Abstract:
A hammer system for automatically carrying out a standard penetration test is disclosed. A cylindrical housing, in which an anvil coupled to a drill rod is received, is supported by a first hydraulic cylinder coupled to boring equipment. A cylindrical hammer is received in the housing. The hammer includes a holding assembly therein, which selectively holds and raises the hammer by a second hydraulic cylinder. Element for limiting a stroke of the hammer is spacedly connected to the holding assembly to be raised and lowered therewith. The limiting element includes a first sensor to detect a slot formed at the housing when the hammer is raised, thereby counting the number of blows. The hammer includes a plurality of protrusions on its outer surface. A wall of the housing includes a second sensor to detect the number of protrusions passed over the second sensor when the hammer is raised, thereby calculating a penetration depth from a difference between the numbers of protrusions detected for two continuous blows.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an apparatus for carrying out a Standard Penetration Test (SPT) to determine the penetration resistance, geological distribution and nature of the soil, and more particularly to an automatic hammer system for a standard penetration test, which enables its hammer to fall from a precise predetermined height regardless of a penetration depth of a sampler, and is able to automatically carry out sequential test procedures such as counting the number of blows by the hammer and a penetration depth of a sampler according to the number of blows. 
     2. Description of the Prior Art 
     To undertake various civil engineering works and construction works, there is a need to first determine the penetration resistance, geological structure and geological composition of the soil by checking consistency and relative density of the soil by testing the soil of an area in question. To this end, a test procedure known as the “Standard Penetration Test” is commonly used. 
     The standard penetration test is a representative geological surveying test for estimating soil constants such as strength, relative density and angle of internal friction of ground in question, which is carried out as follows. A hammer of 63.5 kg is raised to a height of 75 cm and then released to fall and impact a split barrel sampler (referred to merely as a sampler, hereinafter), and this procedure is repeatedly carried out until the soil is penetrated to a depth of 30 cm by the sampler. Subsequently, an N value, which is the number of blows of the hammer counted until the sampler penetrates the soil to the depth of 30 cm, is calculated, and the soil constants of the ground are obtained from the N value. 
     In this test, the number of blows counted until the sampler initially penetrates the soil to a depth of 15 cm is regarded as a number of preliminary blows because the soil sample is believed to be disturbed, and the number of blows counted until the sampler further penetrates the soil to a depth of 30 cm from the level corresponding to the initial depth of 15 cm is determined as the N value for the soil in question. Where the number of blows counted until the sampler penetrates the soil to the depth of 30 cm exceeds 50, a depth of the soil penetrated after the hammer gives the sampler 50 blows is measured. 
     As a rule, though the standard penetration test must be carried out every 1.5 m under the current ground surface, the standard penetration test is carried out only once where the same geological formation continues underground. 
     Referring to FIG. 1, there is shown the most common apparatus for use in the standard penetration test, which uses a winch. 
     As shown in the drawing, a frame  1  is provided at its lower portion with a winding drum  2  fixed thereto, and is provided at its upper portion with a pulley  3 . A rope  4  is wound around the winding drum  2  for several turns and wrapped around the pulley  3  to be directed downwardly. A cylindrical hammer  5  is coupled to one end of the rope  4 , and slidably inserted over a vertical guide rod  6 . 
     The guide rod  6  is coupled at its lower end to a drill rod  8 , which is inserted into a boring hole (not shown) which has been previously drilled. The drill rod  8  is provided at its upper end with an anvil  7  mounted thereon, on which the hammer  5  impacts, and is provided at its lower end with a sampler (not shown) coupled thereto to obtain a disturbed soil sample. The guide rod  6  is provided with a marking which indicates a maximum lifting height at a certain height from the anvil  7 . 
     In an operation of the winch-type apparatus, the drill rod  8 , on which the sampler is mounted, is inserted into the boring hole of the soil, and then coupled to the guide rod  6 . Subsequently, the rope  4  is pulled by an operator to raise the hammer  5  to the lifting height (75 cm), and then released to allow the hammer  5  to free fall. Consequently, the hammer  5  falls along the guide rod  6  and impacts the anvil  7 . 
