Riverbed silty clay foundation pit excavation method

Abstract

The invention relates to a foundation pit excavation method, and particularly discloses a riverbed silty clay foundation pit excavation method. The method comprises the following steps: S1, filling a cofferdam, primarily draining water from a foundation pit, draining underground water and draining surface water from an open trench; s2, testing the hardness of the soil; s3, a foundation pit double-helix road is built; and S4, the excavation area of the foundation pit is measured and lofted, an excavation lineation is determined, and zoned and layered excavation is conducted from the middle to the periphery in a bowl bucket type excavation mode. According to the method, the foundation pit excavation area is lofted, layered excavation is conducted in a bowl-bucket type excavation mode, and the slope stability is maintained through the gradient and the self weight of the excavated slope; and the double-helix road is arranged to avoid vehicle meeting, so that the transportation efficiency is improved.

Description

Technical Field

[0001] The invention relates to a foundation pit excavation method, and in particular to a riverbed silty clay foundation pit excavation method. Background Art

[0002] With the development of the engineering industry, the technology of foundation pit excavation has become more mature, and the methods of foundation pit excavation have also become various. The common ones include slope and layered excavation, basin excavation, supported foundation pit excavation and center island excavation. However, when facing foundation pits with different types of soil, due to the uniqueness of the engineering industry, it is still difficult to find a ready-made and highly applicable construction method.

[0003] The riverbed silty clay foundation pit has high water content, high plasticity, low hardness and poor stability. The foundation pit excavation is deep and construction is difficult. In the excavation of deep foundation pits, maintaining slope stability, safety and high-efficiency excavation are still the key and difficulty of foundation pit excavation.

[0004] Among the above-mentioned foundation pit excavation methods, slope-cutting and layered excavation is more suitable for large riverbed foundation pits. However, the amount of foundation pit excavation work increases, and the slope deformation is large. It is necessary to perform stability analysis on the foundation pit slope and monitor the slope deformation at any time. Therefore, it is necessary to improve the construction method, while reducing the slope gradient, increase the foundation pit excavation and transportation intensity to meet the construction progress requirements.

[0005] In the existing foundation pit excavation methods, most of them use one road in and out of the foundation pit to meet the needs of transport vehicles entering and exiting the foundation pit. This method not only requires a large road width to meet the needs of two vehicles traveling in opposite directions at the same time; it also affects the transportation efficiency because vehicles have to reduce their speed when meeting each other.

[0006] Therefore, a method for excavating foundation pit in riverbed silty clay is proposed. Summary of the invention

[0007] The purpose of the present invention is to provide a riverbed silty clay foundation pit excavation method to solve the problems of high excavation slope and low transportation efficiency during the riverbed silty clay foundation pit excavation process.

[0008] In order to solve the above technical problems, the technical solution adopted by the present invention is: a method for excavating a riverbed silty clay foundation pit, comprising the following steps; Step S1, fill the cofferdam, carry out initial drainage of the foundation pit, drain groundwater and drain surface water; Step S2, testing the soil hardness, and proceeding to the subsequent steps after the soil hardness meets the standard; Step S3, constructing a double helix road in the foundation pit; Step S4, measuring and setting out the foundation pit excavation area, determining the excavation line, and using a bowl-shaped excavation method to excavate in sections and layers from the middle to the surrounding areas.

[0009] As a further technical solution of the above scheme, in step S4, when the foundation pit is excavated, monitoring points are set on the upstream and downstream cofferdam slopes and the left and right bank slopes to continuously monitor the deformation of the slopes and the foundation pit.

[0010] As a further technical solution of the above solution, in step S1, the following specific steps are included: Step S11, after the cofferdam is closed and sealed, the accumulated water in the foundation pit is pumped out using a water pump and a water pipe; Step S12, when the height difference between the weir top elevation and the water level in the foundation pit exceeds the rated lift of the water pump, a transfer sump is set on the slope within the height difference below the lift of the water pump, and the accumulated water in the foundation pit is pumped into the river channel by connecting multiple water pumps in series; Step S13, a circle of pipe wells are arranged around the foundation pit, and the groundwater in the foundation pit and the seepage water of the cofferdam are pumped out by using the pipe well point method; Step S14, digging crisscross open ditches in the foundation pit to collect surface water such as rainfall and construction wastewater, and pumping them out of the foundation pit with a water pump.

