Abstract:
A linear driver is provided and particularly, but not exclusively, to a linear driver for use in the field of civil engineering or construction, for example in the formation of piles or piers as foundations. The driver includes a weighted sleeve slidable relative a driving member, and a valve arrangement to control the linear movement of the weighted sleeve relative to the driving member. Closing the valve arrangement serves to stop movement of the weighted sleeve and transfer momentum to the driving member without physical impact between the weighted sleeve and the driving member.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to PCT Application No. PCT/GB2015/052389, having a filing date of Aug. 17, 2015, based off of GB Application No. 1414554.4 having a filing date of Aug. 15, 2014, the entire contents of which are hereby incorporated by reference. 
     
    
     FIELD OF TECHNOLOGY 
       [0002]    The following relates to a linear driver and particularly, but not exclusively, to a linear driver for use in the field of civil engineering or construction, for example in the formation of piles or piers as foundations. 
       BACKGROUND 
       [0003]    It is conventional to provide a foundation as the lowest supporting layer of a structure to be built or else to shore up the ground. In general, foundations can be split into two different categories, so-called shallow foundations and deep foundations, with the only real difference between the two categories of foundation being the depth to which they are embedded in the ground. 
         [0004]    A well-known form of deep foundation is a so-called pile, or pier, which is typically an elongate structure that is located in the ground. Piles can either be prefabricated, in which case they are generally driven into the ground by a pile driver, or they can be cast-in-situ. There are many existing techniques for forming cast-in-situ piles, very often involving the formation of a hole in the ground, and filling such a hole with cementitious material. Upon setting, the cementitious material solidifies to provide the desired pile. 
         [0005]    One of the challenges faced during the provision of cast-in-situ piles is the formation of the hole in which the pile necessarily sits. Traditional methods include using a rotation auger, or hammering or vibrating an upper end of a tube or mandrel into the ground, and then either removing the tube or mandrel before or during the operation of filling the hole with concrete, or leaving the tube or mandrel in position to form part of the pile. There are, however, several disadvantages associated with such methods. 
         [0006]    In order to reach the depth required in the ground, a large amount of energy needs to be employed. Furthermore, the length of a pile former used to reach the required depth in the ground often necessitates the use of large support structures and associated equipment, which can be expensive, time consuming to mobilise, and which may pose a significant risk to the safety of workers involved in the formation of the required pile. These disadvantages in general make the provision of cast-in-situ piles unsuitable for a number of applications, and thus cast-in-situ piles are generally used only in large scale, or large load-bearing, construction projects. 
         [0007]    A further disadvantage arising from the need to impart a large amount of energy to the pile formers is the large amount of noise and disruption that pile driving generally creates. 
         [0008]    A typical percussive pile driver comprises a mechanical or hydraulic actuation system for repeatedly raising a weighted impact member, or hammer, which is then allowed to fall under the influence of gravity onto the top of a pile former or pile to drive it into the ground. Each impact generates a large amount of noise, which is particularly undesirable if construction is taking place in already built-up areas, eg close to existing residences or office buildings. 
         [0009]    An alternative driving method which avoids such impacts is a so called ‘diesel hammer’, where the driving is achieved by a weight which acts as a piston in what is effectively a large single cylinder two-stroke diesel engine. The weight is initially raised by mechanical means, drawing air into a cylinder formed between the piston and the head of a pile former. Diesel fuel is then injected into the cylinder and the weight is released, compressing the fuel-air mixture to the point where combustion takes place. The combustion drives the pile former downwards, and simultaneously drives the weight upwards drawing more air into the cylinder. The cycle then repeats, driving the pile further into the ground. Diesel hammers thus avoid the physical impacts between the weight and the pile former, but the combustion events mean that they are typically even louder in operation than percussive hammers, so do not overcome this shortcoming. 
         [0010]    One approach to mitigating noise is to use a vibratory pile driver containing a system of counter-rotating eccentric weights designed in such a way that horizontal vibrations cancel out, while vertical vibrations are transmitted into the pile. One major drawback with vibratory pile driving is that soil conditions and the installation procedure can greatly affect the bearing capacity of vibrated piles, so the method is only suitable for use with certain soil types, and even then the bearing capacity of vibratory driven piles is difficult to predict with any degree of certainty. In addition, the intensity of ground vibrations generated can be problematic, and it is extremely important to avoid resonance in the ground or at adjacent structures or structural elements. The method is also relatively costly as it requires special equipment. 
       SUMMARY 
       [0011]    An aspect relates to an improved driving method, suitable for use in a pile formation system, which overcomes or substantially mitigates the aforementioned and/or other disadvantages associated with the prior art. 
         [0012]    The linear driver comprises a driving member, a weighted sleeve slidable relative to the driving member, and a valve arrangement connected to the driving member, wherein the valve arrangement controls fluid pressure in a region between the driving member and the weighted sleeve to linearly move the weighted sleeve in first and second directions relative to the driving member. Closing the valve arrangement during use stops movement of the weighted sleeve in said second direction such that the momentum of the weighted sleeve is transferred to the driving member without physical impact between the weighted sleeve and the driving member. 
         [0013]    When used as a pile driver, embodiments of the invention are oriented vertically and the driving member is engaged with a pile former. The weighted sleeve is then raised, lowered and stopped relative to the driving member using controlled pneumatic or hydraulic pressure. Using the valve arrangement and fluid pressure to stop the sleeve transfers the momentum of the weighted sleeve to the driving member without any physical impact. The valve, seals and hydraulic fluid take the ‘impact’, such that the loud noise (and vibration) of a typical hydraulic hammer is avoided. 
         [0014]    The weighted sleeve may be internal or external to the driving member. 
         [0015]    Preferably, the valve arrangement is located in the region between the driving member and the weighted sleeve. The valve arrangement can thereby divide this region into two separate regions or chambers, and selectively allow flow between the chambers via a relatively short flow path. 
