A coral reef limestone geological foundation pit and its construction method

Abstract

The present invention relates to the field of foundation pit construction, and specifically to a coral reef limestone geological foundation pit and a construction method thereof. The construction method comprises the following steps: obtaining geological data of the depth of the foundation pit to be excavated and the location of the foundation pit excavation; the geological data include: stratum structure and rock and soil mechanical parameters and hydrogeological data such as cohesion, wet density, internal friction angle, etc.; and designing an open excavation and forced drainage scheme according to the depth of the foundation pit to be excavated and the geological data. When designing the scheme, the present invention introduces a groundwater level line that changes with the boundary of the foundation pit and a uniformly distributed load set on the edge of the foundation pit to conduct a stability analysis, and obtains an open excavation and forced drainage scheme with a stable foundation pit and a settlement around the foundation pit that meets the standard. While meeting the drainage requirements of the foundation pit, there is no need to lay a water-stop curtain and an internal support structure, and in the open excavation and forced drainage scheme, the foundation pit slope is built by relying on geological materials on the construction site, eliminating the environmental pollution risk of casting the water-stop curtain, avoiding the increase in cost and construction period, and protecting the environment while ensuring the construction period.

Description

Technical Field

[0001] The present invention relates to the field of foundation pit construction, and in particular to a coral reef limestone geological foundation pit and a construction method thereof. Background Art

[0002] Coral reef limestone is widely distributed in tropical oceans between the Tropic of Cancer and the Tropic of Capricorn, including the South China Sea, the Red Sea, the Indian Ocean, and the Pacific Ocean. Due to differences in diagenetic environment, material composition, and geological evolution, it possesses generally similar yet distinct engineering geological properties. Coral reef limestone possesses relatively low compressive and shear strengths, high porosity, and strong permeability.

[0003] When constructing foundation pits on coral reef limestone, especially deep ones in coastal areas, the rock's high permeability makes it easy for seawater to seep into the excavated pit, often at significant levels. Therefore, specialized drainage measures are necessary during construction. For highly permeable coral reef limestone foundation pits, existing technologies often employ water-stop curtains and ground-connected walls, or combine them with pipe well dewatering.

[0004] For example, a Chinese patent application with publication number CN108118702A discloses a recyclable flexible composite support structure and construction method for coral reef foundation pits. Steel pipe piles are driven around the foundation pit to solve the problem of unbalanced water and soil pressure in water-facing foundation pits. Inner and outer baffles are provided on the inner and outer sides of the support pile fences around the foundation pit, respectively. The inner and outer baffles can disperse the water and soil pressure so that the support pile fences are more evenly stressed. A polymer anti-seepage body is poured between the inner and outer baffles. The polymer anti-seepage body can fill the gaps in the soil between the inner and outer baffles, thereby preventing seawater from seeping into the excavated foundation pit.

[0005] While combining a waterstop curtain with internal supports can ensure the safety of the foundation pit, this approach requires not only the installation of slope supports and the pouring of the waterstop curtain during excavation, but also the construction process itself, given the large number of trenches, the extensive construction workload, and limited personnel and equipment. This approach results in high material costs, a long construction timeline, and difficulty ensuring the project schedule. Furthermore, if the foundation pit is close to the coast (tens to hundreds of meters), and there are coral reefs along the coast, the waterstop curtain installation could potentially pollute the coral reef environment. Summary of the Invention

[0006] The purpose of the present invention is to overcome the problems of high cost and easy pollution of the coral reef environment when constructing deep foundation pits on offshore coral reef limestone geology using the existing solution combining water-stop curtains and internal supports, and to provide a coral reef limestone geological foundation pit and a construction method thereof.

[0007] In a first aspect, the present invention provides a method for constructing a coral reef limestone geological foundation pit, comprising the following steps:

[0008] S1. Obtain the depth of the foundation pit to be excavated and collect geological data of the coral reef limestone geology. Preferably, the geological data includes engineering geological data and hydrogeological data. The engineering geological data includes stratum structure and geotechnical parameters. The geotechnical parameters include at least cohesion, wet density, and internal friction angle.

[0009] S2, determine whether the depth of the foundation pit to be excavated exceeds a depth threshold; if so, execute step S4; if not, execute step S3.

[0010] S3, determining whether the cohesion of the soil in the excavated foundation pit that exceeds the depth threshold from the ground surface reaches a preset cohesion value; if so, executing step S4. Preferably, the preset cohesion value is in the range of 20-30 kPa.

[0011] S4, designing an open-cut forced drainage scheme based on the geological data. Preferably, the open-cut forced drainage scheme design includes: designing a foundation pit enclosure to determine the foundation pit excavation structure, and designing a foundation pit drainage system to determine the structural parameters and drainage capacity of the water collection ditch.

[0012] Preferably, the foundation pit enclosure design includes:

[0013] The water collection ditch is arranged along the bottom edge of the foundation pit.

[0014] The excavation structure of the foundation pit is determined by designing the number of slope layers, the height of each slope layer, the slope rate of each slope layer, and the width of the slope platform based on the thickness of the rock and soil layers, the cohesion and friction angle of each soil layer, and the depth of the foundation pit to be excavated.

[0015] Based on the cohesion, wet density, and internal friction angle of each stratum structure, and in conjunction with the excavation structure, a stability analysis is conducted on a typical profile of the excavation and slope protection of the foundation pit. A groundwater level that varies with the pit boundary is incorporated into the stability analysis, and a uniformly distributed load is applied to the pit edges. Preferably, the groundwater level at the pit bottom is flush with the water level in the drainage ditch, while the groundwater level at the pit slope gradually decreases and remains at a distance from the slope surface that is no less than a preset threshold.

[0016] According to a preferred embodiment, the rock and soil mechanical parameters also include: permeability coefficient. The foundation pit drainage design includes:

[0017] Calculation of foundation pit water inflow, including:

[0018] The total water inflow from the foundation pit is calculated based on the permeability coefficient, the thickness of the phreatic aquifer, the water level in the foundation pit after dewatering, the length of the filter inlet, the equivalent radius of the foundation pit, and the dewatering impact radius. Preferably, the permeability coefficient is the maximum value found in the engineering geological data. If the thickness of the phreatic aquifer is unknown, it can be set based on the site's drilling conditions.

[0019] Several water collection wells are installed in the drainage ditch, and drainage pumps are placed in these wells to pump out the water. The location and number of the water collection wells and the drainage capacity of the drainage pumps are determined based on a redundant foundation pit water inflow that exceeds the calculated foundation pit water inflow. Preferably, the redundant foundation pit water inflow is set to 1.5 times the calculated foundation pit water inflow.

[0020] The open excavation forced drainage scheme design also includes: calculating whether the cost of foundation pit excavation and foundation pit drainage exceeds the preset cost. If so, optimizing the foundation pit retaining design and the foundation pit drainage design, with priority given to optimizing the foundation pit drainage design.

