US3772892A - Process of installing compacted sand columns in the ground - Google Patents

Abstract

Process of installing compacted sand columns in the soft ground wherein the underground depth at the lower end of the casing pipe, the discharged sand volume and the strength of the original or stabilized ground are continuously detected, recorded, indicated in the operation meter and controlled as viewed from the design.

Description

United States Patent [191 Ogawa [111 3,772,892 [451 Nov. 20, 1973 PROCESS OF INSTALLING COMPACTED SAND COLUMNS IN THE GROUND [76] Inventor: Mitsuro Ogawa, 31, l-Chome,Tanabehonmachi, Higashisumiuoshi-ku, Osaka. Japan [22] Filed: June 9, 1972 [21] Appl. No.: 261,540

[30] Foreign Application Priority Data Sept. 18, 1971 Japan 46/72882 [52] U.S. Cl 61/35, 61/11, 61/63 [51] Int. Cl E02b 11/00, E02d 3/08 8] Field of Search ..61/35,11, 13, 63,

[56] References Cited UNITED STATES PATENTS Staunau 61/63 X 3,255 ,592 6/1966 3,300,988 1/1967 3,648,467 3/1972 3,707,848 1/1973 Chelminski 61/35 Primary ExaminerMervin Stein Assistant ExaminerPhilip C. Kannan Attorney-Eric l-l. Waters et al.

[57] ABSTRACT Process of installing compacted sand columns in the soft ground wherein the underground depth at the lower end of the casing pipe, the discharged sand volume and the' strength of the original or stabilized ground are continuously detected, recorded, indicated in the operation meter and controlled as viewed from the design.

17 Claims, 16 Drawing Figures PMENIED mw 2 n 1973 SHEET 3 CF 4 FIG. IO

TOTAL DISCHARGED SAND VOLUME ()ISu) FIG. 7

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0 5&5

A I l 6. M23? Qz w SI 52 (ZSu) FIG. 9

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O 2 l 2 l0 D Am wm2D$ An: Ikmwo AQv mm ozqm PROCESS OF INSTALLING COMPACTED SAND COLUMNS IN THE GROUND DETAILED DESCRIPTION OF THE INVENTION A The present invention relates to a process of install ing compacted sand columns in the ground for the purpose of stabilizing a soft ground, and more particularly it relates to a process of installing compacted sand columns wherein the installing process is performed while detecting and controlling the state of the compacted sand columns being installed in the ground in the installation process for the purpose of installing compacted sand columns as those required basing upon a design.

A process of stabilizing the soft ground by the steps of forming a large number of loose or uncompacted sand columns in the soft ground and then imposing a load on the ground surface thereby causing the water content in the soft ground to be discharged out of the ground surface by way of the drain formed by the sand columns has been known as a so-called Sand Drain Method. This known method has been popularly utilized in order to stabilize the soft ground, but there are drawbacks such that it takes a considerable long time to dehydrate the water content in the soft ground and also to require a great deal of labor to impose a load and remove the imposed load.

As an improved method in the afore-mentioned Sand Drain Method, the method as mentionedbelow has been also known. That is, this known method is carried out by installing a large number of compacted sand columns in the soft ground, each having a diameter larger by about 1.3 to 2.0 times than that of the casing pipe, applying a pressure to the soft ground around the sand columns by said compacted sand columns, and then rapidly discharging the water content contained in the soft ground out of the ground surface through the sand columns thereby stabilizing the soft ground to the consolidated state. Furthermore, this method can be effectively applied on an extremely soft ground in place of methods of dredge or sand replacement. In this case, compacted sand columns are successively installed in a closely neighboring state, and the soft soil at optional portion in the ground is forcibly excluded and replaced with a favorable material such as sand for a desired volume at a predetermined replacement ratio. The abovementioned process of installing the compacted sand columns in the ground is carried out in the following manner: that is, at first the casing pipe in the hollow state is driven into the soil down to a predetermined depth, and thereafter the driven casing pipe is pulled up by an appropriate length upward, and then the casing pipe is filled with sand inside thereof, and the filled sand is caused to flow down into the cavity formed within the soild at the lower portion of the casing pipe by the pulling up process, and the causing pipe is again driven into the soil in order to compact the sand flowed down into said cavity. Then, the casing pipe is pulled up by an appropriate length, and thereafter repeating the same process until compacted short sand columns are installed consecutively from the required depth up to the ground surface.

In the installation of the above-mentioned compacted sand columns, it is possible to estimate the volume of sand supplied into the casing pipe by measurement on the ground, but it is impossible, at the pullingup of the casing pipe in the sand column installation process, to assure that the same volume of sand as that of the cavity formed in the lower portion at the lower end of the casing pipe by the pulling-up is discharged downwardly out of the casing pipe. Thus, when the installation process is not carried out properly, the compacted sand column being installed is liable to be disrupted halfway, or a defective forms of sand column such as having an extremely small diameter due to the insufficient supply of sand volume to be compacted may be produced. Also it is unavoidable that the judgement is not correct as to determine whether or not the installed compacted sand column has a sufficient strength as compared with the required strength. Consequently, it is often necessary to take a measure such as supplement of installing further number of compacted sand columns which are found to be necessary as a result of an investigation of the ground after the installation of the compacted sand columns has been finished.

