Method and device for in-situ survey of water pressure borne by underground structure

Abstract

According to the method and device for in-situ survey of the water pressure borne by the underground structure, the value of the water pressure borne by the outer surface of a structure in an underground water-soil mixture on the unit area is measured according to the Newton's third law, namely the principle that acting force is equal to counter-acting force. Providing a measuring device at least comprising a survey hole, a load system and an underground water level control system; the acting force along the gravity direction is reduced in a step-by-step unloading mode, so that the measuring bottom plate tends to a static unbalance state and enters different self-stabilization balance states; along with the gradual increase of unloading, the counterforce of the soil spring in the reverse gravity direction is gradually reduced; recording the motion state of the measuring bottom plate under each stage of load, and obtaining a previous self-stabilization equilibrium state closest to the state by recording the violent floating motion state of the measuring bottom plate; the tiny soil reaction force in the self-stabilization equilibrium state is ignored, and according to a mechanical equilibrium equation, the total lower pressure value on the measurement bottom plate in the state is approximately taken as the underground water pressure value borne by the measurement bottom plate.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of engineering geological survey, and particularly relates to a method and device for in-situ surveying water pressure on underground structure in soil. BACKGROUND

[0002] Water pressure on underground structure is one of the main loads of underground engineering structure, especially in the area with high underground water level. When the water pressure acts on the structure against gravity, it is called water buoyancy. For underground structure with small self-weight and large volume, the anti-floating design is a key factor affecting the safety of the structure and controlling the construction cost. Since the theoretical research is not clear, the current common practice in the country is to ignore the influence of the existence of soil particles on the water pressure, to simplify the water-soil mixture as water, and to consider the water pressure on the underground structure according to Archimedes' law. When the soil is clayey soil (weakly permeable layer) or relatively complete rock, this simple method overestimates the water pressure, resulting in overlarge design section of underground structure members or excessive anti-floating measures, leading to high construction cost and waste of social resources. In a few areas, such as Chongqing, when the bottom of the basement floor is relatively complete mud and sandstone, the water pressure in soil is directly considered as zero, and the structure design does not consider the water buoyancy on the structure, so occasional engineering accidents caused by water pressure occur. Therefore, there is an urgent need for a reliable and practical survey method to measure the water pressure in soil on underground structure, so as to design reasonable structure members under the premise of ensuring the safety of the structure, significantly reduce the construction cost, and save social resources. SUMMARY

[0003] The present application directly measures the water pressure value according to Newton's third law, i.e. the principle of action and reaction. The measurement target is not the pore water pressure, but the water pressure value on the unit area of the outer surface of the structure in the water-soil mixture, and then the water pressure design of underground engineering structure is optimized. The outer surface of the structure refers to the entire macroscopic outer surface of the structure in the soil, including the sum of the surfaces in contact with solid soil particles and liquid water, rather than specifically referring to the part in contact with water.

[0004] The present application provides a measuring device comprising at least a surveying hole, a loading system and a groundwater level control system, which is centered on a measuring base plate, and first constructs a static steady force system in the soil along the gravity direction, which comprises a loading force system acting downward along the gravity direction on the measuring base plate, and an anti-gravity force system acting on the measuring base plate along the anti-gravity direction, both of which have equal values and opposite directions. The loading force system is composed of a loading force, the self-weight of the measuring base plate and the boundary friction force. The anti-gravity force system is composed of the groundwater buoyancy and the soil spring counterforce. During the measuring process, the groundwater level is kept constant at a high position, i.e. the water pressure acting on the measuring base plate is kept constant; when the loading force system decreases slightly, as long as the total value of the loading force system is greater than the groundwater buoyancy, the soil spring is still in the compressed state, but the pressure decreases, and the balance system will automatically reduce the soil spring counterforce through slight rebound to make the system enter a new balance state, which is called self-stabilizing balance state. However, when the loading force decreases too much so that the total value of the loading force system is less than or equal to the groundwater buoyancy, because the soil spring is a single compression spring, it cannot provide counterforce in the non-compressed state, i.e. the force acting on the measuring base plate by the soil spring is zero, and the measuring base plate enters a static unbalanced force state and moves along the anti-gravity direction at an accelerated speed. The measuring device provided by the present application allows the measuring base plate to gradually approach the static unbalanced state from the initial self-stabilizing balance state through step-by-step unloading, and the soil spring counterforce in the anti-gravity direction also gradually decreases with the gradual increase of the unloading. The movement state of the measuring base plate under each level of load is recorded, and through the recorded accelerated upward movement state, a self-stabilizing balance state closest to the state is obtained, and the total downward pressure value of the loading force system in the state is approximately taken as the groundwater pressure value by ignoring the small soil counterforce of the balance state.

