, with a resilient linkage for connecting the compactor mass to the carriage. The linkage may comprise a drag link connected rigidly to the axle at one end, a drop link which is pinned at one end to the opposite end of the drag link and at an intermediate point to the carriage, and a spring acting between the carriage and the opposite end of the drop link. The spring is conveniently an hydraulic spring. The apparatus may comprise more than one compactor mass. Thus in one embodiment, there are two spaced apart compactor masses mounted rotatably on the axle, the frame being located between the masses and the force applying means acting on the axle between the masses. BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be described in more detail, by way of example only, with reference to the accompanying drawings. In the drawings: Figure 1 (a) and 1 (b) show a typical three-sided impact compactor mass in the impact position and the energy storage position respectively; Figure 2 shows a graph of surface settlement against compactor blows for different energy values of a typical compactor mass such as that seen in Figures 1(a) and 1(b); and Figure 3 shows a partly sectioned plan view of an impact compactor according to the present invention; Figure 4 shows a side elevation of the impact compactor seen in Figure 3, with one compactor mass removed to reveal working components; and Figure 5 shows a detail of hydraulic componentry. DESCRIPTION OF PREFERRED EMBODIMENTS A description has already been given with reference to Figures 1(a) and 1(b) of a conventional three-sided compactor mass and, with reference to Figure 2, the manner in which the compaction characteristics of a compactor mass varies with the amount of potential energy accumulated and delivered by the mass in use. Figures 3 and 4 illustrate relevant parts of an impact compactor according to the present invention. The compactor has a dual mass system with two identical three-sided compactor masses 10 connected to one another by a common axle 14. As in the case of the compactor mass seen in Figures 1(a) and 1(b), each mass 10 has three salient points 11 each followed, in the order of movement, by a re-entrant portion 12 and a radiused compaction zone or compacting face 13. The impact compactor of Figures 3 and 4 includes a frame or carriage 15 fitted with ground engaging wheels 16. The forward end of the carriage 15 is connected solidly to a wheeled traction unit 17. The axle 14 is connected to the carriage 15 by a resilient linkage which includes a draglink 18, a droplink 19 and an hydraulic spring 20 applying a traction force. One end of the draglink 18 is connected fast to the axle 14. The droplink 19 is pinned to the other end of the draglink 18 at a point 9 and to the carriage 15 at a point 21. The hydraulic traction spring 20 acts between the traction unit 17 and the upper end of the droplink 19. In use, the traction spring 20 applies a traction force to the upper end of the droplink 19. It will be appreciated that the illustrated linkage of components 18, 19 and 20 is a resilient linkage which provides a connection between the axle 14 and the traction unit 17 and hence the carriage 15. The linkage enables the axle 14 to move fore and aft as well as up and down relative to the carriage 15 as the compactor masses 10 rotate on the axle 14 in use. Mounted on the carriage 15 is a device 22 which applies an upward force to the draglink 18 and a corresponding downward reaction force to the carriage. In this particular embodiment, the device 22 is an air spring of a type commonly used in heavy duty vehicles for suspension control and is capable of accommodating fore and aft movement of the axle 14 relative to the carriage 15. Air under pressure is supplied via a flexible hose 23 to the air spring 22 by a compressor 42 mounted in practice on the traction unit 17. A multi-position control valve 24 for the compressor 42 is in practice mounted within reach of the traction unit operator. By operating the valve 24, the operator is able to vary the air pressure in the air spring 22, and accordingly the amount of uplift applied via the drag link to the axle with a corresponding reduction in the impact energy applied to the soil surface by the compactor masses. For example, with an impact roller mass of 10 000kg and a lift height of 0,15m as used in the example described previously, and assuming that the inflation of the air spring 22 produces a component of upward force on the axle 14 of 4 000kg, then the residual downward component of force is (10 - 4) x 9,81 kN - 58,86kN and the energy available per blow reduces to 58,86 x 0,15kJ = 8,83kJ. In other embodiments of the invention other devices, such as an hydraulic cylinder arrangement could be used to apply the required upward force to the axle 14. Instead of applying a variable upward force to the axle 14 it is also within the scope of the invention to provide for a variable downward force, so that a relatively light pair of compactor masses could be used, with the operator having the ability to increase the energy per blow as required. For application of a downward force, an hydraulic ram 26 is pivotally connected to the extremity of a cantilever beam 27 forming an integral part of the traction unit 17. The piston rod of the ram 26 exerts a downward thrust upon the drag link 18 at a pivot point 28. The details of a typical ram 26 are illustrated in Figure 5. As seen in Figure 5, the cylinder 25 of the ram 26 is pivoted to the cantilever beam 27 by a pair of stub shafts 29 protruding from the cylinder casing. The piston rod 30 is connected to a piston 31 which is reciprocable in the cylinder and which is fitted with an annular wear strip 32 that centralises the piston rod 30 relative to the cylinder bore. A pressure seal 43 acts between the cylinder and the piston rod. An aperture 33 passes through the piston so as to allow hydraulic fluid in the cylinder unrestricted access to both sides of the piston. Hydraulic fluid is able to flow in and out of the cylinder 25 through a port 34 connected to the port 37 of an hydraulic accumulator 35. The accumulator 35 is of a generally conventional type and accommodates a volume of inert gas 36 under pressure, typically within a neoprene bladder (not shown). The gas ensures that pressure is maintained in the hydraulic fluid 41 with the result that a nett downward force is exerted on the piston 31 and piston rod 30. A pressure sensitive gauge 38 indicates the hydraulic pressure. Alternatively, the gauge may