[0021] According to a preferred embodiment, in the foundation pit enclosure design: if the result of the cross-section stability analysis indicates that the foundation pit is unstable, the range of the unstable foundation pit sliding surface is determined. Preferably, if the range of the unstable foundation pit sliding surface is less than or equal to the preset range threshold, the corresponding foundation pit sliding surface is grouting reinforced, and the stability analysis of the grouting reinforced foundation pit is performed; if the grouting reinforced foundation pit is stable, subsequent design simulation is performed; if the grouting reinforced foundation pit is still unstable, the foundation pit enclosure design is re-performed, and parameters such as the number of foundation pit slope layers, the slope height of each layer, the slope rate of each layer, and the slope platform width are set. Preferably, if the range of the unstable foundation pit sliding surface is greater than the preset range threshold, the foundation pit enclosure design is re-performed, and parameters such as the number of foundation pit slope layers, the slope height of each layer, the slope rate of each layer, and the slope platform width are set.

[0022] According to a preferred embodiment, the stability analysis uses a limit equilibrium method to perform stability analysis of a typical section of the foundation pit slope excavation. Preferably, the limit equilibrium method includes at least one of the following: the Ferenius method, the Bishop method, the Taylor method, the Janbu method, the Morgenstern-Price method, the Spencer method, and the Salma method.

[0023] According to a preferred embodiment, when designing the number of sloping layers, the height of each layer, the slope rate of each layer, and the width of the slope platform, the following design principles are followed: the slope height is designed based on the structural strength of each stratum. For strata with strong structural strength, the slope height can be unlimited while ensuring slope stability; for strata with weak structural strength, the slope height does not exceed the preset single-layer slope height. Preferably, the cohesion of the strata with strong structural strength reaches the preset cohesion value, while the cohesion of the strata with weak structural strength is lower than the preset cohesion value.

[0024] According to a preferred embodiment, the following steps are also included:

[0025] S5: Simulate the open-cut forced drainage scheme. Based on the Biot consolidation theory, use finite element analysis software to create a three-dimensional finite element model and calculate the ground soil settlement around the foundation pit. Determine whether the impact of the foundation pit on the surrounding environment meets the preset indicators. If so, proceed to step S6; if not, return to step S4.

[0026] S6: Excavate a foundation pit according to the open-cut forced displacement scheme and monitor the settlement of the foundation pit. Preferably, determining whether the impact of the foundation pit on the surrounding environment meets a preset indicator includes calculating whether the settlement around the foundation pit is less than a preset settlement indicator, which is 60 mm.

[0027] According to a preferred embodiment, the rock and soil mechanical parameters described in step S2 also include: a compressive elastic modulus and a Poisson's ratio. The simulation of the open-cut forced displacement scheme in step S5 is performed by calculating the ground soil settlement around the foundation pit using the MC calculation model combined with the cohesion, wet density, internal friction angle, compressive elastic modulus, and Poisson's ratio of each stratum structure in the open-cut forced displacement scheme.

[0028] According to a preferred embodiment, the depth threshold is 6m, the uniform load is set at 2m from the edge of the foundation pit, and the uniform load size is set to 20KN / m 2 The water level in the ditch is 0.5m lower than the bottom of the foundation pit, the preset distance threshold is 3m, and the preset single-layer slope height is 3m.

[0029] In a second aspect, the present invention further provides a coral reef limestone geological foundation pit. The foundation pit provided by the present invention is constructed using the coral reef limestone geological foundation pit construction method provided by the present invention, and comprises a side slope, a foundation pit bottom, and a drainage ditch disposed along the edge of the foundation pit bottom. The drainage ditch is provided with a plurality of water collection wells.

[0030] According to a preferred embodiment, at least two drainage ditches are provided along the bottom edge of the foundation pit, wherein the two drainage ditches are a first drainage ditch close to the slope and a second drainage ditch further away from the slope. Preferably, the bottom of the first drainage ditch is lower than the bottom of the second drainage ditch, and the volume of the second drainage ditch is greater than that of the first drainage ditch. The water collection well is provided above the first drainage ditch, and the water level in the first drainage ditch is lower than that in the second drainage ditch. Water from the second drainage ditch infiltrates into the first drainage ditch through the coral reef limestone, thereby reducing the turbidity of the water.

[0031] Compared with the prior art, the present invention has the following beneficial effects:

[0032] For foundation pits constructed on coral reef limestone geology, especially for foundation pits whose depth does not exceed the depth threshold, or whose depth exceeds the depth threshold but whether the cohesion of the soil in the part of the foundation pit that exceeds the depth threshold reaches the preset cohesion value; the present invention carries out foundation pit enclosure design and foundation pit drainage design according to the geological data of the depth of the foundation pit to be excavated and the coral reef limestone geology, and introduces the groundwater level line that changes with the foundation pit boundary and the uniformly distributed load set on the edge of the foundation pit when carrying out the foundation pit enclosure design to carry out stability analysis, and obtains an open-cut forced drainage scheme in which the foundation pit is stable and the settlement around the foundation pit meets the preset indicators, and then the foundation pit is excavated according to the open-cut forced drainage scheme. While meeting the foundation pit drainage needs, there is no need to arrange water-stop curtains and internal support structures, and the slopes in the open-cut forced drainage scheme are built by relying on geological materials on the construction site, eliminating the environmental pollution risks and the increase in costs and construction period brought by casting water-stop curtains, which can protect the environment and ensure the construction period. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 This is a schematic flow chart of a method for constructing a coral reef limestone geological foundation pit according to a preferred embodiment of the present invention;

[0034] Figure 2 This is a schematic cross-sectional diagram of a foundation pit slope according to a preferred embodiment of the present invention;

[0035] Figure 3 This is a schematic diagram of foundation pit stability calculation results according to a preferred embodiment of the present invention;

[0036] Figure 4 This is a schematic diagram of a foundation pit where only part of the sump is close to the drainage channel;

[0037] Figure 5 This is a schematic diagram of setting two drainage ditches in the foundation pit;

[0038] Figure 6 This is a plan view of the drainage channel;

[0039] Figure 7 for Figure 6 Schematic diagram of the cross section taken along the AA section line;

[0040] Figure 8 for Figure 6 Schematic diagram of the cross section along the middle BB section line;

[0041] Figure 9 for Figure 6 Schematic diagram of the cross section along the CC cutting line.

[0042] Reference numerals:

[0043] Fill layer 101, saline sand layer 102, fully weathered coral reef limestone layer 103, strongly to moderately weathered coral reef limestone layer 104,

[0044] Foundation pit bottom 100, water collecting ditch 110, first water collecting ditch 111, second water collecting ditch 112, water collecting well 120, diversion trough 130,

[0045] Drainage channel 200, plastic film 211, geotextile 212, cement mortar layer 213, first drainage channel 210, gravel bank 220, sedimentation tank 230, baffle 231, second drainage channel 240, gravel wall 250, embankment 260, first anti-fouling curtain 270, second anti-fouling curtain 280,

[0046] Drain pump 300, drain pipe 301,

[0047] Ocean 400. DETAILED DESCRIPTION

[0048] The present invention will be further described in detail below with reference to specific embodiments. However, this should not be construed as limiting the scope of the present invention to the following embodiments, as all technologies implemented based on the present invention fall within the scope of the present invention.

[0049] Unless otherwise specified, in the description of the specific embodiments of the present invention, the terms indicating the orientation or positional relationship, such as "upper", "lower", "left", "right", "center", "inside", and "outside", are based on the expressions of the orientation or positional relationship shown in the accompanying drawings, or are the orientation or positional relationship in which the invented product / device / apparatus is placed when it is conventionally used. These terms of orientation or positional relationship are merely for the purpose of facilitating the description of the scheme of the present invention or simplifying the description of the specific embodiments to facilitate the rapid understanding of the scheme by technicians, and do not indicate or imply that a specific device / component / element must have a specific orientation, or be constructed and operated in a specific positional relationship, and therefore should not be understood as limiting the present invention.