         [0016]    For ease of assembly, the driving member may comprise first and second parts which are connected, for example by screw threads, to opposite sides of the valve arrangement. The first and second parts of the driving member may have substantially the same external dimensions, or may have different external dimensions to provide a simple means of providing two different external dimensions/diameters on the driving member. 
         [0017]    The driver may further comprise attachment means, such as a clamp or latch, for attaching the driver to a pile former or similar elongate article. The driving member may be a tube, and may surround the pile former or elongate article in use. Alternatively, driving member may be a solid rod or similar, possibly with a driving flange, for use as an end drive hammer/driver. The attachment means may provide selective engagement with the pile former. 
         [0018]    The driver may further comprise a housing, surrounding and slidably connected to the driving member, which comprises means for attaching the driver to a support structure. Due to the reduced noise generated by the driver of embodiments of the present invention, the housing need not be acoustically sealed, and may be an open sided housing such as a cage to minimise weight of the driver. 
         [0019]    The driver may further comprise an actuator to provide a signal to close the valve arrangement before the weighted sleeve impacts the driving member. The actuator may comprise an actuator rod which is depressed by the weighted sleeve as it moves in the second direction. Engagement of the sleeve with the actuator rod shortly before impact with the elements of the driving member would provide a simple means for ensuring correct timing of the valve closure. 
         [0020]    The driver may also comprise an adjustable valve for controlling the fluid pressure applied during operation and/or one or more sensors to monitor fluid pressures during operation. 
         [0021]    Embodiments of the invention also provides a driving system for a piling operation. The system incorporates a linear driver as previously described. 
         [0022]    In the driving system, a mast or similar support is provided for supporting the driver in a substantially vertical orientation. A lower end of the mast may be held in contact with the ground during use of the driving system. 
         [0023]    An adjustable carrier may be provided, mounted between the mast and the driver, to permit vertical movement of the driver relative to the mast, and movement of the driver relative to the mast may be controlled by the adjustable carrier. The adjustable carrier may comprise, for example, a belt or chain and pulley system or a hydraulic actuator/cylinder. 
         [0024]    The mast may be mounted to a purpose built rig or, due to the weight savings of embodiments of the present invention, to a number of alternative carriers such as the arm of a tracked excavator. 
         [0025]    Finally, embodiments of the invention provides a method for testing soil properties prior to a piling operation as defined by the appended claim  27 . The method uses a particular embodiment of the linear driver previously described. 
         [0026]    The testing method comprises mounting a driver with at least one sensor to a support, repeatedly driving an elongate member into the ground to be tested using the driver, and monitoring and recording the pressure readings during the driving operation. 
         [0027]    Any of the optional features described in relation to any single aspect of embodiments of the invention may be applied to any other aspect of embodiments of the invention. 
     
    
     
       BRIEF DESCRIPTION 
         [0028]    Some of the embodiments will be described in detail, with reference to the following figures, wherein like designations denote like members, wherein: 
           [0029]      FIG. 1  is a schematic view of a pile driver according to a first embodiments of the present invention; 
           [0030]      FIG. 2  is a schematic view of an embodiment of a valve arrangement from the pile driver of  FIG. 1 ; 
           [0031]      FIG. 3  is a cross-section of an embodiment of the valve arrangement shown in  FIG. 2 ; 
           [0032]      FIG. 4  depicts a schematic view of an embodiment of the pile driver and the valve arrangement at a stage of operation; 
           [0033]      FIG. 5  depicts a schematic view of an embodiment of the pile driver and the valve arrangement at another stage of operation; 
           [0034]      FIG. 6  depicts a schematic view of an embodiment of the pile driver and the valve arrangement at another stage of operation; 
           [0035]      FIG. 7  depicts a schematic view of an embodiment of the pile driver and the valve arrangement at another stage of operation; 
           [0036]      FIG. 8  depicts a schematic view of an embodiment of the pile driver and the valve arrangement at another stage of operation; 
           [0037]      FIG. 9  depicts a schematic view of an embodiment of the pile driver and the valve arrangement at another stage of operation; 
           [0038]      FIG. 10  depicts a schematic view of an embodiment of the pile driver and the valve arrangement at another stage of operation; 
           [0039]      FIG. 11  depicts a schematic view of an embodiment of the pile driver and the valve arrangement at another stage of operation; 
           [0040]      FIG. 12  is a side view of a system according to an embodiment of the invention when supported in use; 
           [0041]      FIG. 13  depicts a first side view of an embodiment of the system driving a pile former into the ground; 
           [0042]      FIG. 14  depicts a second side view of an embodiment of the system driving a pile former into the ground; 
           [0043]      FIG. 15A  shows an alternative driver according to embodiments of the present invention; 
           [0044]      FIG. 15B  shows an alternative driver according to embodiments of the present invention; 
           [0045]      FIG. 16A  shows an actuator for use in embodiments of the present invention; 
           [0046]      FIG. 16 b    shows an actuator for use in embodiments of the present invention; 
           [0047]      FIG. 17  depicts a schematic view showing an alternative driver and valve arrangement according to embodiments of the invention in operation; 
           [0048]      FIG. 18  depicts a schematic view showing an alternative driver and valve arrangement according to embodiments of the invention in operation; 
           [0049]      FIG. 19  depicts a schematic view showing an alternative driver and valve arrangement according to embodiments of the invention in operation; 
           [0050]      FIG. 20  depicts a schematic view showing an alternative driver and valve arrangement according to embodiments of the invention in operation; 
           [0051]      FIG. 21  depicts a schematic view showing an alternative driver and valve arrangement according to embodiments of the invention in operation; 
           [0052]      FIG. 22  is a graph indicative of the force transferred to a pile by a conventional hammer/driver during use; 
           [0053]      FIG. 23  is a graph indicative of the force transferred to a pile by a driver according to embodiments of the present invention; and 
           [0054]      FIG. 24  is a graph indicative of the force transferred to a pile by a driver according to embodiments of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0055]    A first embodiment of a pile driver according to the present invention, in the form of a pile driver  1 , is shown in  FIG. 1 . 
