Construction method for preventing hole collapse of pile foundation

Abstract

The application discloses a construction method of pile foundation hole collapse prevention, comprising the following steps: cement soil replacement construction: dynamically adjusting cement mixing amount according to underground water level and soil moisture content; calculating replacement depth according to formula; taking soil sample before replacement to determine optimal mixing amount; guide wall installation and stability monitoring: setting 6-8 settlement observation points on four corners and middle part of the guide wall after precast guide wall installation, monitoring once a day before slotting, monitoring once every 2 hours during slotting, and allowing settlement difference to be less than or equal to 5 mm; steel casing installation: immediately installing steel casing when the excavation depth in the guide wall reaches 1 / 2 of the thickness of soft soil layer or 1.5 times of the height of the guide wall. The application has the beneficial effect that the hole collapse rate is less than or equal to 2.5%, which is far lower than 16.7% of the traditional process, and the technical scheme is especially suitable for complex environments such as high water level, thick soft soil and pipelines.

Description

Technical Field

[0001] This invention relates to the field of pile foundation construction technology, and in particular to a construction method for pile foundation anti-collapse holes. Background Technology

[0002] In the construction of underground projects such as urban rail transit and underground integrated pipe corridors, pile foundation construction often faces complex geological conditions in soft soil strata (such as silty soil and loose sand). The foundation is prone to collapse due to insufficient bearing capacity and unstable borehole walls, resulting in reduced construction efficiency, increased costs, and even safety accidents.

[0003] In existing technologies, foundation treatment often adopts a fixed cement content replacement scheme without considering the dynamic adjustment parameters of groundwater level and moisture content. This often results in insufficient replacement depth or insufficient cement-soil strength, leading to fluctuations in foundation bearing capacity (the measured bearing capacity is often below 100 kPa). Guide wall construction is mostly a rigid structure that does not consider differences in stratum settlement, making it prone to cracks and leakage. Furthermore, it lacks a real-time monitoring mechanism, and the verticality deviation during trenching often exceeds 1 / 400, requiring frequent rework. Steel casing installation relies on experience to judge depth, without clearly defining the relationship with the thickness of the soft soil layer and the height of the guide wall. Moreover, the verticality control accuracy is low (the inclination often exceeds 1 / 250), resulting in poor hole wall protection.

[0004] The aforementioned problems result in a high borehole collapse rate of 8%-12% during pile foundation construction in soft soil strata, and a reduction in trenching efficiency of more than 30%. There is an urgent need for a comprehensive technical solution that integrates dynamic foundation treatment, guide wall stability control, and precise installation of steel casing to solve the pain points of poor parameter adaptability, monitoring lag, and protection failure in traditional construction. Summary of the Invention

[0005] The main technical problem solved by this invention is to provide a construction method for anti-collapse holes in pile foundations, thereby solving one or more of the problems in the prior art.

[0006] To solve the above-mentioned technical problems, the present invention adopts a technical solution as follows: a construction method for anti-collapse holes in pile foundations, the innovation of which lies in the following steps: (1) Cement-soil replacement construction: The cement dosage should be dynamically adjusted according to the groundwater level and soil moisture content: when the groundwater level is higher than 2m or the soil moisture content is greater than 30%, the cement dosage should be 150~200Kg / m³. 3 When the groundwater level is below 2m or the soil moisture content is ≤30%, the cement dosage is 100~150Kg / m³. 3 ; The replacement depth is calculated using the formula: Replacement depth = pipeline burial depth + 2.0m + silt layer thickness × 1.2. After removing the loose backfill sand and silt, cement-soil layered replacement is carried out. Before replacement, soil samples were taken for mix design tests to determine the optimal admixture ratio; after replacement, a light dynamic penetration test (N10) was used to test the bearing capacity of the foundation, which was required to be ≥120kPa. (2) Installation and stability monitoring of guide walls: After the precast guide wall is installed, 6 to 8 settlement observation points are set at the four corners and the middle of the guide wall. The monitoring is carried out once a day before the trench is formed and once every 2 hours during the trench formation. The allowable settlement difference is ≤5mm. The verticality of the top and sides of the guide wall is checked using a total station. If the deviation exceeds 1 / 500, it is reinforced by grouting (cement grout water-cement ratio 1:1) or by adding diagonal supports. A 5mm expansion joint is reserved between the main body of the guide wall and the wing panel, and filled with water-swellable sealing strips. (3) Steel casing installation: When the excavation depth inside the guide wall reaches 1 / 2 or 1.5 times the thickness of the soft soil layer or the height of the guide wall (take the greater of the two), the steel casing should be installed immediately. The length of the steel casing = the thickness of the soft soil layer + 0.5m (depth of embedment in the stable soil layer). If the soft soil layer is followed by a sand layer, the casing should be embedded in the sand layer for ≥1.0m using the vibratory pipe sinking process. During installation, a dual-point hoisting method is used, and the casing is guided by a pre-set positioning bracket (error ≤ 2cm) on the guide wall to ensure that the inclination of the casing is ≤ 1 / 300.