[0005] The groundwater level control system is composed of at least one water pumping and injection hole and a water level observation hole, and a set of water pumping and injection pipe pump assembly. The water pumping and injection hole and the water level observation hole can also be collectively referred to as the water level control hole.

[0006] The loading system comprises a measuring casing, a measuring base plate, a loading rod and a loading box. The measuring casing is tubular, and is placed vertically along the gravity direction during the measurement, with the lower end inserted into the soil to the position to be measured and the upper end exposed on the ground. The measuring base plate is inside the tube at the bottom end of the measuring casing, with the lower surface connected to the soil, the periphery in contact with the inner wall of the measuring casing, and the top surface in contact with the bottom end of the loading rod. The loading rod is a columnar body with a smaller inner diameter than the measuring casing, and is placed concentrically with the measuring casing, with the lower end connected to the measuring base plate and the upper end exposed above the top end of the measuring casing and connected to the loading box. The loading box is located at the uppermost end of the entire measuring system, is an open-top container, and can be loaded by adding water. The bottom end is provided with an unloading faucet, and the system can be unloaded by draining water from the faucet.

[0007] The position relationship of the measuring sleeve of the loading system with the water pumping and injecting hole and the water level observation hole of the water level control system in the horizontal plane is that the three points are on a straight line and the measuring sleeve is in the middle. The advantage of this arrangement is that the underground water level at the survey point, i.e. the measuring sleeve, can be easily determined by the water level of the water pumping and injecting hole and the water level observation hole.

[0008] In order to truly simulate the contact interface between the reinforced concrete bottom plate or the plain concrete cushion and the soil in the actual underground structure engineering, the measuring bottom plate is made of the in-situ cast concrete plate.

[0009] Before pouring the measuring bottom plate concrete on the soil at the bottom end of the measuring sleeve, the underground water level can be controlled at the survey point, i.e. below the bottom end of the measuring sleeve, by the underground water level control system, and the water level is maintained until the measuring bottom plate concrete is solidified, the loading system is installed and the loading is completed.

[0010] When the maximum loading is completed, there is no water in the soil and the loading force is equal to the soil reaction force. After the underground water level is controlled to rise above the measuring bottom plate by the underground water recharge method, the force on the bottom surface of the measuring bottom plate is the water pressure plus the soil reaction force. In the entire subsequent measurement process, the underground water level is controlled at a desired stable high water level, and then the unloading measurement process is entered. With the gradual unloading, the downward pressure on the measuring bottom plate gradually decreases, but because the water level does not change, the water pressure does not change, only the soil pressure decreases, and the measuring bottom plate automatically enters a new equilibrium state through slight rebound. When the loading force system decreases to less than the water pressure, there is an inequality: Water pressure > Loading force + Frictional force , the measuring bottom plate enters an accelerated upward movement state, wherein the friction force is the friction force between the measuring bottom plate, the loading rod and the inner wall of the measuring sleeve. Obviously, if the friction force is too large, even if the loading force is zero, the inequality may not hold true throughout the measurement process, i.e. the accelerated upward state will not occur, so measures should be taken to minimize the friction force between the measuring bottom plate, the loading rod and the measuring sleeve.

