What is the tunnel construction technology in karst areas? What is the impact on tunnel construction? Please read the article by the editor of Zhongda Consulting.
Karst is the product of chemical dissolution and mechanical destruction of soluble rock formations through continuous recharge, runoff, infiltration and circulation of surface water and groundwater. As the basic conditions for karst development, the solubility and fissure of rock as well as the erosion and flowability of water are the difficulties in tunnel construction in karst areas. How to improve tunnel construction technology in karst areas has become an important issue facing the industry.
1. The impact of karst on tunnel construction
Karst is a general term for various surface or underground corrosion phenomena caused by the chemical action and mechanical destruction of soluble rock layers by surface water and groundwater. .
Construction of tunnel projects in karst areas faces great resistance. The karst cave is located at the bottom, because the filling material is soft and loose, which is not conducive to the construction of the tunnel base; it is located at the upper part, and the filling material collapses easily during construction, so it is not suitable for excavation; in some areas, the rock quality of the karst cave is not strong and there are many broken materials, so it is not easy to excavate during construction. If you are not careful, it is easy to collapse; if the filling material filled with water dissolves, when the tunnel is dug to its edge, the water-containing filling material will continue to flow into the tunnel, making it difficult to contain it. The surface may even crack and sink, and the pressure on the mountain will increase dramatically. increase; during construction, it is also possible to encounter large water pockets, causing rocks and soil to rush into the tunnel in large quantities with water; there are also karst caves that are intricate and circuitous, and their branches are widely distributed, making them difficult to deal with. .
2. The main technical methods often used in tunnel construction
1 Geological forecast
The main purpose of adopting geological forecast is twofold: 1) During the construction period , to avoid sudden geological disasters, thereby harming people's lives and property; 2) Forecasting geology, the design content can be adjusted according to the dynamically changing environment. At present, the main means of geological forecasting in my country include geological sketching, geophysical prospecting, drilling and hole exploration, and the reliability of their detection increases with the order of them. However, due to the ever-changing nature of karst geology, it is impossible to accurately predict by just relying on a single detection method. The following are the advantages and disadvantages of geological forecasting techniques summarized by the author, which I hope can be comprehensively applied by people in the industry according to actual needs:
1) TSP202 (TSP203) advance geological forecast
a Advantages . (1) Wide range of use. This prediction method can be monitored in extremely soft rock geology and can also work in extremely hard rock, so it has a wide range of applications; (2) Large span. It can predict the geological conditions within the range of 100~350 m in front of the tunnel face. The harder and more complete the surrounding rock, the longer the prediction length; (3) The interference is small. The advance geological forecast time of TSP202 (TSP203) takes about 30 minutes, and this work can be arranged during breaks in tunnel construction; (4) The forecast data is timely. The report can be issued on the second day of data collection at the site.
b Disadvantages. (1) TSP202 (TSP203) advanced geological forecast is based on the average elastic wave velocity between 24 blast holes and the receiver and the time when the reflected wave of the geological body reaches the receiver, to determine the distance between the predicted geological body and the tunnel face The actual situation is different from the predicted position of the geological body, resulting in some changes in its accuracy; (2) The width of the geological body reflected by the accuracy of the forecast is determined based on the sampling interval and the elastic wave velocity of the rock mass. ; (3) This prediction method is more accurate in predicting planar structures such as faults and weak hard rock contact surfaces, but the radiation signal for point-like geological bodies such as caves is not strong, especially small caves with a width less than 0. 4m. It is difficult to predict.
2) Geo-radar
a Advantages. (1) Detection of underground strata or geological anomalies (such as caves, voids, etc.) within a range of 5 to 30 meters on the surface has obvious reflection signals; using medium to low frequency antennas before tunnel bottoming in limestone areas can be used to detect hidden karst at the bottom of the tunnel. Cavern means; (2) High-frequency antennas can be used for non-destructive testing of tunnel concrete lining quality.
b Disadvantages.
(1) Due to the poor sealing performance of the radar instrument, it is easily damaged and inconvenient to operate, making it unsuitable for long-distance detection; (2) The use environment of the radar should be relatively spacious, and the space conditions in the tunnel are not suitable for radar. Detection, coupled with the interference of metal components such as steel frames and rails in the cave, makes the detection effect poor.
3) Infrared detection
a Advantages. The instrument is small in size, easy to carry, and has simple operating procedures. It can detect the entire space and all angles of the detection area. It can predict whether there is hidden water or water-bearing structures in the peripheral space of the tunnel and within 30m in front of the tunnel. The time occupied by tunnel construction is basically negligible. Later data analysis and summary are concise, easy to understand, and time-consuming.
b Disadvantages. Predicting whether there is water in the tunnel is limited to a range of 30m in front of the detection point, and the specific conditions of the water, such as water volume and specific orientation, cannot be detected. It can be said that infrared detection is in the qualitative analysis stage, and quantitative analysis still needs to be studied.
2 Treatment technology for caves encountered at the bottom of the tunnel
For caves that have not changed much, have small space and no water, the location of the intersection with the tunnel and its filling condition can be used , use concrete, mortar rubble, etc. to backfill and seal. Whether a side wall foundation is needed can be determined based on the actual geological conditions.
1) When encountering large and deep caves, beam arches can be used to span them. When implementing this kind of side wall foundation, attention should be paid to that the beam ends and abutments should be constructed on a relatively stable foundation stone. In order to increase stability, concrete can also be used for reinforcement; 2) When constructing tunnels in karst caves, if you encounter long and narrow For deep caves, the side wall foundation on that side can be deepened to pass through; 3) When the bottom of the tunnel encounters a large cave and there is running water, mortar masonry or concrete support walls can be built below the tunnel bottom to support the tunnel structure, and A culvert is set inside the support wall to drain the cave water; 4) If a large cave is encountered, the tunnel construction process will be more complicated. Depending on the actual situation, side wall beams and driving beams will be used to pass through.
3 Treatment technology for caves encountered above the tunnel arch
When the cave has a relatively stable and complete roof, no obvious water leakage, and its thickness is less than 1/ of the span of the cave. 2 or above, general roadbed and bridge construction plans can be used to construct directly in the cave, and no lining structure is required; if water leakage occurs, a lining structure should be prepared to protect the safety of vehicles passing through the cave. When removing turquoise in a cave, anchor rods, large steel pipes, and rails should be used to reinforce the rock mass to prevent the cave or roof from collapsing.
In short, tunnel construction in cave areas is relatively complex. In order to improve construction quality, construction personnel should constantly summarize construction experience and improve their understanding of various cave conditions. In addition, they should also strengthen their study of theoretical knowledge. , thereby improving their ability to solve problems and contributing to the construction of tunnels in the cavern areas of southwest and central south my country.
The above is collected and compiled by Zhongda Consulting
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