[0007] In some implementations, in step (1), the thickness of each layer of the cement-soil replacement is ≤30cm, and it is compacted by a plate vibrator for a vibration time of ≥30s.

[0008] In some implementations, in step (2), the settlement observation point is made of steel bars with a diameter of 10mm embedded 20cm below the top surface of the guide wall, and the top is ground into a hemispherical shape as an observation marker.

[0009] In some embodiments, in step (3), the steel casing is made of Q235B steel plate with a wall thickness of ≥10mm and three annular stiffening ribs with a spacing of 1.5m on the inner wall.

[0010] In some implementations, in step (1), the spacing between the light dynamic penetration test points is ≤2m, each test point is hammered 3 times consecutively, and the average value is taken as the basis for determining the bearing capacity of the foundation.

[0011] In some embodiments, in step (2), the expansion ratio of the water-swellable sealing strip is ≥200%, the width is 30mm, and the thickness is 20mm.

[0012] In some implementations, in step (3), the positioning bracket is welded from 20# channel steel and tied to the guide wall steel reinforcement skeleton. The installation error is adjusted by shims.

[0013] The beneficial effects of this invention are: High anti-collapse effect: The collapse rate of Examples 1-3 is ≤2.5%, which is far lower than the 16.7% of the traditional process, and is especially suitable for complex environments such as high water level, thick soft soil, and proximity to pipelines.

[0014] Improved construction accuracy: The verticality deviation of the pile foundation is controlled at 1 / 450~1 / 600, which is better than the 1 / 300 requirement of the "Technical Specification for Building Pile Foundations" (JGJ94-2008), ensuring the smooth lowering of the steel cage and the uniform pouring of concrete.

[0015] Cost and efficiency optimization: By dynamically adjusting the cement dosage and quantifying the replacement depth, cement consumption is reduced by 8% to 12%; timely installation of steel casing reduces ineffective excavation, improving construction efficiency by 15% and reducing overall costs by 8% to 20%.

[0016] High safety: Guide wall monitoring and steel casing reinforcement measures can provide early warning of risks, avoiding casualties or equipment damage caused by hole collapse, and significantly improving construction safety. Attached Figure Description

[0017] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort, wherein: Figure 1 This is a flowchart of a construction method for a pile foundation anti-collapse hole according to the present invention. Detailed Implementation

[0018] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0019] This invention includes a construction method for anti-collapse holes in pile foundations, applicable to pile foundation construction under complex geological conditions such as soft soil foundations, high water levels, proximity to underground pipelines, or roadways. The method achieves the anti-collapse goal through the synergistic effect of three core elements: refined cement-soil replacement parameters, dynamic monitoring of guide wall stability, and standardized steel casing installation. The implementation of the technical solution is described below with specific construction details: (a) Essential Constituent Elements Materials and Equipment: Cement-soil mixture: PO 42.5 grade ordinary Portland cement is used, and the soil material is silty clay with a clay content of 15%~30% and a moisture content controlled at 20%~25% (optimal moisture content ±2%).

[0020] Guide wall: C30 reinforced concrete precast structure with a cross-sectional dimension of 1.2m (height) × 0.8m (width) and an internal Φ12mm HRB400 steel mesh (spacing 200mm × 200mm).

[0021] Steel casing: made of Q235B steel plate, with a diameter 200mm larger than the designed pile diameter, a wall thickness of 10~12mm, and 3 ring stiffening ribs welded on the inner wall (thickness 12mm, width 100mm, spacing 1.5m).

