GB2125856A - Improved process and apparatus for the dynamic compacting of earth - Google Patents

Abstract

According to the invention, compacting is improved by the fact that the energy of compacting is delivered in the form of successions of double shocks separated by a very short interval of time. Two masses (M1, M2) suspended one below the other and separated by a small distance d which is adjustable for regulating the interval of time separating the two shocks are advantageously used for this purpose. The invention is applicable to the treatment of any type of ground to be compacted. <IMAGE>

Description

SPECIFICATION Improved process and apparatus for the dynamic compacting of earth The present invention relates to improvement in the dynamic compacting of earth.

As progressively more efficient and powerful lifting cranes are coming on the market, so the dynamic compacting of earth has taken on a development of progressively increasing importance.

The basic principle, which has been known for a very long time, consists of stabilizing a plot of ground on which it is intended, for example, to construct infrastructures, by compacting (or ramming or compressing) the ground by means of repeated dynamic shocks.

As this technique has progressively developed, the compacting masses employed and the height from which they are released have also progressively increased.

Both practice and theory teach that for a given result obtained, the main parameter to be considered is the energy measured, for example, in tonnes x metres per surface of ground to be treated.

The best procedure to be adopted will be determined by mechanical requirements, the performance of the material available and practicai considerations, and will differ from case to case. In particular, a given compacting energy may be employed more or less efficiently for achieving stabilization of the treated ground, which may be measured in particular by the degree of compression obtained, The man of the art will therefore plan his operation of compacting the earth according the compacting material available and will on this basis decide in particular the number of blows to strike in each area of ground and the energy of impact to be imparted with each blow.

It is an object of the present invention to optimize even more perfectly the use of this energy of impact so that the same amount of energy will result in a greater degree of compression, which will translate itself in the ground by an increase in compression generally varying from 30% to 100%.

The process of dynamic compacting of earth according to this invention of the type in which a mass released from a given height and falling on the ground is used and the operation is repeated as many times as is necessary, successively over the whole plot of ground to be treated, is characterised in that, in order to improve the amount of compacting obtained for a given amount of energy expended, the said compacting energy is delivered to the ground in the form of a succession of dynamic shocks, for example double shocks, localized in the same place and separated by a very brief interval of time not exceeding a fraction of a second. The time interval separating the two dynamic shocks would normally be in the range of 1/1 000th to 1/1 ooth of a second, the chosen interval being shorter the more resistant the ground.

The two dynamic shocks delivered would in practice have values of the same order of magnitude.

It is found that when this procedure is adopted, delivering to the ground, for example, two successive shocks of a value of 100 tonnes x metres/m2 following upon one another within a time interval of 1/100th of a second, the resuling compression of the ground is normally improved by at least 50% and possibly even much more, depending upon the nature of the ground, compared with the compression achieved when only one dynamic shock of 200 tonnes x metres/m2 is delivered to the same piece of ground. It would appear, therefore, that two dynamic shocks taking place within a very short interval of time are much more efficient than a single dynamic shock equal to the sum of the two aforesaid shocks.

The explanation which may be advanced to accound for this phenomenon is no doubt that if this procedure is adopted, and provided the time interval separating the two shocks is judiciouly chosen, the wave of shearing stress produced by the first shock and the compression wave produced by the second shock may be caused to substantially coincide in the region of ground to be treated at the moment of the second shock since these two waves do not propagate through the ground at the same velocity.

This superimposition of two different types of waves considerably facilitates ground compression, which is made much more efficient by this superimposition of the two actions of compression and shearing in one and the same locality. Whatever the true explanation, it is found that the amount of compression of the ground obtained from a given amount of compacting energy is effectively improved.

In order to obtain the best results, the first dynamic shock should generally be from one third to three times the value of the second dynamic shock, and this first shock would be all the smaller the more resistant the ground is at the surface and the softer it is below the surface, that is to say at the depth of the layer which is to be treated, which is normally several metres below the surface of the earth. The optimum choice of ratio of the dynamic masses employed and of the time interval separating the two shocks may be found, for example, experimentally either on the site or in the laboratory.

The process according to the invention may be carried out, for example, by means of an apparatus comprising, in combination with a load lifting machine such as a crane equipped with at least one cable and a lifting winch, two masses connected together, arranged one above the other with a slight space between the two masses, the first mass being suspended underneath the second mass which is hooked to the said cable. When the two masses are together dropped from the height of the crane, the first mass delivers a first dynamic shock to the ground at the moment of impact.Shortly thereafter, the second mass strikes against the first mass and delivers a second dynamic shock, the time interval separating the two shocks obviously being a function of the distance between the two masses at the moment when they are released and the height of drop, which determines the velocity at which the masses strike the ground and hence the time taken for the second mass to strike against the first. It is found that with such a system, the time interval between the two shocks can easily be regulated to the nearest 1/1 OOth or 1/1 000th of a second by suitably adjusting, for example to the nearest millimetre, the distance between the two masses, which would be of the order of a few centimetres.The apparatus incorporating the two masses is advantageously in the form of a piston slidably mounted in a cylinder, the whole being suspended from the cable of the winch of the crane, by the piston forming the second mass while the cylinder forms the first. The time interval separating the two shocks may be adjusted by a device adjusting the stroke of the piston in the cylinder.

