GB2288242A

GB2288242A - Ground compaction apparatus and method

Info

Publication number: GB2288242A
Application number: GB9507031A
Authority: GB
Inventors: M C R Davies
Original assignee: University College Cardiff Consultants Ltd; Cardiff University
Current assignee: University College Cardiff Consultants Ltd; Cardiff University
Priority date: 1994-04-05
Filing date: 1995-04-05
Publication date: 1995-10-11
Anticipated expiration: 2015-04-05
Also published as: GB2288242B; GB9406646D0; GB9507031D0

Abstract

In a method and apparatus for compacting ground by repeatedly impacting a mass 18 onto a base plate 10 laid on the ground, on each impact the output signals from a load cell 12 and velocity or acceleration transducers 14, 16 are processed in order to monitor the degree of compaction of the ground. The impacting of one area can then be stopped as soon as the required degree of compaction has been reached, and the apparatus moved on to an adjacent area of the ground. As described the outputs of the transducers 14, 16 are averaged (e.g. to give a signal corresponding to the velocity of the centre of the base plate) and this signal and the output of the load cell 12 are subjected to a fast Fourier transform operation. The transform of the velocity or acceleration signal is then divided by the transformed output of the load cell, and from the mobility curve the processor determines the mass of the vibrating system and the stiffness of the soil influenced by the impact. <IMAGE>

Description

Ground Compaction Apparatus and Method The present invention relates to an apparatus and method for use when compacting soil or other ground.

It is often necessary to compact soil or other ground, for example prior to constructing buildings, roads etc. This compaction can be carried out by repeatedly impacting successive areas of the site with a heavy mass: for example a base plate is laid on the ground and then impacted by repeatedly lifting the heavy mass and then allowing it to fall under gravity onto the base plate. It is known to test each area after compaction and one technique for doing this comprises impacting a small sledgehammer, which is fitted with a force transducer, against a small base plate fitted with a velocity transducer: output signals from the force and velocity transducers of this separate test equipment enable parameters related to the degree of compaction of the soil to be determined.

However, it is time consuming to have to test each compacted area of the site and then if necessary bring back the heavy compacting equipment to each area which requires further impacting.

We have now devised an apparatus and method for use when compacting soil or other ground, which saves time and difficulty relative to the techniques outlined above, and furthermore leads to additional advantages.

Thus, in accordance with this invention, there is provided a method of compacting soil or other ground, comprising repeatedly impacting a mass onto a base plate laid on the ground in order to compact the ground, and for each impact processing output signals provided by a load cell and at least one velocity or acceleration transducer associated with said base plate.

Thus, as compaction proceeds, signals from the load cell and velocity or acceleration transducer are processed to give a quantitative measure of the degree of compaction thus far achieved. The compaction of each area can thus be stopped once a desired degree of compaction has been achieved, and the compaction equipment can be moved on to an adjacent area, successive such areas being compacted in turn until the entire site has been compacted to the required condition. The method thus saves time and difficulty, but also enables the entire site to be compacted to a uniform degree. Moreover, the measurements which are made relate to the full volume of soil influenced by the mass performing the compaction, rather than to the relatively small volume of soil influenced by the separate, light-weight test instrument hitherto used.

Also in accordance with this invention there is provided a compaction apparatus comprising a base plate for lying on the ground, means for repeatedly lifting a mass and then causing or permitting said mass to descend and impact onto said base plate to compact the underlying ground, a load cell and at least one velocity or acceleration transducer associated with said base plate, and a processing unit for processing, on each impact, output signals provided from said load cell and velocity or acceleration transducer.

Typically the mass and means for lifting it are mounted to or carried by a vehicle: typically the base plate is also coupled to the vehicle, so that the vehicle can lift the base plate from one position and then advance and lower the base plate onto the ground at a new position.

An embodiment of this invention will now be described by way of example only and with reference to the accompanying drawing, the single Figure of which is a schematic elevation of an apparatus in accordance with this invention, for use when compacting the soil at a site.

Referring to the drawing, there is shown an apparatus in accordance with this invention for carrying out soil compaction on site. The apparatus comprises a base plate 10 which in use is laid on the ground and impacted a number of times to compact the underlying soil, and is then moved to an adjacent position where the process is repeated, and so on until compaction has been completed over the whole of the required area. The base plate 10 has a load cell 12 on its upper side: typically the load cell may comprise an annular member for example of steel, which is provided with strain gauges for providing an output signal representing the force with which the base plate 10 is impacted via the load cell.

A pair of velocity transducers 14,16 (typically conventional geophones) are also mounted to the base plate 10, at equal and opposite distances from the centre of the base plate. The output signals from the load cell and from the two velocity transducers are passed to a processing unit (not shown).

In use, the base plate 10 is impacted repeatedly by a mass 18 arranged to move vertically in an upright support 20 which is mounted to a vehicle 22. The support 20 includes a mechanism (not shown) for raising the mass 18 and then letting the mass fall under gravity onto the base plate, impacting the latter via the load cell 12: alternatively, the support may include an hydraulic or pneumatic ram for impacting the mass onto the base plate.

