A method and apparatus for in situ surveying of subsurface structures subjected to water pressure

Abstract

The invention measures the water pressure value on the unit area of the outer surface of the structure in the underground water-soil mixture according to the third law of Newton, i.e. the principle of action and reaction. A measuring device is provided, which comprises at least a survey hole, a load system and an underground water level control system; the force acting along the gravity direction is reduced by step-by-step unloading, so that the measuring bottom plate tends to be in different self-stable equilibrium states in the static force unbalance state; with the gradual increase of the unloading, the soil spring counterforce in the direction opposite to the gravity gradually decreases; the motion state of the measuring bottom plate under each level of load is recorded, and the self-stable equilibrium state closest to the previous state is obtained by recording the violent upward motion state of the measuring bottom plate; the small soil counterforce of the self-stable equilibrium state is ignored, and the total downward pressure value on the measuring bottom plate in this state is approximately taken as the underground water pressure value on the measuring bottom plate according to the mechanical equilibrium equation.

Description

Technical Field

[0001] This invention belongs to the field of engineering geological exploration technology, specifically relating to a method and apparatus for in-situ surveying of underground structures subjected to water pressure in the soil. Background Technology

[0002] Groundwater pressure is one of the main loads on underground engineering structures, especially in areas with high groundwater levels. When groundwater pressure acts against gravity on a structure, it is commonly referred to as groundwater buoyancy. For underground structures with relatively low self-weight but large volume, anti-buoyancy design is a key factor affecting structural safety and controlling project costs. Due to the lack of clear theoretical research, the current common practice nationwide is to ignore the influence of soil solid particles on groundwater pressure in structural design, regardless of soil conditions, simplifying the soil-water mixture as water and considering the groundwater pressure on the underground structure according to Archimedes' principle. When the soil is cohesive (weakly permeable) or relatively intact rock, this simplistic approach overestimates groundwater pressure, resulting in excessively large design sections for underground structural components or excessive anti-buoyancy measures, leading to excessively high construction costs and wasting social resources. In a few areas, such as Chongqing, when the bottom surface of the basement slab is relatively intact mud or sandstone, the water pressure in the soil is directly considered to be zero, and the structural design does not consider the buoyancy effect of groundwater on the structure, thus occasionally causing engineering accidents caused by groundwater pressure. Therefore, there is an urgent need for a reliable and practical surveying method to measure the water pressure in the soil on underground structures, so as to design reasonable structural components while ensuring structural safety, significantly reduce construction costs, and save social resources. Summary of the Invention

[0003] This invention, based on Newton's third law (the principle that action equals reaction), directly measures groundwater pressure. The measurement target is not pore water pressure, but rather the water pressure per unit area on the outer surface of a structure situated within a groundwater-soil mixture, thereby optimizing the water pressure resistance design of underground engineering structures. This outer surface refers to the entire macroscopic outer surface of the structure within the soil, including the surface in contact with both solid soil particles and liquid water, rather than specifically referring to the portion in contact with water.

[0004] This invention provides a measuring device comprising at least a survey borehole, a loading system, and a groundwater level control system. Centered on a measuring base plate, it first constructs a static steady-state equilibrium force system in the soil along the direction of gravity. This system includes a loading force system acting downwards on the measuring base plate along the direction of gravity, and an anti-gravity force system acting against gravity on the measuring base plate; the two forces are equal in magnitude but opposite in direction. The loading force system consists of the loading force, the self-weight of the measuring base plate, and boundary friction. The anti-gravity force system consists of groundwater buoyancy and the reaction force of the soil springs. During the measurement process, the groundwater level is maintained at a relatively high position, i.e., the water pressure acting on the measuring base plate remains 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 springs remain under compression, but the pressure decreases. The equilibrium system will automatically reduce the soil spring reaction force through slight rebound, allowing the system to enter a new equilibrium state. This static equilibrium state is called a self-stable equilibrium state. However, when the loading force decreases excessively to the point that the total loading force is less than or equal to the buoyancy of the groundwater, because the soil spring is a single-compression spring, it cannot provide a reaction force in the non-compressed state. That is, the force exerted by the soil spring on the measuring base plate is zero, and the measuring base plate enters a static unbalanced force state, accelerating along the direction against gravity. The measuring device provided by this invention allows the measuring base plate to move from an initial self-stable equilibrium state to a static unbalanced state and enter different self-stable equilibrium states through a step-by-step unloading method. 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 accelerated 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.

[0005] The groundwater level control system consists of at least one pumping / injection well and one water level observation well, along with a set of pumping and injection pipe and pump assembly. The pumping / injection well and the water level observation well can also be collectively referred to as water level control wells.