[0022] Monitoring equipment: total station (accuracy ±2mm+2ppm), level (accuracy ±0.5mm), lightweight dynamic penetrometer (N10 type).

[0023] Construction process: site leveling → cement-soil replacement → guide wall prefabrication and installation → guide wall stability monitoring → guide wall excavation → steel casing installation → trenching construction → subsequent pile foundation construction (reinforcing cage placement and concrete pouring).

[0024] (II) Key Steps Cement-soil replacement construction: Parameter determination: Before construction, take undisturbed soil samples for indoor mix design tests to determine the optimal cement content (e.g., 180 kg / m³ in high-water-level geological conditions). 3 The replacement depth is calculated using the formula: Replacement depth = pipeline burial depth (e.g., 1.5m) + 2.0m (safety distance) + silt layer thickness (e.g., 3m) × 1.2 = 1.5 + 2 + 3.6 = 7.1m. The actual construction depth is taken as 7.5m (rounded up to the nearest 0.5m).

[0025] Layered construction: The layer thickness is 25~30cm. Use a walking roller (excitation force ≥300kN) to compact 4~6 times, or use a plate vibrator (power 2.2kW) to vibrate for 30~45s to ensure compaction degree ≥93%.

[0026] Quality inspection: every 200m 2 Set up one light dynamic penetration test point, hammer it 3 times and take the average value. The requirement is N10≥15 blows (corresponding to bearing capacity≥120kPa).

[0027] Guide wall installation and monitoring: Prefabrication and installation: The guide wall is prefabricated in sections (each section is 6m long). During installation, a 10t crane is used for hoisting, and a 10cm thick C15 subbase is laid at the bottom for leveling. The backfill soil is made of graded sand and gravel (maximum particle size ≤50mm), compacted in layers (30cm), with a compaction coefficient ≥0.95.

[0028] Monitoring Implementation: Settlement observation points are constructed using Φ10mm steel bars (30cm long) embedded 20cm below the top surface of the guide wall, with crosshairs engraved on the top as observation markers. During total station monitoring, 6 points are observed at each station, and the average of 3 measurements is taken as the result.

[0029] Reinforcement measures: If the verticality deviation of the guide wall reaches 1 / 450 (exceeding the 1 / 500 standard), immediately use Φ50mm grouting pipes (1.5m spacing) for grouting, with a cement grout water-cement ratio of 1:1 and a grouting pressure of 0.3~0.5MPa.

[0030] Steel casing installation: Timing control: Hydraulic grab buckets are used for excavation inside the guide wall. When the excavation depth reaches 1 / 2 (2m) of the soft soil layer thickness (e.g., 4m) or 1.5 times the guide wall height (1.2m×1.5=1.8m), take the maximum value of 2m, immediately stop excavation and install steel casing.

[0031] Length and embedding: Length of steel casing = thickness of soft soil layer (4m) + 0.5m (embedding of stable soil layer) = 4.5m; if the underlying layer is medium sand (thickness 1.2m), the casing is embedded into the sand layer by 1.0m using a vibratory hammer (excitation force 150kN), and the total length is adjusted to 5.0m.

[0032] Verticality control: The positioning bracket is welded with 20# channel steel (80cm high), the bracket spacing is 2m, and the installation error is adjusted by 3mm thick stainless steel shims to ensure that the inclination of the casing is ≤1 / 300 (i.e., the deviation of 5m length is ≤16.7mm).

[0033] Examples and Comparative Examples Example 1: High water table soft soil foundation (groundwater level 1.8m, silt layer thickness 3.5m) Construction parameters: Cement dosage 180 kg / m³ 3 Replacement depth = pipeline burial depth (1.2m) + 2.0m + 3.5m × 1.2 = 7.4m (take 7.5m); guide wall monitoring frequency once every 2 hours during trenching; steel casing length = 3.5m + 0.5m = 4.0m (the underlying layer is a clay layer, no additional embedding is required).

[0034] Results: 30 piles were constructed using trenching, with a 0% collapse rate and a maximum verticality deviation of 1 / 550 (better than the standard deviation of 1 / 300). Cement consumption was reduced by 12% compared to traditional methods.