The velocity transducers 14,16 provide output signals representing the vertical velocity of the base plate 10 at the respect locations of these transducers: these output signals are averaged in the processing unit, to provide a signal representing the vertical velocity of the centre of the base plate. The processing unit carries out a fast Fourier transform (FFT) on the time-varying average of the velocity transducer outputs, and also on the load cell output at the instant of each impact: the processing unit then divides the FFT of the velocity transducer outputs by the FFT of the load cell output, to determine (in accordance with known principles) the frequency-varying mobility of the vibrating system (of base plate and the underlying volume of soil which has been influenced by the impact).From the mobility curve, and also in accordance with known principles, the processing unit determines the mass of the vibrating system, and the stiffness (or average stiffness) of the volume of soil influenced by the impact. Further, from the thus-determined mass of the vibrating system, the processing unit determines the depth of influence. In the drawing, the volume of soil influenced by each impact is indicated by the dotted line, and the depth of influence is represented by D.

In a modification, the velocity transducers 14,16 are replaced by acceleromets. For each impact, the processing unit carries out a fast Fourier transform on the time-varying load Fell output as previously, and also caries out a fast Four transform on the time-varying average of the accelerometer outputs: the processing unit then divides the FFT of the average of the accelerometer outputs by the FFT of the load cell output, giving the accelerants. Next the processing unit divides the accelerants by the corresponding frequencies to give the frequency-varying mobility of the vibrating system.

From the mobility curve, the processing unit is able to determine the mass of the vibrating system, the stiffness of the volume of soil influenced by the impact, and also the depth of influence, as previously described. By the use of accelerometers in this modification, the possible effects of noise in the velocity transducer outputs (due to ringing of the base plate) are avoided.

It will be appreciated that the apparatus and method which have been described give output data as soil compaction is carried out, enabling the progress of the compaction to be monitored quantitatively. In particular, this avoids the necessity to test each successive area of soil after compaction, and bring the heavy compaction equipment back to impact again areas which these tests indicate as requiring further compaction. This in itself saves considerable time and difficulty and therefor cost. Moreover however, by monitoring the soil condition as compaction progresses, the impacting of each successive area can be stopped as soon as that area has reached a desired quantitative degree of compaction: this in particular enables the whole site to be compacted to a uniform degree. Also, the measurements made by the apparatus relate to the full depth of influence D of the impacting mass, whereas the measurements from a separate test equipment relate only to the relatively small depth of influence of the light-weight impacting head of that test equipment: the measurement provided by the apparatus of this invention are therefore substantially more truly representative of the degree of compaction which is achieved.

Claims

1) A method of compacting soil or other ground, comprising repeatedly impacting a mass onto a base plate laid on the ground in order to compact the ground, and for each impact processing output signals provided by a load cell and at least one velocity or acceleration transducer associated with the base plate.

2) A method as claimed in claim 1, in which the repeated impacting of said base plate, for compacting one area of ground, is stopped when the processed said output signals indicate a given degree of compaction, and the base plate is moved and the repeated impacting of the base plate is resumed, for compacting another area of the ground.

3) A method as claimed in claim 1 or 2, in which said processing of the output signals comprises carrying out a fast Fourier transform on the time-varying output of the velocity transducer, carrying out a fast Fourier transform on the timevarying output of the load cell, and dividing the first of these transforms by the second to determine the mobility of the vibrating system which consists of the base plate and the underlying volume of ground influenced by the impact.

4) A method as claimed in claim 1 or 2, in which said processing of the output signals comprises carrying out a fast Fourier transform on the time-varying output of the acceleration transducer, carrying out a fast Fourier transform on the time-varying output of the load cell, dividing the first of these transforms by the second and dividing the result by the corresponding frequencies to determine the mobility of the vibrating system which consists of the base plate and the underlying volume of ground influenced by the impact.

5) A compaction apparatus comprising a base plate for lying on the ground, means for repeatedly lifting a mass and then causing or permitting said mass to descend and impact onto said base plate to compact the underlying ground, a load cell and at least one velocity or acceleration transducer associated with said base plate, and a processing unit for processing, on each impact, output signals provided from said load cell and from said velocity or acceleration transducer.

6) A compaction apparatus as claimed in claim 5, in which said processing unit is arranged to carry out a fast Fourier transform on the time-varying output of the velocity transducer and a fast Fourier transform on the time-varying output of the load cell, and divide the first of these transforms by the second to determine the mobility of the vibrating system which consists of the base plate and the underlying volume of ground influenced by the impact.

7) A compaction apparatus as claimed in claim 5, in which said processing unit is arranged to carry out a fast Fourier transform on the time-varying output of the acceleration transducer and a fast Fourier transform of the time-varying output of the load cell, divide the first of these transforms by the second and then divide the result by the corresponding frequencies to determine the mobility of the vibrating system which consists of the base plate and the underlying volume of ground influenced by the impact.

8) A method of compacting soil or other ground, the method being substantially as herein described with reference to the accompanying drawings.

9) A compaction apparatus substantially as herein described with reference to the accompanying drawings.