     Therefore, the impact of the falling hammer  5  is transmitted to the drill rod  8  through the anvil  7 , so that the soil in question is penetrated by the sampler coupled to the lower end of the drill rod  8 . This procedure is repeated until the penetrated depth reaches a desired value. 
     However, since such a conventional winch-type apparatus for use in the standard penetration test is required for an operator to check, with his naked eye, a lifting height of the hammer  5  during every lifting procedure, it is difficult to maintain a constant lifting height throughout all the striking procedures even though the test is carried out by a skilled person. Hence, the drill rod is applied with different impact strengths throughout the striking procedures. 
     Furthermore, since the hammer  5  is raised by the rope  4 , frictional loss is generated between the winding drum  2  and the pulley  3  during the falling of the hammer  5 . The frictional loss varies depending on the properties and age of the rope  4 , and actual impact strength applied to the anvil  7  is reduced to a value lower than the specified value. 
     Therefore, the conventional winch-type apparatus is inadequate to carry out the standard penetration test, and it is difficult to assure a precise measurement of an N value and to assure reliability of test results because of various factors. 
     In addition, since an N value obtained by the test is in an operator&#39;s memory, and a penetration depth of the sampler is obtained by an additional measuring procedure, an operator is apt to obtain incorrect test results, and considerably different test results may be obtained depending on operators even though the tests are carried out on the same soil sample. 
     To overcome the above-mentioned problems, a drive hammer system for a standard penetration test is disclosed in U.S. Pat. No. 4,405,020, which is adapted to enable a hammer to consistently fall from the same height, and to minimize frictional loss generated during the falling of the hammer. 
     The drive hammer system is slidably supported to an outer surface of a hydraulic cylinder via a pivot arm connected to a piston rod of the hydraulic cylinder. The hydraulic cylinder is vertically mounted on a drill rig. The pivot arm is rotated to a working position and raised by the hydraulic cylinder to be positioned over an impact surface of an anvil. When the drive hammer system is positioned over the anvil, a shutoff valve is opened to allow fluid in the hydraulic cylinder to be exhausted. 
     In this state, by actuation of a motor mounted on the cylindrical housing, a sprocket is rotated to cause a chain to be rotated clockwise. Lifting lugs on the chain are raised along a slot axially formed at the cylindrical housing by the rotation of the sprocket. At this point, the lug comes into contact with a lower end of a hammer received in the housing. As the lug pushes the hammer up, the hammer is gradually distanced from the anvil. 
     When the lug reaches the sprocket and begins to move outwardly, the lug moves from under the hammer, permitting the hammer to free fall until it strikes the impact surface of the anvil. By the striking action of the hammer against the anvil, a sampler penetrates the soil, thereby allowing the anvil to be lowered. At this point, the cylindrical housing free falls by the penetration depth of the sampler, and thus is placed on a flange of a drill rod, thereby maintaining a drop height at a certain value. 
     The drive hammer system itself is lowered by the penetration depth after every blow so as to maintain the drop height of the hammer at a certain value. However, since the drive hammer system strikes the flange of the drill rod soon after blows from the hammer (i.e. secondary blows), the sampler further penetrates the soil. 
     In addition, since the hammer is adapted to be raised by the lifting lug of the turning chain and to fall by release from the lug, the hammer may be raised to a position higher than the specified height by being struck by the lug in the course of turning when the chain is rotated at high speed. 
     In addition to this, it is troublesome to measure a penetration depth of the sampler by blows of the hammer by an additional measuring device. 
     Accordingly, this drive hammer system is not able to assure accuracy and reliability of an N value. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention has been made keeping in mind the above problems occurring in the prior art, and an object of the present invention is to provide an automatic hammer system for use in a standard penetration test, which is adapted to enable a hammer to be raised and to fall automatically, and which is adapted to maintain a drop height of a hammer at a certain value, regardless of a penetration depth of a sampler. 
     Another object of the present invention is to provide an automatic hammer system for use in a standard penetration test, which is able to minimize loss of impact energy of a hammer caused by frictional contacts between associated components, and which is adapted to reliably prevent secondary blows against an anvil, thereby permitting the anvil to always be applied with a specified impact energy. 