[0011] As a further technical solution of the above scheme, in step S2, an iron rod is freely dropped into the soil for testing, the cylindrical iron rod is raised, and then freely dropped vertically, and the soil hardness is calculated according to the formula: f′×h′=m×g×(h+h′), In the formula, h is the height from which the iron rod falls to the ground, h′ is the depth of the iron rod inserted into the soil, g is the acceleration due to gravity, f′ is the soil resistance, and m is the mass of the iron rod.

[0012] As a further technical solution of the above solution, in step S3, a double-helix drainage pipe is arranged along the double-helix road to cooperate with the drainage pipe in the foundation pit for drainage.

[0013] As a further technical solution of the above solution, in step S4, the following specific steps are included: Step S41, dividing the foundation pit into sections and layers on the cross section, and excavating from the center to the surrounding areas; Step S42, the slope and the base form a bowl-shaped excavation surface; Step S43: during excavation, longitudinal drainage ditches are set on the surface of the excavation area to remove rainfall during the construction period, construction wastewater and soil surface seepage, and transverse main drainage ditches are set on the lowest excavation surface in the middle to collect water from the drainage ditches into the water collection well and then pumped out of the foundation pit; Step S44, prefabricated concrete pipes are buried at the intersection of the drainage ditch and the double-helix road to pass water, and soil is backfilled on top to facilitate vehicle passage.

[0014] Compared with the prior art, the present invention has the following advantages and beneficial effects: the present invention lays out the foundation pit excavation area, adopts a bowl-shaped excavation method for layered excavation, and utilizes the slope and deadweight of the excavated slope to maintain the stability of the slope; and a double helix road is set to avoid oncoming vehicles, thereby improving transportation efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 This is a schematic diagram of the plan layout of the foundation pit.

[0016] Figure 2 This is a schematic diagram of the plan layout of the open ditch.

[0017] Figure 3 This is a schematic diagram of the plan layout of the double helix road.

[0018] Figure 4 Schematic diagram of soil hardness detection.

[0019] Figure 5 This is a schematic diagram of the foundation pit excavation layers.

[0020] Figure 6 It is a flow chart of the steps of the present invention.

[0021] The meanings of the numbers in the figure are: foundation pit-1; open ditch-2; soil-3; double helix road-4; double helix drainage pipe-5. DETAILED DESCRIPTION

[0022] The technical scheme in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings in the embodiments of the present invention, so as to have a further understanding of the concept of the present invention, the technical problems solved, the technical features constituting the technical scheme and the technical effects brought about.

[0023] like Figure 1-Figure 6 As shown, a method for excavating a riverbed silty clay foundation pit comprises the following steps: Step 1: Construct a cofferdam anti-seepage system and drainage measures for foundation pit 1, and build an excavation method model; Step 2: According to the geological characteristics, flow conditions and flood standards of the river in the construction section, determine the structural type, axis of the cofferdam, anti-seepage system and construction method of the cofferdam, and intercept and build the cofferdam at the axis of the cofferdam upstream and downstream of the pre-excavated riverbed section; Step 3: After the cofferdam is completed, the initial drainage of the foundation pit is carried out. The water in the foundation pit, the seepage water of the weir body and the weir foundation are pumped out to the river outside the foundation pit using water pumps and water pipes. The initial drainage time of the foundation pit is determined according to the nature of the cofferdam and the depth of the foundation pit. For earth cofferdams or overburden slopes, the water level drop rate of the foundation pit is controlled at 0.5-0.8m / d at the beginning, and at 1.0-1.5m / d when it is close to being drained. In general, 5-7d can be used for large foundation pits, and 3-5d for medium-sized foundation pits; Step 4: Drain groundwater and open ditch 2 Drain surface water. Arrange a circle of pipe wells around the foundation pit. Use pipe well points to pump out groundwater in the foundation pit and seepage water from the cofferdam. Dig criss-cross open ditches in the foundation pit to collect surface water such as rainfall and construction wastewater, and pump them out of the foundation pit with a water pump. Step 5: Use a simple method of letting an iron rod fall freely into the soil to a depth of 3 to estimate the soil hardness. When the soil hardness reaches the standard, proceed to the subsequent steps. Step six: Build a temporary double-helix road in and out of the foundation pit. According to the layout of the foundation pit and the layout of the cofferdam road, construct an excavation model, calculate and determine the starting point of the road, and calculate the slope of the upstream and downstream double-helix roads by inversion based on the excavation depth. Form a circular foundation pit road in steps to form one-way traffic for unloaded and heavy-loaded vehicles, solving the problem of difficult traffic in the narrow foundation pit. The double-helix road is constructed by mixing the curing agent mixture and silty clay on site. The thickness is calculated to be 53cm according to the bearing capacity, and the vibratory roller is compacted to meet the traffic requirements; Step 7: Measure and stake out the excavation area in the foundation pit to determine the excavation boundary and excavation partition lines. Set detection benchmarks on the stable bedrock on the left and right banks, and set monitoring points on the upstream and downstream cofferdams and the edge of the foundation pit. Regularly measure and record the displacement and settlement changes between the benchmark points and the monitoring points, and compare them with the warning values ​​to determine the safety of the slope; Step 8: Set up monitoring points on the upstream and downstream cofferdam slopes and the left and right bank slopes to monitor the deformation of the slopes and foundation pit; Step nine, use the bowl-shaped excavation method to excavate the soil from the middle to the surrounding areas in sections and layers. The bowl-shaped excavation of the foundation pit means dividing the foundation pit into sections and layers on the section, and excavating from the center to the surrounding areas; the slope and the base form a bowl-shaped excavation section; during excavation, a longitudinal drainage ditch is set on the surface of the excavation area to remove rainfall during the construction period, construction wastewater and soil surface seepage, and a transverse main drainage ditch is set on the lowest excavation surface in the middle to collect the water from the drainage ditch into the water collection well and pump it out of the foundation pit. At the intersection of the drainage ditch and the temporary road, a φ40 precast concrete pipe is buried to pass the water, and the soil is backfilled on top to facilitate vehicle traffic; Step 10: Continuously monitor the displacement and deformation of slopes and foundation pits, collect and organize monitoring data, and divide them into slope stability and safety.

