System for measuring the penetration depth of an elongated object into the ground

Abstract: System for measuring the penetration depth of an elongated object (13) such as a pile, tube, sheet pile or drill into the ground, to be used in combination with an installation for bringing the elongated object into the ground (10) for instance by pressing, hammering, vibrating, drilling or lowering, or for pulling or otherwise removing said elongated object out of the ground. The system comprises means for measuring the displacement of the elongated object embodied as a flexible elongated element (14,18) the weight of which is uniformly distributed along its length, said length being at least equal to the maximum displacement of the elongated object. At least part of said elongated element is supported by a supporting surface. Means (20) are present to determine the total weight of that section of the flexible elongated element which is supported by said supporting surface.

Description: The invention relates to a system for measuring the penetration depth of an elongated object such as a pile, tube, sheet pile or drill into the ground, to be used in combination with an installation for bringing the elongated object into the ground for instance by pressing, hammering, vibrating, drilling or lowering, or for pulling or otherwise removing said elongated object out of the ground, said system comprising means for measuring the displacement of the elongated object. A system of this type, more specifically destined for driving a pile into the ground by means of a hammer, is described in the laid-open Japanese patent publication JP 58-94525. In this prior art system the displacement of the elongated object is measured by measuring the displacement of a wire or cable of which one end is directly or indirectly connected to the pile. The other end of the wire or cable is wound onto a drum. During operation of the system the amount of wire paid out from said drum is measured. The measured values are recorded and can be used for real time calculations. Thereby especially information is obtained about the speed with which the pile or tube is penetrating into the ground. A specific problem with measurements of this type resides in the fact that installations for driving piles, tubes, sheet piles or drills into the ground are usually operating in a very dirty, even hostile environment. Therefore, the various components of the measuring system have to be embodied such that they will operate in a reliable manner even under said hostile circumstances. Rotating means such as a drum on which a measuring wire is wound (as described in JP 58-94525) should be avoided. A specific disadvantage related to the use of a drum resides in the fact that the error occurring during the successive revolutions of the drum is cumulating in the final measurement value, resulting in many cases in a relatively large absolute divergence between the measured value and the real penetration depth. Furthermore slip and stretching of the wire could lead to an additional deviation. The object of the invention is now to indicate in which way accurate information can be obtained about the penetration of the pile or tube etc. with means which are basically insensitive for a dirty or even hostile environment. In agreement with this object the invention provides a system of the above-mentioned type which is according to the invention characterized in that a flexible elongated element the weight of which is uniformly distributed along its length is directly or indirectly connected to said elongated object, said length being at least equal to the maximum displacement of the elongated object, whereby at least part of said elongated element is supported by a supporting surface, and that means are present to determine the total weight of that section of the flexible elongated element which is supported by said supporting surface. Application of the system according to the invention results in a very easy to use and easy to calibrate measuring facility which is in essence not susceptible to a dirty or hostile environment. In a first preferred embodiment the elongated element comprises a chain made of a plurality of interconnected links. The advantage of such an embodiment is that chains are readily available in all kinds at reasonably low costs. A chain is very insensitive for dirt, oil etc. and forms therewith an elongated element which is perfectly suited for the job. Another embodiment of the system according to the invention makes use of a flexible element which is embodied as a wire or cable to which separate weight elements are connected at mutual distances. It is even possible to use a flexible wire or rope as elongated element as long as its weight per unit length is sufficient to provide an accurate reading in combination with the applied weight determining means even for short displacements of the elongated object. In a preferred embodiment the weight of the section of the flexible elongated element supported on the supporting surface is measured by means of a weight measuring device installed underneath said supporting surface. An advantage of this embodiment is that the weight measuring sensor can be connected to for instance a distant data processor or data logger by means of a connecting cable which can be installed completely out of reach of the personnel operating the system. A disadvantage of this embodiment could be that any dirt, oil, or other strange materials accumulating during the driving or removing process on the supporting surface may have influence on the weight measurement. In another embodiment the means for determining the weight