Patent Publication Number: US-2005129467-A1

Title: Drop mass compaction of soil

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      This application is a continuation of International Application No. PCT/IB2003/002578, filed Jul. 1, 2003, which was published in the English language on Jan. 8, 2004, under International Publication No. WO 2004/003301 A1, and the disclosure of which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION  
      This invention relates to methods and apparatus for achieving drop mass soil compaction and for monitoring soil properties.  
      There are numerous applications where it is necessary to compact a relatively small area of soil but where the use of conventional soil compaction machinery, typically employing rollers of one type or another, is inappropriate. One important example is in the compaction of soil adjacent bridge abutments, where limited space makes it impossible to compact with conventional large rollers or other machines. Another example is in the compaction of soil in relatively narrow trenches for pipes, strip foundations or the like. Yet another example is in road maintenance where local failure of a section of a road may have taken place in a relatively small area.  
      Although small vibratory rollers and impactors are available and are widely used in such applications, the level of soil compaction and the depth of compaction influence which can be achieved with such devices is limited. The result is often that undue settlement and or structural failure can take place after a relatively short period of time.  
      It has been proposed to use drop mass soil compaction to achieve compaction of soil in the kind of situations envisaged above. In drop mass compaction, a substantial mass is repeatedly raised and dropped to apply impacts to the soil surface for the purposes of compacting it. For more detail about one known drop mass soil compactor, reference may for instance be made to International Patent Application Publication No. WO 00/28154 A1 (Cook, et al.).  
      Although the principles of drop mass soil compaction are sound, it is felt that greater control over the compaction process is required to enable this type of compaction technology to achieve its full potential.  
     BRIEF SUMMARY OF THE INVENTION  
      According to one aspect of the present invention, there is provided a method of compacting soil. The method includes repeatedly raising a mass above the soil surface and then dropping the mass to apply impacts to the soil surface, from the impacts on the soil surface periodically deriving indications of one or more instantaneous properties of the soil, and controlling the number of subsequent impacts and/or the energy imparted to the soil at each impact and/or the frequency of the impacts, in accordance with the indicated property or properties, in order to achieve in the soil one or more predetermined properties.  
      The instantaneous properties for indications are periodically derived may include, for instance, mechanical properties such as the stiffness or strength of the soil or the bearing capacity thereof. Alternatively, indications may periodically be derived for a property such as total or incremental soil settlement.  
      Preferably, a controller is used to vary one or more of the above parameters automatically to achieve the predetermined mechanical property or properties. The controller may include a programmable logic controller (PLC) which is pre-programmed with one or more predetermined mechanical properties to be attained.  
      In a preferred embodiment of the method, a sensor is associated with the mass to provide the necessary indications. The output of the sensor is fed to the controller. This sensor may be, for instance, an accelerometer which is arranged to monitor the deceleration of the mass at each impact and which is arranged to output signals related to, for instance, the instantaneous stiffness of the soil to the controller.  
      According to another aspect of the invention, there is provided a soil compaction apparatus that includes a drop mass soil compactor. The drop mass soil compactor has a mass and a lift assembly that repeatedly raises the mass above the soil surface and then drops the mass to apply impacts to the soil surface. The apparatus also includes a sensor arranged to provide periodic indications of one or more instantaneous properties of the soil, and a controller responsive to the indications provided by the sensor in order to control the number of subsequent impacts and/or the energy imparted to the soil at each impact and/or the frequency of the impacts, thereby to achieve in the soil one or more predetermined soil properties.  
      As indicated previously, the controller may include a PLC which is pre-programmed with one or more predetermined soil properties which it is desired to achieve and which is arranged to control one or more of the variable parameters listed above.  
      The apparatus of the invention may also include a position monitor, typically a global positioning system (GPS), which is interfaced with the controller to enable the controller to exercise control over the compaction of soil in accordance with a geographical plan.  
      The apparatus of the invention may be vehicle mounted and may be movable from a transportation orientation in which it is carried on the vehicle to an operative position in which it is located alongside the vehicle.  
      According to yet another aspect of the invention, there is provided a method of measuring one or more physical soil properties on a site. The method includes providing on the site a drop mass soil compactor that has a mass and a lift assembly that raises the mass above the soil surface and then drops the mass to apply an impact to the soil surface. The method also includes measuring one or more instantaneous physical properties of the soil from the impact applied to the soil surface by the mass.  