         [0056]    The pile driver  1  comprises a driving tube  2  which is shown located around a pile former  4  and attached thereto by a clamp  6  at its lower end. A cage  8  is slidably mounted to the driving tube and provides a means for attaching the pile driver to a mast  10  or other supporting fixture. Typically, piling hammers have to incorporate a housing that is fully sealed to attenuate airborne noise. Such housings are heavy, which increases the overall weight of the hammer. The quieter operation of the driver according to embodiments of the invention avoids the need for such housings, so that a simpler, lighter housing, for example a cage type housing, can be used. 
         [0057]    A weighted sleeve  12  is located around the driving tube  2  and within the cage  8 . Both end portions  14 , 16  of the weighted sleeve  12  are in sealing contact with an outer surface of the driving tube  2 , while a length of the sleeve  12  between these portions is spaced from the driving tube  2  to define an annular space  18  for receiving hydraulic fluid. The weighted sleeve  12  comprises a hydraulic cylinder body with a separate heavy collar which is fastened to a flange on the cylinder body. This allows the weight of the sleeve  12  to be adjusted as required by substitution of the collar. 
         [0058]    A valve arrangement  20  is located at approximately the mid-point of the driving tube. The pile driver  1  is assembled such that the valve arrangement  20  is received in the annular space  18  provided between the weighted sleeve  12  and the driving tube  2 . In this regard, the lower end portion  16  of the weighted sleeve  12  is shown as a separate component to allow assembly of the weighted sleeve  12  around the valve arrangement  20 . 
         [0059]    As shown in  FIG. 1 , the weighted sleeve  12  is at its lowermost position such that the valve arrangement  20  is at the upper end of the annular space  18 . However, it will be appreciated that as the weighted sleeve  12  moves relative to the driving tube  2  the valve arrangement will effectively divide annular space  18  into upper and lower chambers. 
         [0060]    The pile former  4  is shown as an elongate ‘I’ section beam with a number of apertures  40  therein. In certain embodiments, the apertures may form a convenient means of attaching the driving tube  2  to the pile former  4  such that substantially all physical impact between components, and thus all associated noise, can be avoided. There will, inevitably, be some physical contact due to mechanical clearances etc, but all avoidable contact is minimised by embodiments of the present invention. 
         [0061]    The cage  8  of the pile driver  1  is attached to the mast  10  by two brackets  50 . The brackets  50  are connected to the mast  10  in such a way that upwards movement of the cage  8  relative to the mast  10  is at least selectively prevented, while downwards movement of the cage  8  relative to the mast  10  is permitted. For example, the brackets  50  may incorporate a ratchet or similar, or may be mounted to a carrier, such as a chain or belt, which is selectively movable relative to the mast  10 . The cage  8  would be rigidly connected to such a drive mechanism, permitting the pile driver  1  to be driven up and down the mast. When not in active use the assembly can be locked in position to prevent any unexpected movement. In a mechanical system this could be achieved by a mechanical brake, and if a hydraulic actuator is fitted, then hydraulic lock valves should be fitted. 
         [0062]    The valve arrangement  20  is shown in greater detail in  FIGS. 2 and 3 . The valve arrangement  20  comprises two galleries in the body of the valve arrangement  20 . The two galleries effectively take the form of pipes, which for ease of reference will be referred to as a return pipe  22  and a feed pipe  24 , each connected to a fluid line  26 , 28  from a three position control valve  30 . The position of the control valve  30  is set by pilot operation valve feeds  31 , 33 . The control valve  30  is shown in  FIG. 2  in a central ‘float’ position, with no pressure applied to either the feed or return pipes  22 , 24 . The return pipe  22  leads to an upper return port  32  and a lower return port  36 , and a first valve spool  42  is provided to selectively open or close the ports  32 , 36 . Similarly, the feed pipe  24  leads to upper and lower feed ports  34 , 38 , and a second valve spool  44  is provided to open or close these ports  34 , 38 . Both valve spools  42 , 44  are shown in their neutral, central, position in  FIG. 2  with all four ports  32 , 34 , 36 , 38  open. Further pilot feeds  43 , 45  are employed within the valve arrangement  20  to move the valve spools  42 , 44  as required. 
         [0063]    For ease of manufacture, the valve arrangement  20  is formed as a standalone component and is assembled into the driving tube  2  as follows. The driving tube  2  is provided in two sections  2 A, 2 B as indicated in  FIG. 2 . The valve arrangement  20  is joined to the upper section  2 A of the driving tube  2  in the region indicated at  46  in  FIG. 2  via an external thread on the valve arrangement  20  and a corresponding internal thread on the upper section  2 A of the driving tube  2 . The lower section  2 B of the driving tube  2  is similarly joined to the valve arrangement  20  in the region indicated at  48 . 
         [0064]      FIG. 2  shows the weighted sleeve  12  in a raised position compared to that shown in  FIG. 1 , so that the annular space  18  is divided into an upper chamber  18 A and a lower chamber  18 B by the valve arrangement  20 . The ends  14 , 16  of the weighted sleeve  12  are not shown in  FIG. 2 , and the actual weight is also omitted for clarity. 