[0050] In addition, if the terms "horizontal", "vertical", "overhanging", "parallel" and the like appear, it does not mean that the corresponding devices / components / elements are required to be absolutely horizontal or vertical or overhanging or parallel, but may be slightly tilted or have deviations. For example, "horizontal" only means that its direction is more horizontal relative to "vertical", and does not mean that the structure must be completely horizontal, but may be slightly tilted. Alternatively, it can be simply understood that the corresponding devices / components / elements are set in directions such as "horizontal", "vertical", "overhanging", and "parallel", and can have an error / deviation of ±10% relative to the corresponding direction setting, more preferably an error / deviation within ±8%, more preferably an error / deviation within ±6%, more preferably an error / deviation within ±5%, and more preferably an error / deviation within ±4%. As long as the corresponding device / component / element is within the error / deviation range, it can still achieve its role in the solution of the present invention.

[0051] In addition, the expressions “first”, “second”, “third”, etc. in the terms are merely used to distinguish the description of the same or similar components, and should not be understood as emphasizing or implying the relative importance of specific components.

[0052] In addition, in the description of the embodiments of the present invention, "several," "plurality," and "a number" represent at least two. It can also be any number such as two, three, four, five, six, seven, eight, nine, or even more than nine.

[0053] Furthermore, in the description of the technical solution of the present invention, unless otherwise expressly specified, defined, or limited, the terms "disposed," "installed," "connected," "connected," "provided with," "laid," and "arranged" should be understood broadly. For example, they may refer to fixed connections, detachable connections, or integral connections. They may be welded, riveted, bolted, threaded, or other commonly used connection methods in the art. Such connections may be mechanical, electrical, or communicative; they may be direct, indirect via an intermediate medium, or internally connected between two components.

[0054] Example 1

[0055] This embodiment provides a method for constructing a coral reef limestone geological foundation pit. Figure 1 The construction method of coral reef limestone geological foundation pit includes the following steps:

[0056] S1. Obtain the depth of the foundation pit to be excavated and collect geological data of the coral reef limestone. Preferably, the geological data includes engineering geological data and hydrogeological data. Preferably, the engineering geological data includes stratum structure and geotechnical parameters. Geotechnical parameters include at least cohesion, wet density, and internal friction angle.

[0057] S2, determine whether the depth of the foundation pit to be excavated exceeds a depth threshold; if so, execute step S4; if not, execute step S3.

[0058] S3, determining whether the cohesion of the soil in the excavated foundation pit that exceeds the depth threshold from the ground surface reaches a preset cohesion value; if so, executing step S4. Preferably, the preset cohesion value is in the range of 20-30 kPa.

[0059] S4, designing an open-cut forced drainage scheme based on geological data. Preferably, the open-cut forced drainage scheme design includes: designing a foundation pit enclosure to determine the foundation pit excavation structure, and designing a foundation pit drainage system to determine the structural parameters and drainage capacity of the water collection ditch 110.

[0060] S5: Simulate the open excavation forced drainage scheme to determine whether the impact of the foundation pit on the surrounding environment meets the preset indicators. If so, proceed to step S6; if not, return to step S4.

[0061] S6, excavate the foundation pit according to the open excavation and forced drainage plan, and monitor the settlement and displacement of the foundation pit.

[0062] Preferably, the foundation pit retaining wall design includes:

[0063] A drainage ditch 110 is provided along the edge of the foundation pit bottom 100 .

[0064] According to the thickness of rock and soil layers, the cohesion and friction angle of each layer of soil, and the depth of the foundation pit to be excavated, the number of slope layers, the slope height of each layer, the slope rate of each layer, and the width of the slope platform are designed to determine the foundation pit excavation structure.

[0065] Based on the cohesion, wet density, and internal friction angle of each stratum structure, and in conjunction with the excavation structure, a stability analysis of a typical excavation profile of the foundation pit enclosure and slope is conducted. The stability analysis incorporates a groundwater level that varies with the pit boundary, and a uniformly distributed load is applied to the pit edge. Preferably, the groundwater level at the pit bottom 100 is flush with the water level in the drainage ditch 110, while the groundwater level gradually decreases along the pit slope, maintaining a distance from the slope surface that is no less than a preset threshold.

[0066] Preferably, this embodiment carries out foundation pit retaining design and foundation pit drainage design based on the engineering geological data and hydrogeological data of the foundation pit depth to be excavated and the coral reef limestone geology, and introduces the groundwater level line that changes with the foundation pit boundary and the uniformly distributed load set on the foundation pit edge to carry out stability analysis when carrying out foundation pit retaining design, and obtains an open-cut forced drainage scheme in which the foundation pit is stable and the settlement around the foundation pit meets the preset indicators. The foundation pit is then excavated according to the open-cut forced drainage scheme. While meeting the foundation pit drainage needs, there is no need to lay water-stop curtains and internal support structures. In addition, the slopes in the open-cut forced drainage scheme are built with geological materials on the construction site, eliminating the environmental pollution risks and the increase in costs and construction period brought by casting water-stop curtains, which can protect the environment and ensure the construction period.

[0067] Example 2

[0068] This embodiment further improves upon embodiment 1, and repeated content will not be repeated. Preferably, the geotechnical parameters include: wet density, salinity, SPT (standard penetration test) value, cohesion, internal friction angle, permeability coefficient, compressive elastic modulus, porosity, and Poisson's ratio. Hydrogeological data include the depth of the groundwater level. Methods for acquiring geological data include survey results and supplementary drilling data. Preferably, the survey results can be historical survey data of the local hydrogeology.

[0069] Preferably, the foundation pit retaining wall design includes: designing the number of slope layers, the slope height of each layer, the slope rate of each layer, and the width of the slope platform according to the thickness of the rock and soil layer and the design depth of the foundation pit; and conducting stability analysis on the typical profile of the foundation pit retaining wall slope excavation designed for the foundation pit.

[0070] Preferably, when designing the number of slope layers, the slope height of each layer, the slope rate of each layer, and the width of the slope platform, the design principles followed include: designing the slope height according to the structural strength of each stratum; for strata with strong structural strength, the slope height may not be restricted while ensuring the stability of the slope; for strata with weak structural strength, the slope height shall not exceed the preset single-layer slope height; wherein, the cohesion of strata with strong structural strength reaches the preset cohesion value, and the cohesion of strata with weak structural strength is lower than the preset cohesion value.

[0071] Preferably, the preset single-layer slope height is: after obtaining the geological data of the coral reef limestone geology, taking the minimum value of the cohesion, and calculating the maximum height value of the single-layer slope that can ensure the stability of the slope when the slope width is the maximum allowable width of the single slope.

[0072] Preferably, the depth threshold is: after obtaining the geological data of the coral reef limestone geology, the cohesion of each layer of soil is taken as the minimum value of the cohesion, and the maximum foundation pit depth value that can ensure the stability of the slope is calculated when the slope height of the single layer slope does not exceed the preset single layer slope height, and the sum of the width of each layer of slope and the width of the slope platform is the maximum allowable width of the multi-slope.

