Construction technology of pile foundation in karst area

2.9. 1 Common pile foundations in karst areas of Guilin

Due to the existence of caves and collapses in the foundation, the surface of limestone foundation generally has a layer of soft soil in soft flow plastic state. When at least one of the checking calculation of the strength of the local foundation bearing layer, the checking calculation of the strength of the soft substratum, the checking calculation of the foundation deformation or the evaluation of the foundation stability fails to meet the requirements, and the foundation treatment cost may be high, it is best to choose the pile foundation at this time. In Guilin karst area, bored pile foundation is mainly used. The cast-in-place pile is drilled directly by machinery or manpower. According to the different hole-forming methods and means, it is also called bored pile, immersed pile and manual digging pile. Other pile types are rarely used in Guilin karst area.

2.9.2 Construction technology of immersed cast-in-place pile in karst area of Guilin

Immersed cast-in-place pile refers to the method of sinking the steel pipe or precast reinforced concrete pile tip with petal-shaped pile shoes into the soil, and then pulling out the conduit by hammering or vibration while pouring concrete.

In Guilin karst area, most of the immersed cast-in-place piles are pebble beds in the first terrace of Lijiang River and pebble clay (mixed soil) in the second terrace as the bearing layer at the pile end, and some of them choose limestone as the bearing layer at the pile end. The advantage of this foundation is that the pile diameter and length can be adjusted appropriately with the change of geological conditions, and the construction progress of this pile foundation is faster, the construction period is relatively short and the unit cost is lower. When limestone is used as the bearing layer at the pile end, there are the following shortcomings: ① In the area with large fluctuation of bedrock surface, inclined piles or even sliding piles are easy to appear; (2) When the pile tip is placed in incomplete rock mass such as olecranon and stone bud, its bearing capacity cannot be guaranteed; (3) If pebbles are distributed on the upper part of bedrock, the construction of foundation piles is more difficult.

preliminary work for construction

2.9.2. 1. 1 material

Mainly including cement, sand, stone, steel and so on.

(1) Cement: Portland cement shall be used, and the cement shall have the factory certificate when entering the site. The construction unit shall conduct sampling inspection according to the variety and batch number of cement entering the site, and it can only be used after it is qualified;

(2) Medium-coarse sand: river sand with good gradation, hard texture and clean particles is adopted, and its silt content is not more than 3%;

(3) Stones: hard gravels or pebbles are used, the maximum particle size should not be greater than 40 mm, and should not be greater than 1/3 of the minimum net spacing of steel bars, and the silt content should not be greater than 2%;

(4) Reinforcement: Reinforcement shall have the ex-factory quality certificate when entering the site. Should check whether its specifications meet the requirements, whether there is damage, corrosion, oil pollution, and in accordance with the provisions, sampling, compression, bending and welding test before use (imported steel bar should be chemical composition and welding test, in line with the relevant provisions before use in engineering). The diameter of the reinforcing cage should meet the design requirements, and should be 60 ~ 80mm； smaller than the inner diameter of the casing;

(5) Pile tip: generally, reinforced concrete pile tip is adopted, and the structure and quantity of reinforcement must meet the requirements of design or construction specifications.

Operating conditions

(1) Site survey shall be conducted before construction. If there are overhead wires, underground wires, water supply and drainage pipelines and other facilities that hinder construction or have an impact on safe operation, they should be removed, displaced or properly handled before construction;

(2) Before construction, the site should be leveled. Effective drainage measures should be taken during construction in rainy season;

(3) There should be engineering geological data of the construction area, construction drawings determined by joint review, construction organization design (or scheme), factory certificates of various raw materials and precast pile tips, sampling test reports, concrete mix design reports and related materials;

(4) The performance of pile driver must meet the design requirements of piles;

(5) It is best to choose 1 ~ 2 pile together with the design unit to carry out the piling process test (i.e. pile test) to check whether the site geological conditions, pile foundation equipment and construction technology meet the requirements of the design drawings.

