Karst terrain, also known as karst topography, is formed by soluble rocks under the long-term action of erosive groundwater. This type of terrain is widely distributed in southwestern China, with the largest areas in Yunnan, Guizhou, Sichuan, Jiangxi, and Guangxi. These regions contain large amounts of carbonate rocks, sulfate rocks, and halide rocks. Under the long-term erosion and scouring of flowing water, these soluble rocks have formed various complex geological features on the surface and underground.

　　At present, foundation construction in karst terrain in China mainly relies on manual excavation with drilling and blasting for muck removal and manual hole digging. This construction method is extremely inefficient and carries significant safety hazards, making it difficult to meet modern requirements for efficiency, safety, and environmental protection. Advanced construction methods are urgently needed to replace it. As a recognized efficient and environmentally friendly piling equipment, the rotary drilling rig is naturally regarded as the preferred choice for solving drilling challenges in karst terrain. However, due to the complex and variable forms of karst terrain, rotary drilling rigs are prone to serious accidents such as inclined holes, drill jamming, and slurry loss during drilling. Therefore, there are currently few successful cases of rotary drilling rigs in karst terrain.

　　Case Analysis

　　In June 2010, a SANY SR280R rock-entry rotary drilling rig participated in construction at the Pingxiang Houpingle Grand Bridge on the Jiangxi section of the Shanghai-Kunming High-Speed Railway. The borehole diameter was 1.0 m, and the hole depth ranged from 27 to 34 m. Considering that karst geology is relatively common in the Pingxiang area, the project required the rotary drilling rig to be capable of drilling through limestone and karst caves. Since there had been no previous domestic precedent for rotary drilling rigs in limestone and karst cave strata, the machine owner requested that the SANY Construction Method Research Institute send personnel for on-site construction assistance.

　　Problem Analysis

　　The main difficulties in karst geology construction lie in the complex geological conditions and the frequent incidental accidents during construction. In this section, the upper layer is silty clay, and the base layer is slightly weathered limestone; some pile locations contain breccia layers that are prone to collapse; and many pile locations contain karst caves. Due to heavy rainfall during the construction season, groundwater is abundant, with groundwater emerging about 1 m below the surface, which also increased construction difficulty. This project mainly required solving the following construction difficulties.

　　Slurry loss and hole collapse Due to the complexity of karst cave strata and the unpredictability of the shapes and sizes of underground karst caves, slurry loss and hole collapse are highly likely during construction, especially when underground karst caves are interconnected.

　　Drill burial and jamming During construction in karst cave strata, drill burial and jamming are highly likely when the rig lifts the drill. When there is no fill in the karst cave, the top plate of the drilling tool can easily become stuck against the cave roof during lifting, causing drill jamming; when the karst cave contains fill such as limestone, crushed stone, and soft-plastic or flow-plastic clay, drill burial accidents are more likely during construction.

　　Inclined holes In the limestone layer at the top of karst caves, because the limestone rock surface varies greatly, the limestone at most pile locations may present a large inclined surface, and any slight carelessness during drilling can cause inclined hole accidents.

　　Solutions

　　Pre-treatment for slurry loss and hole collapse accidents For small karst caves with no fill, rubble, clay, and cement can be filled into the cave so that the fill is 1 m above the cave roof. The drilling rig adopts slow drilling without penetration, squeezing the fill into the karst cave to seal possible leakage, form a stable hole wall, and prevent drilling deviation.

　　For karst caves with massive slurry loss, concrete can be pumped in to fill up to 1 m above the karst cave, and drilling can continue after initial setting. For exceptionally large interconnected karst caves, to reduce the amount of concrete poured, when drilling reaches the large karst cave, rubble can be thrown into the hole and then filled with low-grade concrete, and drilling can continue once the concrete has gained a certain strength.

　　For larger karst caves, the method of following with a steel casing can be adopted, but the construction cost is relatively high.

　　The key to solving slurry loss and hole collapse accidents mainly lies in good accident prevention and response plans: ① Prepare sufficient auxiliary materials in advance, such as rubble + clay; the clay should be bagged for standby use. ② Prepare an enlarged mud pit and sufficient water source, and ensure that the mud has good viscosity and colloid fraction. ③ After approaching and entering the karst cave layer, closely monitor changes in the mud level in the hole. Once slurry loss occurs, quickly replenish the slurry, lift the drill, and throw the prepared auxiliary materials listed in item ① into the hole.

　　Pre-treatment for drill burial and jamming accidents For karst caves with fill, where the fill is limestone, crushed stone, and soft-plastic or flow-plastic clay, maintain the mud water level to prevent the fill from collapsing when the drill bit drills into the bottom limestone, causing drill burial accidents.

　　For karst caves with no fill, as described in the pre-treatment for slurry loss and hole collapse accidents, fill materials such as rubble, clay, or concrete can be added into the karst cave to prevent the top plate of the drilling bucket from becoming stuck against the cave roof during lifting and causing drill jamming.

　　For conventional drilling tools, guide devices can be welded on to prevent drill jamming accidents.

　　Pre-treatment for inclined hole accidents A correct and reasonable drilling method should be adopted: where the limestone rock surface at the top of the karst cave varies greatly and the limestone presents an inclined surface, it is necessary to switch to a core barrel when drilling into the limestone karst cave layer, lower the drill after reaming, and use "reduced-pressure drilling" when initially drilling into the limestone stratum. In actual operation, in addition to judging the location of karst caves in advance based on geological reports, it is also necessary to carefully observe the drill cuttings and the vibration of the drilling rig to judge the stratum.

　　Select a suitable drill bit; the drill barrel should be a straight cylindrical type; under permissible conditions, increase the height of the drill barrel as much as possible; and the drill bit should have a guiding structure.

　　For drilling in the bottom limestone of karst caves whose height is less than the core barrel height, reduced-pressure drilling should also be used at the beginning; after passing through the inclined top layer, conventional drilling methods can be used. For karst caves whose height is greater than the core barrel height, because the bottom layer may encounter stalagmites, karst cave sidewall rock, half rock and half soil, etc., fill materials such as rubble, clay, or concrete can also be added into the karst cave to 1 m above the cave roof (above stone buds and inclined surfaces), and then a specially made extended core barrel can be used for drilling.

　　Results

　　Through improvements to on-site drilling tools and guidance on correct operating methods, no accidents such as slurry loss and hole collapse, drill burial, drill jamming, or inclined holes occurred during drilling in the karst cave strata at this site, and drilling efficiency was relatively high. The drilling of rotary drilling rigs in karst cave strata was successfully achieved, reducing the construction cost of the original manual hole digging and greatly improving construction efficiency. Taking pile hole No. 4 as an example, the pile hole depth was 34.8 m, with a karst cave from 26 to 30.5 m, spanning about 4.7 m. The hole was completed in less than 20 hours, more than 20 m3 of concrete was poured, the reinforcement cage was lowered smoothly, and the hole verticality was good.