The formed diameter of a compacted sand column, the discharging volume of sand downwardly of the casing pipe at each of the depths, the volume of presscharging sand into the ground, the density of compacted sand column being installed, strength of the sand column, etc., are important factors to be controlled for installation as viewed from the design and the effect of the ground improvement of the compacted sand columns in the stabilizing construction work of the soft ground. In the installation of the conventional compacted sand columns, however, since the installation of the sand columns is carried out under the ground, the'desired installation of the compacted sand columns could only be obtained by relying upon the experience and skill of the operators of the sand column installation apparatus.

The principal object of the present invention is to provide an installation process of compacted sand columns by improving the process of incorrect installation process which relies upon the so-called intuition of the operators of the apparatus for above-mentioned con-' ventional'compacted sand column, namely, by carrying out, in the process of installation of compacted sand column, in detecting and controlling the press-charged sand volume or strength of the sand column, or both thereof, which are the principal values determined from the design of the compacted sand columns, whereby it is possible to obtain the installation of the compacted sand columns which is favorable and reliable just as that designed and, at the same time, by judging and correcting each time in the installation processes for a variation of the ground under installation against the designed values, thus making it possible to install compacted sand columns which are highly accurate and which conform to the designed object.

Other objects, construction and effects of the embodimental process of the present invention will clearly be understood by the following description referring to the drawings, in which FIG. 1 is a view for explaining successively the process of installation of a compacted sand column,

FIG. 2 is a view for explaining briefly an example of a depth detecting device used in the process according to the present invention,

FIG. 3 is a view for explaining briefly an example of a sand volume detecting device used in the process of the present invention,

FIG. 4 is a view for explaining briefly an example of a strength detecting device used in the process of the present invention,

FIG. 5 is a view for explaning briefly an example of a recording device used in the process of the present invention,

FIG. 6 is a view for explaining briefly an example of a recording device used in the process of the present invention,

FIG. 7 is a view for explaining a behavior of the sand in one operation cycle in the installation of the compacted sand column,

FIG. 8 is a graph showing an example of a recorded graph depicting the depth and the sand volume respectively against the time axis,

FIG. 9 is an enlarged view of the portion of one operation cycle in a part of the recorded graph shown in FIG. 8,

FIG. 10 is a graph showing an example of a depth total discharged sand volume, 1

FIG. 11 is an enlarged view of the portion of one operation cycle in a part of the recorded graph shown in FIG. 10,

FIG. 12 is a graph showing an example of a recorded graph depicting the depth and the strength of the sand column respectively against the time axis,

FIG. 13 is a graph showing an example of the graph of depth strength of sand column,

FIG. 14 is a block diagram showing controlling equipments used in the process of the present invention,

FIG. 15 is a front elevation showing the meter portion of an example of an operation meter used for controlling press-charging sand volume, and

FIG. 16 is a front elevation showing the meter portion of an example of an operation meter used for controlling the strength of the sand column.

The outline of the installation of the compacted sand columns has already been mentioned above, and now the description thereof will be given in detail referring to FIG. 1. In the installation of the compacted sand column, a motor-driven driver 2 utilizing a vibrator force or an impact forcehaving a rapid cycle is mounted on the top portion, and together therewith a steel casing pipe 1 provided with a sand supplying hopper 3 at the upper portion and with a neck portion 4 or an opening and closing cover for preventing the backflow of the sand at the lower end portion are used. A compacted sand column So having a diameter larger than that of the casing pipe 1 extending from the predetermined depth to the ground surface is installed by driving the casing pipe 1 into the ground up to the predetermined depth by means of the vibratory force or the impact force (refer to FIG. 1, I), supplying sand 5 injo the easing pipe 1 through a hopper 3 (refer to FIG. 1,11), pulling up the casing pipe 1 by an appropriate length and discharging the sand 4 contained in the casing pipe into a space formed below the lower end of the casing pipe (refer to FIG. 1, III), then driving again the casing pipe 1 for an appropriate length by the vibratory or impact force so as to press-compacting the sand which has been discharged into said space at the lower end portion of the casing pipe and at the same time expanding the sand into the surrounding soil (refer to FIG. 1, IV), and by repeating this operation cycle successively up to the ground surface (refer to FIG. 1, V).