[0011] Taking the previous self-stabilizing equilibrium state boundary as the unsteady state, there is a mechanical equation: Water pressure + Soil reaction force = Loading force + Frictional force , ignoring the small soil reaction force, and further obtaining: Water pressure = Loading force + Frictional force . The loading force is known, the friction force can be prepared with the same condition formula in the mechanical laboratory, and then the water pressure value is obtained. The water pressure value divided by the bottom area of the measuring bottom plate can obtain the water pressure value acting on the unit outer surface of the measuring bottom plate.

[0012] Through the equation: Water pressure = Loading force +Frictional force It is found that when the water pressure is small, the loading force must also be small, so the parts of the loading system are designed to be detachable to unload the force exerted on the measuring base plate by its own weight to adapt to the case of small groundwater pressure.

[0013] For easy operation, the loading system of the present technology is made of light and high-strength materials.

[0014] The survey method is similar to the foundation pile bearing capacity detection method, which can quickly and accurately measure the water pressure value of the underground structure at any depth of the underground soil layer. The survey method is simple and intuitive, and the measuring tools used are easy to obtain, the exploration cost is low, and the problem of overestimating the groundwater pressure effect by simply applying Archimedes' law in the case of weak water permeable layer foundation due to the lack of exploration technology means, resulting in excessive anti-floating measures, oversized structure, high cost, and resource waste. Or conversely, avoid the risk of insufficient structural anti-floating capacity caused by completely ignoring the groundwater pressure in underground engineering design, resulting in engineering accident risk of underground structure damage.

[0015] A preferred embodiment of the present application will be described in further detail below with reference to the accompanying drawings. BRIEF DESCRIPTION OF DRAWINGS

[0016] Figure 1 is a schematic diagram of force analysis of the measuring base plate in a stable equilibrium state in the unloading measurement stage;

[0017] Figure 2 is a schematic diagram of the completed site drilling and dewatering to the lowest water level elevation before placing the measuring sleeve;

[0018] Figure 3 is a schematic diagram of the completed installation of all parts and loading to the maximum loading force state;

[0019] Figure 4 is a schematic diagram of the control of the groundwater level rising to the expected measurement control high water level state;

[0020] Figure 5 is a schematic diagram of the unloading measurement process of the loading box;

[0021] Figure 6 is a schematic diagram of the maximum unloading state that may occur;

[0022] Figure 7 is Figure 3 1-1 cross-sectional view in the

[0023] Figure 8 is a schematic diagram of the unloading segment loading force-displacement curve drawn according to the measurement data record, where the loading force is denoted as N i , and the displacement is denoted as D i .

[0024] Reference signs:

[0025] 0: foundation soil; 1: measuring base plate; 2: single pressure soil spring; 3: survey hole; 4: water pumping and injection hole; 5: water level observation hole; 6: water pumping and injection pipe; 7: low groundwater level line; 8: measuring casing; 9: loading pipe; 10: loading box; 11: loading box water level line; 12: connecting casing; 13: drain pipe and faucet; 14: stabilizing ring; 15: high groundwater level line; 16: jointing material;

[0026] The technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor fall within the scope of protection of the present application.

[0027] In the description of the present application, it should be understood that the terms "center", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer" and the like indicate the orientation or positional relationship based on the orientation or positional relationship shown in the drawings, and are only for the convenience of describing the present application and simplifying the description, and do not indicate or imply that the device or element referred to must have a particular orientation, be constructed and operated in a particular orientation, and therefore cannot be understood as a limitation on the present application. In the description of the present application, unless otherwise specified, the meaning of "a plurality of" or "several" is two or more.