[0035] Example 2: Nearby underground pipeline (DN800 water supply pipeline, burial depth 2.0m, backfill sand layer thickness 2.5m) Construction parameters: Cement dosage 160 kg / m³ 3(Moisture content 28%), replacement depth = 2.0m + 2.0m + 2.5m × 1.2 = 7.0m; the expansion joint waterstop of the guide wall adopts a rubber strip with an expansion ratio of 250% (30mm × 20mm); the steel casing is embedded in the sand layer for 1.0m (total length = 2.5m + 1.0m + 0.5m = 4.0m).

[0036] Results: 25 piles were constructed in the trench, with a 0% collapse rate, pipeline settlement ≤3mm (the allowable value is 10mm), and verticality deviation ≤1 / 600.

[0037] Example 3: Thick sand layer geology (soft soil layer 2.0m thick, underlying medium sand layer 3.0m thick, adjacent to the roadway) Construction parameters: Cement dosage 150 kg / m³ 3 (Groundwater level 2.5m), replacement depth = 1.0m (no pipeline) + 2.0m + 2.0m × 1.2 = 5.4m (take 5.5m); the steel casing is embedded in the sand layer for 1.2m using the vibratory pipe sinking process (total length = 2.0m + 1.2m + 0.5m = 3.7m); the guide wall monitoring adds lateral displacement observation (allowable value ≤ 5mm).

[0038] Results: 40 piles were constructed in the trench, with a collapse rate of 2.5% (1 pile collapsed due to local loosening of the sand layer, which was corrected after grouting). The verticality deviation was ≤1 / 450. The construction efficiency was improved by 15% compared with the traditional process (due to timely installation of the casing, which reduced waiting time).

[0039] Comparative example: Traditional construction methods (without employing the measures of this invention) Construction parameters: Cement dosage fixed at 150 kg / m³ 3 The replacement depth is 5m based on experience; there are no monitoring measures for the guide wall; the steel casing is installed after excavation to the design pile bottom elevation (12m), with a length of 3.0m (only covering the soft soil layer).

[0040] Results: Of the 30 piles constructed in the trench, the collapse rate was 16.7% (5 piles), the maximum verticality deviation was 1 / 250 (exceeding the standard), rework resulted in a 20% increase in cost and a 12-day delay in the construction period.

[0041] Comparison Table of Example and Comparative Data The principle of this technical solution is as follows: Cement-soil replacement enhances foundation bearing capacity: By dynamically adjusting the cement content (based on water level and moisture content) and quantifying the replacement depth (calculated by formula), the loose soil layer (backfill sand, silt) is ensured to be replaced by high-strength cement-soil (fcu≥2.5MPa), thereby improving the foundation's resistance to deformation and preventing hole collapse due to hollowness at the bottom during trenching.

[0042] Guide wall monitoring ensures structural stability: As a trenching guide structure, the settlement and verticality of the guide wall are monitored in real time through high-frequency monitoring (every 2 hours during trenching). Combined with grouting reinforcement and flexible expansion joint design, it can offset uneven settlement of the foundation and temperature stress, ensuring accurate trenching trajectory (verticality deviation ≤ 1 / 500).

[0043] Standardized anti-collapse hole installation with steel casing: The steel casing is installed in time at the "critical depth" (1 / 2 or 1.5 times the height of the guide wall in soft soil layer), which can use the rigidity of the casing to resist shallow soil pressure; when embedded in the stable soil layer (0.5m + 1.0m sand layer), it forms a "rigid barrier" to prevent the hole from collapsing due to deep sand flow or piping. At the same time, the double lifting point hoisting and positioning bracket ensure the installation accuracy.

[0044] The advantages of this technical solution are: High anti-collapse effect: The collapse rate of Examples 1-3 is ≤2.5%, which is far lower than the 16.7% of the traditional process, and is especially suitable for complex environments such as high water level, thick soft soil, and proximity to pipelines.

[0045] Improved construction accuracy: The verticality deviation of the pile foundation is controlled at 1 / 450~1 / 600, which is better than the 1 / 300 requirement of the "Technical Specification for Building Pile Foundations" (JGJ94-2008), ensuring the smooth lowering of the steel cage and the uniform pouring of concrete.

[0046] Cost and efficiency optimization: By dynamically adjusting the cement dosage and quantifying the replacement depth, cement consumption is reduced by 8% to 12%; timely installation of steel casing reduces ineffective excavation, improving construction efficiency by 15% and reducing overall costs by 8% to 20%.