of that section of the flexible elongated element carried by the supporting surface comprises a weight measuring device installed between said one end of the flexible elongated element and the connection element or the wire or cable. Thereby in fact the weight of the section of the elongated element hanging above the supporting surface is measured. However, a simple subtraction from the initial weight provides the required weight value. An advantage of this embodiment is that any dirt, oil, or other strange materials present on the supporting plate are not influencing the measurement. A disadvantage, however, is that the measurement device should be embodied such that it is mechanically able to carry the weight of the flexible elongated element hanging from this measurement device. Furthermore the signal communication between the measurement device and the remote data logger/processor could be more complicated. The invention will now be explained in more detail with reference to the attached drawings. Figure 1 illustrates schematically an installation for driving an elongated object such as pile into the ground combined with a system according to the invention for measuring the penetration depth of said elongated object into the ground. Figure 2 illustrates a sample of the signals, derived by the weight sensor in the system according to figure 1. Figure 3 illustrates another embodiment of a system according to the invention. Figure 4 illustrates a further embodiment of a system according to the invention. Figure 5 illustrates the use of a system according to the invention in combination with an installation for driving a pile non-vertically into the ground. The figures 6, 7, 8 illustrate various modifications of chains which can be used in the system according to the invention. Figure 9 illustrates a sample of a signal derived from the weight sensor in case a vibratory hammer is used in the system. Figure 10 illustrates a signal sample obtained from the weight sensor in case the system is used for driving a drill into the ground. Figure 1 illustrates schematically an installation for driving a pile or tube into the ground in which system the invention is embodied. The pile driving frame, comprising a lot of well-known components, is in general indicated by 10 and will not be discussed in detail assuming that the average expert is familiar with such piling rigs. The hammer unit 11 is guided along a leader 12 and is acting upon a foundation pile 13. One end of a wire or cable 14 is in this embodiment attached to a part of the hammer 11. The installation 10 is equipped with a system according to the invention comprising as essential components the wire 14, the flexible elongated element 18 and the weight sensor 20. The wire or cable 14 runs over a pulley 15 just underneath the crown post 16 of the leader 12. From the pulley 15 the cable 14 runs downwards and ends into a connection device 17. Through this connection device 17 the respective end of the cable 14 is connected to a flexible elongated element embodied in this case as a chain 18. As is illustrated in figure 1 a section of the chain 18 is hanging from the wire or cable 14, another section of the cable is laying on a support surface which in this case is embodied as the bottom of a container or reservoir 19 near the lower end of the leader 12. A weight sensor 20 is installed between the container or reservoir 19 and the lower section of the leader 12 to measure the weight of that section of the cable which is still resting on the bottom of the container 19. The weight sensor in this embodiment is connected through a wireless communication link 23a, 23b to a data logger/processor combination 21 which can be installed in the cabin 22 of the system unit 10. It will be clear that for implementing the wireless communication link 23a, 23b the weight sensor 20 should be combined with at least a transmitter and the data logger/processor should be combined with a receiver. Details thereof are considered well known to the average expert. Before the installation can be used the system according to the invention has to be calibrated. For that purpose the hammer unit 11 is moved to a first height H1, whereby preferably almost the complete weight of the chain rests upon the bottom of the container 19. In this position the weight W1 of the chain in the reservoir 19 is measured and stored in the data logger 21. Thereafter the hammer unit 11 is moved to a second height H2, whereby preferably only a small section of the chain is still resting upon the bottom of the container 19 and the weight W2 of that small section is measured and stored in the data logger 21. If the height difference H1-H2 is accurately known, then it will be clear that a displacement of the pile 13 (= a corresponding displacement of the hammer unit 11) over one unit of length corresponds with a measured weight difference of Δ W = (W1-W2)/(H1-H2). As an alternative the calibration can be carried out with a non-active piling rig without moving the hammer unit. The end of the wire 14 is moved by hand to a first height H1 and the weight W1 is measured. Thereafter the end of the wire is moved by hand to a second height H2 and the weight W2 is measured. After performing the above mentioned calculation the calibration procedure is finished. During operation first of all the pile 13 is directed alongside the leader 12 and the hammer unit 11 is brought into position on top of the pile 13. Thereafter, but before the hammer unit is activated, the weight of that section of the chain 18, which is still resting on the bottom of the container 19 is measured by means of the sensor 20 to get a weight