[0028] Figure 1 is a schematic diagram of force analysis of the measuring base plate from the self-stable equilibrium state in the unloading measurement stage of the present application, embodying the core idea of the present application, comprising a measuring base plate 1, a single pressure soil spring 2, and a balance force system composed of a plurality of forces. The balance force system is divided into two force systems according to the direction of action, a loading force system in the direction of gravity and an anti-gravity force system. The loading force system includes a loading force F i transmitted by the loading pipe, the self-weight G of the measuring base plate, the friction force f, and the resultant force N i . The two f in the figure do not mean that there are two f acting on the measuring base plate 1, but only express that the friction force acts on the periphery of the measuring base plate. The anti-gravity force system includes a water pressure W, a single pressure soil spring counterforce S i , and the resultant force U i . There is N i = F i + G + f, U i = S i + W. According to Newton's third law, there is the equation N i = U i , i.e. F i + G + f = Si +W. In the above formula, the variable F i , S i The letter i in the formula represents the number of unloading times, i.e. F i , S i respectively represent the loading force after the i-th unloading and the value of the soil spring counterforce. The study of the equilibrium formula F i +G+f=S i +W shows that during the measurement process, G and f are constant, so the equation has only three variables, W, F i and S i . According to the recognized theory, the soil spring is a single compression spring. When the load rod pressure F i is gradually unloaded to the point where the upper compression force N i is exactly equal to W, the solid soil is not under compression, and the solid soil particle spring counterforce S i is also equal to zero, i.e.:

[0029] N i =F i +G+f=S i +W=0+W=W→F i +G+f=W. At this time, the two force systems are in a state of static unstable equilibrium, which is easily lost due to disturbance. Because of the stepwise unloading, i.e. the jump change of F i , it is difficult to have N i =F i +G+f=W, and it is more likely that W is between two levels of load, which can be recorded as: F i -1+G+f>W>F i +G+f. Since the soil spring is a single compression spring, when W>F i +G+f, the soil spring is not under compression, and the counterforce S i =0, at this time the upper and lower forces cannot be balanced, i.e. U i =W>F i +G+f=N i , the measurement base plate will produce upward acceleration displacement, i.e. the measurement base plate will have accelerated floating motion. The study of the inequality F i-1 +G+f>W>F i +G+f shows that as long as the difference between F i -1 and F i is small enough, W will approach F i-1 +G+f. In other words, as long as the load value of each unloading is small enough, a stable equilibrium state very close to the static unstable state can be measured according to the accelerated floating of the measurement base plate, and a sufficiently accurate water pressure W can be measured accordingly.

[0030] This embodiment provides a method and apparatus for in-situ surveying of water pressure on underground structures based on the above principle, specifically including the following structure and steps:

[0031] 1) Figures 2-4 This is a front elevation view of the measuring device during the measurement preparation stage of the present invention. Figure 2 As shown, firstly, at least one exploration borehole (marked as 3 in the diagram) is drilled into the ground at the proposed site, reaching the expected exploration location at a depth h0; one pumping / injection borehole (marked as 4) is drilled, with a depth greater than h0; and one water level observation borehole (marked as 5) is drilled, with a depth greater than h0. All boreholes are arranged in a straight line on the plane, with the exploration borehole located in the center. In a preferred embodiment, the diameter of each borehole is small enough to prevent personnel from entering. Next, a pumping / injection pipe (6) is inserted into borehole 4 to pump out groundwater until the groundwater level drops below the bottom surface of exploration borehole 3. Then, as... Figure 3 As shown, a measuring sleeve 8 is lowered into the survey hole, and a gap-filling material 16 (see figure 3) is placed in the gap between its outer wall and the hole 3. Figure 7 The measuring sleeve 8 is fixed and placed in the direction of gravity. Next, the measuring base plate 1 is poured sequentially, followed by the placement of the loading pipe 9, stabilizing ring 14, and loading box 10. Water or other liquid is then added to the loading box to the expected maximum water level 11. The loading pipe applies the maximum loading force F0 to the measuring base plate. Part 14 is the stabilizing ring, designed to prevent the loading pipe 9 from becoming unstable under pressure. Its number can be increased or decreased depending on the situation, but at least one should be installed at the top of the measuring sleeve to ensure its stability. A drag-reducing structure, such as a micro-rolling bearing, is provided at the contact edge between the stabilizing ring 14 and the inner wall of the measuring sleeve to reduce motion friction to a negligible level. Part 12 is the connecting sleeve, located in the center of the bottom surface of the loading box. It is a tubular structure fixedly connected to the loading box, slightly larger than the measuring sleeve, facilitating the installation of the loading box onto the measuring sleeve. Part 13 is the drain pipe and faucet, used for subsequent staged drainage and unloading. Then... Figure 4 As shown, water is injected into borehole 4 through six pumping and draining pipes to control the groundwater level and raise it to the required high groundwater level of 15. The injection rate is adjusted to maintain the stability of this high groundwater level, completing the preparatory work before unloading and measurement. To prevent... Figure 4 The stage shown indicates the occurrence of unstable acceleration and buoyancy of the measuring base plate, where the maximum loading force F0 = γ can be taken. w h0A, where γ w Let h0 be the specific weight of water, h0 be the depth of the measuring point, and A be the area of ​​the bottom surface of the measuring base plate. To reduce the friction between the measuring base plate and the measuring sleeve, lubricating oil or paraffin wax can be applied to the inner surface of the bottom of the measuring sleeve beforehand to reduce friction.