     A further object of the present invention is to provide an automatic hammer system for use in a standard penetration test, which is adapted to automatically carry out a series of test procedures for counting the number of blows by a hammer and a penetration depth of a sampler according to the number of blows, thereby affording a precise N value. 
     In order to accomplish the above object, the present invention provides an automatic hammer system for a standard penetration test, comprising: a first vertical hydraulic cylinder rotatably coupled to boring equipment; a cylindrical housing positioned to be parallel to the first hydraulic cylinder and coupled thereto, the cylindrical housing being connected to a piston rod of the first hydraulic cylinder and adapted to receive therein an anvil of a drill rod, wherein the drill rod is provided at its lower end with a sampler to be inserted in a boring hole of the soil; a cylindrical hammer with a blind lower end, which is movably received in the housing to be disposed over the anvil; a holding assembly received in the hammer and adapted to hold the hammer at its lower dead point and to release the hammer at its upper dead point to allow the hammer to fall; a second hydraulic cylinder concentrically coupled to an upper end of the housing and adapted to raise and lower the holding assembly; means for limiting a lifting height of the hammer, which is received in the housing to be disposed over the hammer and integrally coupled to the holding assembly with a spacing therebetween, the limiting means being raised and lowered within a certain range; means for counting the number of blows of the hammer against the anvil; means for measuring a penetration depth of the sampler by blows of the hammer; and a control unit for carrying out control of the striking action of the hammer and calculation of an N value according to data obtained by the counting means and the measuring means, and for carrying out record and display of test results. 
     According to an aspect of the present invention, the holding assembly includes a cylindrical casing which is radially provided at its wall with a plurality of fitting slots at a certain angular spacing, a plurality of holding blocks slidably fitted in the fitting slots of the casing and adapted to selectively press an inner surface of the hammer, and a pusher unit received in the casing and connected to the piston rod of the second hydraulic cylinder, the pusher unit being adapted to outwardly push or release the holding blocks in the course of axial movement. 
     The pusher unit is adapted to outwardly push and release the holding blocks when the pusher unit is further lowered and raised after the limiting means is stopped. 
     According to another aspect of the present invention, the counting means comprises a detection slot formed at an upper portion of the housing, and a first sensor mounted on the plunger to detect the detection slot to count the number of blows by the hammer. 
     According to a further aspect of the present invention, the measuring means comprises a plurality of protrusions axially formed along an outer surface of the hammer at a certain pitch, and a second sensor mounted on a wall of the housing to detect the number of protrusions passed over the second sensor during every lifting motion, thereby enabling a penetration depth to be obtained from the number of protrusions. 
     According to the present invention, the holding assembly is actuated to outwardly press an inner surface of the elongated cylindrical hammer, thereby firmly holding the hammer. The holding assembly engaging the hammer is raised by the second hydraulic cylinder and then releases the hammer to fall freely. After a blow by the hammer, since the holding assembly holds the hammer at a position which is higher than the previous holding position by a penetration depth of the previous blow, a drop height of the hammer is uniformly maintained for every blow, regardless of a penetration depth of the hammer. 
     Furthermore, since the hammer is adapted to be raised to a certain height and then to fall therefrom without lowering displacement of the hammer system itself, it is possible to reliably prevent secondary blows caused by lowering of a conventional hammer system. Therefore, the anvil can always be applied with specified impact energy. 
     In addition, since the number of blows by the hammer and penetration depths according to the number of blows are automatically calculated and accumulated, an N value can be precisely obtained, thereby affording improvements in reliability of test results and convenience in testing. 
     Therefore, the automatic hammer system for a standard penetration test according to the present invention can contribute to improvements in the accuracy, reliability and convenience of a standard penetration test. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features and advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a perspective view of a conventional hammer system for use in a standard penetration test; 
     FIG. 2 is a perspective view of an automatic hammer system for a standard penetration test according to the present invention, which is mounted on a boring machine; 
     FIG. 3 is a front elevation view of the automatic hammer system for a standard penetration test according to the present invention; 
     FIG. 4 is a side elevation view taken along line IV—IV of FIG. 3; 
     FIG. 5 is a cross-sectional view taken along line V—V of FIG. 