[0073] Preferably, the maximum allowable width of a single slope and the maximum allowable width of multiple slopes are determined by factors such as the foundation pit construction area and earth excavation cost.

[0074] Preferably, this embodiment introduces a groundwater level that changes with the foundation pit boundary in the stability analysis. Preferably, if the result of the profile stability analysis indicates that the foundation pit is unstable, the range of the unstable foundation pit sliding surface is determined. Preferably, if the range of the unstable foundation pit sliding surface is less than or equal to the preset range threshold, the corresponding foundation pit sliding surface is grouting reinforced, and the stability analysis of the grouting reinforced foundation pit is performed; if the grouting reinforced foundation pit is stable, subsequent design simulation is performed; if the grouting reinforced foundation pit is still unstable, the foundation pit enclosure design is re-performed, and parameters such as the number of foundation pit slope layers, the slope height of each layer, the slope rate of each layer, and the slope platform width are set. Preferably, if the range of the unstable foundation pit sliding surface is greater than the preset range threshold, the foundation pit enclosure design is re-performed, and parameters such as the number of foundation pit slope layers, the slope height of each layer, the slope rate of each layer, and the slope platform width are set.

[0075] Preferably, in the stability analysis, the uniform load is set at 2m from the foundation pit edge, and the uniform load size is set to 20KN / m 2, the water level in the ditch 110 is 0.5m lower than the bottom of the foundation pit 100, and the preset distance threshold is 3m.

[0076] Preferably, the stability analysis is performed using a limit equilibrium method to analyze the stability of a typical section of the foundation pit slope excavation. Preferably, the limit equilibrium method includes at least one of the following: the Fellenius method, the Bishop method, the Taylor method, the Janbu method, the Morgenstern-Price method, the Spencer method, and the Salma method.

[0077] Preferably, the foundation pit drainage design includes: setting a ditch 110 along the edge of the foundation pit bottom 100 and calculating the foundation pit water inflow; setting a plurality of sump wells 120 on the ditch 110 and placing drainage pumps 300 in the sump wells 120 to pump out the water; setting the dimensional parameters of the ditch 110 and sump wells 120, setting the location and number of sump wells 120, and setting the drainage capacity of the drainage pumps 300. Preferably, the ditch 110, sump wells 120, and drainage pumps 300 are set based on 1.5 times the foundation pit water inflow. Preferably, the redundant foundation pit water inflow is set to 1.5 times the calculated foundation pit water inflow.

[0078] Preferably, the calculation of the foundation pit water inflow includes: calculating the total foundation pit water inflow based on the permeability coefficient, the thickness of the phreatic aquifer, the water level in the foundation pit after dewatering, the length of the filter inlet, the equivalent radius of the foundation pit, and the dewatering impact radius. Preferably, the permeability coefficient is the maximum value found in the engineering geological data; if the thickness of the phreatic aquifer is unknown, the thickness of the phreatic aquifer is set based on the site drilling conditions.

[0079] Preferably, the open excavation forced drainage scheme design also includes: calculating whether the cost of foundation pit excavation and foundation pit drainage exceeds the preset cost. If so, optimizing the foundation pit retaining design and foundation pit drainage design, with priority given to optimizing the foundation pit drainage design.

[0080] Preferably, the preset cost can be the estimated cost of constructing the same foundation pit using the water-stop curtain scheme. The cost items mainly include: cement quantity, number of drill holes, bentonite quantity, steel sections inserted in the water-stop curtain, seamless steel pipes, steel beams, drilling volume, drainage wells, earth excavation, earth backfilling, design fees, and labor costs.

[0081] Preferably, the cost items of the open-cut forced drainage plan mainly include generator rental, water pumps, diesel, earth excavation, earth backfilling, design fees, and labor costs.

[0082] Preferably, in step S5, determining whether the impact of the foundation pit on the surrounding environment meets a preset index includes calculating whether the settlement around the foundation pit is less than a preset settlement index. Preferably, the preset settlement index is 60 mm.

[0083] Preferably, the simulation in step S5 includes: using finite element analysis software to establish a three-dimensional finite element model based on the Biot consolidation theory formula to calculate the ground soil settlement around the foundation pit. Preferably, the simulation of the open-cut forced displacement scheme in step S5 includes: using an MC calculation model (Monte Carlo model) to combine the cohesion, wet density, internal friction angle, compressive elastic modulus, and Poisson's ratio of various strata structures in the open-cut forced displacement scheme to calculate the ground soil settlement around the foundation pit.

[0084] Preferably, when obtaining the depth of the foundation pit to be excavated, relevant foundation pit engineering parameter indicators are also obtained, specifically parameter indicators such as the depth, length, width, and floor area of ​​the deep foundation pit. Preferably, a deep foundation pit generally refers to a foundation pit with an excavation depth exceeding 5 meters.

[0085] Preferably, if it is determined that the depth of the foundation pit to be excavated exceeds the depth threshold and it is determined that the cohesion of the soil in the part of the foundation pit to be excavated that is deeper than the depth threshold from the ground surface does not reach the cohesion preset value, then the open excavation forced drainage scheme is not adopted. In the case that the depth of the foundation pit to be excavated exceeds the depth threshold and the cohesion of the soil in the part of the foundation pit to be excavated that is deeper than the depth threshold from the ground surface does not reach the cohesion preset value, if the open excavation forced drainage scheme is insisted on for foundation pit drainage, in order to prevent slope collapse, it will inevitably lead to an increase in the slope rate, an increase in the slope width, and an increase in the construction area. On the one hand, the increase in the construction area will lead to an increase in the costs of earth excavation, earth backfilling, labor, etc., reducing the cost-effectiveness of the project. On the other hand, it may be limited by the size of the construction site and it may not be possible to excavate a slope that is too wide.

[0086] Preferably, several deep foundation pit engineering parameter indicators that need to be excavated in coastal areas are shown in Table 1.

[0087] Table 1:

[0088]

[0089] Preferably, geological data is collected from several deep foundation pits excavated in coastal areas. Preferably, based on survey results and supplementary drilling data, the coral reef limestone at the location of the deep foundation pit is relatively shallow, and the upper part of the coral reef limestone comprises: a fill layer 101 with a thickness of 0.5 to 2 meters; a saline sand layer 102 with a thickness of 2 to 4 meters, and an overall cover layer buried at a depth of 3 to 5 meters.

[0090] Coral reef limestone can be divided into three types based on core structure: fully weathered coral reef limestone, strongly weathered coral reef limestone, and moderately weathered coral reef limestone. Fully weathered coral reef limestone consists of coral sand interbedded with coral reef calcareous nodules, typically located 0-2 meters above the surface of the coral reef limestone. Strongly weathered coral reef limestone is massive and short columnar. In most boreholes, the interface between strongly and moderately weathered rock is difficult to distinguish, with interbedded layers of strongly and moderately weathered rock often seen.

[0091] Preferably, in this embodiment, the strongly weathered coral reef limestone and the moderately weathered coral reef limestone are merged into the strongly-moderately weathered coral reef limestone.

[0092] Preferably, the engineering geological data and hydrogeological data of the deep foundation pit construction area are obtained by collecting geological data of several deep foundation pits excavated in the coastal area.