      Still further, the invention provides an apparatus that measures one or more physical soil properties on a site. The apparatus includes a drop mass soil compactor that has a mass and lift assembly that raises the mass above the soil surface and then drops the mass to apply an impact to the soil surface. The apparatus also includes a sensor that measures from the impact applied by the mass, one or more instantaneous physical properties of the soil. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS  
      The foregoing summary, as well as the following detailed description of the invention, will be better understood when read in conjunction with the appended drawings. For the purpose of illustrating the invention, there are shown in the drawings embodiments which are presently preferred. It should be understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown.  
      In the drawings:  
       FIG. 1  shows a side elevational view of a vehicle mounted drop mass soil compaction apparatus according to this invention, the apparatus being at an operative orientation; and  
       FIG. 2  shows a perspective view of the apparatus of  FIG. 1 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      The drawings illustrate a drop mass soil compaction apparatus  10  according to this invention. In this embodiment, the apparatus  10  is mounted on the chassis  12  of a truck  14 , but it will be understood that the apparatus could equally well be mounted on a wheeled trailer chassis drawn by a truck or tractor, or on a track-type self-driven vehicle or trailer.  
      The apparatus  10  includes a lift assembly  23  that raises and drops a compaction mass  28 . The lift assembly  23  includes a frame  16  mounted for side to side traversing movement on a laterally extending traverse beam  18  which is pivoted to the rear end of the truck  14  at a pivot point  20 . The frame  16  includes a pair of upright support members  22  supporting a rearwardly directed suspension member  26 . The rearward end of the member  26  is located vertically above the compaction mass  28  and a hydraulic cylinder  30  acts between the suspension member  26  and the mass as illustrated. Extending vertically from the mass  28  are guide rods  32  which slide in guide cylinders  34  connected to the members  22  by inclined braces  35 .  
      A bracket  36  connected to the support members  22  is engaged in slidable manner with the traverse beam  18  in order to achieve the side to side traversing movement of the frame as mentioned above. The apparatus  10  includes a drive (not shown) for driving the frame  16  from side to side. The drive could take any one of a number of conventional forms. It may, for instance, be a chain and sprocket drive, at least some components of which are mounted within the bracket  36 , and/or the traverse beam  18 .  
      The drop mass soil compactor apparatus  10  is shown at an operative orientation with the mass  28  resting on the ground. The compactor apparatus  10  will operate at this orientation during soil compaction activities. When the compactor apparatus  10  is to be transported from one location to another, hydraulic cylinders  38  acting between lugs  40  extending from the traverse beam  18  and the chassis  12  are contracted to pivot the frame  16  to a prone, generally horizontal orientation on the chassis  12 .  
      In use, soil beneath the mass  28  is compacted by the lift assembly  23  repeatedly lifting the mass  28 , by contracting the cylinder  30 , and then dropping the mass  28  to apply impacts to the soil surface.  
      Attached to the mass  28  is a sensor  42  or specifically an accelerometer  42 , the output of which is fed to a controller such as a programmable logic controller (PLC)  44 . The PLC  44  is shown diagrammatically in  FIG. 1  as a remote component, but it will be understood that in practice the PLC  44  and other ancillary equipment will form an integral part of the apparatus  10  itself. The controller or PLC  44  may include other ancillary control circuitry without departing from the invention.  
      As the mass  28  impacts on the soil, the deceleration which the mass  28  undergoes, as measured by the accelerometer  42 , provides an indication of the instantaneous stiffness of the soil, and this in turn provides a reliable, real time indication of the level of compaction of the soil. In this example, the PLC  44  is pre-programmed for the apparatus  10  to achieve in the soil a predetermined level of soil stiffness. The PLC  44 , in turn, automatically controls various parameters of the compaction operation. For instance, the PLC  44  may control the impact energy applied to the soil at each impact. This is achieved by varying the height to which the mass  28  is raised prior to each blow, and accordingly, the potential energy which is stored and which is available for delivery to the soil, by controlling the hydraulic operation of the cylinder  30  and/or by means of appropriate mass position sensors to sense the position of the mass.  