     FIG. 6 is a cross-sectional view taken along line VI—VI of FIG. 4; 
     FIG. 7 is an enlarged cross-sectional view of a holding assembly according to the present invention; 
     FIG. 8 is a cross-sectional view taken along line VIII—VIII of FIG. 7; 
     FIG. 9 is an enlarged cross-sectional view of means for limiting a stroke of a hammer according to the present invention; 
     FIGS. 10A to  10 D are cross-sectional views showing a holding operation of the holding assembly according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     This invention will be described in further detail by way of example with reference to the accompanying drawings. 
     As shown in FIGS. 2 to  6 , an automatic hammer system for a standard penetration test according to the present invention comprises a first hydraulic cylinder  10 , a cylindrical housing  20  adapted to be raised and lowered by the first hydraulic cylinder  10 , a hammer  30  received in the housing  20  to be raised and lowered to impact against an anvil  91  coupled to a drill rod  90 , a holding assembly  40  adapted to raise the hammer  30  by gripping action and to allow the hammer  30  to fall, a second hydraulic cylinder  50  adapted to raise and lower the holding assembly  40 , means  60  for limiting a lifting height of the hammer  30  to a certain height, means  70  for counting the number of blows of the hammer  30 , means  80  for measuring a penetration depth of a sampler by blows of the hammer  30 , and a control unit for controlling striking action of the hammer  30  and for recording and displaying test results such as N values. 
     As shown in FIG. 2, the first hydraulic cylinder  10  is disposed parallel to a vertical support shaft  110 , and rotatably coupled to the support shaft  110  via an arm bracket  120 . The support shaft  110  is mounted on boring equipment  100 , which is adapted to excavate boring holes (not shown) to be used in a soil test. 
     The housing  20  is coupled to the first hydraulic cylinder  10  by a carrier  21  such that the housing  20  is disposed parallel to the first hydraulic cylinder  10  and is raised and lowered with respect to the first hydraulic cylinder  10 . The carrier  21  is slidably inserted at its one end on the first hydraulic cylinder  10 , and fixedly coupled at its other end to the housing  20 . 
     A support rod  22  is vertically positioned and fixed to the carrier  21  at its lower end. The upper end of the support rod  22  is connected to an upper free end of a piston rod  11  of the first hydraulic cylinder  10  by a connector  23 . 
     The anvil  91  coupled to the upper end of the drill rod  90  is slidably received in the housing  20  and normally disposed at its lower portion. 
     The hammer  30  is shaped as an elongated cylindrical form, and is movably received in the housing  20 . The hammer  30  is comprised of a striking part  31  positioned at its lower portion to provide blow to the anvil  91 , and an elongated cylindrical holding part  32  disposed on the striking part  31  and opened at its upper end to receive the holding assembly  40 . 
     The holding part  32  of the hammer  30  is sized to be longer than a sum of a penetration depth (15 cm) of a sampler (not shown) and a penetration depth (30 cm) of the sampler corresponding to an N value, in which the penetration depth (15 cm) of the sampler is believed to be a depth corresponding to preliminary blows. 
     As illustrated in FIGS. 7 and 8, the holding assembly  40  includes a casing which is movably received in the holding part  32  of the hammer  30 , a pair of push blocks  42  adapted to radially and outwardly press and release an inner surface of the holding part  32 , and a pusher unit  43  adapted to actuate the push blocks  42 . 
     The casing  41  is comprised of a cylindrical body with a blind lower end in which the pusher unit  43  is operatively received. The casing  41  is provided at its upper end with a cap  44  to limit an upward movement of the pusher unit  43  and to prevent separation of the pusher unit  43 . The casing  41  is formed with a pair of fitting slots  41   a  at diametrically opposite sides in which the pair of push blocks  42  are fitted. 
     The pair of push blocks  42  are slidably inserted in the pair of fitting slots  41   a  of the casing  41 , so that the outer ends of the push blocks  42  are selectively engaged to an inner surface of the holding part  32  of the hammer  30 . Each of the push blocks  42  is sized to be longer than a wall thickness of the casing  41  so that an inner end of the push block  42  is slightly and inwardly protruded from an inner surface of the casing  41 . 