2.9.2.2 operation technology

(1) The construction method of cast-in-place pile with hammer sinking pipe is generally "single-click method", but "multi-hit method" can also be adopted according to design requirements or soil conditions.

(2) Hammer sinking cast-in-place pile shall be driven backward in the order of flowing water. For pile groups and piles whose center distance is less than 3.5 times the pile diameter, technical measures should be taken that do not affect the quality of adjacent piles;

(3) When the pile driver is in place, the pile pipe should be vertically aligned and inserted into the embedded pile tip, and the pile frame base should be horizontally and stably positioned, and the allowable deviation of pile frame verticality should not be greater than 0.5%;

(4) After the pile tip is buried, the pile axis shall be rechecked. The surface of the pile tip should be cleaned, and the contact between the pile pipe and the shoulder of the pile tip should be padded with straw rope or sack;

(5) Pay attention to the inspection before the formal construction to ensure that there is no deviation in the verticality of the pile pipe. At the beginning of piling, generally use a low hammer and a slow hammer first. If there is any deviation in the pile pipe during piling, measures should be taken to correct it. If the deviation is too large to be corrected, it should be solved in time with the construction director and the technical and design departments;

(6) The depth of immersed tube shall be controlled according to the design requirements and the bearing stratum at the pile end and the penetration of the last three rows of ten hammers, which shall be determined by the test pile and based on the depth of buried tube. In Guilin karst area, when the bearing stratum at the pile tip is pebbles, the penetration depth of ten hammers in each row of the last three rows is generally not more than 20 mm, and the penetration value of ten hammers in each row should not be increased;

(7) When pulling out the pipe, the method of pulling out the pipe shall be adopted, with a small drop of free fall hammer hitting not less than 40 times /m in, and the speed of pulling out the pipe shall be uniform, generally, the soil layer shall not be greater than 65438+/-0m/min;

(8) Part or all of the piling construction according to the design requirements must be carried out before the initial setting of the first poured pile concrete.

Matters needing attention in the construction of 2.9.2.3

2.9.2.3.1Avoid common faults in engineering quality.

(1) In order to prevent necking, broken piles, concrete falling off, steel bar sinking and mud inclusion in piles, the engineering geological report should be studied in detail and effective technical measures should be formulated;

(2) When pouring concrete, it is necessary to accurately judge whether the total amount of concrete poured into the pile can meet the design calculation. In the process of tube drawing, the speed of tube drawing should be strictly controlled, and measures should be taken in time when necking is found;

(3) If the pile jumping method is adopted for construction, the pile jumping can only be carried out when the adjacent piles reach more than 60% of the design strength;

(4) Try to avoid turning and inserting when pulling out the pipe, so as to avoid mud around the hole wall squeezing into the pile body and causing mud inclusion in the pile body.

Main safety technical measures

(1) In the construction scheme, carefully formulate practical safety technical measures;

(2) Remove the high-altitude and underground obstacles that hinder the construction, level the site within the piling scope and tamp the road where the pile driver walks;

(3) Seriously investigate the adjacent original buildings and underground pipelines, and study effective safety measures to avoid damaging the original buildings and causing casualties;

(4) During piling, in case of ground heave or sinking, the pile driver shall be leveled at any time and the pile frame shall be righted;

(5) When piling, it is forbidden to straighten the pile head pad by hand, and it is forbidden to hammer or brake before the pile hammer hits the pile top, so as not to damage the piling equipment;

(6) Strictly implement the relevant provisions of Technical Specification for Temporary Electricity Safety on Construction Site (JGJ 46—2005).