      By way of example, the impact energy which is applied may be controlled by the PLC  44  to have the same value for a number of impacts, then a different value for a succeeding series of impacts, and so on. Alternatively, the impact energy may be varied from impact to impact or the same energy level may be maintained throughout the compaction operation.  
      The PLC  44  may also control, in accordance with its program, the number of blows applied to the soil. As yet another possibility, the PLC  44  may control the frequency of the impacts, in this case by varying the duration of each impact cycle and/or by varying the time lapse between cycles. Although not specifically mentioned, other variable parameters may also be controlled instantaneously by the PLC  44 .  
      In the drawings, the numeral  50  indicates a flexible cable tray which carries signal transmitting wiring and/or hydraulic hoses for the cylinder  30  and its ancillary equipment such as hydraulic pumps and so on. It will also be understood that in practice a hydraulic power pack (not illustrated) will be mounted on the chassis  12 .  
      In practice, the soil compaction apparatus  10  described above may be used to compact soil at various locations on a site, or to compact the entire site. It will be understood that the vehicle  14  can be driven from one location to another on the site to enable the soil compaction apparatus to be used to compact different regions of the site. At each stationary location of the vehicle  14 , the apparatus  10  can be used to compact a limited strip of soil, this being permitted by the ability of the frame  16  to traverse from side to side on the traverse beam  18  and the maneuverability of the vehicle  14 .  
      The soil compaction apparatus  10  described above can also be interfaced with a positioning system, typically a global positioning system (GPS), so that indications of soil stiffness can be correlated with geographical position. This in turn enables a site plan to be derived which gives real time information related to the levels of soil compaction at different locations on the site. It will be understood that integrated information regarding geographical position and level of soil compaction can be stored digitally and/or represented graphically to provide a substantially complete analysis of the state of soil compaction across a site or selected regions thereof.  
      Although mention has been made of compacting soil regions over a site, the principles of the invention are equally applicable to compaction of single, small areas where a specific level of compaction is required. A typical example here would be soil compaction adjacent a fixed structure such as a bridge abutment or building.  
      Also, although specific mention has been made of monitoring soil stiffness and of varying the operating parameters to achieve a desired level of soil stiffness, it is within the scope of the invention for the apparatus  10  to monitor a variety of other soil properties and to control the operating parameters to achieve one or more specific, desired properties. The apparatus  10  could be arranged to monitor any one or more of a number of other mechanical soil properties other than stiffness, for instance soil strength and/or bearing capacity, and to control the operating parameters to achieve desired properties. Still further the apparatus could be arranged to monitor a soil property such as total soil settlement at each point in time or incremental soil settlement, i.e., the amount of soil settlement in a given period of time or as a result of one or more impact blows.  
      Instead of an accelerometer  42  to monitor any particular soil property, in the above example the soil stiffness, any other suitable form of sensor  42  could also be used. Other viable forms of sensor  42  include a velocity sensor to measure the velocity of the mass during impact, a displacement sensor such as a displacement transducer to measure soil settlement or a force or pressure transducer to measure the force or pressure applied to the soil surface by the mass at impact.  
      As mentioned previously, the principles of the invention can be used not only to achieve and control soil compaction, but also to measure instantaneous soil properties on a site. In this role, the apparatus  10  described above may be arranged to raise and drop the mass  28  only once at each location on the site where a measurement of instantaneous soil properties is required. The monitoring equipment, including the accelerometer  42  and/or additional transducers described above, are then arranged to provide, at each such location, a measurement of a selected soil property, such as stiffness, settlement and so on. As in the case of the apparatus described above, the apparatus in this role may be interfaced with a geographical positioning system such as a GPS, thereby to provide soil property measurements for different site locations. The information obtained in this way may, with appropriate signal processing equipment and associated software, be presented in the form of a map, contoured or otherwise, or in tabular form. This application may for instance be employed by soils engineers to assist them in performing site certification functions.  
      It will be appreciated by those skilled in the art that changes could be made to the embodiments described above without departing from the broad inventive concept thereof. It is understood, therefore, that this invention is not limited to the particular embodiments disclosed, but it is intended to cover modifications within the spirit and scope of the present invention as defined by the appended claims.