value representing the initial situation. Thereafter the hammer unit 11 is brought into operation and with short time intervals the weight of the section, still remaining on the bottom of the reservoir is measured. In this embodiment the measured weight values will show a gradual decrease until the moment that the pile reaches a firm bottom layer. From that moment on the decrease in the series of measurement values will stop or at least slow down significantly indicating to the operating personnel, monitoring the measured values on the display of the processor 21, that the firm ground layer is reached. As will be explained in more detail with reference to figure 2, preferably, the weight measurements are carried out in synchronisation with the moment at which the hammer 11 strikes the pile 13. As is described for instance in the above-mentioned Japanese specification JP 58-94525 a detecting means can be used including a vibration sensor to detect every stroke made by the hammer 11. In figure 2 the signal, derived from the weight sensor 20 and received through the wireless communication link 23a, 23b in the processor 21 is illustrated in figure 2 in which the signal amplitude, corresponding with the measured weight and therewith with the penetration depth of the pile 13 is illustrated as a function of the time. It is assumed that in the initial situation the signal starts in rest in the origin with an initial amplitude a. At time T1 the hammer 11 strikes the top of the pile 13 for the first time and the blow results into a strong oscillatory signal at the output of the weight sensor 20. In the following time period the oscillations in the signal of the weight sensor are mainly damped out such that just before the moment T2 the signal has reached approximately a steady state with an average amplitude b. It will be clear the amplitude difference a-b represents the weight of that section of the chain 18 which as result of the first blow is lifted from the supporting surface, i.e. the bottom of the reservoir 19, and represents therewith the penetration of the pile 13 as a result of the first blow. At time T2 the hammer 11 strikes again, resulting again in a damped oscillatory signal reaching after sometime the steady state with an average amplitude c. Just before the further moment T3 the penetration depth is represented by (a-c). Preferably the processor 21 comprises a circuit for detecting the first relatively high amplitude pulse directly following each blow. These blows are counted and the number of blows over a certain penetration depth yields the so-called "blow count". The "blow count" is a measure for the soil resistance. Furthermore the computer preferably measures the time interval between two successive blows. From this time measurement the so-called "blow rate" (the number of blows per time unit) can be calculated. In some cases the blow rate forms a measure for the hammer energy. The actual peak value of each high amplitude pulse following a blow forms a measure for the intensity of the hammer blow and the energy delivered by the hammer. It will be clear from the above description that no additional vibration sensor is necessary for obtaining information about the number of blows, the intensity of each blow and about the number of blows per time interval. An alternative embodiment of the system according to the invention is illustrated in figure 3. The system according to figure 3 comprises in fact the same components as the system illustrated in figure 1 with the difference that the weight sensor 20 is now combined with the connection element 17. In this embodiment the weight sensor 20 measures in fact the weight of that section of the chain 18 which is actually hanging through the connection element 17 on the cable 14. It will be clear for the expert in this field that the weight sensor 20 can be attached to the connection element 17 or even incorporated therein in various ways. A further difference between this embodiment and the embodiment illustrated in figure 1 is residing in the fact that the weight values, measured by the sensor 20 are transferred through a wire 23 to the processor/data logger 21. The wire 23 runs in a suitable way between the sensor 20 and the processor 21. Various ways of implementing such a connection are considered known to the expert in this field, so that it is considered superfluous to provide details thereof. It will be clear that the sensor 17 can also be installed on the hammer unit 11 such that the wire 14 is connected to the sensor 17. To illustrate the various ways in which the invention can be implemented the wire 14 is in figure 3 connected to the lower part of the hammer unit 11. It is also possible to connect the wire 14 directly to the pile 13 near the top thereof as is illustrated in figure 4. In this figure the elongated flexible measuring element 18 is embodied as a flexible rope having a sufficient weight per length unit to enable the weight sensor 20 underneath the reservoir 19 to measure weight differences with acceptable accuracy. The signals generated by the sensor 20 are transferred through a cable 23 to the data logger/processor 21, which in this embodiment is a handhold device operated by a person 24. In this embodiment the upper end of the flexible rope 18 is connected to a clamp 28 which is fixed around or onto the top section of the pile 13. It will be clear that the upper end of the rope 18 could also be attached to the lower part of the hammer unit 11 with the same results. In both embodiments illustrated in figures 1 and 3 a protective tubing is positioned around that section of the cable or wire 14 which runs from the pulley 