2.9.3 Construction of rock-socketed cast-in-place pile with drilled (punched) holes in Guilin karst area

In the karst area of Guilin, limestone is selected as the bearing layer at the pile tip for rock-socketed bored piles. Bored (punched) rock-socketed cast-in-place pile can not only pass through the locally dense pebble layer, but also pass through the limestone bud with small thickness and the roof of karst cave, and reach the complete limestone section with sufficient bearing layer thickness; It also solves the problems of sliding pile and inclined pile caused by the fluctuation of bedrock surface in other pile types, and can ensure high bearing capacity. This kind of pile foundation has high bearing capacity and can be used for one column and one pile. Safe and reliable, and flexible design of pile length and diameter. The construction noise is relatively small, but the disadvantage is that the unit cost is high, the construction time of single pile is long, and mud and sewage are easy to cause adverse effects on the surrounding environment during construction.

Preparation before construction

(1) Level the site, remove sundries and tamp the backfill soil;

(2) Dredging pond, sedimentation pond and water storage pond to prepare qualified clay;

(3) water and electricity are connected;

(4) Straight frame to ensure that displacement and uneven settlement will not occur during drilling;

(5) Cohesive soil is generally used for mud that meets the requirements of indicators.

2.9.3.2 borehole

(1) The connection of drilling tools should be vertical, and the initial drilling speed should not be too fast. The footage should be properly controlled to make the initial hole vertical and round, so as to prevent the hole position from being eccentric and the orifice from collapsing, and the drilling speed should be adapted to the mud displacement.

(2) During drilling, always check the change of mud index, and pay attention to adjust the mud height in drilling. Generally, bentonite, CMC, PHP and soda ash are selected to prepare high-quality mud. Adjust mud properties in time according to formation conditions. Refer to the following mud performance indicators:

Mud relative density:1.1~1.15;

Viscosity: general formation 16 ~ 22 s, loose and easy to collapse formation19 ~ 28s;

Sand content: fresh mud is not more than 4%;

Colloidal rate: not less than 95%;

PH value: 7 ~ 9.

(3) Filter slag in time during drilling, and pay attention to the change of stratum at any time. Take a slag sample from the stratum change place, judge the stratum type, record it in the record table, and compare it with the geological profile provided by the design. Drill residue samples should be numbered and kept for future analysis.

(4) Regularly check whether the machines and tools are operating normally, and report immediately if any abnormality is found. The parts that need lubrication must be checked once per shift.

(5) After the final hole is completed, the hole shall be cleaned according to the specifications, and the sediment at the bottom of the pile shall generally not exceed 10 cm.

Fabrication and Placement of Reinforcing Cage in 2.9.3.3

(1) reinforcement must have a certificate of approval when it comes into play, and each batch of materials and specifications can only be used after passing the sampling inspection; (2) The specification should be strictly observed in the process of steel binding;

(3) In order to prevent the deformation of steel bars during hoisting, the reinforcing cage should be firmly positioned after entering the hole, and the reinforcing cage must be prevented from being pulled up when lifting the conduit;

(4) Weld the steel hoop and steel hanger to ensure that the steel cage is not deformed during installation.

2.9.3.4 concrete pouring

(1) Before pouring concrete, put a hole-protecting funnel at the orifice, then put a reinforcing cage, and measure the thickness of virtual soil in the hole again. When pouring concrete into the belled pile, it should be poured to the top surface of belled pile first, and then vibrated and compacted; When pouring concrete within 5 m below the pile top, it should vibrate with the pouring, and the height of each pouring should not be greater than1.5 m;

(2) During construction, the site shall be kept clean and sanitary, mud shall not overflow everywhere, and sediments shall be removed in time;

(3) After the construction is completed, chisel out the pre-heightening concrete at the pile head.

Matters needing attention in foundation construction of rock-socketed cast-in-place pile with drilled (punched) holes in 2.9.3.5;

(1) During construction, it is necessary to ensure that the whole section of the pile tip is embedded in the complete bedrock, the minimum rock-socketed depth must be ≥0.5 m, and there must be complete limestone with sufficient thickness below the pile tip;

(2) During construction, local soft soil sections and loose pebble sections are prone to shrinkage and hole collapse, and inclined holes and stuck holes will appear in karst areas such as steep rock faces and stone buds. When mud is used to protect the wall, slurry leakage will occur, and effective measures must be taken to prevent it.