Firstly, the installation of the compacted sand column 50 will be described with respect to the installation process according to the present invention which is carried out while detecting and controlling the volume of the sand, that is, the press-charged sand volume for the compacted sand column. The installation of the compacted sand column is carried out as described above referring to FIG. 1, and a depth detcting device is used for detecting, at each moment, the underground depth D of the lower end of the casing pipe 1. Virous types of the depth detecting device have been contrived. The depth detecting device illustrated in FIG. 2 is so constructed that a tension wire 6 connected at the upper portion of the casing pipe 1 is passed around a depth detecting drum 7, thus changing the up and down movement of the casing pipe 1 into the rotation of the depth detecting drum 7, further changing rotation into a change in resistance value of a potentiometer 8 associated with the depth detecting device 7, taking out an output voltage Vd out of the potentiometer 8 which is proportional to the depth of the casing pipe 1, whereby the depth of the casing pipe 1 is detected. Next, in order to detect, at each moment, the volume of sand discharged from the lower end of the casing pipe into the cavity formed at the lower portion of the casing pipe 1 by its pulling-up, a sand volume detecting device is employed. Although various 'types of sand volume detecting devices are considered, the sand volume detecting device shown in FIG. 3 is found to show a favorable result. This device employs an electrode plumb 9 as the detecting end of the varying sand surface 50 in the casing pipe 1, the electrode plumb is suspended by a cable wire 10, and the cable wire 10 is adapted to be wound and unwound by a winding drum 11 fixed at a proper position. At the same time forming an electric circuit consisting of the electrode plumb 9 cable wire 10 control unit 12 conductive pipe wall of the casing pipe 1 sand 5 containing some water electrode plumb 9 so as to efl'ect a switching action by whether the electrode plumb 9 and the sand surface 50 coming into contact or not. Thus, by controlling the operation of a driving motor 13 of the winding drum using said control unit 12 so that the electrode plumb 9 may follow the sand surface 50 which fluctuates constantly, and taking out the output voltage Vs proportional to the position of the sand surface 50 (the height from the lower end of the casing pipe) by converting the quantity of movement of said cable wire 10 into the change of resistance value of a potentiometer 14 associated with the winding drum 11, whereby the position of the sand surface in the casing pipe 1 can be detected continuously. In this drawing, the device 15 is a reduction gear.

As will be described later, the change is sand volume which has a significant meaning on the control of installing compacted sand column is only the discharge of sand from the lower end of the casing pipe or the diminishing process of the sand volume in the pipe. Thus, as for the sand volume detection, the sand volume detecting device is not restricted to the above type where the detection is effected continuously, but it is also possible to employ such a sand volume detecting device which has a construction, for example, that a plumb suspended by a wire is lowered only at the discharge of sand and detect the sand volume from the length of the wire.

FIG. 5 shows an example of a recording device for plotting a graph, change obtained as mentioned above from both the depth detecting device and sand volume detecting device. In this example, in order to obtain two records of said depth of the casing pipe and the sand volume in the casing pipe, the recording device has two driving elements for pens. Firstly, the pen-driving element 16 for detecting the depth of the casing pipe will be explained. The voltage Vd obtained from the depth detecting device is compared, by a comparing amplifier, with the voltage of a potentiometer which electrically represents the position of the recording pen 17 on a recording paper. When there is a difference between both of the voltages, a balancing motor is rotated and which moves the potentiometer, and the recording pen always assumes a position corresponding to the voltage Vd obtained from the depth detecting device. On the other hand, since the recording paper is moved at a constant speed by a synchronous motor 18, the depth D of the casing pipe is plotted against the time axis 1. Nextly, as

'for the driving portion of the pen driving element 19 concerning the sand volume in the casing pipe, it is similar to that mentioned above, but this element is further provided with an operating element 20, which calculates and plots the sand volume S in the casing pipe on the time axis ton the recording paper, with a recording pen 17, corresponding to the output voltage V, corresponding to the height of the sand surface SL in the casing pipe 1 obtained from the sand volume detecting device. The recorded graph continuously plotted by this recording device is shown in FIG. 8.

FIG. 7 shows a behavior of sand in one operating cycle for installation of compacted sand column shown in FIG. 1, which is supply of sand into the casing pipe 1 discharge of sand from the lower end of the pipe due to the pulling-up by an appropriate length of the casing pipe 1 compacting of sand discharged by redriving of the casing pipe 1 That is, in FIG. 7, D,, D D show respectively the depths of the lower end of the casing pipe 1 in said one operating cycle, SL shows a height of sand surface of the sand 5 in the pipe immediately before the pulling-up of the casing pipe 1, SL shows a height of sand surface of the sand 5 in the pipe after the pulling-up of the casing pipe 1. Accordingly, the sand colume S in the casing pipe 1 is plotted, as

with a recording pen, of voltage volume or press-charged sand volume, in the recording graph shown in FIG. 8, with the reference values of predetermined depthpress-charged sand volume required by design, it is possible to obtain compacted sand columns to be installed having a desired presscharged sand volume as designed.

Nextly, assuming that the weight per unit volume of sand in the compacting process became A times the weight per unit volume of sand in the casing pipe, the diameter R of the compacted sand column formed in one operation cycle can be obtained from the following equation by a simple calculation:

R V X 1 id/( 1 3) (1) Since the value of A in the above-shown equation can practically be estimated, through the results of various measurements in the past, from the initial strength of the ground, kind of sand to be used as the material of the sand column, kind of driver for easing pipe used for the installation, etc., the diameter of the compacting sand column can readily be determined gy applying the above equation on FIG. 9.