[0006] The loading system includes a measuring sleeve, a measuring base plate, a loading rod, and a loading box. The measuring sleeve is tubular and is placed longitudinally along the direction of gravity during measurement, with its lower end inserted into the soil until the intended measurement position is reached, and its upper end protruding above the ground. The measuring base plate is located inside the bottom of the measuring sleeve, with its lower surface connected to the soil, its sides in contact with the inner wall of the measuring sleeve, and its top surface in contact with the bottom end of the loading rod. The loading rod is a cylindrical body smaller than the inner diameter of the measuring sleeve, longer than the measuring sleeve, and placed concentrically with the measuring sleeve. Its lower end is connected to the measuring base plate, and its upper end protrudes above the top of the measuring sleeve, where it is connected to the loading box. The loading box is located at the top of the entire measuring system and is an open container. Water is added to initiate loading, and an unloading tap is installed at the bottom to drain water and unload the load.

[0007] The positional relationship between the measuring casing of the loading system and the pumping / injection holes and water level observation holes of the water level control system in the horizontal plane is such that the three points are all on a straight line, with the measuring casing located in the middle. The advantage of this arrangement is that it is easy to measure the groundwater level at the measuring casing, i.e., the survey point, by using the water levels at the pumping / injection holes and water level observation holes.

[0008] In order to realistically simulate the contact interface between reinforced concrete slabs or plain concrete cushion layers and soil in actual underground structural engineering, this technical solution uses in-situ cast-in-place concrete slabs for measuring the base plate.

[0009] Before pouring the concrete for the measuring base plate located on the soil at the bottom of the measuring casing, the groundwater level can be controlled at the measuring point, i.e. below the bottom of the measuring casing, through the groundwater level control system, and this water level can be maintained until the concrete for the measuring base plate solidifies and the loading system is installed and loaded.

[0010] At the point of maximum loading, there is no water in the soil, and the loading force equals the soil reaction force. Afterwards, the groundwater level is controlled to rise above the measuring base plate through groundwater recharge. The force on the bottom surface of the measuring base plate is the water pressure plus the soil reaction force. Throughout the subsequent measurement process, the groundwater level is controlled at a predetermined stable high level before the unloading measurement process begins. As unloading proceeds in stages, the downward pressure acting on the measuring base plate gradually decreases. However, because the water level and water pressure remain constant, only the soil pressure decreases. The measuring base plate automatically enters a new equilibrium state through slight rebound. When the loading force decreases to less than the water pressure, the following inequality applies: Water pressure > Loading force + friction The measuring base plate enters a state of accelerated upward motion, where the frictional force is the frictional force between the measuring base plate, the loading rod, and the inner wall of the measuring sleeve. Obviously, if the frictional force is too large, even if the loading force is zero, the inequality may not hold during the entire measurement process, that is, the accelerated upward state will not occur. Therefore, measures should be taken to minimize the frictional force between the measuring base plate, the loading rod, and the measuring sleeve.

[0011] Taking the previous stage of the unsteady state as the boundary of the self-stabilizing equilibrium state, we have the following mechanical equation: Water pressure + Soil reaction force = Loading force + friction Ignoring the minute soil reaction force, we then obtain: Water pressure = Loading force + friction Given the applied force, the frictional force can be measured in a mechanics laboratory using a sample prepared under the same conditions, thus obtaining the water pressure value. Dividing this water pressure value by the bottom area of ​​the measuring base plate yields the water pressure value acting on a unit outer surface of the measuring base plate.

[0012] Through the equation: Water pressure = Loading force + friction Analysis shows that when the water pressure is low, the loading force must also be low. Therefore, all parts of the loading system in this technology are designed to be detachable to unload the force applied to the measuring base plate by their own weight, in order to adapt to the situation of low groundwater pressure.

[0013] For ease of operation, the loading system of this technology is made of lightweight and high-strength materials.

[0014] This survey method is similar to the pile bearing capacity testing method. It can quickly and accurately measure the water pressure value of underground structures at any depth in the underground soil layer. The survey method is simple and intuitive, and the measuring tools used are readily available. The exploration cost is low. It solves the problem of simply estimating the groundwater pressure by Archimedes' law in the case of weak permeable soil layers due to the lack of exploration technology, which leads to excessive anti-buoyancy measures, oversized structural components, excessive costs, and waste of resources in engineering design. Conversely, it avoids the risk of engineering accidents caused by insufficient anti-buoyancy capacity and damage to underground structures due to the complete disregard for groundwater pressure in underground engineering design.

[0015] A preferred embodiment of the present invention will be described in further detail below with reference to the accompanying drawings. Attached Figure Description

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

[0017] Figure 2 This is an elevation view showing the completion of on-site drilling and dewatering to the lowest water level before the installation of the measuring sleeve;

[0018] Figure 3 This is a schematic diagram of the elevation after all parts have been installed and the load has been applied to the maximum load force.