[0032] 2) Figures 5-6 This is the front elevation view of the measurement stage equipment. (For example...) Figure 5As shown, the high groundwater level line 15 is maintained unchanged throughout the unloading measurement process; normal unloading is carried out by draining the water in the loading tank, which is convenient and easy to control the unloading accuracy through the water tank liquid level scale. The control value of each unloading stage is the same, and the unloading time of each stage is long enough and the same, and the loading system is unloaded step by step and the rebound displacement of the loading system under each load is recorded. The method refers to the provisions of the slow maintenance load unloading section of the vertical bearing capacity detection of foundation pile. After each unloading stage is completed, the corresponding displacement is recorded, and the next unloading stage can be carried out. If the upward displacement of the loading system under a certain load does not converge or accelerates upward, the corresponding record can be made and the measurement is ended. In the extreme case where the groundwater buoyancy is very small, even if the water in the loading tank is drained, the loading system still does not accelerate upward or the displacement does not converge. At this time, the last two special unloading stages and the corresponding measurement can be carried out by unloading the loading tank and the loading pipe. The front elevation view of the measuring device after unloading is shown in Figure 6 As shown, if the measuring bottom plate still does not accelerate upward or the displacement does not converge in the case of Figure 6 , it indicates that the groundwater pressure value is still less than G+f, and the water pressure exceeds the measurement range of the device.

[0033] 3) Figure 7 is the section Figure 3 in Figure 1-1 . The measurement sleeve 8, the water injection and drainage hole 4, and the water level observation hole 5 are arranged on the same straight line.

[0034] 4) In order to measure the friction force f between the measuring bottom plate and the measuring sleeve of the in-situ survey device, a measuring sleeve segment with the same size, material and surface finish is cut off, the inner surface of the sleeve is treated with the same lubrication as the in-situ surveying and detecting sleeve, and the same size and formula concrete is poured inside. By using appropriate equipment and detection methods, the friction force f of the measuring bottom plate of the detection device is measured and determined in the laboratory, and then the total loading force N i value of each stage is calculated.

[0035] 5) After the measurement is completed, the recorded measurement data is processed, and the total loading force N i and the rebound displacement D i curve are drawn, as shown in Figure 8 . The floating motion state of the measuring bottom plate can be judged by the sharp decline segment of the curve shown in Figure 8 , and the groundwater pressure is inferred. Taking the curve of Figure 8 as an example, the loading system can still maintain stability under the action of N6 load, and the sharp rise motion occurs at N7. The groundwater pressure is N6, and the corresponding groundwater level is Figure 5h2, the water head is h. The accurate underground water level h2 at the measuring point can be calculated according to the water head measured by the water level control system and the water injection test, or h2=(h1+h3) / 2 can be approximately taken. When expressed in pressure, the water pressure acting on the outer surface of the structure is

[0036] The underground water pressure per unit area is:

[0037] W p =N6 / A

[0038] In the above formula, A is the bottom area of the measuring base plate 1, and W p is the water pressure acting on the unit area of the bottom surface of the measuring base plate.