Nextly, in the pulling-up process of the casing pipe, when the sand in the pipe is not discharged continuously, but discharged discontinuously, a cavity is produced at the lower portion of the casing pipe at the time when the discharge is disrupted, and a soft and weak soil of the original ground such as waterly mud (I-IEDORO) enters into the cavity, which deteriorates the function of the compacted sand column remarkstated above, in the recording device by calculation of SL X K (where K is a constant related to the sectional area of the casing pipe) by means of the calculation element 20.

Thus, in this one operation cycle, the discharged sand volume discharged downwardly from the lower end of the casing pipe 1 is shown by (SL SL X K, and the length of the compacted sand column portion by the redriving of the casing pipe 1, is shown by D D The press-charged sand volume of the compacted sand column portion is (SL SL X K, and the presscharged sand volume per unit depth is shown by (SL 2) X 1 3)- FIG. 9 is a graph showing briefly enlarged parts corresponding to one operation cycle of FIG. 8. In this graph, D D shows the length of the compacted sand column portion formed in one operation cycle, and S 8, shows the discharged sand volume in one operation cycle. Accordingly, when a redriving operation of the casing pipe 1 is carried out in each of the operation cycles confirming the coincidence of the discharged sand ably. For this reason, it is necessary to judge whether the discharging condition of sand in the pipe is carried out properly in the pulling-up process of the casing pipe. For this judgement, the pullingup process of FIG. 9 is to be noted, and the pulling-up speed dD/dt of the casing pipe in FIG. (a) and the discharging speed dS/dt of the sand in the pipe shown in FIG. (b) are compared. This comparison shows, when the gradient of graph in FIG. (a) and that in FIG. (b) are compared on each of the time in the pulling-up process, that the discharge of the sand is favorable if both of the gradients are equal, and if the latter gradient is smaller than the former, the discharge of the sand becomes more unfavorable.

As described above, by processing the operation of the casing pipe using the recorded graph shown in FIG. 8 recorded by the recording device shown in FIG. 5, it is possible to install desired compacted sand columns which have a predetermined press-charged sand volume and a diameter as well as a favorable form, but practically it is comparatively difficult to control the operation of casing pipe basing on the recorded graph depicting the depth and the sand volume respectively on the time axis t shown in FIG. 8.

Accordingly, as for the more desirable recorded graph, a depth total discharged sand volume graph is employed. To plot this recorded graph, a recording device shown in FIG. 6 is used. In this recording device, the recording paper is fixed, and a lever 21 is moved up and down by a pen driving element 16 controlled by the output voltage Vd from the depth detecting device mentioned above; a recording pen 22 is so constructed that it moves right and left on the lever 21 by a pen driving element 23 controlled by the output voltage Vs from said sand volume detecting device. The pen driving element 23 is provided with a calculating element 24, which is so constructed that a total discharged sand volume ESu is plotted against the depth axis D on the recording paper by means of the recording pen 22.

An example of the depth total discharged sand volume graph obtained by the above-mentioned recording device is shown in FIG. 10, the total discharged sand volume ESu is recorded against the depth D in a saw-toothed shape. FIG. 11 shows a graph which is a brief, enlarged view of parts corresponding to one operation cycle in FIG. 10. In this graph, the part m-n shows the discharged sand volume Su at the pulling-up of the casing pipe 1, n-p shows the volume of the compacted sand column made by one operating cycle. In FIG. 10, therefore, the discharged sand volume at each of the operation cycles is totaled, and the graph shows the total discharged sand volume ESu as a whole. In FIG. 10, the broken line shows a predetermined reference line for depth press-charged sand volume obtained from the design, and this line is previously to be included on the recording paper.

Thus, in re-driving of the casing pipe in each of the operation cycles from the above-mentioned recording graph, by effecting each of the re-driving operation of the casing pipewhile confirming the coincidence of the press-charged sand volume with the predetermined standard value of the depth press-charged sand volume, it is possible to ensure quantitatively the presscharging volume of sand at each of the depths of the compacted sand column being installed under the ground, and the compacted sand columns can be installed which are favorable and which confirm with the predetermined design.

Next, to estimate the diameter of the installed, compacted sand column from the hove-shown FIG. 11, since (S S )/(D D in the radical sign on the right side of said equation (1) becomes the gradient of the broken line m-p in FIG. 11, the diameter of the compacted sand column can be estimated using said equation (1) as it is. Next, in order to assertain the suitability of sand discharge at the pulling-up process of the casing pipe, it is possible to apply the method for interpreting said suitability of discharge taking notice that the gradient, at each point on the pulling-up process m-0, represents dS/dD. That is, in the above-mentioned method for interpretation of the suitability of discharge, the values of dS/dt and dD/dt are compared, but by eliminating the time t therefrom, the value of dS/dD can be compared with 1. Thus, the suitability of sand discharge can be estimated by comparing the gradients at each point on m-o with the gradient of 1 (45).