         [0065]    Although not clearly shown in  FIG. 2 , the outside diameter of the upper section  2 A of the driving tube  2  is larger than the outside diameter of the lower section  2 B. The internal diameter of the weighted sleeve  12  is constant, with the result that the cross-sectional area of the upper annular chamber  18 A is smaller than that of the lower chamber  18 B. The reasons for this will be explained later, but the use of separate upper and lower sections  2 A, 2 B simplifies the inclusion of two different outer diameters on the driving tube  2  to achieve this result. In the illustrated example, the outside diameter of the upper section  2 A is 212 mm and the outer diameter of the lower section  2 B is 211 mm, while the inside diameter of the weighted sleeve  12  is 225 mm, so the difference in area between the upper and lower chambers  18 A, 18 B is relatively small. It will be readily appreciated, however, that alternative dimensions could be used and/or that a larger difference could easily be achieved through the provision of an upper section  2 A with a larger outer diameter and/or a lower section  2 B with a smaller diameter. 
         [0066]      FIG. 3  shows a cross-section taken through the valve arrangement  20 , the lower section  2 B of driving tube  2  and the weighted sleeve  12  of  FIG. 2 . Again, the weight is omitted for clarity.  FIG. 3  also shows the ‘I’ section pile former  4  around which the valve assembly is positioned in use. The feed and return pipes  22 , 24  of the valve arrangement  20  are accommodated in the spaces between the web and the flanges of the ‘I’ section pile former  4  (it should be noted that the views of  FIGS. 2 and 3  are taken at 90° to that of  FIG. 1 ). This helps to minimise the outside diameter of the apparatus, although different arrangements would be possible for differently shaped pile formers. 
         [0067]    Operation of the pile driver  1  will now be described with reference to  FIGS. 4 to 11 . 
         [0068]      FIG. 4  shows the pile driver  1 , attached by its clamp  6  to a pile former  4 , with the weighted sleeve  12  still resting in its lowermost position as in  FIG. 1 , and the valve arrangement  2  at an upper end of the annular space  18 . 
         [0069]    The pile former  4  is first pushed into the ground by forcing the pile driver  1  vertically downwards. It will be understood that any upwards movement of the cage  8  relative to the mast  10  will be resisted by the brackets  50  and/or the carrier such that the entire pile driver  1  will move downwards as a unit. The pile former  4  is forced into the ground in this way until the ground resistance equals the downward force applied to effectively ‘set’ the pile former  4  prior to operation of the pile driver  1 . 
         [0070]    Once the pile former  4  is set, the weighted sleeve  12  is raised from its rest position. As shown in  FIG. 5 , the control valve  30  is actuated to apply a low pressure to the feed pipe  24 , and the second valve spool  44  has been moved to close the lower feed port  38  while leaving the upper feed port  34  open. Simultaneously, the first valve spool  42  has been moved to close the upper return port  32  while leaving the lower return port  36  open. An upper port  34  of the valve arrangement  20  is therefore pressurised while a lower port  36  is connected to tank, which will force the weighted sleeve  12  to rise relative to the driving tube  2 . 
         [0071]    This lifting operation generates a reaction that could force the driving tube  2  and pile former  4  downwards. Setting the pile former  4  as outlined above provides a reaction force which prevents this uncontrolled movement and ensures a known position prior to the driving operation. This is important for calibration and monitoring of the driving operation. 
         [0072]    The weighted sleeve  12  is raised to a position approximately 25 mm below its fully raised position within the cage  8 , and end damping is then applied to the lower port  36  to retard movement of the weighted sleeve  12  and prevent hard impact with the lower face of the valve arrangement  20 . This prevents any impact damage and reduces noise emitted from the driver. 
         [0073]    After damping the weighted sleeve  12  is taken to its fully raised position and sensors check when the sleeve  12  has come to rest. This monitoring of the fully raised position before the weight is fired downwards is important to ensure that a desired stroke length of the weighted sleeve is reliably and repeatably applied. It should be appreciated that both the start of the damping and the fully raised position of the weighted sleeve  12  can be adjusted and controlled hydraulically if desired to alter the stroke length and/or the size of the damping region. 
         [0074]      FIG. 6  shows the weighted sleeve  12  at its fully raised position. The annular space  18 , full of hydraulic fluid, can now be seen above the valve arrangement  20 . The relatively small cross-sectional area of the annular space  18  in the illustrated example means that relatively little hydraulic fluid must be moved in order to raise the weighted sleeve from the position shown in  FIG. 4  to the position shown in  FIG. 6 . For a 100 mm section pile former  4 , typically less than one litre of hydraulic fluid is required. Furthermore, the small size and central location of the valve arrangement  20  provides a short path for the transfer of fluid from the lower chamber  18 B to the upper chamber  18 A. The cross-sectional area is dictated by the hydraulic forces necessary to raise and lower the weighted sleeve  12  with maximum efficiency and reduce flow rates through the valves and pipework to minimize back pressure and efficiency losses. 
         [0075]    From the position shown in  FIG. 6 , pressure is applied to both sides of the valve arrangement  20 , as illustrated in  FIG. 7 . The control valve  30  is shown moved to a third position applying pressure to both the feed pipe  24  and the return pipe  22 . The pressure applied is higher than that applied to raise the weighted sleeve  12  as described in relation to  FIGS. 4 and 5 . Both the first and second valve spools  42 , 44  are centrally located so that all ports  32 , 34 , 36 , 38  of the valve arrangement are open. Equal pressure is therefore applied above and below the valve arrangement  20 , and free flow through the valve arrangement  20  is permitted. 
         [0076]      FIG. 8  shows the weighted sleeve  12  moving downwards from its fully raised position. An upper annular chamber  18 A is shown separated from a lower annular chamber  18 B by the valve arrangement  20 . As described previously, the cross-sectional area of the upper chamber  18 A is smaller than that of the lower annular chamber  18 B. Accordingly, the equal pressure applied to both chambers  18 A, 18 B creates a force imbalance that acts along with gravity to drive the weighted sleeve  12  downwards. During this driving step the valve arrangement  20  remains open as shown in  FIG. 9 . 