be calibrated to indicate the downward thrust applied by the piston rod or even to indicate actual energy per blow. Pressure in the hydraulic system can be increased by opening a valve 39, accessible to the operator, to admit hydraulic fluid from a pressure source 40, typically an hydraulic pump. The valve 39 has three positions. By selection of the second, or neutral position, the fluid flow is shut off so that system pressure is maintained constant, and by selection of the third position, fluid is drained back to the reservoir tank 44, thereby reducing the system pressure and hence the thrust exerted by the piston rod 30. SUBSTITUTE SHEET (RULE 26J As the piston 31 moves up and down in the cylinder 25 during rotation of the compactor masses 10, hydraulic fluid is displaced into and out of the accumulator 35 respectively. Because of the small volume of fluid displaced by the piston at each up stroke relative to the gas volume 36 in the accumulator, there is only a small variation in the pressure of the hydraulic fluid. The accumulator thus ensures that there is an almost constant downward force applied to the drag link 18 at the point 28, and hence an almost constant downward thrust on the axle 14. By means of the manual controls accessible to the operator, it is possible at any time during a compacting operation to vary the upward (or downward) force applied to the axle 14, thereby adjusting the energy value of the impact for effective and economic compaction. A further practical benefit which arises from being able to effect a rapid change in energy is that with a reduction of energy by reducing the downward force of the masses on the ground the drawbar pull required for the traction wheels to pull the masses up from a compacting face onto a salient point is correspondingly reduced. Once the masses pass over the top dead centre position traction becomes easier, it being possible to restore the full load of the masses while the impact roller is in motion. By use of this facility of reducing the demand for a high pulling force of the traction wheels at start-up, an economy of tractive power and hence cost can be achieved. The downwardly acting thrust means, in this example provided by the hydraulic system described above, can be provided on its own or in conjunction with the upwardly acting thrust means, in this example provided by the air spring 22. Likewise, the upwardly acting thrust means may be provided on its own. It is possible to provide sufficient lift capacity with the air spring 22 to lift the masses 10 clear of the ground such that the masses are supported by the carriage 15. The lift force provided by the air spring can thus be used to raise the masses clear of the ground for the purposes of transportation of the machine without damaging the road surface by application of compaction blows. Although the specific example described above and illustrated in the drawings has a pair of side by side compactor masses, it will be appreciated that the principles of the invention are applicable to single mass machines as well as machines having more than two masses.

Claims: CLAIMS 1. An impact compaction apparatus for compacting a soil surface, the apparatus comprising a wheeled frame, at least one impact compactor mass connected rotatably to the frame for delivering periodic impact blows to the soil surface when the frame is moved over the surface, and means acting between the frame and the compactor mass for applying a variable vertical force to the impact compactor mass, thereby to vary the blow energy delivered to the soil surface at each impact blow. 2. An impact compaction apparatus according to claim 1 wherein the force applying means includes uplift means for applying a variable upward force to the impact compactor mass. 3. An impact compaction apparatus according to claim 2 wherein the uplift means is operable to apply sufficient upward force to lift the compactor mass clear of the soil surface so that transportation thereof can take place without the application of impact blows to the soil surface. 4. An impact compaction apparatus according to claim 3 wherein the apparatus comprises an automatic sensor which is operatively associated with the steering mechanism of a traction unit used to move the frame over the soil surface and which is sensitive to the direction in which the traction unit is steered, the sensor being arranged to cause to the uplift means to raise the compactor mass clear of the soil surface in response to a predetermined change in steering direction. 5. An impact compactor mass according to claim 2 or claim 3 wherein the uplift means comprises an air spring acting between the frame and the impact compactor mass, and means for charging the air spring with a variable pressure. 6. An impact compactor mass according to any one of claims 2 to 4 wherein the uplift means comprises an hydraulic cylinder. 7. An impact compaction apparatus according to any one of the preceding claims wherein the force applying includes downward loading means for applying a variable downward force to the impact compactor mass. 8. An impact compaction apparatus according to claim 6 wherein the downward loading means comprises an hydraulic cylinder acting between the frame and the impact compactor mass, a gas-charged accumulator for supplying the hydraulic cylinder with hydraulic fluid under pressure, and means for varying the pressure of the hydraulic fluid supplied to the hydraulic cylinder by the accumulator. 9. An impact compaction apparatus according to any one of the preceding claims wherein the frame is in the form of a drawn or self-powered carriage, with a resilient linkage for connecting the compactor mass to the carriage. 10. An impact compaction apparatus according to claim 9 wherein the impact compactor mass is mounted rotatably on an axle and the linkage comprises a drag link connected at one end to the axle and coupled at the other end to a traction unit, and wherein the means which acts between the frame and the impact compactor mass to apply a variable vertical force to the impact compactor mass is arranged to act between the frame and the drag link. 11. An impact compaction apparatus according to claim 10 comprising a pair of impact compactor masses mounted rotatably on the axle.

Inventors: BERRANGE AUBREY RALPH, BERRANGE, AUBREY, RALPH
Applicants: BERRANGE AUBREY R, COMPACTION TECH SOIL LTD, BERRANGE, AUBREY, RALPH, COMPACTION TECHNOLOGY (SOIL) LIMITED
Application year: 1995
Main classification: E02D3
IPC codes: E02D3