(3) Before pouring concrete, the sediment must be cleaned up, and the thickness of the sediment must be ≤ 50 mm When there is groundwater, the underwater concrete pouring process must be adopted for construction;

(4) The bearing capacity of single pile should be determined by static load or high strain dynamic test.

2.9.4 Construction of Manual Digging Cast-in-place Pile in Karst Area of Guilin

Manual digging pile is a kind of pile with manual digging, steel cage placed in the hole and concrete poured. The common construction method is to manually dig down the earth (rock) into a round hole, pour a circle of concrete retaining wall every excavation of 1 m, and so on until the depth required by the design, and then put a reinforcing cage in the hole and cast-in-place concrete. Manual digging piles are mainly suitable for geological conditions where karst development is not strong, the overlying soil layer is not thick, and bedrock can be used as a stable bearing layer at the pile end. For places with deep bearing stratum, rich groundwater (such as sandy pebble stratum in the first terrace of Lijiang River), soft soil or strong karst development, if forced excavation is carried out, it will often be difficult or even impossible to construct because of the difficulty of retaining wall or the sudden increase of water inflow, and forced drainage of groundwater will have adverse effects on the surrounding environment. Use it with caution or not at this time. The diameter of manual digging pile (excluding retaining wall) in karst area of Guilin is generally greater than 1.0 m and less than 2.5 m; ; Hole depth is generally not more than 20 m.

preliminary work for construction

(1) Be familiar with the site construction drawings and hydrogeological data, and make a feasible construction plan;

(2) Before full-scale excavation, selectively dig two test pile holes, and analyze relevant conditions such as soil quality and hydrology, so as to modify the original construction scheme;

(3) Before excavation, temporary facilities such as lighting, electricity and safety facilities shall be completed;

(4) according to the foundation plan, determine the axis and the anchor point of the pile position;

(5) Preparation of construction machines and tools for hand-dug cast-in-place piles: such as concrete mixer, hoist, bucket, formwork for retaining wall, lifting pump, ventilation and oxygen supply equipment, pickaxe, spade, soil basket, waterproof lighting (low voltage 36V, 100W), safety helmet, safety belt, etc.

2.9.4.2 building technology

Technological process: survey control → unlocking the hole → checking the axis (center) of the pile position → installing construction machines and tools → circulating down layer by layer → acceptance → hanging the reinforcing cage → pouring concrete into the pile.

Measurement control

Axis pile location should be set on the ground cross control network and datum.

Keyhole opening

(1) Earthwork excavation of the first pile hole: the pile hole excavation should be carried out layer by layer from top to bottom, and the earthwork in the middle part should be excavated first, and then extended to the periphery, so as to effectively control the section size of the excavated hole;

(2) Pouring the first section of retaining wall concrete: pouring the retaining wall concrete immediately after the pile hole is dug, and tamping it manually.

2.9.4.2.3 Check the pile position (center) axis.

After the completion of the wall protection of each pile hole, the cross axis and elevation of the pile position must be set at the top of the wall protection, then the crosshair is used to center, the messenger wire falls to the bottom of the well, the vertical flatness of the hole wall is checked with a radius ruler, and then it is trimmed. Well depth must be based on datum points, and putright shall be carried out one by one to ensure that the axis position, elevation and section size of pile hole meet the design requirements.

Installation of construction machines and tools

(1) Erection of vertical transport frame: it is required to be stable and firm;

(2) installing an electric hoist or crane on the vertical transportation frame;

(3) Install buckets, movable covers, lighting, water pumps, ventilators, etc.

Cycle down step by step.

Dig the pile hole to the design depth, remove the virtual soil or slag, and the pile bottom should be supported on the bearing layer specified in the design.

Inspection and acceptance

After the hole is formed, the diameter of the pile body, the elevation of the bottom of the hole, the center line of the pile position, the verticality of the shaft wall and the thickness of the virtual soil must be comprehensively measured, and the construction records should be made.