15 downwards inside the leader 12. This protective tubing is to be considered as an option and is not necessary for bringing the invention into practice. In figure 4 for instance such a tubing is not used. In figure 1, figure 3 and figure 4 the leader 12 takes an upright position such that from the pulley 15 the cable 14 extends vertically. Although this situation is ideally suited to make very accurate measurements there are conditions under which piles are driven into the ground under a specified predetermined angle. However, also under these circumstances the invention can be used with very good results. Tests have been carried out with piles which were driven under an angle into the ground and the results of those tests were very satisfying. Figure 5 illustrates a practical situation whereby the pile is driven in a non-vertical direction into the ground. Under these circumstances it is possible (although not necessary) to locate the reservoir 19 outside the leader 12. The cable 14 runs from the pulley 15 eventually along a further pulley 15' and extends from there downwards until the connection 17 with the chain 18 which also runs in a vertical direction. The reservoir 19 is installed in a suitable position such that the weight of the whole reservoir can be measured by means of the weight sensor 20. The weight sensor 20 is through a cable 23 connected to the data processor/data logger 21 which in this case is embodied as a hand-held device, operated by a person 24. Although the container 19 in the illustrated configuration rests upon constructional parts of the drilling rig 10 it will be clear that the container can also be placed on the ground as long as the weight sensor 20 is able to carry out its function. The elongated element 18 can be embodied as a generally known chain made of a plurality of interconnected links of the type which is very schematically illustrated in figure 6. Therein the chain 18a comprises a number of ellipsoidal, round or otherwise suitable shaped interconnected links 22a, 22b, ..... 22n. Another embodiment of an elongated flexible element is illustrated in figure 7. In this embodiment the flexible elongated element comprises a series of weight elements 23a, 23b, .... 23n which are interconnected by means of small eyes 24a, 24a', 24b, .... at both sides such that in fact a chain is formed. Another embodiment is illustrated in figure 8 and consists of a wire or robe 25 carrying weight elements 26a, 26b, ..... 26n at mutually equal distances. Instead of a chain also a wire, cable or rope can be used as elongated flexible element (see embodiment illustrated in figure 4) as long as the uniformly distributed weight thereof is sufficient to enable accurate weight measurements by the sensor 20. All the above-described embodiments were specifically directed to installations for driving a pile into the ground. As is already remarked in the introductory part of this specification the system can also be used in combination with installations for driving a sheet pile, a tube, a drill or other elongated objects into the ground. Furthermore the invention is not restricted to systems, in which impact hammers are used as tool for driving the elongated object into the ground. Instead of an impact hammer also a vibratory hammer can be used or, in case of drilling, a drill rotating head. In case a vibratory hammer is used the signal derived from the sensor 20 has a somewhat different shape. Figure 9 illustrates the respective signal consisting of an essentially monotonous descending oscillatory signal, the oscillations corresponding with the vibratory movement of the hammer unit. At successive time intervals the average value over a short time period is taken. If (a) is the reference level, at which the system was initiated, corresponding with ground level of the under surface of the pile, then (a-b), (a-c), etc. represents the penetration of the pile at each successive measurement. Again the penetration increment per time interval yields the penetration rate. The penetration rate is a measure for the resistance of the soil. The amplitude of the oscillating signal forms a measure for the power supplied by the vibratory hammer. It is remarked that in this embodiment the time moments at which the main value are determined, are not related or synchronized with each blow of the hammer unit. In this case preferably the processor or data logger includes a timer which determines regular intervals at which the main values are determined. In case the system is used for driving a drill into the ground, for instance for forming a bore hole in which a pile can be casted in situ, or for other purposes, then the signal will be a more or less smoothly decreasing signal as illustrated in figure 10. In the areas a, b, c, etc. at successive time intervals the mean value of a short time interval is taken. If (a) is the reference level (for instance ground level) then (a-b), (a-c), etc. represents the penetration of the pile at each successive measurement. The penetration increment per time interval yields then the penetration rate and the penetration rate is a measure for the resistance of the soil. In the above-described embodiments the elongated object was driven into the ground. However, in most cases the same rigs or other rigs can be used to pull or otherwise remove elongated objects from the ground. Also under these circumstances the measuring system according to the invention can be applied with the same accurate results. It is remarked that the invention is not restricted to the shown embodiments and that various other embodiments are conceivable without leaving the scope of the invention.