[0093] Preferably, the average wet density of the fill layer 101 and the saline sand layer 102 is 18KN / m 3 The average salt content is 1%, the pH value is 8.6-9.5, the average SPT values ​​are 25 and 31 respectively, the cohesion is in the range of 1-7KPa, the average internal friction angles are 30° and 32° respectively, and the permeability is 1×10 -5 ~1×10 -3 m / s. Wet density of fully weathered coral reef limestone is 18-22.5KN / m 3 The average SPT is 48, the cohesion is 5-10 kPa, the average internal friction angle is 35°, the unconfined compressive strength is mostly concentrated between 2 and 7 MPa, and the Young's modulus is mostly concentrated between 200 and 350 MPa. The weight of strongly to moderately weathered coral reef limestone is basically the same as that of fully weathered coral reef limestone, with unconfined compressive strength mostly concentrated between 2 and 8 MPa, with some strengths reaching 12 MPa, and Young's modulus mostly concentrated between 200 and 400 MPa.

[0094] Preferably, the exploration results are calculated according to the formula of Hoek and Deiderichs. The cohesion of strongly-medium weathered coral reef limestone is 90-150 kPa, and the internal friction angle is uniformly 27.6°. Coral reef limestone has well-developed pores, with a porosity ratio of 0.5-0.65; the permeability is 1×10 -4 ~1×10 -2 m / s, where the permeability coefficient obtained by the borehole pumping test is 7.5×10 -4 ~4.03×10 -3 m / s. The average depth of the groundwater level is 3.0m or the elevation is +1.0m.

[0095] Preferably, the geotechnical parameters of each stratum in the deep foundation pit construction area are as shown in Table 2.

[0096] Table 2:

[0097]

[0098] Preferably, taking the fourth foundation pit as an example, the design of the open excavation and forced drainage scheme in the coral reef limestone geological foundation pit construction method provided in this embodiment is explained.

[0099] Preferably, the preset single-layer slope height is set to 3m. Preferably, the depth threshold is set to 6m.

[0100] Preferably, the deepest part of the fourth foundation pit is 13.4m, the depth threshold is set to 6m, and the cohesion of the soil in the part of the foundation pit to be excavated that is more than 6m deep from the ground surface is determined to be 90KPa, which exceeds the preset cohesion value.

[0101] See also Figure 2 According to the thickness of the rock and soil layer and the geotechnical parameters of each layer, a three-layer slope is adopted. The height of each layer from top to bottom is 2.5, 3, and 7.9m respectively, and the slope rate (the ratio of slope height to slope width, the larger the slope rate, the gentler the slope, and the smaller the slope rate, the steeper the slope) is 1.5, 2.5, and 0.25 respectively. The width of the upper two layers of slope platform is 2.0m; the ground construction overload value is 20KPa. The excavation section of the fourth foundation pit is as follows Figure 2 shown.

[0102] Optimally, when designing the number of sloping layers based on the thickness of the rock and soil layers and the geotechnical parameters of each stratum, the slope height should be designed based on the structural strength of each stratum (primarily the compressive elastic modulus and cohesion). For strata with strong structural strength, the slope height can be unlimited while ensuring slope stability. For strata with weak structural strength, the slope height should not exceed the preset single-layer slope height.

[0103] See also Figure 2 The foundation pit slope is divided into three layers from top to bottom. The thickness of the fill layer 101, the saline sand layer 102 and the fully weathered coral reef limestone layer 103 is 5.5m in total. Since the structural strength of the fill layer 101, the saline sand layer 102 and the fully weathered coral reef limestone layer 103 is relatively weak, two layers of slopes with a slope height not exceeding the preset single layer slope height are set. The strongly-medium weathered coral reef limestone layer 104 at the bottom layer has a strong structural strength, so a third layer of slope is set separately. The cohesion of the strongly-medium weathered coral reef limestone layer 104 at the bottom layer is 90KPa, which exceeds the preset cohesion value.

[0104] See also Figure 3Preferably, the Morgenstern-Price method was used to analyze the stability of a typical section of the fourth foundation pit during slope excavation using the Slope / W module in Geo-Studio software. Based on trenching experience at this site, when excavation reaches the coral reef limestone layer, groundwater seeps through the coral reef limestone, while the overburden tends to dry out. This groundwater level was set based on this information. The calculated overall stability of the fourth foundation pit was 1.823, greater than 1.2, indicating that the foundation pit is stable.

[0105] Preferably, the groundwater level line at the bottom of the foundation pit 100 is flush with the water level in the ditch 110, the water level in the ditch 110 is 0.5m lower than the bottom of the foundation pit 100, and the groundwater level line gradually decreases at the slope of the foundation pit and the distance from the slope surface is not less than 3m.

[0106] Preferably, when conducting stability analysis, a uniform load is set at the edge of the foundation pit. The uniform load is set 2m away from the edge of the foundation pit, and the uniform load size is set to 20KN / m 2 .

[0107] Preferably, if the results of the cross-section stability analysis indicate that the foundation pit is unstable, the range of the unstable foundation pit sliding surface is determined. Preferably, if the range of the unstable foundation pit sliding surface is less than or equal to a preset range threshold, the corresponding foundation pit sliding surface is grouting reinforced, and the stability analysis of the grouting reinforced foundation pit is performed; if the grouting reinforced foundation pit is stable, subsequent design simulation is performed; if the grouting reinforced foundation pit is still unstable, the number of foundation pit sloping layers, the height of each layer, the slope rate of each layer, the width of the slope platform and other parameters are reset. Preferably, if the range of the unstable foundation pit sliding surface is greater than the preset range threshold, the number of foundation pit sloping layers, the height of each layer, the slope rate of each layer, the width of the slope platform and other parameters are reset. Preferably, a stability analysis is required after any of the parameters such as the number of foundation pit sloping layers, the height of each layer, the slope rate of each layer, the width of the slope platform and other parameters are changed.

[0108] Preferably, a drainage ditch 110 is excavated along the edge of the foundation pit bottom 100 at a position 2 to 3 meters away from the connection line between the foundation pit bottom 100 and the slope. The drainage ditch 110 is 1 to 2 meters lower than the foundation pit bottom 100.

[0109] Preferably, a water collection well 120 is provided on the water collection ditch 110 . The water collection well 120 has a length and width of not less than 2 meters and a depth 2 to 3 meters lower than the foundation pit bottom 100 .

[0110] Preferably, the drainage pump 300 is placed in the water collection well 120 to pump out the precipitation. Preferably, the drainage pump 300 has a power of 1500m 3 / h mainly.

[0111] Preferably, the total water inflow of the foundation pit is calculated. Preferably, the calculation formula is as follows:

[0112] (1)

[0113] (2)

[0114] Where: Q is the total water inflow of the foundation pit; k is the permeability coefficient; H is the thickness of the phreatic aquifer; h is the water level in the foundation pit after dewatering; l is the length of the water inlet part of the drainage pump 300 filter; r0 is the equivalent radius of the foundation pit; and R is the dewatering impact radius.

[0115] Preferably, the permeability coefficient is the maximum value k=4.03×10 -3 m / s. The thickness of the phreatic aquifer of the fourth foundation pit is unknown. According to the current drilling situation on the site, the depth to 30m is still coral reef limestone, and the thickness of the phreatic aquifer H is taken as 30m. The hourly water inflow is 22244m 3 .