         [0077]    The force applied to the sleeve  12  is dependent both on the relative areas of the upper and lower chambers  18 A, 18 B and on the pressure applied during the driving step. Therefore, for a given apparatus the force can still be varied by increasing or reducing the pressure applied using an adjustable ‘force control’ valve  52 . In the illustrated example the additional force is sufficient to cause the sleeve  12  to accelerate at around 15 ms −2 , to a velocity of around 4.5 ms −1 . 
         [0078]    The weighted sleeve  12  is driven downwards until it reaches a position about 25 mm from impact with the upper face of the valve arrangement  20 , as shown in  FIG. 10 . Depending on the particular operational requirements, the set distance may be greater or less than 25 mm. At this point, a predetermined induced back pressure is applied between the weighted sleeve  12  and the valve arrangement  20  in the upper chamber  18 A. 
         [0079]      FIG. 11  shows the valve spools  42 , 44  in position to apply the back pressure. The first valve spool  42  is moved to close the upper return port  32 , thereby cutting off the return path for fluid from the upper chamber  18 A. Fluid trying to exit the upper chamber via the open upper feed port  34  is blocked from flowing back to source by the non-return valve  53 , and is forced instead towards the force control valve  52 . 
         [0080]    The blocking of return flows effectively closes off the upper chamber  18 A, and the pressure of the fluid remaining in this region resists further downward movement of the weighted sleeve  12 . 
         [0081]    The applied back pressure stops the movement of the weighted sleeve  12  before it contacts the valve arrangement  20 . As movement of the weighted sleeve  12  is stopped, its momentum is transferred to the driving tube  2  and thereby to the pile former  4  to drive the pile former into the ground. The transfer of momentum occurs without any physical contact between components of the pile driver  1 , with the ‘impact’ instead being taken by the hydraulic fluid and seals within the valve arrangement  20 . 
         [0082]    Assuming the ground resistance is less than the induced force applied to the driving tube  2 , then all the force will be transferred to the pile former  4 , moving it downwards until the ground resistance equals the applied force. A ‘push’ will be applied throughout the driving cycle rather than relying on a sudden impact blow. 
         [0083]    By virtue of the brackets  50  allowing downward movement of the cage  8  relative to the mast, the cage is free to move downwards as the driving tube  2  is driven downwards. If the ground resistance is greater than the applied force, or at the point where the forces are balanced during normal operation, hydraulic fluid will be forced through the force control valve  52  which acts as a relief valve. The relief part of the force control valve  52  will be set to a particular pressure rating, such that fluid will pass through the valve  52  only if the back pressure exceeds a certain predetermined value, chosen based on numerous criteria relating to the particular driving process, and limited by the rating of the seals and other hydraulic components. Once the back pressure drops below this value, the system will again be effectively ‘locked’ such that force is again transferred to the driving tube  2 . In the case of extremely hard ground, the hydraulic fluid will be forced through the force control valve  52  until exhausted. Movement of the weighted sleeve  12  into contact with the valve assembly is thereby damped, avoiding the noise and potential damage associated with a sudden impact. 
         [0084]    In some circumstances, such as in very soft ground, where a void exists below a hard crust, or in circumstances where the pile former has not been correctly ‘set’, the driving tube  2  may move downwards more quickly than the cage  8 . In these circumstances, an upper stop  54  connected to the driving tube  2  will engage with an upper surface  56  of the cage  8  to pull down the cage until the free movement of the cage  8  overtakes the driving tube  2  and returns it to the rest position. In the illustrated example contact between the stop  54  and the upper surface  46  of the cage  8  occurs after 160 mm of relative movement between the driving tube  2  and the cage  8 . 
         [0085]    This failsafe is important in ensuring that embodiments of the invention continues to function in the event of ‘runaway’ of the driving tube  2  as set out above. However, relying on this is undesirable since the impacts between the stop  54  and the cage  8  generate precisely the type of noise which embodiments of the invention seeks to avoid, and can also cause damage to the cage  8  and the pile driver  1  as a whole. 
         [0086]    The need to rely on the failsafe can be largely avoided by ensuring the pile former  4  is correctly set before operation. However, in some circumstances the ground conditions are such that very little resistance is provided to the pile former  4 , and in such circumstances ‘runaway’ can still occur even if the pile former  4  is set correctly. In these circumstances it is possible to adjust the back pressure applied and increase the size of the damped impact zone using the adjustable valve  52 . This results in a longer and softer hit or push being applied to the driving tube  2 , reducing the risk of runaway. The same adjustment may also be used to for the first few blows of any operation to give an indication of the ground strength and resistance. 
         [0087]    As described above, the valve arrangement  20  of embodiments of the invention is designed to transfer the force/momentum of the sleeve to the driving tube  2  to avoid physical contact between metal components and reduce, or eliminate, the associated noise. The hydraulic system also allows all variables, eg speed, stroke length, damping etc. to be readily controlled/adjusted as required, even during operation of the pile driver  1 , and by incorporating appropriate sensors the pressures throughout operation can be monitored and recorded to provide a record of the energy imposed. This allows properties of the ground to be demined and recorded during the pile driving operation. 
         [0088]    Indeed, the simple control, measuring and recording of forces applied using embodiments of the invention permit use of the device as a standalone investigation or analysis tool, for example for use in cone penetration testing. This beneficially allows a single apparatus to be used to first test the properties of the ground at a worksite and then subsequently to undertake the piling operation. 
         [0089]    The relatively small size and weight of the pile driver  1 , and the relatively small volume of hydraulic fluid required for its operation, allows the mast to be fitted onto a quick hitch of a 9 to 13 tonne tracked excavator rather than requiring a large dedicated piling rig.  FIG. 12  shows this arrangement, with the pile driver  1  mounted on a mast  10  which is attached to the arm  56  of such a vehicle  60 . Investigations have revealed that a JCB JS130 will safely lift 1.4 tonnes with its arm  56  fully extended. The use, in embodiments of the present invention, of hydraulic pressure to actively drive the pile former  4  downwards means that the weight of the pile driver  1  can be kept within allowable limits while still being able to generate as much force as a much larger piling rig. 