     The pusher unit  43  includes an actuating rod  45  slidably received in the casing  41 , a drop head  46  which is fitted in a hole formed at the lower end of the casing  41  to be axially slid, and a dog  47  pivotally connected to a lower end of the actuating rod  45  by a hinge pin  47   a.    
     The actuating rod  45  is connected to a piston rod  51  of the second hydraulic cylinder  50 , and is raised and lowered in the casing  41 . 
     The drop head  46  is fitted in the hole  41   b  formed at the lower end of the casing  41 . The drop head  46  is provided at its outer surface with a flange  46   a , so that the drop head  46  is hung on the lower end of the casing  41  and properly protruded upwardly and downwardly to open the dog  47 . 
     The dog  47  is elastically biased by a torsion spring (not shown) in a closing direction, and is adapted to be opened by a lowering motion of the actuating rod  45  to receive the drop head  46  at its mouth, thereby pushing the push blocks  42  outwardly. 
     The second hydraulic cylinder  50  is concentrically connected to an upper end of the housing  20 . The piston rod  51  of the second hydraulic cylinder  50  is received in the housing  20 , and is connected to the actuating rod  45  of the pusher unit  43  via a connecting pipe  48 . 
     As shown in FIG. 9, the limiting means  60  includes a plunger unit  61  received in the housing to be positioned over the hammer  30  and to be raised and lowered in a certain range, and a pair of guide slots  62 , which are axially formed at the wall of the housing  20  to face each other. 
     The plunger unit  61  comprises a bush-type body  64  which includes a flange  64   a  having an external diameter corresponding to an internal diameter of the housing  20  and a guide hole  64   b  formed at its center, a connector  65  slidably fitted in the guide hole  64   b  of the body  64  to connect the piston rod  51  of the second hydraulic cylinder  50  to the connecting pipe  48 , and a pair of guide protrusions  63  formed on an outer surface of the bush-type body  64  and slidably fitted in the corresponding guide slots  62 . 
     The body  64  of the plunger unit  61  is integrally coupled to the casing  41  of the holding assembly  40  by a joint pipe  66 , and is thus raised and lowered together with the holding assembly  40  with a certain spacing therebetween. The body  64  of the plunger unit  61  is adapted to be raised and lowered in a height range corresponding a drop height (75 cm) specified in the standard penetration test. 
     The body  64  of the plunger unit  61  is securely connected to the casing  41  of the holding assembly  40  by means of a plurality of connecting rods  67 . 
     The housing  20  is provided at its outer surface with a pair of upper stoppers  68   a  and a pair of lower stoppers  68   b  such that the upper stoppers  68   a  are axially spaced from the lower stoppers  68   b , so as to more stably limit axial movement of the plunger unit  61 . The pair of upper stoppers  68   a  and the pair of lower stoppers  68   b  are disposed at positions corresponding to the guide slots  62  of the housing  20 , which come into contact with the guide protrusions  63  of the plunger unit  61 . 
     The spacing defined between the upper stoppers  68   a  and the lower stoppers  68   b  is set to equal to the drop height specified in the standard penetration test, and is also set to be smaller than a stroke length of the second hydraulic cylinder  50 , so that the pusher unit  43  can be raised and lowered in the casing  41 . 
     The means  70  for counting the number of blows comprises a detection slot  71  formed at an upper portion of the housing  20 , and a first sensor  72  mounted on an upper end of the connecting rod  67  projected from the plunger unit  61  to detect the detection slot  71  during axial movement of the plunger unit  61 . 
     The means  80  for measuring a penetration depth of the sampler, comprises a plurality of annular protrusions  81  formed on an outer surface of the hammer  30  at a certain pitch, and a second sensor  82  mounted on a wall of the housing  20  to detect the annular protrusions  81 . 
     The control unit stores various data such as a pitch of the annular protrusions  81  required for a standard penetration test, and controls the action of the hammer  30 . 
     An operation of the automatic hammer system for a standard penetration test according to the present invention will now be described with reference to FIGS. 10 a  to  10   d.    