[0047] High safety: Guide wall monitoring and steel casing reinforcement measures can provide early warning of risks, avoiding casualties or equipment damage caused by hole collapse, and significantly improving construction safety.

[0048] This invention solves the problems of ambiguous parameters, lack of monitoring, and arbitrary installation of casing in traditional pile foundation construction by multi-stage collaborative control, and provides an efficient, economical and safe anti-collapse hole solution for pile foundation engineering under complex geological conditions.

[0049] The above description is merely an embodiment of the present invention and does not limit the patent scope of the present invention. Any equivalent structural or procedural transformations made based on the content of the present invention specification, or direct or indirect applications in other related technical fields, are similarly included within the patent protection scope of the present invention.

Claims

1. A construction method for anti-collapse holes in pile foundations, characterized in that: Includes the following steps: (1) Cement-soil replacement construction: The cement dosage should be dynamically adjusted according to the groundwater level and soil moisture content: when the groundwater level is higher than 2m or the soil moisture content is greater than 30%, the cement dosage should be 150~200Kg / m³. 3 When the groundwater level is below 2m or the soil moisture content is ≤30%, the cement dosage is 100~150Kg / m³. 3 ; The replacement depth is calculated using the formula: Replacement depth = pipeline burial depth + 2.0m + silt layer thickness × 1.

2. After removing the loose backfill sand and silt, cement-soil layered replacement is carried out. Before replacement, soil samples were taken for mix design tests to determine the optimal admixture ratio; after replacement, a light dynamic penetration test (N10) was used to test the bearing capacity of the foundation, which was required to be ≥120kPa. (2) Installation and stability monitoring of guide walls: After the precast guide wall is installed, 6 to 8 settlement observation points are set at the four corners and the middle of the guide wall. The monitoring is carried out once a day before the trench is formed and once every 2 hours during the trench formation. The allowable settlement difference is ≤5mm. The verticality of the top and sides of the guide wall is checked using a total station. If the deviation exceeds 1 / 500, it is reinforced by grouting (cement grout water-cement ratio 1:1) or by adding diagonal supports. A 5mm expansion joint is reserved between the main body of the guide wall and the wing panel, and filled with water-swellable sealing strips. (3) Steel casing installation: When the excavation depth inside the guide wall reaches 1 / 2 or 1.5 times the thickness of the soft soil layer or the height of the guide wall (take the greater of the two), the steel casing should be installed immediately. The length of the steel casing = the thickness of the soft soil layer + 0.5m (depth of embedment in the stable soil layer). If the soft soil layer is followed by a sand layer, the casing should be embedded in the sand layer for ≥1.0m using the vibratory pipe sinking process. During installation, a dual-point hoisting method is used, and the casing is guided by a pre-set positioning bracket (error ≤ 2cm) on the guide wall to ensure that the inclination of the casing is ≤ 1 / 300.

2. The construction method for a pile foundation anti-collapse hole according to claim 1, characterized in that: In step (1), the thickness of each layer of the cement-soil replacement is ≤30cm, and it is compacted by a plate vibrator for ≥30s.

3. The construction method for anti-collapse holes in pile foundations according to claim 1, characterized in that: In step (2), the settlement observation point is made by embedding a 10mm diameter steel bar 20cm below the top surface of the guide wall, and the top is ground into a hemispherical shape as an observation marker.

4. The construction method for a pile foundation anti-collapse hole according to claim 1, characterized in that: In step (3), the steel casing is made of Q235B steel plate with a wall thickness of ≥10mm and three annular stiffening ribs with a spacing of 1.5m on the inner wall.

5. The construction method for a pile foundation anti-collapse hole according to claim 1, characterized in that: In step (1), the spacing between the light dynamic penetration test points is ≤2m, and each test point is hammered 3 times continuously. The average value is taken as the basis for determining the bearing capacity of the foundation.

6. The construction method for a pile foundation anti-collapse hole according to claim 1, characterized in that: In step (2), the expansion ratio of the water-swellable sealing strip is ≥200%, the width is 30mm, and the thickness is 20mm.

7. The construction method for a pile foundation anti-collapse hole according to claim 1, characterized in that: In step (3), the positioning bracket is welded from 20# channel steel and tied to the guide wall steel reinforcement skeleton. The installation error is adjusted by shims.

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