When drilling in karst areas, the slurry is prone to leakage, causing a sudden drop in the slurry level inside the hole. This can lead to the failure of the casing and result in borehole collapse. The fill material in karst areas is loose and has collapsed, and if the volume of the karst area is too large, it can easily cause severe overloading of the concrete during injection. If not handled properly, this can result in accidents such as drill burial, drill jamming, and rod burial. Therefore, when encountering a karst area, each pile should have its geological conditions, depth, height, and fill material type meticulously recorded, and a corresponding construction plan should be formulated. For each treatment plan, detailed calculations must be conducted. In the case of large karst areas, the plan must be reviewed by the supervising engineer and the design unit, clarified, and approved by the supervisor before implementation.

According to the height of the karst cave and the condition of the filling material in the cave, different treatment methods are adopted for the karst cave. The main ones are: throwing, grouting consolidation, concrete injection construction, steel casing laying, etc. The following is a brief introduction of the construction scheme for several forms of karst cave for reference.

The cave is small in scope

If the height is less than 1m, there is no filling material or the cave is filled with water, if the hole collapses, first inject water into the hole to raise the water level, and then put the clay and stone pieces into woven bags and sink them to the bottom of the hole. The number of soil bags is determined according to the amount of slurry leakage, so that the cave is filled with filling material and a protective wall is formed, and then continue construction.

The height of the cave is 1-3m

The cave is either fully or partially filled. When the slurry level drops rapidly, it indicates that the cave has been successfully connected. After injecting slurry to the hole opening, a clay bag is placed at the front of the hole, followed by adding bags of cement and rubble. The drill bit stirs the mixture inside the hole to form a cement paste and gravel wall on the eastern side. Once the cement paste has initially sealed the gaps between the rubble, it is left undisturbed for 24 hours until it reaches the initial setting stage. At this point, drilling continues through the cave.

The cave is higher than 3m

The rotary drilling can be combined with the pipe piling machine for construction. A fully steel casing is used to sink into the bottom of the karst cave to form a protective wall. This method allows the steel casing to be dry-formed without the need for slurry lining, and the casing can be removed after pouring. Although using a fully steel casing may increase costs, it is still more economical compared to filling the hole with concrete. For karst caves where the hole depth exceeds 3m but can be formed after stone throwing, yet cannot be filled with underwater concrete, a 4mm thick casing can be welded to the steel cage at the karst cave location and placed together with the steel cage, followed by the pouring of underwater concrete.

The above are several commonly used treatment methods of rotary drilling in the case of karst geology.

Common problems of rotary drilling rig in limestone karst cave strata

1. Tooth breaking and drill bit damage occur due to uneven force during drilling

2. The hole is inclined when drilling on a sloping rock surface or half side of the rock

3. Drilling is stuck in the roof of the cave

4. Leaking slurry occurs in geological formations with cavities and fractures

5. Caving holes in the strata above the karst cave due to leakage of slurry in the karst layer or unstable filling layer of the karst cave

6. Large amounts of super square are injected into the karst cave

2. Construction preparation

The accuracy of the karst cave situation in the preliminary survey directly impacts the selection of construction plans, construction costs, and the construction schedule. Therefore, in areas with karst cave development, each hole must be surveyed. Based on the different karst cave development conditions identified in the geological survey results, different construction plans should be adopted, and preparations before drilling should be thorough to ensure clear planning during the drilling process.

3. Prevent excessive damage to the drill bit and tooth breaking

Before drilling into the cave, the drill teeth need to be reinforced, and the parts of the drill bit need to be checked frequently for weld cracking. When drilling in the cave stratum, the principle of "low drilling speed and variable pressure" can be adopted to avoid accidents such as tooth breaking and drill bit damage as far as possible.