         [0090]    The pile driver  1  is shown in  FIG. 12  attached to a pulley system  58  which is in turn attached to a mast  10  supported by the arm  56 . With the mast  10  held in contact with the ground  62 , the pulley system  58  allows easy adjustment of the height of the pile driver  1  relative to the pile former  4 . It should be noted that the height of the mast  10  and pulley system  58 , and therefore the maximum height  66  of the pile driver is 5.5 m, which is significantly lower than the length, and thus overall height  64 , of the 8 m long pile former  4 . The combination of the lower weight and operating height of the pile driver  1  of embodiments of the present invention are both clearly beneficial for the safety of site workers. 
         [0091]      FIG. 13  shows the pile former  4  partly driven into the ground. The pile driver  1  has been allowed to move downwards during operation by the pulley system  58 . At this point, the clamp  6  is detached from the pile former  4  and the pile driver  1  is raised to a higher point on the pile former  4  using the pulley system  58  and reattached using the clamp  6 . The driving operation is then repeated to continue driving the pile former  4  into the ground  62  as illustrated in  FIG. 14 , at which point the pile former  4  has been driven to a depth  68  of 5 metres. At this point the pile former  4  can be removed, using the pulley system  58 , to leave a hole  70  for filling with cementitious material, or the pile driver  1  can be raised again relative to the pile former  4  to allow for further driving. 
         [0092]    Throughout the operation shown in  FIGS. 12 to 14 , it can be seen that there need be no movement of the arm  56  of the tracked excavator  60 , or of the excavator itself. The mast  10  at the end of the arm  56  can held in contact with the ground to provide a stable platform during the piling operation. 
         [0093]    The operation of embodiments of the invention, along with its resulting small size and weight, allow the piling operation to be completed using a tracked excavator  60  rather than a dedicated piling rig. Tracked excavators  60  are almost inevitably required for other building operations, so this minimises the number of machines required on site. In addition, the size and weight of typical piling rigs means that there is a requirement to provide a piling mat, generally constructed from stone and waste spoil, to support and stabilise the weight of the rig. By avoiding this requirement a developer would benefit from significant time and cost savings. 
         [0094]    Even if not optimised for use with a standard tracked excavator, minimising size and weight is further beneficial in avoiding the costs and administration associated with moving large heavy loads/machinery by road. 
         [0095]    The above description relates to a preferred embodiment of the invention and is not intended to limit the scope of the appended claims. Various modifications would be possible. For example, alternatives to the pulley system  58 , such as a slider arrangement, could be used to allow adjustment of the height of the pile driver. 
         [0096]    Although the illustrated embodiment shows the pile driver  1  clamped to a pile former  4 , the principles of embodiments of the invention would apply equally to a similar device configured to rest on top of a pile former in a more conventional manner. For example, a flat plate could be included in the place of the clamp  6 , and the arrangement could be pushed down against the pile former in the initial step to set the pile former as outlined above. The lack of a firm connection between the pile driver  1  and the pile former  4  has a number of drawbacks, including the likelihood of additional noise generation between these components. However, by allowing the driver to start from a position of contact with the pile former, the overall noise level is still expected to be lower than that of a conventional system. 
         [0097]      FIGS. 15A and 15B  provide an example of an alternative driver  101  in a configuration as a top drive hammer. The top drive hammer  101  is similar in certain respects to the pie driver  1  shown in  FIG. 1 , and shares many of the same benefits including quiet operation and light weight for versatile mounting and ease of transportation etc. However, the top drive hammer  101  comprises a solid cylinder rod  102  and driving flange  106  rather than a hollow driving tube and clamp for securing around a pile former. 
         [0098]    A cylinder body  111  is provided with a removable/exchangeable weighted collar  113 , these components together forming a weighted sleeve similar to that previously described. A housing  108  is provided around the weighted sleeve  111 , 113 , and is provided with upper and lower slider bearings  107 , 109 . 
         [0099]      FIG. 15A  shows the top drive hammer in its raised position, while  FIG. 15B  shows the lowered or dropped position, with damping provided by the fluid in an upper chamber  118 A as in  FIG. 10 . 
         [0100]      FIGS. 16A and 16B  show a pilot actuator  72  for use with a driver  1 , 101  according to embodiments of the present invention. The actuator briefly comprises an actuator rod  74  and return spring  76  housed within an actuator body  78 . The actuator body  78  is housed within a swivel housing  80  which is provided with a height adjustment thread  82 . A pilot pressure port  84  is provided in the lower end of the actuator  72 , and an air bleed valve and filter  86  is located at the upper end. 
         [0101]    The actuator  72  is shown in its free, unactuated, state in  FIG. 16A , while  FIG. 16B  shows the actuator compressed in its energised state. The difference between the free height  88  and the compressed height  90  of the actuator rod  74  can be clearly seen. 
         [0102]    The actuator  72  is located, in use, at the lower end of the cage  8  or housing  108  surrounding the driver. For example, with reference to  FIG. 15A , on the lower part of the hammer support housing  108 , just outboard of the lower slider bearing  109 . The precise radial location will vary depending on the location of components such as hydraulic pipes and the associated available clearances. The role played by the actuator  72  will be explained during the following explanation of the driving operation as illustrated in  FIGS. 17 to 21 . 
         [0103]      FIG. 17  shows a driver/hammer at rest. A piston head/regeneration valve body  120  is shown which shares certain features and characteristics with the valve arrangement  20  described with reference to  FIG. 2 , along with hydraulic schematics for operation of driver. The valve body  120  is secured between an upper cylinder rod  102 A and a lower driving rod  102 B. The cylinder body  111  surrounds this arrangement and abuts the external surface of the valve body  120  so that an upper chamber  118 A is defined by the cylinder body  111  the valve body  120  and the upper cylinder rod  102 A, and a lower chamber  118 B is defined by the cylinder body  111  the valve body  120  and the driving rod  102 B. 
