Karst is formed by the long-term chemical dissolution of soluble rocks such as limestone by water. Karst strata have karst channels, karst troughs, and karst caves, especially the top (bottom) plates of the caves are interlocked, with significant differences in height. The rock layers have uneven hardness or are composed of half rock, half soil, half rock, half solution, and slope rocks. The difficulty of bridge drilling and pile construction in complex karst formations due to complex geological factors is a technical challenge. This article combines engineering examples to introduce the construction technology of drilling piles in complex formations with strong karst development, and points out corresponding construction treatment methods and construction precautions.
Project Overview:
The Jinghe Extra Large Bridge of the third section of the Beijing Shanghai High speed Railway is located in Tengzhou City, Shandong Province. The total length of the bridge is 11.445km, and the span arrangement includes 20 × 24 double track simply supported box girders, 328 × 32 double track simply supported box girders, and 2- (32+48+32) double track continuous box girders. The foundation design of all bridge piers and abutments is based on drilled pile foundations, with a diameter of 1.0m, 1422 piles, a total length of 44272.77m, a diameter of 1.25m, 1538 piles, a total length of 83527.39m, a diameter of 1.50m, 108 piles, and a total length of 14325.16m. Pier 203 has a diameter of 1.50m and a maximum pile length of 85.5m.
Engineering geological features:
The Jinghe Grand Bridge is mainly composed of limestone, with a monocline structure in the rock strata. The rock strata have an attitude of 30 °∠ 12 °, and the rock joints and fissures in the bridge site area are developed. Dk580+229~dk582+100 segment; The rate of encountering karst caves during drilling reaches 76.9%, and in areas with strong karst development, it reaches 90-100%. Dk582+100 to dk583+100 segments; The rate of encountering karst caves during drilling is 60.8% to 73.7%, and the caves are distributed in a bead like pattern. Among them, there are as many as 13 bead shaped karst caves at Pier 203, with cave heights generally ranging from 2 to 8 meters. The cave height of Pier 286 is 25.5m, and the section from dk583+100 to dk586+100; The rate of encountering karst caves during drilling is 66.1% to 71.9%. Dk586+100~dk587+100 section; Drilling encountered karst caves, accounting for 42.5%, which developed in a bead like shape. The soil caves in the bridge site area are mainly formed by the interaction of subsurface erosion and negative pressure suction caused by the infiltration of surface water and groundwater in the overlying soil layer of karst strata.
Drilling pile construction technology:
1. Selection of drilling rig type
The drilled piles of Jinghe Grand Bridge pass through clay, silty clay, medium coarse sandy soil, mudstone, and limestone. According to the geological conditions, choose a mast type impact drilling rig, which can adjust the lifting height (stroke) of the drill bit according to the hardness of the rock layer at any time and has the characteristic of high impact energy.
2. Drilling sequence
Bridge drilling pile construction is carried out in highly developed and complex karst strata, and the complex geological structure makes drilling and pile formation difficult. Before the construction of drilled piles, it is necessary to accurately grasp the geological conditions. We adopt the principle of combining drilling and geophysical exploration. Based on the geological drilling of each pier and pile, we also conducted detailed geophysical exploration of the entire bridge pier by pier to clarify the thickness of the covering layer, the location, quantity, and filling condition of each pier and pile. It is particularly important to select the drilling sequence for pile positions based on the geological structure of drilling and geophysical data during drilling construction. The basic principle for selecting the drilling sequence of pile positions in the construction of the Jinghe Extra Large Bridge is that for unequal length pile piers, the drilling construction follows the construction sequence of long piles first and then short piles. The drilling sequence for the same pier is from deep to shallow and from many (bead like) to few, with edge and corner piles being constructed first. This construction is beneficial for timely sealing of the cave, blocking the cave passage, and facilitating the construction of the next pile.
3. Mud Wall Protection
Mud wall protection is the most basic technical measure in the construction of drilled piles. In order to strengthen the mud wall protection, it is necessary to use clay with good viscosity to prepare mud. During the impact drilling, the clay is directly injected into the hole, and the mud is produced by punching with a drill bit. In the construction of drilled piles for the Jinghe Extra Large Bridge, for the key and difficult piles with complex geology and frequent leakage and collapse of slurry, in order to improve the wall protection effect of slurry, special bentonite for geological drilling was also used to make slurry and achieve significant wall protection effect. When the filling material in the cave is fine sand or flow plastic material, adding an appropriate amount of cement to the mud can improve the anti permeability ability of the mud, increase the wall protection effect, and effectively prevent hole collapse and leakage. The cement dosage generally accounts for 10% to 20% of the clay dosage.
4. Artificial pore walls
Mud wall protection is the most basic technical measure in the construction of drilled piles. In order to strengthen the mud wall protection, it is necessary to use clay with good viscosity to prepare mud. During the impact drilling, the clay is directly injected into the hole, and the mud is produced by punching with a drill bit. In the construction of drilled piles for the Jinghe Extra Large Bridge, for the key and difficult piles with complex geology and frequent leakage and collapse of slurry, in order to improve the wall protection effect of slurry, special bentonite for geological drilling was also used to make slurry and achieve significant wall protection effect. When the filling material in the cave is fine sand or flow plastic material, adding an appropriate amount of cement to the mud can improve the anti permeability ability of the mud, increase the wall protection effect, and effectively prevent hole collapse and leakage. The cement dosage generally accounts for 10% to 20% of the clay dosage.
5. Karst caves with leakage of grout
The Jinghe Grand Bridge has strong karst development strata, with karst caves distributed in a bead like pattern. Among them, there are as many as 13 bead shaped karst caves at Pier 203, with cave heights generally ranging from 2 to 8 meters. The cave height of Pier 286 is 25.5m, with a cave encounter rate of 66.1% to 71.9% during drilling. The area with strong karst development reaches 90-100%, and the connectivity of semi filled and unfilled empty caves is good. In order to accelerate the construction progress and ensure the construction period, 2-3 drilling machines are used simultaneously for drilling construction on the same pier position, which often leads to frequent leakage of grout in karst cave holes. The use of rubble clay backfill to treat the leakage of grout in karst cave holes is ineffective. The treatment method for leakage of grout in karst caves can be to inject underwater concrete to seal the caves and block the channels of the caves; Lift the drill bit out of the hole and pour underwater concrete into the borehole. The pouring surface should be 2-3m higher than the top of the cave. Wait for 0.5-1.0 hours for the underwater concrete pouring surface to stabilize before pulling out the conduit to ensure that the underwater concrete fully seals the cave. Wait for the underwater concrete to reach a certain strength (1-2 days) before drilling. The frequent leakage of grout in the karst caves of the bridge piers such as 289 # and 290 # of Jinghe Extra Large Bridge was addressed by pouring underwater concrete to seal the caves, blocking the cave passage and achieving significant results.
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