The principle of two methods of interpretation mentioned above can be used independent of the manner of taking the axes of recording. Also, in these methods of interpretation, the change in sand volume is necessary only at the discharge of sand when the casing pipe is pulled up, so that the interpretation is never hindered even when such a type of sand volume detecting device is used, where the measurement is carried out only at the discharge of sand.

With the above-mentioned graphical record of the depth total discharged sand volume, when the predetermined reference line for depth discharged sand volume, which has been obtained from the design, is shown on the recording paper as stated above, it is possible to control the installing operation easily and surely, in observing the graph being plotted, at each of the operation cycle of the compacting sand column installation, by re-driving the casing pipe 1 by the operator until the vertical lowering tip end of the graph coincides with the abovementioned reference line. As stated above, the depth total discharged sand volume graphical record is not only utilized directly for installation process of compacted sand columns in which the press-charged sand column is controlled by measurement, but also it is necessary as a past record of the installation process of the compacted sand column.

As described above, it is more or less difficult practically for the operator of the casing pipe to carry out the installation while observing directly the recording process in the recording device. Because, the operator of the casing pipe 1, in general operates on a transferable driving machine which effects the process and transfer of the casing pipe 1, and the recording device is subjected to an unfavorable vibration or the like. Therefore, it is desirable to effect the recording of the graph by a recording device at a separate location, and as shown in FIG. 15, providing an operation meter 26 associated with the above-mentioned recording device 25 at a proper location on the transferable driving machine, whereby the operator can carry out the installation of the compacted sand columns as before in controlling the installation by means of the indication of the meter. FIG. 15 shows briefly an embodimental example of the operation meter 26 mentioned above, and the reference value of depth press-charging sand volume obtainable from the design can be memorized in the operation meter 26. On the left side of the meter, there is shown the depth D of the casing pipe 1 which is detected by a depth detecting device, and on the right side thereof, the re-driving depth Dd of the casing pipe 1 which satisfies the reference value of the presscharging sand volume at different depths. Accordingly, the operation of the casing pipe 1 may be effected while observing the indication of the depth D at the left side until it conforms with the indicated depth Dd shown at the right side. It is needless to say that various kind of indication type may be used as the operation meter beside that mentioned above.

The above is the explanation of the installation process of the compacted sand column carried out while controlling the press-charged sand volume, but it is also necessary in controlling the installation to effect the installation of the compacted sand column while controlling the strength of the sand column.

In'the case where the ground to be stabilized is a sandy soil, the effect of the compacted sand column is an increase in strength due to a diminution of void ratio of ground considered as average, and when the ground to be stabilized is a clayey soil, the effect is the increase in shearing resistance of the so-called composite ground that is the combination of the increase in the strength due to consolidated dehydration of surrounding clayey soil by the expansion of the sand column and the large internal friction angle of the compacted sand column itself. Accordingly, what is important in the stabilization of soft ground by the installation of the compacted sand column is to proceed the design grasping correctly th e strength of the original ground and to install the sand columns having a predetermined strength (degree of compactness) basing upon the design.

An explanation will be given in the following on the process according to the present invention which carries out the installation of the compacted sand column while measuring and controlling the strength of the compacted sand column.

For detecting the strength of the above-mentioned compacted sand column, a strength detecting device is used. Various types of the strength detecting device have been considered, and the strength detecting device for a sand column shown in FIG. 4 is employed in a case where a motor-operated driver 2 is used, wherein, in applying the fundamental conception of the formula for dropping hammer used in pile driving, the strength C of the ground or the sand column can be estimated from the formula:

where E electric power consumed in a motor M for the driver 2,

V penetrating speed in the driving or re-driving process of the casing pipe,

m, n, a, B constants.

Thus, the penetrating speed of the casing pipe 1 is converted into a voltage by a penetration speed detecting device 27 connected to the depth detecting drum 7. On the other hand, the electric power consumed in the motor M of the driver 2 is converted into an electric voltage by a power detecting device 28. Both of the voltages are operated basing on said formula (2) by means of an claculating element 29, and a voltage Vc corresponding to the strength C of the sand column is worked out of the calculating element 29. As for the strength detecting device, it is also possible to use those of type in which the strength is obtained from the strain of the casing pipe.

Next, as for the recording device for plotting a graph of the voltage changes Vc, Vd, with recording pens, from the strength detecting device obtained as above and from the above-mentioned depth detecting device, a recording device corresponding to those shown in FIGS. 5 and 6 described above is used. This recording meter is so contrived that meaningless measured values of the strength such as those during the pull-up will not be recorded. A recorded graph shown in FIG. 12 is obtained by a recording device corresponding to that shown in FIG. 5, and a recorded graph shown in FIG.