2.9.4.2.7 Hoisting the reinforcing cage.

When hoisting the reinforcing cage, aim at the hole position, hang it straight and hold it steady, and sink slowly to avoid collision with the hole wall. When the reinforcing cage is placed in the design position, it should be fixed immediately.

2.9.4.2.8 pouring pile concrete

According to the specification, concrete is poured into the pile hole.

2.9.4.3 pile hole quality requirements guarantee.

(1) Check the verticality and aperture of each pile, and correct the deviation at any time to ensure the correct position;

(2) The bearing layer at the bottom of the pile should meet the design bearing capacity requirements;

(3) After pouring concrete, the pile top elevation and laitance treatment must meet the design requirements and construction specifications.

Experience and measures to ensure the safety of digging holes in 2.9.4.4

(1) When digging a hole, the workers outside the well shall serve as the safety officer of the hole to monitor the safety of the personnel in the well and underground at all times;

(2) Carefully study drilling data, analyze geological conditions, and formulate targeted safety measures for possible quicksand, piping, water gushing, harmful gases, etc.

(3) The excavated earthwork must be transported away in time. Around the wellhead/within the range of kloc-0/m, it is forbidden to pile earthwork, and the pile height shall not be greater than 0.8 m. It is forbidden to overload and not pour piles within the range of 3 m.:

(4) Excavators and guardians must wear safety helmets, and it is forbidden to wear slippers, barefoot and work at their posts after drinking;

(5) The personnel in the well must go up and down in a special cage, and may not go up and down the wellhead in a bucket or with their feet on the protective wall. Safety ropes and ladders used in emergencies must be set in the borehole;

(6) When the hole is deep, ventilation should be made under the hole to strengthen air convection, and oxygen should be delivered when necessary to prevent the harm of toxic gases. During operation, the upper and lower personnel rotate, and the personnel on the pile hole pay close attention to the personnel under the pile hole to prevent safety accidents;

(7) When the amount of underground water is not large, the mud water will be transported out with a bucket while digging. When the underground water seepage is large, the bucket can't satisfy the drainage. First, dig a sump at the bottom of the pile hole and pump water with a high-lift pump. Operators must leave the well, and it is forbidden to work in the same pit. Observe the change of borehole wall at any time;

(8) Install on-site power supply lines and electrical equipment in strict accordance with relevant regulations, and the licensed electrician shall be responsible for installation and maintenance. Only after passing the inspection can it be put into use; The installation and removal of all power supplies and circuits in the construction site must conform to the current industry standard "Technical Specification for Temporary Electricity Safety in Construction Site" (JGJ 46—2005).

(9) When the adjacent hole piles are filled with concrete, the excavation should be stopped in principle to prevent the shaft from collapsing under large lateral pressure;

(10) The pile hole must have a certain depth of retaining wall, and it is strictly forbidden to dig without retaining wall; When it rains heavily, it is not allowed to work underground.

2.9.5 hole wall stability of manually dug cast-in-place pile [35]

Manual digging cast-in-place pile is a kind of pile foundation formed by manually digging a hole at the designed pile position, then placing a reinforcing cage and pouring concrete. It has many advantages: it can directly observe the stratum, the bottom of the hole is easy to clean, the equipment is simple, the noise is low, the piles in the site can be constructed at the same time, the pile diameter is large, and it is economical. Therefore, it is very popular when the site conditions permit (for example, the amount of underground water is not large). However, without the protection of concrete wall, it is easy to collapse, causing hidden dangers to construction safety. At present, some production units and designers can successfully carry out excavation without concrete retaining wall for the sake of engineering economy, but sometimes the excavated wall collapses, making it impossible to construct and even unsafe. Therefore, we should be careful not to dig holes with retaining walls. At present, many designers and constructors only determine the stability of the hole wall of digging piles by experience, and there is no exact theoretical basis. This paper attempts to study and discuss the quantitative calculation and analysis method of hole wall stability (without retaining wall) of manual digging cast-in-place pile by using elastic theory.