[0039] The survey method and device can accurately survey the water pressure value of the underground structure at any depth of the underground soil layer under the expected underground water level, the survey method is simple, and the measuring tools used are convenient and easy to obtain, thereby solving the problem of lack of basis for the water pressure value in the underground structure design. The above examples are only illustrative of the present application and do not constitute a limitation on the protection scope of the present application, and any design identical or similar to the present application falls within the protection scope of the present application.

Claims

1. A method and apparatus for in-situ surveying of underground structures subjected to groundwater pressure, characterized in that: 1) The method described above is based on Newton's third law, namely the principle that action equals reaction, to directly measure the groundwater pressure value. 2) The measurement target of the method and apparatus is not the pore water pressure value in the soil, but the water pressure per unit area on the outer surface of the structure in the groundwater-soil mixture. 3) The method and apparatus include at least a survey borehole, a loading system, and a groundwater level control system. 4) The method and apparatus take the measuring base plate as the object. First, a steady-state two-force equilibrium system is constructed. Then, through a step-by-step unloading method, the measuring base plate is made to tend towards a static unbalanced state and enter different self-stable equilibrium states. As the unloading gradually increases, the soil spring reaction force against gravity also gradually decreases. The motion state of the measuring base plate under each level of load is recorded. By recording the violent upward motion state, the self-stable equilibrium state closest to this state is obtained. The small soil reaction force in this equilibrium state is ignored, and the total downward pressure of the loading force system in this state is approximately taken as the groundwater pressure value.

2. The method and apparatus for in-situ surveying of underground structures under water pressure as described in claim 1, characterized in that, The loading system includes a measuring sleeve, a measuring base plate, a loading rod, at least one stabilizing ring, and a loading box.

3. The method and apparatus for in-situ surveying of underground structures under water pressure as described in claim 1, characterized in that, The groundwater level control system consists of at least one pumping and injection hole and one water level observation hole, as well as a set of pumping and injection pipe and pump assembly.

4. The loading system as described in claim 2, characterized in that, The measuring sleeve is tubular and is placed along the direction of gravity during measurement. The lower end is inserted into the soil until the intended measurement position is reached, while the upper end protrudes above the ground. The measuring base plate is located inside the tube at the bottom of the measuring sleeve. Its lower surface is connected to the soil, its periphery is in contact with the inner wall of the measuring sleeve, and its top surface is in contact with the bottom end of the loading rod. The loading rod is a cylindrical body smaller than the measuring sleeve but longer. It is placed concentrically with the measuring sleeve, with its lower end connected to the measuring base plate and its upper end protruding above the top of the measuring sleeve. It is connected to a loading box. The loading box is located at the top of the entire measuring system. It is an open container that can be loaded by adding water. An unloading tap is installed at the bottom, and unloading is achieved by draining water from the tap.

5. The loading system as described in claim 2, characterized in that, The measuring base plate at the bottom of the measuring sleeve is a cast-in-place concrete slab to ensure that the contact state between the measuring surface and the soil is consistent with the actual engineering situation.

6. The groundwater level control system as described in claim 3, characterized in that, The function of the groundwater level control system is to initially control the groundwater level to be lower than the bottom of the measuring casing so that the concrete measuring base plate can be poured and loaded. In the later stage, the system controls and raises the groundwater level to a high level close to the ground surface, and maintains this high level throughout the entire subsequent unloading and measuring process.

7. The loading system as described in claim 4, characterized in that, To increase measurement accuracy, measures were taken to reduce the friction between the measuring base plate and the measuring sleeve. The friction was measured in the laboratory by preparing a sample under the same conditions, and then the groundwater pressure was calculated.

8. The loading system as described in claim 4, characterized in that, The loading rod, stabilizing ring, and loading box of the loading system can all be easily disassembled to unload the loading force applied to the measuring base plate by their own weight when necessary, so as to measure a smaller groundwater pressure.

9. The loading system as described in claim 2, characterized in that, The loading system is made of lightweight, high-strength materials for ease of operation.

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