[0024] In the process of calculating soil hardness, first measure the length of the cylindrical iron rod, lift the iron rod to a certain height, and measure the height from the ground; then let the iron rod fall freely into the soil and measure the depth of the pit; assuming that the resistance of the iron rod when it falls into the soil is balanced, it can be known that the resistance of the soil f′×the depth of fall h′=the potential energy of the iron rod=the mass of the iron rod m×the height difference of the iron rod after falling into the soil (h+h′)×the acceleration of gravity g; that is, f′×h′=m×g×(h+h′), where h is the height of the iron rod falling to the ground. The mass m of the iron rod can be calculated from the volume and density ρiron of the iron rod. According to the formula, the compressive strength of the soil (the pressure it can withstand) can be calculated as: p=77×103N / m³×h″*(h+h') / h''; In the formula, g=9.8N / m2; ρiron=7.86g / cm3, h is the height of the iron rod falling to the ground; g is the acceleration due to gravity; h'' is the length of the iron rod itself.

[0025] Finally, based on the above data and formula, the compressive strength of the soil is calculated. Through multiple sets of tests, the average strength of the soil can be obtained to determine whether it will sink when the car is driving and whether it can be excavated. When the calculated result is greater than 105 Pa, excavation can be carried out.

[0026] When constructing the double helix road 4, the spiral road shall have a slope of no more than 10% from the top of the cofferdam to the foundation pit, from high to low; the turning radius of the road shall not be less than 15m; the road surface width shall be 4.5m or above; a 20m long and 6.5m wide passing section shall be set up every 200m or 1 turn. When the road approaches the foundation pit, the slope shall be gradually reduced, not exceeding 5% at the edge of the foundation pit, and not exceeding 3% in the fully loaded car starting section inside the foundation pit; the temporary road shall use concrete pavement, and the surface water and scattered soil and gravel on the road shall be cleaned regularly, and regular maintenance and repair shall be carried out; and after the double helix road is built, a double helix drainage pipe 5 shall be set up along the double helix road. The double helix drainage pipe serves as an auxiliary drainage device, and cooperates with the open ditch in the foundation pit to drain water together.

[0027] When surveying and staking out the excavation area in the foundation pit, determine the excavation boundary line and excavation partition line, and measure the coordinates and elevation data in the foundation pit based on geological data and design drawings. Based on the measurement results, draw the excavation area and make obvious marks by spraying paint or spreading lime.

[0028] At the edge of the foundation pit, in the middle and at the bottom of the cofferdam, set up foundation pit displacement and deformation monitoring points to observe the settlement, deformation and displacement data of the foundation pit and cofferdam. Set up two reference points on the rock foundation on the left and right banks, pour concrete piers, and embed steel bars in the center of the piers; Three rows of monitoring points are set up on the upstream and downstream cofferdams, with no less than three monitoring points in each row. The horizontal displacement monitoring points are observed by traverse measurement method and total station. The settlement monitoring points are observed by geometric leveling method and level instrument.