13 is obtained by one corresponding to that shown in FIG. 6. Namely, on the recorded graph shown in FIG. 12, there are plotted the depth D, the strength Gs of the original ground and the strength C of the compacted sand column, respectively, against the time axis t, and on the recorded graph shown in FIG. 13, there are plotted the strength Gs of the original ground and the strength C of the compacted sand column, that is a depth sand column strength graph.

Thus, by effecting a re-driving operation of the casing pipe 1 while confirming whether the sand column strength has reached to a reference value of the predetermined depth sand column strength required by the design from the above-mentioned recorded graph, it is possible to ensure quantitatively the strength of the sand column at different depths of the compacted sand column being installed in the ground, and the compacted sand column being installed will conform with the predetermined design. In the depth sand column strength graph shown in FIG. 13, the broken line shows a reference value of the predetermined depth sand column strength. By arranging the record of the depth sand column strength graph in such a manner that the operator can observe it directly, and by effecting the operation of the casing pipe I while confirming whether the sand column strength line being recorded on the graph reaches to the predetermined reference line of the depth sand column strength, it is possible to satisfy the strength of the sand column of the compacted sand column which is installed easily and surely.

As stated above, the graphical record of the depth sand column strength is utilized directly on the measurement control of the compacted sand column being installed, and at the same time it is of course a requisite as a historic record of the installation process of the compacted sand column.

By the same reason as above, the recording of the depth sand column strength graph is effected as a record, and by providing separately an operation meter 31 connected to a recording device 30 as shown in FIG. 14, causing to memorize therein a predetermined reference value of the depth sand column strength obtained from the design. It is preferable that the operation of the re-driving of the casing pipe in the installation of compacted sand column as stated above be carried out by the operator while controlling the re-driving operation of the casing pipe in the installation of the compacted sand column referring to the indication of the operation meter 31. FIG. 16 shows briefly an embodimental example of the abovementioned operation meter 31, and the meter is so constructed that the predetermined reference line Cd of the sand column strength obtained from the design at various depths D is shown by a broken line, and the sand column strength C at the re-driving of the casing pipe 1 is shown at the lower portion. Accordingly, the operator has only to carry out the re-driving of the casing pipe 1 while observing the meter until the sand column strength C reaches to the position shown by the broken line of the meter. It is needless to say that other indicating types may also be employed.

In the above, the process is explained in'which the operation of casing pipe is effected in measuring each of the two factors of the press-charged sand volume and the sand column strength which are the most important controlling factors in the installation of the compacted sand columns. Generally,-in the stabilizing work of the soft ground by compacted sand column, the designed value of the compacted sand column is determined basing upon the prior ground investigation, but there may often occurs a case in which the ground condition at the field location of the sand column installation difiers to some degree from the presumed condition at the design. Namely, in the execution of the individual installation of the impacted sand column, following cases may be produced: (I) both the presscharged sand volume and the sand column strength are satisfied, (II) the press-charged sand volume is sufiicient but the sand column strength is not sufficient, (III) the press-charged sand volume is insufficient but the sand column strength is sufficient. In the above cases, (II) is a case which is experienced at the installation of the compacted sand column in the clavey stratrim, and it is necessary to reinforce the insufficient sand column strength by effecting an increment in excess of the planned value of the press-charged sand volume, and the case (III) is experienced in the installation of compacted sand column at the sand stratum, and because of the excess of the designed press-charged sand volume for the ground, there occurs an inconvenience that unnecessary press-charging of sand is effected when the compacting has already attained to the designed strength of the sand column. Furthermore, such an inconvenience is liable to occur in a case where the ground strength at the neighboring unfinished part is increased so much as the influence of the installation of compacted sand columns at a part of the predetermined area where a stabilization is to be effected. In consideration of the above-mentioned cases, it is necessary to select the above two factors always comparing in conjunction with the designed values at the installation of the compacted sand columns. Thus, it is needless to mention that it is most rational to use jointly the process of installation control by the aforesaid presscharging sand volume and the sand column strength for the installation of the compacted sand column which fits the purpose.

One of the characteristic features of the present invention is to detect the strength of the original ground in the depth at the lower end of the casing pipe during the driving of the casing pipe (by regarding it as a pile) for installation of the compacted column'down to the initially predetermined depth (refer to FIG. 1, I). The strength of the original ground can be found by the aforementioned strength detecting device. That is, in the recorded graph shown in FIGS. 12 and 13, Gs shows the strength of the original ground. Accordingly, it is possible to know, from this graph, the ground strength at the field location of execution of the compacted sand column in the driving of the casing pipe down to the initial predetermined depth, so that if the ground strength at each of these installing positions differs considerably from the presumed strength of the original ground at the ground investigation in the designing stage of the compacted sand column in the plan of the ground stabilization, there is an advantage that it is possible to change immediately the design in the installation of each of the compacted sand columns, whereby the stabilization of thesoft ground by the compacted sand columns can be performed more surely and more economically.

