Method and apparatus for measurement of in-situ horizontal stress of non-coherent soil

The present invention relates to a laboratory test method and apparatus, and a field test method and apparatus, for measuring the in-situ horizontal stress of non-coherent soil such as sand and gravel.

Measurement of the in-situ stress in soil is of major importance in a wide variety of geotechnical problems, but is difficult to carry out accurately and reproducibly in the case of non-coherent soil deposits such as sand, gravel or like loose particulate material.

The present invention sets out to make such measurement by studying the thawing of a frozen core sample, or of the walls of a borehole extending within a frozen soil volume.

In one form the invention sets out to provide a laboratory test method for determining the in situ lateral stress of non-coherent soil in which a high quality core sample is removed from a frozen volume of the soil at the required location and permitted to thaw within a pressurised test chamber under pressure conditions such as to retain its dimension, the exerted pressure being a measure of the lateral stress; and sets out moreover to provide suitable apparatus for this method.

In another form the invention sets out to provide a field test method for determining the in situ lateral stress of non-coherent soil, in which a high quality core sample is removed from a bore to leave a high quality bore within a frozen volume of the soil, and the walls of this bore are permitted to thaw about a pressurised cylindrical test member under pressure condition such that the walls retain their shape, the exerted pressure being a measure of the lateral stress; and sets out moreover to provide suitable apparatus for this method.

In one aspect the invention consists in a laboratory test method for measuring the in-situ lateral stress of non-coherent soil comprising:

• (a) freezing a volume of soil in situ at a predetermined depth
• (b) coring the frozen volume to provide a test sample
• (c) surrounding the sample with a rubber membrane
• (d) enclosing the membrane with a cell to which liquid is admitted to surround the membrane
• (e) applying to the specimen a vertical load equivalent to the effective vertical stress at the core depth
• (f) allowing the frozen specimen to thaw
• (g) adjusting the pressure of surrounding liquid to maintain liquid level constant despite radial strain exerted during thawing, and
• (h) noting the internal cell pressure when the sample is thawed as a measure of the in-situ lateral stress in the non-coherent soil.

In another aspect the invention consists in equipment for measuring in situ lateral stress of non-coherent soil, by the above method, comprising:

• (a) an outermost cell housing
• (b) means for holding a membrane-covered frozen core specimen within the cell
• (c) means for exerting pressure on the specimen in an axial direction to reproduce the effective vertical stress at the core depth
• (d) air supply means to the cell
• (e) an inner cell construction surrounding the specimen location
• (f) a float for floating on the surface of liquid in the inner cell
• (g) sensor means to detect the vertical position of the float, and
• (h) means responsive to the float position for controlling air supply to the air supply means:

In a further and related aspect the invention consists in a field test method for measuring the in situ lateral stress of non-coherent soil, comprising:

• (a) freezing a volume of the soil in-situ at a predetermined depth
• (b) boring a hole in the frozen soil,
• (c) lowering into the hole a test apparatus surrounded by a rubber membrane
• (d) inflating the membrane into conformity with the walls of the hole by admitting water to the test apparatus
• (e) adjusting the pressure exerted by the water at a membrane to compensate for displacement due to thawing of the walls of the hole
• (f) noting the said pressure when thewalls are thawed as a measure of the in-situ lateral stress in the non-coherent soil.

In a yet further aspect the invention consists in field equipment for measuring in-situ lateral stress of non-coherent soil, using the method of the preceding paragraph, comprising

• (a) a cylindrical test member;
• (b) a rubber membrane speced from the surface of the test member to define an inflatable gap and the constitute an outer surface
• (c) water supply means for the inflatable gap
• (d) heater means within the inflatable gap for heat supply to water held therein
• (g) air-pressure-exerting means for said water supply and
• (f) means for determining air pressure:

Preferably, a pore water pressure transducer is fixed to the apparatus, e.g. at the base, so that the pressure due to water in the pores of the non-coherent soil can be separately measured and distinguished from the lateral stresses of the soil itself.

The method and equipment for measuring the in-situ lateral stress of non-coherent soil according to the present invention will become more apparent from the following description taken in conjuction with the accompanying drawings, in which:

• Fig.1 shows diagrammatically in-situ ground freezing of moist non-coherent soil and coring of the frozen ground,
• Fig.2 is cross sectional diagram of the test apparatus used in the laboratory test method of measuring the in-situ lateral stress of non-coherent soil by thawing of a frozen test specimen prepared from a high quality undisturbed sample cored from the ground frozen in-situ as described in relation to Figure 1,
• Fig.3 shows the essentials of a field test to measure in-situ horizontal (lateral) stress of the non-coherent soil, and
• Fig.4 is a cross section of the test apparatus used in the field test.

To obtain a test sample a borehole is first drilled in the ground 1 to about the desired sample depth 11. A freezing pipe 2 (e.g. of about 76 mm external diameter) is placed in the borehole and a coolant such as liquid nitrogen, or ethanol and crushed dry ice mixed with brine, is pumped down the pipe 2. The coolant causes a volume 3 of soil, shown shaded, to freeze and thus cohere. The freezing speed can be controlled by reference to thermocouples (not shown) in the soil. A typical overall lateral dimension of the frozen volume, if gravel, is about 120 cm, and if sand is about 50 cm.

At this stage there are alternative ways of proceeding. One method is to remove essentially the whole volume 3 of frozen soil in a large diameter steel pipe drilled down to a suitable extent and then to core samples from this removed and stored block. The other, as shown in Figure 1 of the drawing, is to core a small-diameter sample directly from a convenient region of the innermost frozen volume 3. In each case, to get a good sample, the regions nearest the freezing pipe 2 or nearest the surface of the frozen volume 3 should be avoided. A typical gravel sample is say 30 cm in diameter: a typical sand sample is 5-10 cm in diameter.