2.9.5. 1 stress state of the soil around the hole wall of the digging pile

Before artificial excavation, the stress state of the foundation at H below the ground is: the vertical pressure is γh, and the horizontal pressure is k0 γ h (k0 is the static lateral pressure coefficient of soil and γ is the weight of soil). It is assumed that the foundation soil layer is a homogeneous isotropic elastic body, and the stress state of each point on the same horizontal plane at h depth below the ground is the same. After the foundation is excavated manually, according to the elastic theory, the stress state of the soil around the digging pile changes, which will lead to stress concentration.

Figure 2.8 Stress Diagram of Soil around Manual Digging Pile 2.8 Stress of Soil around Manual Digging Pile

The stress state of the soil around the hole wall of the digging pile is analyzed by taking the foundation horizontal plane with the depth of h under the foundation. Let the radius of manual digging pile be a, and the horizontal stress on the foundation far away from digging hole be k0γ h. Therefore, the stress distribution of the soil around the digging pile can be regarded as the stress distribution of an infinite plate hole under biaxial compression (Figure 2.8), and the stress of the soil around the soil hole can be solved by polar coordinates. The plane problem solution of this problem obtained in elastic theory, the formula for solving stress is:

Theory and practice of geotechnical engineering in Guilin karst area

Where: σ r is the radial stress in the soil around the digging hole;

σ θ —— tangential stress in the soil around the digging hole;

τ r θ-shear stress in the soil around the digging hole;

P—— horizontal stress acting on soil; p=K0 γh

θ-the angle with the horizontal axis.

According to formula (2.27), the stress generated by the soil around the hole wall of the digging pile, that is, the stress when r=a, can be obtained by the following formula:

Theory and practice of geotechnical engineering in Guilin karst area

According to formula (2.28), the tangential stress σ θ around the hole wall is the largest, the radial stress σr=0, and the shear stress τ r θ = 0. σ θ and σr are principal stress and secondary principal stress.

Stability discrimination of soil around the hole wall of 2.9.5.2 digging pile

According to Mohr-Coulomb limit equilibrium criterion, it can be judged whether a point in foundation soil is damaged or not. According to the geometric relationship between the limit stress circle and the shear strength envelope (Figure 2.9), the shear failure condition of a point in the soil, that is, the limit equilibrium condition of the soil, can be established by σ 1 and σ3.

Figure 2.9 Limit equilibrium conditions of soil Figure 2.9 Limit equilibrium conditions of soil

As can be seen from the geometric relationship in Figure 2.9:

R O = c × cotφ

Theory and practice of geotechnical engineering in Guilin karst area

Simplifying Equation (2.29), the limit equilibrium condition can be obtained as follows:

Theory and practice of geotechnical engineering in Guilin karst area

or

Theory and practice of geotechnical engineering in Guilin karst area

Around the soil hole, because τ r θ = 0, σ θ and σr are principal stresses, σ 1 = σ θ, σ 3 = σ r = 0, the limit equilibrium conditions of soil are obtained as follows:

Theory and practice of geotechnical engineering in Guilin karst area

It can be seen from formula (2.28) that σ θ and σr are respectively the major principal stress and the minor principal stress in the soil around the hole wall. Substitute σ θ = 2k0 γ h and σr=0 into formula (2.32) to judge the stability.

In order to distinguish whether any point in the hole outside the wall (r> no matter a) is damaged, the stresses σr, σ θ and τrθ at this point can be obtained according to formula (2.27) (σr and τrθ are not 0 at this time), and then the obtained σr, σ θ and τrθ are substituted into formula (2.30) or formula (2.3 1).

Theory and practice of geotechnical engineering in Guilin karst area

Finally, the major and minor principal stresses σ 1 and σ3 are judged by the Mohr-Coulomb criterion (2.32).

2.9.5.3 application example

It is proposed to use φ 1200mm hand-dug cast-in-place pile foundation for the pile foundation. There is no groundwater in the site, and the bearing layer at the pile end is slightly weathered limestone. The buried depth of limestone surface is10.6m, and concrete wall protection is not required. See Figure 2. 10 for the stratum.