     After a boring operation by the boring equipment  100  is carried out to form a boring hole to a target depth, the drill rod  90 , which is connected to the sampler at its lower end, is coupled to an anvil  91 , and then inserted into the boring hole. Subsequently, the automatic hammer system is rotated about the support shaft  110  of the boring equipment  100  until the automatic hammer system is precisely positioned over the boring hole, as indicated by dotted lines in FIG.  2 . 
     The housing  20  is raised or lowered by activation of the first hydraulic cylinder  10 , so that the hammer  30  received in the housing  20  is placed on the anvil  91 , as shown in FIG.  10 A. The holding assembly  40  is then controlled to be positioned at a lower portion of the holding part  32  of the hammer  30 . At this point, the plunger unit  61  of the limiting assembly  60  is disposed at the lowermost position and comes into contact with the lower stoppers  68   b.    
     In this state, since the drop head  46  of the holding assembly  40  is hung on the lower end of the casing  41 , and is not bitten by the dog  47 , the push blocks  42  are not applied with pressing force, so that the hammer  30  is free of engagement with any component. 
     Thereafter, as the hammer system is driven, the piston rod  51  is lowered by actuation of the second hydraulic cylinder  50 . Consequently, the actuating rod  45  of the pusher assembly  43 , which is coupled to the piston rod  51  via the connecting pipe  48 , is lowered in the casing  41 . 
     Consequently, the dog  47  pivotally coupled to the lower end of the actuating rod  45  is engaged to the top of the drop head  46  hung on the lower end of the casing  41 , and thus opened, followed by biting the drop head  46  by elastic force of the torsion spring, as shown in FIG.  10 B. 
     After the drop head  46  is bitten by the dog  47 , the piston rod  51  of the second hydraulic cylinder  50  is raised, as shown in FIG.  10 C. In this state, since the drop head  46  is merely hung on the hole  41   b  of the casing  41 , the drop head  46  is also raised together with the actuating rod  45  in a state of being bitten by the dog  47 . 
     As the dog  47  is raised, the push blocks  42  are radially and outwardly pushed by the opened dog  47 , and come into close contact with the inner surface of the hammer  30 , as indicated by a phantom line in FIG.  8 . Accordingly, the hammer  30  is integrally coupled to the holding assembly  40  via the push blocks  42 , and then raised in the housing  20  together with the piston rod  51 . 
     At this point, since the plunger unit  61  disposed over the hammer  30  is connected to the casing  41  of the holding assembly  40  via the joint pipe  66 , the plunger unit  61  is also raised therewith. 
     When the guide protrusions  63  of the plunger unit  61   60  come into contact with the upper stoppers  68   a , the upward movement of the plunger unit  61  is stopped, and the holding assembly  40  connected to the plunger unit  61  is also stopped at the upper dead point. 
     When the plunge unit  61  is positioned at the upper dead point, the first sensor  72  of the count means  70  mounted on the connecting rod  67  is positioned to face the detection slot  71  of the housing  20 , thereby detecting the detection slot  71 . The detection signal is sent to the control unit, so that the control unit counts the number of detections. 
     At the same time, the second sensor  82  mounted on the wall of the housing  20  detects the number of the annular protrusions  81  passed over the second sensor  82 , and sends a signal corresponding to the number to the control unit. More specifically, the second sensor  82  detects the annular protrusions  81  which pass over the sensor  82  during one lifting action of the hammer  30 , and send a signal corresponding to the number of the protrusions  81  to the control unit. 
     Since a stroke length of the piston rod  51  of the second hydraulic cylinder  50  is set to be longer than the spacing between the upper stoppers  68   a  and the lower stoppers  68   b , the piston rod  51  is further raised even after the guide protrusions  63  of the plunger unit  61  have been caught by the upper stoppers  68   a.    
     More specifically, since the piston rod  51  passes through the connector  65  slidably fitted in the guide hole  64   b  of the plunger unit  61 , and is connected to the actuating rod  45  of the pusher unit  43  via the connecting pipe  48 , the piston rod  51  can be further raised until the actuating rod  45  is raised to the top of the casing  41  and thus caught by the cap  44 , as shown in FIG.  10 D. 