[0019] Figure 4 This is a schematic diagram illustrating the control of groundwater level rise to the expected high water level.

[0020] Figure 5 This is a schematic diagram of the loading box's step-by-step drainage and unloading measurement process.

[0021] Figure 6 This is a diagram illustrating the possible maximum unloading state;

[0022] Figure 7 yes Figure 3 Section 1-1 in the diagram.

[0023] Figure 8 This is a schematic diagram of the loading force-displacement curve of the unloading section, drawn based on measurement data records, where the loading force is denoted as N. i The displacement is denoted as D. i .

[0024] Explanation of reference numerals in the attached figures:

[0025] 0: Foundation soil; 1: Measurement base plate; 2: Single-pressure soil spring; 3: Survey hole; 4: Pumping / injection hole; 5: Water level observation hole; 6: Pumping / injection pipe; 7: Low groundwater level line; 8: Measurement sleeve; 9: Loading pipe; 10: Loading box; 11: Loading box water level line; 12: Connecting sleeve; 13: Drainage pipe and tap; 14: Stabilizing ring; 15: High groundwater level line; 16: Gap filling material;

[0026] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0027] In the description of this invention, it should be understood that the terms "center," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," and "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention. In the description of this invention, unless otherwise stated, "a plurality of" or "several" means two or more.

[0028] Figure 1 This is a schematic diagram illustrating the force analysis of the self-stabilized equilibrium state of the measuring base plate during the unloading measurement stage of this invention. It embodies the core concept of this invention, including the measuring base plate 1, a single soil-pressing spring 2, and a balanced force system composed of multiple forces. The balanced force system is divided into two force systems according to their direction of action: a loading force system along the direction of gravity and an anti-gravity force system. The loading force system includes the loading force F transmitted from the loading pipe. i Measure the weight of the base plate G, the frictional force f, and the resultant force N. i The two marked 'f' in the diagram do not represent two 'f' forces acting on the measuring base plate 1; they only indicate that this frictional force acts around the perimeter of the measuring base plate. The anti-gravity force system includes the water pressure W and the single-pressure earth spring reaction force S. i The resultant force is denoted as U. i In algebraic terms, N = i =F i +G+f,U i =S i +W. According to Newton's third law, we have equation N. i =U i That is, F i +G+f=Si +W. In the above formula, the variable F i S i The letter 'i' in F represents the number of times the program was uninstalled. i S i Let F represent the loading force and the soil spring reaction force after the i-th unloading, respectively. The equilibrium equation F is studied. i +G+f=S i +W, where G and f are constants during the measurement process, then the equation only requires W and F. i and S i Three variables. According to accepted theory, a soil spring is a single-compression spring. When the load is gradually unloaded, the pressure F under the rod decreases. i The combined pressure N i When W is exactly equal to the solid soil mass, the solid soil is not under pressure, and the spring reaction force S of the solid soil particles is... i That is, it equals zero, which means:

[0029] N i =F i +G+f=S i +W=0+W=W→F i +G+f=W. At this point, the two-force system is in a statically unstable equilibrium state, and is extremely prone to losing equilibrium due to disturbances. This is because of the staged unloading, i.e., F i The changes are abrupt, so it is difficult for N to appear. i =F i The case of +G+f=W is not necessarily the case where W is between two load levels, which can be denoted as: F i -1+G+f>W>F i +G+f. Since the earth spring is a single-compression spring, W>F. i When +G+f, since the soil spring is no longer under pressure, the reaction force S i =0, at this point the forces acting from above and below cannot be balanced, i.e., U i =W>F i +G+f=N i The measuring base plate will experience an upward accelerating displacement, meaning it will undergo accelerated buoyancy. The inequality F is then investigated. i -1+G+f>W>F i +G+f, and we find that as long as F... i -1 and F i If the difference is small enough, W will approach F. i-1 +G+f. In other words, as long as the load value unloaded each time is small enough, a stable equilibrium state that is very close to the static unstable state can be obtained by measuring the dynamic state of the accelerated floating of the bottom plate, and the water pressure W can be measured with sufficient accuracy.