         [0104]    The upper and lower chambers  118 A, 118 B are connected by ‘C’ shaped regeneration paths  123 . This relatively short length of the regeneration paths, along with the central location of the valve body  120  within the system, minimises back pressure in the system during operation and permits high efficiency regeneration during the active part of the driving cycle. 
         [0105]    To comply with the safety requirements of this type of installation, two independent controls have to be activated for the hammer to function. Accordingly, a hammer select valve  129  is provided upstream of a hammer operation valve  130 . These two valves  129 , 130  control the flow of fluid to and from accumulators  125  upstream of the hammer select valve  129 . 
         [0106]    A pilot signal control valve  192  is included to provide a pilot signal to a pair of valve spools  142 , 144  within the valve body  120 , to close off the regeneration path  123  as required. 
         [0107]    A damping actuation signal  194  is provided by the pilot actuator  72  when the cylinder body  111  approaches a damping zone during operation. The damping pressure is controlled by a 400 bar pressure relief and unloading valve  196  which also permits unloading of this pressure, when the weight comes to rest. The system also includes a pilot relief valve  198 . 
         [0108]      FIG. 18  shows priming of the system ready for use. The hammer select valve  129  is actuated by solenoid or pilot signal, or possibly a combination, if high flows are involved. This directs pressure and flow to the hammer operation valve  130 , which defaults to both signal ports being connected to the input. Oil is directed from here through the fluid lines  127  to both high pressure inlet ports on the piston head/valve body  120 . With no pressure applied to either valve spool  142 , 144  pilot port, the valve then permits free flow to either side of the piston head  120 . 
         [0109]    As previously described in relation to  FIG. 2 , the cross sectional area of the lower chamber  118 B is greater than that of the upper chamber  118 A, and therefore the lower piston head area exceeds the upper piston head area. As a result, the hammer weight remains in position at the bottom of its stroke, held in position by the force generated by the piston head differential area. This information is signalled to the electrical/electronic control system, which indicates that the system is ready to commence the hammer cycle. 
         [0110]    The hammer weight raise is illustrated in  FIG. 19 . The hammer operation valve  130  is selected automatically by the electric/electronic control system, whilst simultaneously the control system selects the pilot signal control valve  192 . Pilot oil is delivered to both spools  142 , 144  of the piston head valve assembly  120 , moving the two spools  142 , 144  in opposite directions. Specifically, as shown, the right hand valve spool  142  is moved upwards, and the left hand valve spool  144  is moved downwards. This results in the regeneration path  123  between the two chambers  118 A, 118 B being cut off to allow pressurization of one chamber. 
         [0111]    Main system pressure is now diverted through the fluid line  127  to the right hand inlet port of the piston head valve  120 , while the left inlet port is connected to the tank return line. On the right inlet port to the piston head valve  120 , the path to the lower cylinder chamber  118 B is blocked, and pressurized oil is diverted to the upper chamber  118 A. On the left port the path to the upper chamber  118 A is blocked, therefore oil being ejected from the lower chamber  118 B is allowed to return back to tank. 
         [0112]    Due to the pressure difference between the upper and lower chambers  118 A, 118 B, the cylinder body  111  accelerates upwards at a rate dictated by the pressure difference. Electronic sensors then signal the control system to arrest the movement at a pre-set height. 
         [0113]      FIG. 20  shows the driving phase, or the hammer ‘fall’. The electrical/electronic control system deselects the pilot signal control valve  192  and the hammer operation valve  130 , connecting both piston head  120  valve spool pilot ports to tank. This returns the two spools  142 , 144  to their original balanced position, allowing free flow between the two chambers  118 A, 118 B of the cylinder. 
         [0114]    The hammer operation valve  130  is returned to its original position to divert pressurized oil via fluid lines  127  to both inlet ports of the piston head  120 . Since the lower piston head area exceeds the upper piston head area, the cylinder body  111  and weight  113  accelerate downwards as the pressure is applied to both sides, due to the forces generated by the differential area and the acceleration due to gravity. This acceleration can be adjusted by varying the pressure applied to the cylinder. The adjustment can be executed manually or automatically, using inputs from pressure and speed sensors suitably positioned in the hammer. 
         [0115]    Finally, the damping operation is illustrated in  FIG. 21 . At a predetermined position the actuator rod  74  of the pilot actuator  72  shown in  FIG. 2  is depressed by the hammer weight/cylinder body  111 , 113 , shown in  FIG. 1 . This movement of the pilot actuator rod  74  generates a damping actuation signal  194  causing pressure and flow from the pilot pressure port towards the piston head  120  valve spool pilot ports, via a shuttle valve  199 . This results in the left hand valve spool  144  moving sharply downwards, while the right hand valve spool  142  moves sharply upwards, synchronous with the movement of the weight  113 . Balanced spools  142 , 144  are shown to permit this operation to be carried smoothly. 
         [0116]    The right inlet to the lower chamber  118 B is quickly blocked, stopping any oil regeneration from the upper chamber  118 A via the regeneration path  123 . Similarly, the left inlet to the upper cylinder chamber  118 A is quickly blocked, preventing any oil regeneration through that route. Ejected oil from the upper cylinder chamber  118 A is now forced through the 400 bar relief valve  196  (this pressure limit can be adjusted to vary the applied force by the hammer). Make up oil is still supplied to the lower cylinder chamber  118 B by the pressure feed through the left inlet, supplemented if necessary by the accumulator. 
         [0117]    The pressure setting of the relief valve  196  should be mirrored by the pressure in the upper chamber  118 A, thus imparting an equal and opposite force to the driving rod  102 B and pile, which will be maintained until the energy has been dissipated. Accordingly, movement of the cylinder body  111  to a position where damping is required automatically generates the pressure signal  194  to initiate back pressure to stop the relative movement and provide damping to the system. This ensures, in a simple way, that movement is stopped and damping is applied at the correct time, regardless of differences in the movement of the cylinder due to varying conditions of, eg, ground during a piling operation, and is only provided when required. 