According to another embodiment of this invention, for example, a load lifting machine such as a crane equipped with two cables and two lifting winches could be used. The two masses would then be arranged close to one another, each supported by one of the said cables, and the two winches would be arranged to be controlled simultaneously to release their masses from a slightly different height or with a slight shift in time so that the masses, after being released from the winches, will reach the ground at successive instants separated by the aforesaid time interval found to be the optimum for the treatment of the particular ground under consideration.

The invention will be understood more clearly with the aid of the description given below with reference to the attached drawings, in which Figure 1 is a schematic view of an apparatus for carrying out the process according to the invention; Figure 2 is a schematic view on an enlarged scale of the double mass used for compacting; Figure 3 is a view similar to that of Figure 2 showing another, specially adapted construction which may be used; Figure 4 indicates schematically how the invention is carried out, employing dynamic compacting by the dropping of a mass and explosion of an explosive charge.

Referring first to Figure 1 ,this shows a gantry crange 1 substantially comprising a tractor 2, a mast 3 and a winch 4 for lifting a load 6 above the ground 7 by means of a cable 5.

According to the conventional method of compacting, the mass 6 is dropped on the ground to treat the area around the point of impact 8. In order to obtain the desired compacting of the ground, it is necessary to deliver to the ground an energy measured in tonnes x metres per m2 of ground depending upon the particular type of ground, and this energy may be delivered in one or several blows by ramming the ground at the same point one or several times, depending upon the characteristics of the engine employed which determines the energy of impact which can be delivered with each blow.

According to this invention, the compacting yield obtained for a given quantity of energy used is improved by each impact being converted into a double dynamic shock, the two shocks being separated by a very brief time interval not exceeding a fraction of a second.

To achieve this and according to the embodiment illustrated in Figure 2, the mass 6 which is made to fall on the ground is composed of two masses M1 and M2, respectively, placed one above the other.

The mass M1 is integrally connected with a skirt 9 forming a sort of cylinder having an internal volume 10 inside which the mass M2 forming the piston is displaceable. The two masses are connected together by ties 11 allowing for a certain displacement d of the piston formed by the mass M2 in the cylinder 1 0. The ties 11 may be force fitted into the apertures 12 of the mass M1 while engaging with some clearance in the apertures 1 3 of whe mass M2. The heads 14, 1 5 of the ties prevent separation of the two masses.

The distance d may be adjusted by placing split washers 1 6 under the heads 1 5 of the ties to form spacers.

The mode of operation of such a mass is obvious.

The double mass is lifted by the cable 5, the mass M1 being suspended underneath the mass M2 at the distance ddetermined by the mounting arrangement, in the position illustrated in Figure 2. When the mass is released from the height of the crane, it falls on the ground at a speed which increases progressively according to the law of gravity.

When the double mass reaches the ground, a first dynamic shock D1 is produced by the impact of the mass M1 arriving on the ground. A short interval thereafter, the mass M2 catches up with the mass M1 and the ground receives its second dynamic shock D2. The delay between the two shocks is equal to the time required for the mass M2 to travel the distance dat the velocity which the mass has acquired at the point of fall under consideration.

A simple calculation will shown that for a height of drop of 5 metres, the velocity of impact of the mass with the ground is approximately 1 Om/s whereas when the mass is released from a height of 20 metres, the velocity of impact is of the order of 20m/s. At a height of 5 metres, therefore, a space d of 10 centimetres would correspond to a time interval of 1/1 OOth of a second whereas a space dof 1 centimetre would correspond to a time interval of 1/1 000th of a second. For a value of 20 metres, a space d of 20 cm would correspond to 1/1 OOth of a second while a space of 2 cm would correspond to a time interval of 1/1 000th of a second. The time interval separating the two dynamic shocks D1 and D2 can therefore easily be regulated, for example, in the range of from 1/10th of a second to 1/1 000th of a second to within the nearest 1/1 000th of a second by using spacers 1 6 of the required thickness.

According to the embodiment illustrated in Figure 3, the mass Ml is attached to a wall 9 forming the skirt of a cylinder 10 in which the mass M2 is displaceable. The difference between this arrangement and that of Figure 2 is that the skirt 9 in this case has a head 9a which limits the excursion dof the piston M2 in the cylinder 10. Furthermore, the cylinder 10 has a conical cover 17 perforated by an aperture 18 through which extends a central part 19 of the mass M2 by which the whole arrangement is suspended. The apparatus is thus protected against any dirt or earth thrown up during compacting. The distance d is in this case adjusted by means of nuts 20 on the disc 9a and screws 21 turned into these nuts and bearing against the upper surface of the mass M2 to reduce the length of the distance d.

