To ensure that there is no slurry leakage during the construction of bored piles in karst development areas, or that there is no slurry leakage but no hole collapse, and to ensure that the pile holes are smoothly formed into holes and piles and meet the bearing capacity requirements, the construction should be based on the detected geological conditions. The formation conditions of the cave, the size of the cavity, and the filling conditions of the cave.
Treatment Plan for Pile Foundation of Karst Cave
Rubble Backfill
Use the normal hole-forming method for construction. When drilling through the cave and leaking slurry, repeatedly put in loess and debris, and use the drill bit to impact the loess and debris into the cave and karst fissures. Cement, caustic soda, and sawdust can also be added to enlarge the hole. The self-stabilizing ability of the wall.
(1) Mixing ratio: To effectively utilize the flakes, the ratio of flakes to clay is 3:7. When adding cement, the mixing ratio is 2 bags/m. When adding sawdust, the mixing ratio is 10% of the clay. to
(2) Adding method: Limestone with a strength of ≥30MPa is used as the flaked stone, and the particle size of the stone is preferably 15~50cm. The effect of adding clay is better after being packaged in cement bags. The best way to put cement is to put it in the whole bag. The mixing method is: gravel and clay bags (you can choose to add cement, sawdust, or not) are added at intervals in layers, and the backfill height is 1m above the roof of the cave. to
(3) Construction precautions: Pay close attention to the working conditions of the drilling rig, surrounding surface settlement, and water level changes in the casing to prevent abnormal situations and slurry leakage from occurring, and handle them immediately. According to the geological data, observe and check frequently when approaching a karst cave. Use the feel of the main impact rope in your hand, the sound of impacting the rock formation, and the extracted rock samples to determine whether it is close to the karst formation. When approaching karst, the amount of slack in the main rope should be 1~2cm to prevent the drill from getting stuck when penetrating the rock shell. When drilling through the upper crust of karst strata, once slurry leaks out, clay blocks and gravel should be put in time and water should be added to maintain the water level in the hole.
(4) Main scope of application and advantages: Suitable for smaller caves (cave height less than 3m), without filler or half-filled, simple construction method, and low cost.
Plain Concrete Backfill
The normal hole-forming method is used for construction. When the cave is drilled and the grout leaks out or when the inclined surface is encountered, low-strength grade concrete is filled in. After a certain period, the hole is drilled by impact drilling.
(1) Concrete strength grade and mix ratio Concrete can use C20 (C15, C10 is also acceptable) plain concrete. To increase the strength of concrete as soon as possible and save construction time, a certain early strength agent should be added to the concrete to improve the early strength of plain concrete.
(2) Adding method: For inclined rock surfaces, to correct the hole position, the backfill plain concrete should be flush with the top surface of the inclined rock surface. For karst caves, the top backfill surface should be 50cm higher than the roof of the cave. to
(3) Construction precautions: After backfilling for 48 hours, when the backfill concrete strength reaches 70%, cores should be taken within the pile foundation range to check the filling effect. Pile foundation construction can only be carried out after the cave is filled and the strength is reached. to
(4) Main scope of application and advantages: It is suitable for medium and small caves. This solution can be used with or without fillers. The construction method is simple and the cost is relatively high.
Grouting Reinforcement
By comparing the geological data, the caves passed by the piles are filled and reinforced. to
(1) Layout of grouting holes: Find the largest cave according to the borehole histogram. If the cave is connected, only the largest cave needs to be reinforced with grouting. to
(2) Grouting pressure control: Use a grouting pump for grouting. The grouting pressure should not be too high. The specific pressure value is determined by on-site testing. The grouting speed is 15~20L/min. The purpose is to make the slurry penetrate the filling material (including poured sand and gravel) and then consolidate. The minimum penetration diameter is positioned at 3.0m to ensure that the punched hole can be drilled. Sufficient consolidation. During grouting, the grouting pipe must be inserted into the bottom of the filler and slowly lifted up while grouting. The pipe lifting speed should not be too fast. to
(3) Construction precautions: To prevent the slurry from losing too far and causing waste, use the intermittent grouting method, so that the slurry and sand (or gravel) are initially injected and then grouted, and the grouting is cycled multiple times until the specified requirements are reached. until the grouting volume and grouting pressure control values. After grouting one hole, continue grouting the remaining holes. The final grouting pressure must be increased, and finally, the hole is sealed. The grouting sequence is controlled by the site. And cores should be taken within the pile foundation area to check the filling effect. Pile foundation construction can only be carried out after the cave is filled and the strength is reached. to
(4) Main scope of application and advantages: It is mostly suitable for small (height less than 1m) or multi-layered caves (multi-layered bead-shaped small caves). It can also be used for various caves with fillers, but the cost is higher.
Steel Casing Follow-Up Reinforcement
Use the steel casing jumping method: punch holes on one side, connect the steel casing on the other, and press or vibrate it to sink into the drilled hole. to
(1) Selection of steel casing. The diameter of the steel casing should be accurate, the connection should be straight, and it should be formed by a winch. The steel casing must have a certain stiffness. A single large karst cave should use a single-layer casing, two large karst caves should use a double-layer casing, and so on. to
(2) Settlement of the inner casing: When the impact passes through the top of the cave, lift the ram repeatedly, slowly lower it up and down within the top thickness range, and lift it gently. When the ram is not blocked, tie the inner casing with a wire rope and use The crane (or the pile driver’s weight) to put the inner casing into the outer casing and sink it to the bottom of the hole. If necessary, use a vibrating hammer to sink it. When using impact drilling to drill holes into the steel casing for follow-up construction, the hole expansion performance of the impact drill should be fully utilized during construction so that the steel casing can sink smoothly. If the hole expansion coefficient of the impact drill in hard rock is small and cannot meet the sinking requirements of the steel casing, backfill and increase the diameter of the drill bit for secondary punching to ensure that the steel casing sinks smoothly. To ensure that the steel casing slides down smoothly, the pile hole is required to be vertical without skew or necking. to
(3) Main scope of application and advantages: Suitable for extra large caves (cave height greater than 3m), unfilled caves, semi-filled caves, or when there is a thick gravel layer above the cave.
Key Points for Construction of Karst Cave Pile Foundation
The shape of the cave is relatively complex, and certain problems are likely to occur during construction, and certain treatment measures need to be taken.
1. When processing small caves, grouting can be used to pass through the small caves through hole formation. If there is no filler such as gravel or dry sand in the cave, it needs to be filled before proceeding. Grouting treatment.
2. When filling an empty cave, you can select a suitable hole position and pour sand and gravel through a steel casing. After pouring, you need to use a compressor to process the dry sand. You can use other holes as a way to reduce pressure. Makes filling more stable. After filling, the grouting process will be carried out until the construction requirements are met.
3. For some dissolution grooves, dissolution ditches, small fissures, etc., when punching, you can put in chips, clay, or even a whole bag of cement to block the wall to ensure that the mud is not lost, so that the borehole can pass through the karst area smoothly.
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