Fig. 2. Schematic diagram of10 foundation soil layer Fig. 2. Schematic diagram of10 foundation soil layer

In order to judge whether the construction without concrete retaining wall is feasible, firstly, the stresses σr, σ θ and τrθ of soil at different depths of the hole wall can be calculated according to formula (2.28), and then the value of σ 1 required to reach the limit equilibrium can be calculated according to formula (2.30).

From the calculation results in Table 2.2 1, it can be seen that the soil on the hole wall of manually dug pile will collapse within the range of 6.3~7.8 m and 8.8 ~ 10.6 m, and relevant measures (such as concrete wall protection) need to be taken.

2.9.5.4 conclusion

Hand-dug cast-in-place pile is favored because of its many advantages, but the stability of its hole wall is the key to the success or failure of the construction without retaining wall. To evaluate its stability, we can first calculate the stress state of the hole wall soil at any depth by using the relevant formula derived from the elasticity theory in this paper, and then calculate and judge the stability of the hole wall soil according to the Mohr-Coulomb limit equilibrium condition. If the soil on the hole wall is unstable, relevant measures (concrete wall protection) can be taken to ensure the smooth construction of manual digging piles.

Table 2.2 1 Discrimination Table of Soil Stress and Stability in Hole Wall of Manual Digging Piles Table 2.2 1 Discrimination Table of Soil Stress and Stability in Hole Wall of Manual Digging Piles

2.9.6 Common accidents in pile foundation construction in Guilin karst area and their treatment

The common accidents in pile foundation construction in Guilin karst area are mainly caused by bored pile foundation, because its construction technology is more complicated than that of immersed cast-in-place pile and manual digging cast-in-place pile. The following mainly analyzes and summarizes the causes and treatment methods of common accidents of bored pile foundation.

In the process of piling, the pile hole is inclined, the hammer is stuck and falls into the pile hole, and the concrete is lost when pouring concrete.

2.9.6. 1 The pile hole is inclined and bent.

The main reason for the inclined bending of the pile hole may be that there are karst forms such as stone buds, karst grooves, karst grooves and caves in the bedrock or the slope of the rock surface is large. During the construction of impact hole, the impact speed is too fast, which leads to the impact hammer sliding in the weak direction of rock and soil, forming inclined hole. If it is impacted by the vertical irregular cavity position, it is easy to form a curved hole along the cavity development position, which sometimes causes a hammer jam accident.

If the bored pile foundation is used in Guilin karst area, it is generally a large-diameter bored pile, that is, its diameter is generally greater than 800 mm The geotechnical investigation in Guilin karst area is in accordance with the requirements of Article 5. 1.6 of the Code for Geotechnical Engineering Investigation (GB 500265438-200 1), that is, the exploration points are required to be arranged one by one. In this way, we can fully understand the distribution of underground caves and take reasonable measures. When the impact drilling hole reaches the top of the cave, use a low hammer to knock. For a single stone bud or boulder, impact alternately with high and low strokes to crush or squeeze the stone bud or boulder into the hole wall; For ditches, grooves and holes, throw stones and flaky into the pile hole, and the filled stones and flaky should be dozens of centimeters higher than the inclined plane. When backfilling, tap lightly with a low hammer, and then carry out normal impact after the full section of the impact hammer enters the rock mass.

If the pile hole is inclined during the hole-forming process of pile foundation construction, it means that there may be slope rock or pile bottom rock in the karst cave section, which cannot be rushed in due to the false filler in the punching process; When this happens, you can stop piling and rush in, lift the hammer out of the pile hole, then pour concrete into the pile hole (concrete can be mixed with accelerator), and then continue the construction after the concrete is solidified. If this is repeated several times, the problem of pile hole inclination can be solved by repair.

The above treatment method is also suitable for the construction of bored pile foundation.