         [0118]    When the cylinder body/weight  111 , 113  has come to rest, high pressure will be locked into the upper cylinder chamber  118 A which could cause malfunction of the hammer operation valve  130  and possibly the pilot signal control valve  198  and prevent the piston head  120  reaching the end of its stroke. The pressure relief valve  196  is therefore fitted with an unloading feature to permit controlled venting of this trapped pressure, permitting the piston head  120  to return to its original position in a controlled manner. Once this has been executed, the hammer cycle can be commenced again. 
         [0119]    Certain benefits of the drive provided by embodiments of the present invention will now be described. 
         [0120]    A more traditional ‘drop’ hammer or driver, consisting of a weight accelerated either under gravity or hydraulic pressure, produces a certain amount of kinetic energy which is suddenly transferred to a pile or column at the point of impact. This kinetic energy is converted to work done on the pile or column, defined as the average force applied to the pile or column multiplied by the distance over which it is applied. In a graph plotting force against distance, the area under the curve is the total work done. 
         [0121]      FIG. 22  shows an example trace  200  as would be expected for a typical drop hammer striking a pile at the set condition or striking very hard ground.  FIG. 23  shows a trace  202  indicative of embodiments of the present invention operating with the same weight. In both Figures, the force F is plotted on the vertical axis and the distance s on the horizontal axis. 
         [0122]    The first point to note is that the area under each graph is similar, but the force distribution is quite different. The present invention will deliver a more constant force over a longer distance, with a much lower peak than the conventional hammer. This provides a smoother ‘push’ than the sudden impact provided by a typical hammer/driver. 
         [0123]    Another problem with the conventional systems is that the maximum force generated is not generally known. As a result, it is common to have to produce an SPT (standard penetration test) analysis of the ground, to establish the ground resistance and the depth to be driven. 
         [0124]    Embodiments of the invention advantageously allows a known particular driving force  204  to be selected for a driving operation. In the piling industry, the force may be selected to be appropriate for a particular size of pile. If the hammer force and pile specification are matched, there is much less possibility of damaging the pile. 
         [0125]    Also, at set, if the driving force is known, the pile resistance, and hence the pile load capacity, can be calculated from this information. In other words, the hammer/driver  1 , 101  can self-certificate each pile capacity. 
         [0126]    To adjust the force applied, a known constant force could be pre-set and different relief cartridges provided to provide a different generated force from the same driver  1 , 101 . For example, three different force cartridges could be supplied with a hammer/driver  1 , 101 , each correlated/tuned to a different pile cross-section within a range. Additional cartridges could be supplied for special pile cross-sections to ensure the perfect driver is provided for each pile size and type. 
         [0127]    Taking  FIG. 23  as indicative of a the set force for a typical driver/hammer  1 , 101  according to the invention, a lower set force  204 A would likely provide a trace  206  as shown in  FIG. 24 , and a larger set force  204 B would likely provide a trace  208  as shown in  FIG. 25 . The trace  206  of  FIG. 24  might be more suitable for smaller diameter piles and the trace  208  in  FIG. 25  might be better for larger piles. However, in each case, the area under each curve, and thus the total work done, would be similar. 
         [0128]    A summary of some of the benefits provided by embodiments of the present invention is provided below:
   The hydrostatic drive system with instrumentation can deliver a predetermined level of kinetic energy and force to the driven member. These values can be adjustable to suit a user&#39;s specific requirements.   The unique piston head/valve body  20 , 120  with close coupled valve arrangement which automatically closes off the regeneration oil path to divert the oil through a pressure control valve at the predetermined location, thereby creating a predetermined driving force to the driving member.   The unique actuating system  72  which uses the hammer weight momentum to generate the pressure signal  194  to initiate the back pressure to permit the transmission of energy to the driving member.   The unique piston head/valve body  20 , 120  with close coupled valve arrangement which permits a direct oil path from inlet to outlet through the valve, offering minimal resistance and back pressure to regenerated oil during the working stroke, giving enhanced operating efficiencies.   The hydrostatic drive system which transmits an energy and force to a driven member without any metal to metal contact, greatly reducing sound generation from the impact.   The cylinder body forms part of the overall hammer weight, this dual function component saves on the weight of the driver assembly. Savings created by their being no need for soundproof cladding further reduces the overall weight.   The drive system which can generate energy by accelerating a weight with hydraulic pressure or gravity or both, so can thereby operate at any attitude.   The through rod configuration lends itself to equally end drive  101  or through drive  1  configurations.   
 
         [0137]    Finally, although developed as a pile driver for use in construction work, the general principles underlying embodiments of the invention are also applicable to other smaller or larger scale devices for use wherever force transfer, in particular controllable measurable force transfer, is required. For example, smaller scale drivers for alternative uses, or percussive hammers for braking or compacting materials could make use of the driver according to embodiments of the invention. Other specific uses of embodiments of the invention include, but are not limited to, a top drive piling hammer for concrete or steel piles, a sheet pile driving hammer, a through drive piling hammer for a mandrel or poker and a soil displacement driver or boring tool such as a Grundomat (RTM). 
         [0138]    The active driving of the weighted sleeve makes use of embodiments of the invention possible in orientations other than vertical, since gravity is not required for the driving process. 
         [0139]    Although the present invention has been disclosed in the form of preferred embodiments and variations thereon, it will be understood that numerous additional modifications and variations could be made thereto without departing from the scope of the invention. 
         [0140]    For the sake of clarity, it is to be understood that the use of ‘a’ or ‘an’ throughout this application does not exclude a plurality, and ‘comprising’ does not exclude other steps or elements.