Fig.2 is a cross section of laboratory test apparatus for measuring the in-situ horizontal stress of non-coherent soil. In this apparatus a frozen test specimen obtained as described above was evaluated.

A pressure cell consists of a transparent plastic cylinder 10 and top plate 11 and base plate 12. Top plate 11 and base plate 12 are connected tightly by three stainless steel bars 21. The frozen test specimen 5 is covered with an impermeable rubber membrane 6 sealed to a pedestal 14 and to top a cap 13.

The top cap 13 is connected to a loading rod 16 and axial load transducer 15. The pedestal 14 is fixed on the base plate 12. There is a porous stone installed on the surface of the top cap 13 and pedestal 14 at the side faced to the test specimen. The top cap 13 and the pedestal 14 are connected to the drain pipe 18. The cell is filled with water 19 to a level somewhat higher than the top surface of the test specimen. The space 20 above the water 19 in the cell is filled with air. The air pressure is supplied from the compressor 22 as adjusted by regulator 23 and applied to the space 20 through plastic tube 24 which is connected to the inlet 25 in the top plate 11.

An inner cell 26 is fixed on the base plate 12 in the Pressure cell. The float 27 is placed on the surface of the water 19 inside the inner cell 26. The gap sensor 28 for measuring the vertical movement of the water level in the inner cell 26 is fixed just below the float 27 to the inner cell 26. The vertical movement of the level of the water 19 can be indicated by the vertical displacement of the float 27, which will induce voltage change in the gap sensor 28. The induced voltage is amplified by amplifier 29 and is input to the controller 31 which consists of servo controller 30, servo motor 32 and regulator 33 for adjusting the air pressure to keep the level of the water 19 constant. The lateral strain of the test specimen 5 exerted during thawing is then restrained. The air pressure in the inner cell (measured by pressure transducer 34) when the frozen test specimen is completely thawed indicates the lateral stress of the soil in-situ.

Fig.3 shows an arrangement for effecting a field test method to measure the in-situ horizontal stress of non-coherent soil is performed within a bored hole 7 prepared for instance by coring the ground as frozen by in-situ freezing to obtain a high quality undisturbed sample for the laboratory test described above, and as described in relation to Fig. 1.

A steel pipe 8 as inserted into the bore hole 7 to a depth somewhat shallower than the depth for testing this keeps the inside surface of the hole from collapse. The test apparatus 9, about 60 cm in length, having a rubber balloon-like structure about its surface is installed in the bored hole at the test depth. Suitable air pressure is applied to ensure that this rubber balloon fits to the wall surface of the frozen bored hole 7.

Fig.4 is a cross section of the test apparatus 9. The rubber membrane or balloon is e.g. about 40 cm in length and of almost the same diameter as the bored hole. it is fixed to both ends of the cylindrical shape housing 41 by a steel ring 44 associated with steel ring 45 and a back-up ring 43 made of rubber. Thus, closed space 42 is sealed off as a balloon. The space 42 is filled with deaerated water through the pipe 47 connected to the inlet 46 inside of the housing 41. The pipe 47 for supplying water is connected to water pump 48. The space 42 is tapered in the upper part 49 in order to ensure that the space 42 is completely filled. Initial air can be easily expressed from the tapered upper part 49 through drilled hole 50 and air valve 51. A coil heater 52 in the space 42 keeps the temperature of the water in the space 42 constant. A water head pipe 55 is connected to a two-way electro-magnetic valve 54 located at the upper part of the inside of the housing 41. The upper part of the water head pipe 55 and differential pressure transducer 56 are connected to the same air pressure supply pipe 57. A pore water pressure transducer 58 is mounted at the base of the housing 41.

The test apparatus 9 is attached to the lower end of a rod and installed at the desired depth for testing as in Fig. 3. The air pressure from the source 60 is adjusted by servo motor 61 and supplied to the pipe 47 through the pipe 57. The air pressure can be monitored from the pressure meter 62. The pressure regulator 66 was operated by servo motor 65 and servo motor regulating valve 61 according to the input from servo controller 64. The input electrical signal to the servo controller 64 is transmitted from the differential pressure measured from the pressure transducer 58 is used for determination of the effective lateral stress in-situ which is needed to calculate the coefficient of earth pressure at rest.

On practice, the space 42 was filled with water and the valve 51 on the ground surface was shut. Then the test probe 9 was installed into the frozen bored hole at a depth to be tested. A small pressure was applied to the space 42 to conform the rubber membrane 40 to the inside surface of the frozen bored hole. The radial displacement of the bored hole 7 during thawing will cause the water level in the water head pipe 55 to rise. The movement of the water level in the pipe 55 will be detected by differential pressure transducer 56, and the regulator 61 will then adjust the air pressure to maintain the level of the water in the pipe 55 constant, which means the no lateral strain in the soil.

The pressure in the space 42, as monitored from the pressure meter 62 at the stage where the frozen bore 3 was completely thawed, indicates the in-situ lateral stress in the soil. The true lateral effective stress in-situ can be calculated from the above monitored air pressure and pore water pressure measured by pressure transducer 58.

The Young's modulus of non-coherent soil in-situ in the horizontal direction can be measured using the test apparatus as used in the field test.

The above detailed description indicates examples of working procedures of the present invention. Various modifications may be made in actual working procedures by soil engineers without deviating from the scope of the attached claims.