[0029] When excavating the foundation pit, the middle area is excavated first, and then the surrounding areas are excavated from the inside to the outside, so that the excavated section is like a bowl shape, which is convenient for building temporary construction roads and setting drainage ditches. The excavation slope ratio is controlled between 1:2 and 1:3, and the excavation thickness is 3m.

[0030] Each area is excavated in layers, and after each layer is excavated, the next layer is excavated; after each layer is excavated, a drainage ditch is excavated on the surface of the next layer to drain rainfall during the construction period, construction wastewater and moisture in the soil surface; a main drainage ditch is dug in the middle of the foundation pit to collect water and pump it out of the foundation pit.

[0031] While excavating the foundation pit, the settlement, deformation and displacement of the foundation pit are monitored. The earthwork excavation adopts the backward excavation method, where the backhoe excavator and dump truck drive on the dry soil surface to excavate the earth below the working surface to avoid wheel sinking.

[0032] The terms “connection” and “fixation” mentioned in the description of the present invention may refer to fixed connection, processing and forming, welding or mechanical connection. The specific meanings of the above terms in the present invention shall be understood according to the specific circumstances.

[0033] In the description of the present invention, the terms "center", "upper", "lower", "horizontal", "inside", "outside", etc., which indicate the orientation or position relationship, are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying a specific orientation that the device or element referred to must have, and therefore cannot be understood as a limitation on the present invention.

[0034] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that the technical solutions described in the aforementioned embodiments can still be modified, or some or all of the technical features therein can be replaced by equivalents. However, these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for excavating a riverbed silty clay foundation pit, characterized in that: The steps include: Step S1, fill the cofferdam and carry out the initial drainage of the foundation pit (1) to remove groundwater and open ditch (2) to remove surface water; Step S2, testing the hardness of the soil (3); Step S3, constructing a double helix road in the foundation pit (4); Step S4, measuring and setting out the excavation area of ​​the foundation pit (1), determining the excavation line, and excavating in sections and layers from the middle to the surrounding areas using a bowl-shaped excavation method.

2. A riverbed silty clay foundation pit excavation method as claimed in claim 1, characterized in that: In step S4, when the foundation pit is excavated, monitoring points are set on the upstream and downstream cofferdam slopes and the left and right bank slopes to continuously monitor the deformation of the slopes and the foundation pit.

3. A riverbed silty clay foundation pit excavation method as claimed in claim 1, characterized in that: In step S1, the following specific steps are included: Step S11, after the cofferdam is closed and sealed, the accumulated water in the foundation pit is pumped out using a water pump and a water pipe; Step S12, when the height difference between the weir top elevation and the water level in the foundation pit exceeds the rated lift of the water pump, a transfer sump is set on the slope within the height difference below the lift of the water pump, and the accumulated water in the foundation pit is pumped into the river channel by connecting multiple water pumps in series; Step S13, a circle of pipe wells are arranged around the foundation pit, and the groundwater in the foundation pit and the seepage water of the cofferdam are pumped out by using the pipe well point method; Step S14, digging crisscross open ditches in the foundation pit to collect surface water such as rainfall and construction wastewater, and pumping them out of the foundation pit with a water pump.

4. A riverbed silty clay foundation pit excavation method as claimed in claim 1, characterized in that: In step S2, an iron rod is freely dropped into the soil for testing. The cylindrical iron rod is raised and then freely dropped vertically. The soil hardness is calculated according to the formula: f′×h′=m×g×(h+h′), In the formula, h is the height from which the iron rod falls to the ground, h′ is the depth of the iron rod inserted into the soil, g is the acceleration due to gravity, f′ is the soil resistance, and m is the mass of the iron rod.

5. A method for excavating a riverbed silty clay foundation pit as claimed in claim 1, characterized in that: In step S3, a double-helix drainage pipe (5) is arranged along the double-helix road to cooperate with the drainage pipe in the foundation pit for drainage.

6. A method for excavating a riverbed silty clay foundation pit as claimed in claim 1, characterized in that: In step S4, the following specific steps are included: Step S41, dividing the foundation pit into sections and layers on the cross section, and excavating from the center to the surrounding areas; Step S42, the slope and the base form a bowl-shaped excavation surface; Step S43: during excavation, longitudinal drainage ditches are set on the surface of the excavation area to remove rainfall during the construction period, construction wastewater and soil surface seepage, and transverse main drainage ditches are set on the lowest excavation surface in the middle to collect water from the drainage ditches into the water collection well and then pumped out of the foundation pit; Step S44, prefabricated concrete pipes are buried at the intersection of the drainage ditch and the double-helix road to pass water, and soil is backfilled on top to facilitate vehicle passage.

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