Hammer Jam and Drop of Punched Piles in 2.9.6.2

The impact hammer enters the crevice of the karst cave, especially the crevice of the vertical karst cave. If the crevice of the karst cave is large, the impact hammer topples, and the upper karst development area falls off due to impact vibration or falls into foreign objects to block the impact hammer; Or hammer jam accident caused by high mud concentration and plum blossom hole during impact. So that the impact hammer cannot be lifted. Drop hammer means that the impact hammer wire rope is old, the joint is slack, and the welding parts such as steering ring and steering sleeve are broken, which leads to the impact hammer falling into the pile hole.

In order to prevent hammer jamming and hammer dropping accidents during construction, the following preventive measures should be taken during construction:

(1) After entering the cave, put stones, gravel and soil into it, and then impact intensively to fill the cave. Low-grade concrete can be used for filling, and after the concrete reaches a certain strength, it can be impacted to form holes.

(2) When percussion drilling is carried out in the cave, firstly, the wire rope should be kept stable and the swing is small, and secondly, the relationship between the height of the cave and the stroke should be paid attention to. In general, when the cave is high, the stroke impact less than the height of the cave can be used.

(3) The rock surface of the ditch and groove is irregular and has a large inclination angle. When the casing has been lowered, it is easy to get stuck at the bottom of the casing. After backfilling pebbles, the drilling method of "heavy hammer tapping" shall be adopted, and the drilling speed shall be slowed down appropriately.

(4) In order to prevent the hammer from falling into the pile hole, preventive measures should be taken first. A wire rope should be attached at the end of the hammer to prevent the hammer from falling. In the process of piling at the top of the cave and in the cave, the method of light hammer and heavy hammer should be adopted, and the hammer should not be raised too high.

Well collapse and mud leakage in 2.9.6.3.

Hole collapse refers to the collapse of the hole wall in different degrees during or after hole formation, and even causes surface collapse in serious cases. Bubbles are constantly appearing in the mud discharged from the hole, and sometimes the water level in the casing suddenly drops, which is a sign of hole collapse. Slurry leakage refers to the phenomenon that the slurry in the retaining wall suddenly loses along the underground crevice channel during the hole forming process, which leads to the sudden drop of the slurry level in the hole. In the process of punching pile construction in karst area, slurry leakage and hole collapse accidents are often caused by * * *, and hole piles with serious slurry leakage are prone to hole collapse accidents.

The prevention and control measures of collapse in karst areas mainly include:

(1) Bury the hole casing, keep the mud with high density, high viscosity and reasonable head height in the hole all the time, and increase the liquid column pressure.

(2) For series-connected karst caves, pile holes with smaller or isolated karst caves should be constructed first, and the underground water activity channel should be blocked after the piles are formed, and then the karst caves and the pile holes to the lower end solution tank should be constructed to avoid serious mud loss and unstable hole walls.

(3) Mud performance parameters shall be measured and controlled at any time during impact. In the process of percussive drilling, the role of mud is to stabilize the hole wall and carry cuttings.

(4) It is not advisable to penetrate the roof of the cave too quickly, and the method of "heavy hammer knocking" should be adopted. On the one hand, don't leak slurry too quickly, and replenish slurry in time; On the other hand, the pile hole should be smooth, and the impact bit can freely pass through the roof of the cave to avoid sticking.

(5) When the hole collapses, the mud should be replenished in time to maintain the head height in the hole. At the same time, clay and flaky with the slurry leakage of 1.2 ~ 1.5 times should be added, and the hole wall should be fixed by short-stroke punching to prevent the hole from expanding.

(6) In order to ensure the integrity of the pile and the occurrence of concrete loss, the construction method of steel casing can be adopted. Hoist the prefabricated steel casing (welded by sections) into the hole, and the verticality of the pile center must be ensured during hoisting. For example, in the pile foundation construction of Guilin Zhou Nan Bridge across Lijiang River in 2005, steel pipe liners were used to prevent the loss of cement concrete, and the pile foundation construction was smooth.