Thus far, the installation process of the compacted sand column according to the present invention has been described, and it isneedless to mention that, in the stabilizing work of the soft ground, appropriate sand-like granular material such as crushed stone, gravel, slag, granulated slag and their mixed material may also be used in place of sand as the material for the sand column, and in these cases, too, the installation process of the present invention can of course be applied.

I claim:

1. Process of installing compacted sand columns in the ground comprising the steps of continuously detecting the underground depth at the lower end of a casing pipe using a depth detecting device related to the casing pipe, continuously detecting the strength of sand column during re-driving of the casing pipe at each operation cycle in the installation of the compacted sand column using a strength detecting device, recording the detected underground depth at the lower end of the casing pipe and the strength of the sand column as a graph of depth sand column strength in recording device, and effecting re-driving operation of the casing pipe, in each of the operation cycle of the compacted sand column installation, while confirming that the strength of the sand column detected on said recording graph has reached to the predetermined reference value of the depth sand column strength required by the design.

2. Process according to claim 1 wherein the underground depth at the lower end of the casing pipe and the strength of the sand column respectively detected by means of the depth detecting device and the sand volumn detecting device are recorded respectively against the time axis in the recording device.

3. Process according to claim 1 wherein additionally providing an operation meter constructed to memorize the reference values of the predetermined depth sand column strength required by the design on said recording device, and effecting re-driving operation of the casing pipe, in each of the operation cycle of the compacted sand column installation while confirming that the strength of the sand column coincides with the reference value of the predetermined depth sand column strength required by the deisgn by means of the indication of said operation meter.

4. Process as claimed in claim 1 wherein the strength of the original ground continuously detected by the strength detecting device during the initial driving of the casing. pipe down to the predetermined underground depth is recorded together with said depth sand column strength in the graph by means of the recording device.

5. Process as claimed in claim 1 wherein a strength detecting device is used which is so constructed as to detect the strength of the original ground strength and the sand column strength from the electric power consumed in the motor for the driver and from the penetrating speed in the driving and re-driving process of the casing pipe.

6. A process for installing compacted sand columns in the ground comprising the steps of:

driving a casing pipe into the ground;

detecting continuously the underground depth of the lower end of said casing pipe;

charging sand into the interior of said casing pipe;

raising said casing pipe a predetermined amount after having been driven downward;

discharging sand from the lower end of the casing pipe into the cavity left in the ground after raising said casing pipe; I

detecting the volume of sand discharged from the lower end of said casing pipe;

recording the detected depth of said casing pipe and the detected volume of sand discharged from said casing pipe; and

driving cyclically said casing downward after having been raised upward to press the discharged sand into said cavity so that the recorded volume of discharged sand corresponds to a predetermined quantity in relation to the recorded depth.

7. A process as defined in claim 6 wherein said detected depth of said casing pipe and said detected volume of sand discharged from said casing pipe are recorded as a function of time.

8. The process as defined in claim 6 including the step of indicating a reference value denoting the volume of sand to be discharged to correspond to said predetermined quantity, said indicated reference value being indicated in conjunction with said recorded detected depth of said casing pipe and said detected volume of sand discharged from said casing pipe.

9. A process as defined in claim 6 wherein the volume of sand discharged from said casing pipe is detected by detecting the surface level of sand within said casing pipe, said level of sand within said casing pipe being detected by a servo electrode plum means continuously following the level of sand within said casing pipe.

10. The process as defined in claim 6 including the steps of continuously detecting the strength of the sand column resulting from the discharge of sand from the lower end of said casing pipe; recording the detected strength of said sand column; and driving cyclically said casing downward after having been raised upward to press the discharged sand into aid cavity so that the recorded strength of the sand column corresponds to a predetermined reference value corresponding to a pre-.

determined column strength.

1. A process as defined in claim 10 wherein said detected depth of said casing pipe and said detected volume of sand discharged from said casing pipe are recorded as a function of time.

12. A process as defined in claim 10 including the step of indicating a reference value denoting the quantity of sand to be discharged into said cavity to correspond to said predetermined quantity, said indicated reference value being recorded in conjunction with said recorded detected depth of said casing pipe and said detected volume of sand discharged from said casing pipe.

13. A process as defined in claim 10 wherein said volume of sand discharged from said casing pipe is detected by detecting the level of the sand surface within the interior of said casing pipe, said sand level within the interior of said casing pipe being detected by a servo electrode plumb following continuously the level within the interior of said casing pipe.

14. A process as defined in claim 10 including the step of recording the strength of the sand column as a function of time.

15. A process as defined in claim 10 including the step of indicating a reference value denoting a predetermined column strength, said casing being driven cyclically downward after having been raised upward to press the discharged sand into said cavity so that the recorded column strength corresponds to said predetermined indicated reference value corresponding to said predetermined column strength.

16. The process as defined in claim 10 including the step of detecting continuously and recording the strength of the original ground during driving of said casing pipe downward to a predetermined underground depth.