[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, during the entire unloading measurement process, the groundwater level was maintained constant at 15 meters through the water injection holes. Normal unloading stages were performed by draining water from the loading tank, facilitating unloading and allowing for easy control of unloading accuracy via the tank level gauge. The unloading value was kept constant at each stage, and the unloading duration was sufficiently long and consistent. Unloading was performed stage by stage, and the rebound displacement of the loading system under each load was recorded, following the method specified in the slow-maintained load method for pile vertical bearing capacity testing. After each unloading stage, the corresponding displacement was recorded before proceeding to the next stage. If, under a certain load level, the upward displacement of the loading system did not converge or accelerated upward movement, the corresponding record was made, and the measurement was terminated. In extreme cases where groundwater buoyancy is very low, even if the water in the loading tank is drained, the loading system may not accelerate upward movement or the displacement may not converge. In such cases, the loading tank and loading pipe can be unloaded to perform the final two stages of special unloading and corresponding measurements. The final elevation view of the measurement device after unloading is shown below. Figure 6 As shown. Clearly, if in Figure 6 If the measuring base plate does not accelerate upward or the displacement does not converge under these circumstances, 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 yes Figure 3 cross section Figure 1-1 The measuring sleeve 8, the water injection / extraction hole 4, and the water level observation hole 5 are arranged on the same straight line.

[0034] 4) To measure the frictional force f between the measuring base plate and the measuring sleeve of the in-situ surveying device, a section of the measuring sleeve of the same size, material, and surface finish can be cut. The inner surface of the sleeve is lubricated in the same way as that of the in-situ surveying and testing sleeve. Then, concrete of the same size and formula is poured inside. Using appropriate equipment and testing methods, the frictional force f on the measuring base plate of this testing device is measured and determined in the laboratory, and then the total loading force N at each stage is calculated. i value.

[0035] 5) After the measurement is completed, process the recorded measurement data and plot the total applied force N. i Springback displacement D i Curves, such as Figure 8 As shown. It can be accessed via... Figure 8 The sharp drop in the curve shown indicates the floating motion of the measuring base plate, and thus the groundwater pressure can be estimated. Figure 8 Taking the curve as an example, the loading system can still maintain stability under load N6, but experiences a steep rise at load N7. Taking the groundwater pressure as N6, the corresponding groundwater level is... Figure 5In this context, h2 represents the water head, h. The precise groundwater level h2 at the measuring point can be calculated based on the water head measured at each borehole of the water level control system and the water injection test. Alternatively, it can be approximated as h2 = (h1 + h3) / 2. When expressed as pressure, the groundwater pressure acting on a unit area of ​​the outer surface of the structure is:

[0036] W p =N6 / A

[0037] In the above formula, A represents the bottom area of ​​the measured base plate 1, and W... p This refers to the water pressure acting on a unit area of ​​the bottom surface of the measuring plate.

[0038] This surveying method and apparatus can accurately measure the soil water pressure on underground structures at any depth below the expected groundwater level. The surveying method is simple, and the measuring tools used are readily available, solving the problem of lack of basis for determining groundwater pressure values ​​in underground structure design. The above examples are merely illustrative of the invention and do not constitute a limitation on the scope of protection of the invention. All designs that are the same as or similar to the invention fall within the scope of protection of the invention.

Claims

1. A method and apparatus for in-situ surveying of water pressure on underground structures, characterized in that, include: Measurement holes and water level control holes are drilled in the soil at the survey site. A loading system is installed in the measurement holes, and a groundwater level control system is installed in the water level control holes. During the measurement process, the groundwater level is stabilized by the groundwater level control system, and a downward external force is applied to the measurement base plate by the loading system, and the load is gradually unloaded to measure buoyancy. The motion state of the measurement base plate under each load is recorded. By analyzing the recorded significant upward motion state of the measurement base plate, the self-stabilizing equilibrium state that is closest to that state is identified. The total downward pressure of the loading system under this equilibrium state is taken as the groundwater pressure value measured on the bottom plate at the corresponding groundwater level.

2. The method and apparatus for in-situ surveying of underground structures under water pressure according to claim 1, characterized in that, The object of the measurement is the water pressure per unit area on the outer surface of the soil of the structure.

3. The method and apparatus for in-situ surveying of underground structures under water pressure according to claim 1 or 2, characterized in that, The method and apparatus include at least a measuring borehole, a loading system, and a groundwater level control system.

4. The method and apparatus for in-situ surveying of water pressure on underground structures as described in claim 1 or 3, 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.

5. The method and apparatus for in-situ surveying of underground structures under water pressure as described in claim 1 or 3, 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.

6. The loading system as described in claim 4, 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.

7. The loading system as described in claim 4, 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.

8. The groundwater level control system as described in claim 5, characterized in that, The function of the groundwater level control system is to control the groundwater level below the bottom of the measuring casing before measurement so that the concrete measuring base plate can be poured and loaded. During measurement, the system controls the groundwater level to be raised and maintained at a high level close to the ground surface until the measurement is completed.

9. 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.

10. 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 weight applied to the measuring base plate when necessary, so as to measure a smaller groundwater pressure.

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