[0116] The recorded daily / hourly rainfall at the fourth foundation pit is 111 mm. The water catchment area of ​​the fourth foundation pit top and its surroundings is 20,000 m 2 Calculation shows that the water volume per hour is 2220m 3 , the maximum water volume of the foundation pit during excavation is 24464m 3 .

[0117] Preferably, in order to ensure the safety and emergency response of subsequent drainage construction, the maximum water volume is 1.5 times that of 36696m 3 The water volume is used for drainage facilities configuration, and a total of 15 units with a power of 1500m 3 / h drainage pump 300. Preferably, four drainage pumps 300 are installed for each water collection well 120. Four water collection wells 120 can meet the requirements. Preferably, the arrangement of water collection wells 120 and drainage pumps 300 can be adjusted according to actual site needs. Preferably, additional water collection wells 120 or drainage pumps 300 can be added to achieve effective precipitation.

[0118] For deep foundation pits in coastal coral reef limestone geology, open excavation and forced drainage are adopted. The groundwater is reduced synchronously with the excavation of the foundation pit, the pore water pressure of the rock and soil is transferred and dissipated, and the effective stress increases. The increased effective stress acts on the porous soil, causing the soil to undergo infiltration and consolidation, which in turn causes soil settlement.

[0119] Preferably, the open excavation and forced drainage scheme designed for the fourth foundation pit is simulated to calculate the settlement displacement of the soil around the fourth foundation pit.

[0120] Preferably, the foundation pit soil settlement is calculated using Biot's consolidation theory formula, and a three-dimensional finite element model is created using Midas software to calculate the surrounding ground settlement. The calculated maximum ground settlement on the north side of the fourth foundation pit is 11.70 mm, the maximum ground settlement on the west side is 10.19 mm, the maximum ground settlement on the south side is 11.54 mm, and the maximum ground settlement on the east side is 10.89 mm, all of which are less than the preset limit of 60 mm.

[0121] Preferably, the actual foundation pit construction is carried out according to the open excavation and forced drainage scheme designed for the fourth foundation pit; and the settlement displacement of the foundation pit is monitored, as well as the groundwater level and water inflow of the foundation pit.

[0122] Monitoring data during actual construction proved that the foundation pit was stable, the ground settlement around the foundation pit was within the range of simulation results, the groundwater could drop to below 0.5m below the base, and the actual water inflow from the foundation pit was close to the calculated results.

[0123] Preferably, the coral reef limestone geological foundation pit construction method provided in this embodiment is used to construct the first foundation pit, the fourth foundation pit and the fifth foundation pit. Compared with the construction scheme of the water-stop curtain combined with the pipe well dewatering, the construction period and construction cost are saved by about 2.0 months, 2.5 months and 2.0 months, 6.1 million yuan, 16.6 million yuan and 20.8 million yuan respectively, directly saving about 38% of the construction cost.

[0124] Preferably, the coral reef limestone geological foundation pit construction method provided in this embodiment is effective when constructing deep foundation pits in coral reef limestone geology in coastal areas by calculating foundation pit stability, water inflow, and ground subsidence caused by strong drainage and designing an open-cut forced drainage scheme. Preferably, the coral reef limestone geological foundation pit construction method provided in this embodiment not only saves construction time and cost compared to the construction scheme of combining water-stop curtains with pipe well dewatering, but also eliminates the environmental pollution risks and increased costs and construction time brought about by casting water-stop curtains.

[0125] Chinese patent application publication number CN108118702A mentions pouring a polymer impermeable material between the inner and outer baffles to prevent seawater from seeping into the excavated foundation pit, which is equivalent to installing a water-stop curtain. However, when the foundation pit is only a few hundred meters or even tens of meters from the coast, the high permeability of coral reef limestone geology and the highly corrosive nature of seawater pose a high risk of leakage, easily contaminating the coral reef environment in coastal areas. The coral reef limestone geology foundation pit construction method provided in this embodiment can both protect the environment and ensure a timely construction schedule.

[0126] Example 3

[0127] This embodiment provides a coral reef limestone geological foundation pit. Preferably, the foundation pit of this embodiment is constructed using the coral reef limestone geological foundation pit construction methods described in Examples 1 and 2, and includes a slope, a foundation pit bottom 100, and a drainage ditch 110 disposed along the edge of the foundation pit bottom 100. Several drainage wells 120 are disposed on the drainage ditch 110.

[0128] Preferably, the slope is sloping in three layers. The drainage ditch 110 is 2-3 meters away from the connecting line between the foundation pit bottom 100 and the slope. The bottom of the drainage ditch 110 is 1-2 meters lower than the foundation pit bottom 100. The length and width of the water collection well 120 are not less than 2 meters, and the depth of the water collection well 120 is 2-3 meters lower than the foundation pit bottom 100.

[0129] Preferably, the water collection well 120 can also be arranged according to the position of the drainage channel 200. Preferably, the drainage channel 200 is used to discharge the water pumped from the water collection well 120 by the drainage pump 300. Preferably, the water inlet of the drainage pump 300 is arranged in the water collection well 120, and the discharge outlet of the drainage pump 300 is arranged next to the drainage channel 200. The drainage pump 300 pumps the water in the water collection well 120 into the drainage channel 200.

[0130] See also Figure 4 When only some of the sump ditches 110 in the foundation pit are close to the drainage channel 200, the sump wells 120 are installed on the sump ditches 110 close to the drainage channel 200. For the sump ditches 110 far from the drainage channel 200, since installing the sump wells 120 and drainage pumps 300 would cause the laid drainage pipelines to interfere with the construction of the foundation pit bottom 100, the sump ditches 110 far from the drainage channel 200 are not equipped with sump wells 120 and drainage pumps 300. As a result, the groundwater collected in the sump cannot be pumped out in time, causing the water level in the sump ditches 110 far from the drainage channel 200 to be higher than that in the sump ditches 110 close to the drainage channel 200. To prevent water from overflowing from the ditch 110 away from the drain channel 200 to the foundation pit bottom 100, diversion grooves 130 are provided in the ditch 110 away from the drain channel 200 and the ditch 110 near the drain channel 200 to increase the flow rate of water from the ditch 110 away from the drain channel 200 to the ditch 110 near the drain channel 200. Preferably, the diversion grooves 130 are filled with gravel, thereby constructing a channel with significantly higher flow efficiency than coral reef limestone without affecting the flatness requirements of the foundation pit bottom 100, allowing water from the ditch 110 away from the drain channel 200 to flow more quickly to the ditch 110 near the drain channel 200.

[0131] See also Figure 5The drainage ditch 110 set at the edge of the foundation pit bottom 100 is configured into two, including: a first drainage ditch 111 close to the slope and a second drainage ditch 112 away from the slope. Preferably, the depth of the first drainage ditch 111 exceeds that of the second drainage ditch 112, and the volume of the second drainage ditch 112 is greater than that of the first drainage ditch 111. Preferably, the bottom of the first drainage ditch 111 is lower than the bottom of the second drainage ditch 112. Preferably, the water collection well 120 is set on the first drainage ditch 111. Preferably, the drainage pump 300 pumps water from the first drainage ditch 111 and discharges the pumped water into the drainage channel 200 through the drainage pipe 301, so that the water level of the first drainage ditch 111 is lower than the water level of the second drainage ditch 112, and the water of the second drainage ditch 112 penetrates into the first drainage ditch 111 through the strongly-medium weathered coral reef limestone layer 104, thereby reducing the turbidity of the water.