17. A process as defined in claim 16 wherein the column strength and the original ground strength are detected by means using electric power consumed in the motor for driving said casing pipe and the speed of said casing pipe during cyclical operation.

Claims (17)

1. Process of installing compacted sand columns in the ground comprising the steps of continuously detecting the underground depth at the lower end of a casing pipe using a depth detecting device related to the casing pipe, continuously detecting the strength of sand column during re-driving of the casing pipe at each operation cycle in the installation of the compacted sand column using a strength detecting device, recording the detected underground depth at the lower end of the casing pipe and the strength of the sand column as a graph of depth - sand column strength in recording device, and effecting re-driving operation of the casing pipe, in each of the operation cycle of the compacted sand column installation, while confirming that the strength of the sand column detected on said recording graph has reached to the predetermined reference value of the depth - sand column strength required by the design.

2. Process according to claim 1 wherein the underground depth at the lower end of the casing pipe and the strength of the sand column respectively detected by means of the depth detecting device and the sand volumn detecting device are recorded respectively against the time axis in the recording device.

3. Process according to claim 1 wheRein additionally providing an operation meter constructed to memorize the reference values of the predetermined depth - sand column strength required by the design on said recording device, and effecting re-driving operation of the casing pipe, in each of the operation cycle of the compacted sand column installation while confirming that the strength of the sand column coincides with the reference value of the predetermined depth - sand column strength required by the deisgn by means of the indication of said operation meter.

4. Process as claimed in claim 1 wherein the strength of the original ground continuously detected by the strength detecting device during the initial driving of the casing pipe down to the predetermined underground depth is recorded together with said depth - sand column strength in the graph by means of the recording device.

5. Process as claimed in claim 1 wherein a strength detecting device is used which is so constructed as to detect the strength of the original ground strength and the sand column strength from the electric power consumed in the motor for the driver and from the penetrating speed in the driving and re-driving process of the casing pipe.

6. A process for installing compacted sand columns in the ground comprising the steps of: driving a casing pipe into the ground; detecting continuously the underground depth of the lower end of said casing pipe; charging sand into the interior of said casing pipe; raising said casing pipe a predetermined amount after having been driven downward; discharging sand from the lower end of the casing pipe into the cavity left in the ground after raising said casing pipe; detecting the volume of sand discharged from the lower end of said casing pipe; recording the detected depth of said casing pipe and the detected volume of sand discharged from said casing pipe; and driving cyclically said casing downward after having been raised upward to press the discharged sand into said cavity so that the recorded volume of discharged sand corresponds to a predetermined quantity in relation to the recorded depth.

7. A process as defined in claim 6 wherein said detected depth of said casing pipe and said detected volume of sand discharged from said casing pipe are recorded as a function of time.

8. The process as defined in claim 6 including the step of indicating a reference value denoting the volume of sand to be discharged to correspond to said predetermined quantity, said indicated reference value being indicated in conjunction with said recorded detected depth of said casing pipe and said detected volume of sand discharged from said casing pipe.

9. A process as defined in claim 6 wherein the volume of sand discharged from said casing pipe is detected by detecting the surface level of sand within said casing pipe, said level of sand within said casing pipe being detected by a servo electrode plumb means continuously following the level of sand within said casing pipe.

10. The process as defined in claim 6 including the steps of continuously detecting the strength of the sand column resulting from the discharge of sand from the lower end of said casing pipe; recording the detected strength of said sand column; and driving cyclically said casing downward after having been raised upward to press the discharged sand into said cavity so that the recorded strength of the sand column corresponds to a predetermined reference value corresponding to a predetermined column strength.

11. A process as defined in claim 10 wherein said detected depth of said casing pipe and said detected volume of sand discharged from said casing pipe are recorded as a function of time.

12. A process as defined in claim 10 including the step of indicating a reference value denoting the quantity of sand to be discharged into said cavity to correspond to said predetermined quantity, said indicated reference value being recorded in conjunction with said recorded deteCted depth of said casing pipe and said detected volume of sand discharged from said casing pipe.

13. A process as defined in claim 10 wherein said volume of sand discharged from said casing pipe is detected by detecting the level of the sand surface within the interior of said casing pipe, said sand level within the interior of said casing pipe being detected by a servo electrode plumb following continuously the level within the interior of said casing pipe.

14. A process as defined in claim 10 including the step of recording the strength of the sand column as a function of time.

15. A process as defined in claim 10 including the step of indicating a reference value denoting a predetermined column strength, said casing being driven cyclically downward after having been raised upward to press the discharged sand into said cavity so that the recorded column strength corresponds to said predetermined indicated reference value corresponding to said predetermined column strength.

16. The process as defined in claim 10 including the step of detecting continuously and recording the strength of the original ground during driving of said casing pipe downward to a predetermined underground depth.

17. A process as defined in claim 16 wherein the column strength and the original ground strength are detected by means using electric power consumed in the motor for driving said casing pipe and the speed of said casing pipe during cyclical operation.

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