Claims: System for measuring the penetration depth of an elongated object such as a pile (13), tube, sheet pile or drill into the ground, to be used in combination with an installation for bringing the elongated object into the ground for instance by pressing, hammering, vibrating, drilling or lowering, or for pulling or otherwise removing said elongated object out of the ground, said system comprising means for measuring the displacement of the elongated object, characterized in that a flexible elongated element (18) the weight of which is uniformly distributed along its length is directly or indirectly connected to said elongated object, said length being at least equal to the maximum displacement of the elongated object, whereby at least part of said elongated element is supported by a supporting surface (19), and that means (20) are present to determine the total weight of that section of the flexible elongated element which is supported by said supporting surface. System according to claim 1, characterized in that the flexible elongated element is embodied as a chain made of a plurality of interconnected links. System according to claim 1, characterized in that the flexible elongated element is embodied as a wire or cable to which separate weight elements are connected at mutual distances. System according to claim 1, characterized in that the elongated element consists of a relatively flexible wire, rope or cable with a substantial weight per unit of length. System according to one of the preceding claims, characterized in that the section of the elongated element not supported by said supporting surface extends substantially vertical during operation of the system. System according to one of the preceding claims, characterized in that the flexible elongated element is connected to the elongated object by means of a connection element such as a clamp. System according to one of the preceding claims 1-5, characterized in that the flexible elongated element is connected to the device for hammering, vibrating or drilling the elongated object into the ground by means of a connection element such as a clamp. System according to one of the preceding claims 1-5, characterized in that one end of the flexible elongated element is connected to one end of a wire or cable of which the other end is connected to said elongated object. System according to one of the preceding claims 1-5, characterized in that one end of the flexible elongated element is connected to one end of a wire or cable of which the other end is connected to the device for hammering, vibrating or drilling the elongated object into the ground. System according to one of the preceding claims, characterized in that the means for determining the weight of said section of the flexible elongated element supported by said supporting surface comprise a weight measuring device installed underneath said supporting surface. System according to claim 10, characterized in that the supporting surface comprises a plate which forms part of a container of which the dimensions are selected such that the complete elongated flexible element will fit within said container. System according to one of the claims 1-10, characterized in that the means for determining the weight of said section of the flexible elongated element supported by said supporting surface comprise a weight measuring device installed between the respective one end of the flexible elongated element and the connection element or the wire or cable. System according to one of the preceding claims, characterized in that at least part of the elongated element not supported by the supporting surface is surrounded by a protective tubing. System according to one of the preceding claims, characterized in that the weight measuring device is connected through a data communication link to a data processor/data logger. System according to claim 14, characterized in that the data communication link is established by means of a wire connection between the measuring device and the data processor/data logger. System according to claim 14, characterized in that the communication link is established by means of a wireless communication path for which at least the weight measuring device is combined with a transmitter and at least the data processor/data logger is combined with a receiver.

Inventors: MIDDELDORP FREDERIK MARIA, MIDDELDORP, FREDERIK MARIA
Applicants: TNO, NEDERLANDSE ORGANISATIE VOOR TOEGEPAST-NATUURWETENSCHAPPELIJK ONDERZOEK TNO
Application year: 1993
Main classification: E02D13
IPC codes: G01B21, E02D13