[0132] During construction, a first drainage ditch 111 and a second drainage ditch 112 away from the slope are excavated at the bottom of the foundation pit 100, and a coral reef limestone layer between the first drainage ditch 111 and the second drainage ditch 112 is reserved as a natural filtration layer.

[0133] See also Figure 6 The drainage channel 200 includes a first drainage channel 210, a gravel bank 220, a sedimentation tank 230, a second drainage channel 240 and a sea outlet.

[0134] Preferably, the first drain channel 210 and the second drain channel 240 are connected to both ends of the sedimentation tank 230; the end of the second drain channel 240 away from the sedimentation tank 230 is connected to the sea outlet. The cross-sections of the first drain channel 210 and the second drain channel 240 can both be inverted trapezoidal. This inverted trapezoidal cross-section design provides greater stability for the walls of the first drain channel 210 and the second drain channel 240.

[0135] Preferably, one or more gravel ramps 220 are provided in the first drainage channel 210. Preferably, several gravel ramps 220 are provided in the first drainage channel 210. Each of the gravel ramps 220 is provided in the first drainage channel 210 and arranged sequentially along its length. The particle size of the gravel in the gravel ramps 220 decreases from the direction away from the sedimentation tank 230 to the direction closer to the sedimentation tank 230. Specifically, there are three gravel ramps 220, and the particle sizes of the three gravel ramps 220 are 30-40 mm, 20-30 mm, and 15-20 mm, respectively. Each gravel ramp 220 is arranged along the cross-sectional direction of the first drainage channel 210, i.e., the length direction of each gravel ramp 220 is aligned with the cross-sectional direction of the first drainage channel 210. Furthermore, each gravel ramp 220 has a trapezoidal or triangular cross-sectional shape.

[0136] Preferably, the sedimentation tank 230 may include a plurality of baffles 231, which are alternately fixed to the walls of the sedimentation tank 230 on opposite sides of the tank. The bottoms of the baffles 231 are connected to the bottom of the sedimentation tank 230. Specifically, the sedimentation tank 230 may be a cast-in-place concrete structure. The baffles 231 may be concrete slabs cast together with the sedimentation tank 230; alternatively, the baffles 231 may be made of steel plates and fixed with anchor bolts, or welded to embedded parts within the sedimentation tank 230 to achieve connection to the sedimentation tank 230.

[0137] Several baffles 231 are alternately fixed to the walls of the sedimentation tank 230 on opposite sides. The specific method is as follows: one end of the first baffle 231 is fixed to the left wall of the sedimentation tank 230, and the other end is kept at a distance from the right wall, forming a channel. The second baffle 231, which is adjacent to the first baffle 231, has one end fixed to the right wall of the sedimentation tank 230 (offset from the first baffle 231), and the other end is kept at a distance from the left wall. Subsequent baffles 231 are fixed to the left and right walls in this alternating pattern, forming a continuous S-shaped flow channel.

[0138] Preferably, adsorption material can be provided on the walls and bottom of the sedimentation tank 230, as well as on the baffles 231. The adsorption material can be diatomaceous earth or activated carbon. Diatomaceous earth has a naturally porous structure (porosity of 80%-90%), can intercept particles sized 1-100 μm, and is low-cost.

[0139] Preferably, the material used for the rubble bank 220 can be at least one of pumice, porous tuff, and scoria. Pumice is a porous volcanic glass rock with a light texture and high porosity. Porous tuff is a pyroclastic rock that can form a porous structure with high porosity after weathering or artificial processing. Scoria is a lightweight porous rock formed by cooling after a volcanic eruption, with well-developed pores and a loose texture.

[0140] Preferably, the gravel slope 220 is provided using the strongly to moderately weathered coral reef limestone layer 104 excavated from the foundation pit.

[0141] Preferably, the top surfaces of the plurality of gravel steps 220 may be 0.3-0.5 m lower than the top of the first drainage channel 210 . The specific height differences may be 0.3 m, 0.35 m, 0.4 m, 0.45 m, or 0.5 m.

[0142] Preferably, the first drainage channel 210 and the second drainage channel 240 are both formed by excavating on the ground.

[0143] Preferably, if Figure 7 and Figure 8As shown, the walls of the first drainage channel 210 and the second drainage channel 240 are, from the outside inward, covered with a plastic film 211, a geotextile 212, and a cement mortar layer 213. Specifically, the thickness of the plastic film 211 can be 200-500 μm. The geotextile 212 can be a polyester filament geotextile, and the unit area mass of the geotextile 212 can be 300 g / m², 350 g / m², 400 g / m², 450 g / m², or 500 g / m². The cement mortar layer 213 can be 8-15 mm thick.

[0144] Preferably, the sea outlet includes a gravel wall 250, a first anti-fouling curtain 270, and a second anti-fouling curtain 280. The gravel wall 250 is connected to a dike 260 at both ends, and one side of the gravel wall 250 is connected to the second drainage channel 240. The first anti-fouling curtain 270 and the second anti-fouling curtain 280 are both arranged in the ocean 400. The first anti-fouling curtain 270 is connected to the dike 260 at both ends of the gravel wall 250, and the second anti-fouling curtain 280 is connected to the dike 260 at both ends of the gravel wall 250. The first anti-fouling curtain 270 is located between the gravel wall 250 and the second anti-fouling curtain 280. Specifically, the gravel wall 250 is composed of gravel with a particle size of 300 to 600 mm, and the pores formed between the gravel allow water to flow through.

[0145] Anti-fouling curtains are environmentally friendly devices used in sea outlets. Their main components include a corrosion-resistant and strong curtain body, a float to stabilize the curtain and maintain its vertical position, a counterweight to ensure the curtain's bottom is stable and in contact with the water to prevent particles from escaping from below, and accessories such as ropes and clips that connect the various components. These components work together to allow the curtain to be positioned vertically in the water, effectively preventing the outward spread of suspended particles in the drainage water, trapping them within a limited area and preventing pollution to the surrounding marine ecosystem.

[0146] Preferably, if Figure 9 As shown, the cross section of the gravel wall 250 may be trapezoidal, and the slope of the side of the gravel wall 250 connected to the second drainage channel 240 is greater than the slope of the side of the gravel wall 250 facing the ocean 400 .

[0147] Figure 6 and Figure 9 The hollow arrows in the figure indicate the direction of water flow.

[0148] Preferably, a drain pipe 301 is installed on one side of the first drain channel 210. Preferably, the drainage pump 300 pumps water from the sump 110 and discharges it into the first drain channel 210 through the drain pipe 301. The water then flows sequentially through the gravel bank 220, the sedimentation tank 230, the second drain channel 240, and the sea outlet, ultimately draining into the sea. Preferably, the water pumped from the foundation pit undergoes multiple filtrations in the drain channel 200 before being discharged into the sea, ensuring that it meets discharge standards.