     In this way, since the actuating rod  45  is further raised after the upward movement of the hammer  30  is stopped, the dog  47  in the casing  41  is raised with respect to the push blocks  42 , thereby allowing the drop head  46  to be released from the dog  47 . 
     At this point, since the pressing force applied to the push blocks  42  which are in state of pressing the inner surface of the hammer  30  radially and outwardly is removed, the hammer  30  falls by its own weight and thus impacts against the anvil  91 , thereby causing the sampler coupled to the drill rod  90  to penetrate the soil. 
     After the anvil  91  is applied with a blow by the hammer  5 , the piston rod  51  of the second hydraulic cylinder  50  is lowered again, so that the holding assembly  40  is lowered together with the plunger unit  61 . 
     Subsequently, when the guide protrusions  63  of the plunger unit  61  are caught by the lower stoppers  68   b , the plunger unit  61  and the casing  41  of the holding assembly  40  are stopped but the actuating rod  45  of the pusher unit  43  is further lowered because the actuating rod  45  is connected to the piston rod  51  of the second hydraulic cylinder  50  via the connector  65  of the plunger unit  61  and the connecting pipe  48 . 
     Consequently, the dog  47  is relatively lowered in the casing  41  with respect to the push blocks  42 , as shown in FIG.  10 A. Thereafter, the dog  47  is opened by forcible engagement with the drop head  46  and thus bites the drop head  46 . In this state, as the piston rod  51  is raised, the push blocks  42  are outwardly pushed by the opened dog  47  with a larger width, thereby causing the hammer  30  to be firmly held by the push blocks  42 . 
     At this point, since the inner surface of the hammer  30  is pressed by the push blocks  42  at a position which is disposed to be higher than the previous pressed position by a distance corresponding to a penetration depth by the previous blow, drop heights of the hammer  30  can be maintained at a predetermined value for every blow, regardless of a penetration depth of the sampler. 
     In other words, since the hammer  30  is held by engagement of its inner surface and the push blocks  42 , and a lifting distance of the holding assembly  40  is defined by the upper and lower stoppers  68   a  and  68   b , a substantial lifting height of the hammer  30  is uniformly maintained even though the hammer  30  is lowered with respect to the hammer system, with only a variation in a holding position of the hammer  30  to which the push blocks  42  are engaged. 
     Therefore, the automatic hammer system according to the present invention can basically prevent secondary blows generated by lowering of an overall hammer apparatus caused by increase of penetration depth, as in a conventional system. 
     When the hammer  30  is raised again by the holding assembly  40  as the piston rod  51  of the second hydraulic cylinder  50  is raised, the second sensor  82  mounted on the wall of the housing  20  detects the annular protrusions  81  formed on the outer surface of the hammer  30  which pass over the second sensor  82 , and outputs a signal corresponding to the number of the annular protrusions  81 . 
     At this point, the control unit calculates the number of protrusions corresponding to a penetration depth corresponding to one blow by subtracting the current number of the protrusions  81  from the previous number of the protrusions  81 , and then finally calculates the penetration depth corresponding to one blow by multiplying the calculated number of protrusions by a pitch of the protrusions. 
     By accumulating penetration depths obtained in every blow in the above manner, it is possible to conveniently obtain a precise N value. 
     As described above, the present invention provides an automatic hammer system for a standard penetration test, which enables drop heights of its hammer to be uniformly maintained for every blow, regardless of a penetration depth of the hammer, since the hammer is held at its inner surface by a holding assembly adapted to be raised and lowered in a predetermined distance range. 
     Furthermore, since the hammer is adapted to be raised to a certain height and then to fall therefrom without lowering displacement of the hammer system, it is possible to reliably prevent secondary blows caused by lowering of a conventional hammer system. 
     In addition, since the number of blows by the hammer and penetration depths according to the number of blows are automatically calculated and accumulated, an N value can be precisely obtained. 
     Therefore, the automatic hammer system for a standard penetration test according to the present invention can contribute to improvements in accuracy, reliability and convenience of a standard penetration test. 
     Although a preferred embodiment of the present invention has been described for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.