4. Prevention of hole skew

When drilling through karst formations, it is essential to use a tubular drill and a sand scoop in conjunction with the drilling process. First, the tubular drill should be used to form a guide and create a free surface. Then, the sand scoop should be used to drill and remove debris, preventing hole deviation. Additionally, raising the height of the drill bucket can enhance guidance and prevent hole deviation. Operationally, use light pressure or self-weight pressure for drilling until the drill bit is fully embedded in the rock layer, then proceed with normal drilling to prevent hole deviation.

5. Drill prevention

Before construction, guide plates can be welded onto the square head rib plates to prevent the area from the top of the square head to the top of the drill pipe from having a noticeable step from top to bottom, ensuring a smooth transition. If the rib plates are strong enough, cutting teeth can also be welded to one set of diagonal ribs, facing the rock body that is being drilled, to cut through the rock and remove the drill bit.

Sixth, slurry treatment

To address slurry leakage, it is essential to first estimate the approximate volume of the underground karst cave. Then, ensure that the slurry tank has a capacity at least twice the volume of the cave. The slurry pump and water replenishment pump should be larger than standard models to facilitate rapid slurry and water replenishment during leakage. Additionally, sufficient filling material should be prepared near the drilling site. The excavator model should be at least 200 to ensure efficient filling.

Filler material:

1. A soft plastic mixture of bentonite, sawdust, clay and cement with water added.

2. The size of the stone and the bulk clay packed should be considered according to the size of the soil inlet of the drill bucket used. The stone should be mudstone blocks, and the hardness should be avoided too large.

3. Block clay, which is impermeable and has good plasticity and viscosity, can be fully filled in the karst cave space after filling and compacted by reverse drilling with a drill bucket. It is not easy to collapse the hole when drilling again.

When drilling, it is found that the mud in the hole drops slowly. It is necessary to replenish the mud in time, and the drill bit is put forward, and the filling material is added to the hole, and the drill bit is put in. The floating and reversing drilling machine will squeeze the filling material 1 into the rock fissure to block the loss of mud.

If the mud surface drops rapidly and the leakage is too fast, it is difficult to achieve the purpose of filling by scattering. In time, a large amount of mud should be added and water should be replenished, and the drill bit should be quickly raised, and 2 or 3 fillings should be put in a large amount in a short time, and the drill bit should be floated and reversed until the mud surface does not drop any more.

7. Large amount of super square treatment

For low karst caves (less than 1 meter), and in the depth of the cave, there is no karst distribution around the pile holes, after drilling, concrete is directly poured to fill, a small amount of excess is normal.

When the karst cave is slightly higher (more than 1 meter but less than 4 meters) or there are karst caves distributed around the pile holes at the depth of the karst cave, and the distribution depth of the karst caves is relatively shallow (such as within 15 meters), the empty karst cave can be filled with filler 2 or 3 and then drilled again.

For high karst caves (over 4 meters), the above fill materials are insufficient to fully fill the voids, leading to low backfilling efficiency and complex construction. In such cases, cement mortar or low-grade concrete can be used for backfilling, and drilling should only proceed after the initial setting of the material. This requires that the strata above the karst cave remain stable during this period. Additionally, if the karst cave contains water, it is crucial to consider how the cement mortar might be diluted by a large amount of groundwater, which could affect its solidification.

For interconnected karst caves (which can be identified by the overlapping distribution depth of adjacent pile holes), pile construction can be carried out in groups. First, drill all the pile holes in a group that are interconnected to the bottom of the karst cave, then backfill and compact them uniformly. This ensures that the interconnected karst caves are fully filled, maximizing the efficiency of concrete use. When constructing in groups, it is essential to calculate the completion time for this group of pile holes to ensure the stability of the hole walls before pouring the concrete.

8. Sludge treatment

Ensure the quality of the mud. Before the reinforcement cage is lowered, the slag must be cleaned with a bottom cleaning drill to ensure that the concrete is poured immediately after the reinforcement cage is lowered and the time of preventing the pile hole is minimized. When the conventional measures cannot completely clean the slag, the gas lift reverse circulation process can be used.