According to the variation of embodiment indicated in Figure 4, the mass 6 which strikes the ground 7 is a single mass such as that, for example, conventionally used. In order to achieve the effect of a double shock, an explosive range 22 is placed at the locality of impact 8 so that the explosion will be produced by the impact of the mass 6 against the charge. The detonator of the explosive charge 22 will be adjusted so that the optimum time interval At will be obtained between the dynamic shock D1 produced by the mass 6 striking the ground and the dynamic shock D2 produced by explosion of the charge. In general terms, the mass of explosive will be chosen so that the dynamic shock D2 will be of substantially the same order of magnitude as the dynamic shock D1 produced by the fall of the mass 6.

To illustrate the application of the invention, an example of treatment of a piece of ground is given below.

Example: the ground is formed by various kinds of filling material to a depth of O to 8 metres below which are 8 to 10 metres of sand and gravel followed by 10 metres of soiid clay.

The earth is first compacted with a mass of 12 tonnes having a surface area of impact with the ground 4 m2. The energy of impact transmitted locally to the ground in terms of m2 of ground treated amounts to, respectively, 50 tonnes x metres/m2, 100 tonnes x metres/m2 and 200 tonnes x metres/m2.

The same experiment is repeated on similar ground, again with a mass of 1 2 tonnes having the same surface area of impact with the ground but composed of two superimposed masses M1 and M2 each equal to 6 tonnes. The distance between the two masses is adjusted according to the height from which they are dropped so that the time interval between the two shocks will be of the order of 1/1 OOth of a second. The ground is again treated as before by transmitting to it a compacting energy in terms of m2 of the same value as previously.

The results are shown in the table below:

Compression Compression Compression Compression E (Single mass) (two masses) % Improvement 50 Txm/m2 5 cm 7 cm 40% 100 Txm/m2 7.5 cm 10 cm 33% 200Txm/m2 9cm 14cm 55% It is found that for a given amount of energy delivered, the results are improved by 33 to 55%, depending on the individual case.

The improvement yields may even exceed 1 00%, depending upon the nature of the ground.

Numerous variations may be applied to the embodiments which have been illustrated and described. In particular, the value of the masses M1 and M2 may be modified, for example by incorporating internal masses in the apparatus of Figure 3, which masses may be connected either with the mass M1 or with the mass M2 so that the ratio of masses M1, M2 may be varied according to the nature of the ground treated while the overall mass, which is basically a function of the characteristics of the lifting crane, remains constant.

It may also be noted that as the ground is normally treated in several passes and is modified from one pass to the next, the parameters such as the ratio of the two masses and the time interval between the two shocks may advantageously be adjusted on each occasion according to the state of the ground as treatment progresses.

In the examples given above, two successive dynamic shocks are used each time, but the process may also be generalized in that three or a larger number of successive shocks separated by time intervals determined to give the optimum results according to local applications may be given according to the same principle. To produce n shocks in succession, for example, all that is required is to attach n masses one above the other at the required distance apart to fall the crane.

Claims (9)

1. Process for the dynamic compacting of soil, according to which a mass is released from a predetermined height to fall onto the ground and the operation is repeated as many times in succession as necessary over the whole plot of ground to be treated, characterised in that, in order to improve the compacting obtained from a given quantity of energy expended, the compacting energy is delivered in the form of a succession of dynamic shocks applied to the same location and separated by a very brief time interval not exceeding a fraction of a second.

2. Compacting process according to Claim 1, characterised in that the time interval separating the successive dynamic shocks is from 1/1 000th to 1/1 ooth of a second, this interval being shorter the more resistant the ground.

3. Compacting the process according to Claim 1 or Claim 2, characterised in that the dynamic shocks applied have values of the same order of magnitude.

4. Compacting process according to Claim 3, characterised in that the first dynamic shock D1 and the second dynamic shock D2 are chosen so that the relation D2/3 < D1 < 3D2.

5. Compacting process according to Claim 4, characterised in that the first dynamic shock D1 is chosen to be the smaller the more resistant the ground is at the surface and the softer it is at depth.

6. Compacting process according to any of the preceding Claims, characterised in the first dynamic shock D1 used is that of a mass failing on the ground and the second dynamic shock D2 is that of an explosive charge released by the first dynamic shock D1.

7. Apparatus for carrying out the process according to any one of Claims 1 to 5, characterised in that it comprises, in association with a load lifting machine such as a crane equipped with at least one cable and a lifting winch, at least two masses connected together so as to be arranged one above the other with a slight space between the two masses, the first mass being suspended underneath the second which is attached to the said cable.

8. Apparatus according to Claim 7, characterised in that the second mass is formed as a piston slidably mounted within the first mass which is formed as a cylinder.

9. Apparatus according to Claim 7 or Claim 8, characterised in that adjustment means are provided for adjusting the distance separating the two masses.

1 0. Apparatus for carrying out the process according to any one of Claims 1 to 5, characterised in that it comprises, associated with a load lifting machine such as a crane equipped with at least two cables and two lifting winches, at least two masses arranged close together and each supported by one of the said cables, the said winches being capable of being controlled simultaneously to release the masses with a slight difference in height or in time so that after being released by the winches, the said masses reach the ground at successive instants separated by the aforesaid time interval.