[0149] The above are only preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for constructing a coral reef limestone geological foundation pit, characterized in that: The following steps are involved: S1, obtaining the depth of the foundation pit to be excavated and collecting geological data of the coral reef limestone geology; wherein the geological data includes engineering geological data and hydrogeological data; the engineering geological data includes stratum structure and rock and soil mechanical parameters; the rock and soil mechanical parameters include at least cohesion, wet density, and internal friction angle; S2, determine whether the depth of the foundation pit to be excavated exceeds the depth threshold; if so, execute step S4; if not, execute step S3; S3, determining whether the cohesion of the soil in the portion of the foundation pit to be excavated that is deeper than the depth threshold from the ground surface reaches a preset cohesion value; if so, executing step S4; wherein the preset cohesion value ranges from 20 to 30 kPa; S4, designing an open excavation forced drainage scheme based on the geological data; wherein the open excavation forced drainage scheme design includes: designing a foundation pit enclosure to determine the foundation pit excavation structure, and designing a foundation pit drainage to determine the structural parameters and drainage power of the water collection ditch (110); The foundation pit support design includes: The water collection ditch (110) is arranged along the edge of the foundation pit bottom (100); The excavation structure of the foundation pit is determined by designing the number of slope layers, the height of each slope layer, the slope rate of each slope layer, and the width of the slope platform according to the thickness of the rock and soil layers, the cohesion and friction angle of each soil layer, and the depth of the foundation pit to be excavated; According to the cohesion, wet density and internal friction angle of each stratum structure, a stability analysis is performed on a typical section of the foundation pit retaining slope excavation in combination with the foundation pit excavation structure, and a groundwater level line that changes with the foundation pit boundary is introduced into the stability analysis, and a uniformly distributed load is set on the edge of the foundation pit; wherein, the groundwater level line at the bottom of the foundation pit (100) is flush with the water level in the drainage ditch (110), and the groundwater level line gradually decreases at the foundation pit slope and the distance from the slope surface is not less than a preset distance threshold.

2. The coral reef limestone geological foundation pit construction method according to claim 1, characterized in that: The rock and soil mechanical parameters also include: permeability coefficient; The foundation pit drainage design includes: Calculation of foundation pit water inflow, including: The total water inflow of the foundation pit is calculated based on the permeability coefficient, the thickness of the phreatic aquifer, the water level in the foundation pit after precipitation, the length of the water inlet part of the filter, the equivalent radius of the foundation pit, and the precipitation impact radius; The permeability coefficient is the maximum value in the engineering geological data; if the thickness of the phreatic aquifer is unknown, the thickness of the phreatic aquifer is set according to the drilling conditions on site; A plurality of water collection wells (120) are provided on the water collection ditch (110), and drainage pumps (300) are placed in the water collection wells (120) for pumping out precipitation; The position and number of the water collection wells (120) and the drainage power of the drainage pump (300) are set according to the redundant foundation pit water inflow whose numerical value exceeds the calculated foundation pit water inflow; wherein the redundant foundation pit water inflow is set to 1.5 times the calculated foundation pit water inflow; The open excavation forced drainage scheme design also includes: calculating whether the cost of foundation pit excavation and foundation pit drainage exceeds the preset cost. If so, optimizing the foundation pit retaining design and the foundation pit drainage design, with priority given to optimizing the foundation pit drainage design.

3. The method for constructing a coral reef limestone geological foundation pit according to claim 2, characterized in that: In the foundation pit retaining wall design: If the results of the section stability analysis indicate that the foundation pit is unstable, the range of the unstable foundation pit sliding surface is determined; If the range of the unstable foundation pit sliding surface is less than or equal to the preset range threshold, the corresponding foundation pit sliding surface is grouting reinforced, and the stability analysis of the foundation pit after grouting reinforcement is performed; if the foundation pit after grouting reinforcement is stable, subsequent design simulation is performed; if the foundation pit after grouting reinforcement is still unstable, the foundation pit enclosure design is re-performed; If the range of the unstable foundation pit sliding surface is greater than the preset range threshold, the foundation pit retaining wall design is performed again.

4. A coral reef limestone geological foundation pit construction method according to claim 3, characterized in that: The stability analysis adopts the limit equilibrium method to perform stability analysis of typical sections of foundation pit slope excavation, wherein the limit equilibrium method includes at least one of: Ferenius method, Bishop method, Taylor method, Janbu method, Morgenstern-Price method, Spencer method, and Salma method.

5. The coral reef limestone geological foundation pit construction method according to claim 4, characterized in that: When designing the number of sloping layers, the height of each layer, the slope rate of each layer, and the width of the slope platform, the design principles to be followed include: The slope height is designed according to the structural strength of each stratum. For strata with strong structural strength, there is no restriction on the slope height while ensuring the stability of the slope; for strata with weak structural strength, the slope height does not exceed the preset single-layer slope height; wherein, the cohesion of the stratum with strong structural strength reaches the preset cohesion value, and the cohesion of the stratum with weak structural strength is lower than the preset cohesion value.

6. The coral reef limestone geological foundation pit construction method according to claim 5, characterized in that: The following steps are also included: S5, simulate the open excavation and forced drainage scheme, and use finite element analysis software to establish a three-dimensional finite element model based on the Biot consolidation theory formula to calculate the ground soil settlement around the foundation pit; Determine whether the impact of the foundation pit on the surrounding environment meets the preset indicators; if so, execute step S6; if not, return to step S4; S6, excavate the foundation pit according to the open cut forced displacement scheme and monitor the settlement displacement of the foundation pit; Among them, judging whether the impact of the foundation pit on the surrounding environment meets the preset indicators includes: calculating whether the settlement around the foundation pit is less than the preset settlement index; the preset settlement index is 60mm.

7. The method for constructing a coral reef limestone geological foundation pit according to claim 6, characterized in that: The rock and soil mechanical parameters in step S2 also include: compressive elastic modulus and Poisson's ratio; The simulation of the open-cut forced drainage scheme in step S5 is as follows: the ground soil settlement around the foundation pit is calculated by combining the cohesion, wet density, internal friction angle, compression elastic modulus and Poisson's ratio of each stratum structure in the open-cut forced drainage scheme with the MC calculation model.

8. The method for constructing a coral reef limestone geological foundation pit according to claim 7, characterized in that: The depth threshold is 6m, the uniform load is set at 2m from the foundation pit edge, and the uniform load size is set to 20KN / m 2 The water level in the water collection ditch (110) is 0.5 m lower than the bottom of the foundation pit (100), the preset distance threshold is 3 m, and the preset single-layer slope height is 3 m.

9. A coral reef limestone geological foundation pit, characterized in that: The foundation pit is constructed using the coral reef limestone geological foundation pit construction method according to any one of claims 1 to 8, comprising: a slope, a foundation pit bottom (100), and a water collection ditch (110) arranged along the edge of the foundation pit bottom (100); A plurality of water collection wells (120) are provided on the water collection ditch (110).

10. The coral reef limestone geological foundation pit according to claim 9, characterized in that: The drainage ditches (110) provided at the edge of the foundation pit bottom (100) are configured as at least two, wherein the two drainage ditches (110) are a first drainage ditches (111) close to the slope and a second drainage ditches (112) away from the slope; The bottom of the first water collecting ditch (111) is lower than the bottom of the second water collecting ditch (112), and the volume of the second water collecting ditch (112) is greater than the volume of the first water collecting ditch (111); The water collection well (120) is arranged on the first water collection ditch (111), the water level of the first water collection ditch (111) is lower than the water level of the second water collection ditch (112), and the water in the second water collection ditch (112) penetrates into the first water collection ditch (111) through the coral reef limestone.

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