Gan Long Railway (hereinafter referred to as Gan Long High-speed Railway, the same below) is a national first-class trunk line with a design speed of 200 km/h, which is an important national construction project. The main line of Gan Long High-speed Railway is 250.4km long, and the total length in Fujian Province is 139.095km. It is a high-standard trunk railway built in mountainous areas with complex terrain and geological conditions in China, and it is one of the difficult areas for railway construction in mountainous areas in our province at present. In particular, there are many bridges and tunnels, accounting for 77.8% of the line length. The engineering geological and hydrogeological conditions are complex, and various adverse geological processes are developed, so the investigation process is difficult and there are many problems. The experience and lessons are worth summarizing.
1 Geological Survey of Fujian Section of Gan Long High-speed Railway
This section is located in the middle and low mountainous area of northwest Fujian, with hills and river terraces and developed river valleys. The structure is developed, carbonate rocks are widely distributed, karst is developed, the terrain is steep, the valley is cut deeply, and the valley shape is mostly V-shaped. Since the late Proterozoic, the development of geological structure in the survey area has experienced multi-cycle development. The tectonic system is mainly composed of four major structures and "mountain" structures: north-south, east-west, northeast and northeast. These structures control the distribution and occurrence of intrusive rocks and the direction of rivers in the area, and have great influence on the engineering geological conditions of rock masses in the area.
There are many exposed strata along the line with complex lithology, mainly Proterozoic, Sinian, Cambrian, Ordovician-Silurian shale and argillaceous shale (mixed with metamorphic siltstone); The sedimentary rocks from Devonian to Tertiary are mainly sandstone, siltstone and glutenite. There are occasional coal lines. Intrusive rocks are widely distributed along the line, accounting for about 40% of the line. The main lithology is biotite granite, fine-grained granite, Indosinian-Variscan gneiss biotite adamellite, Caledonian mixed granite, granodiorite and dikes in different periods. The genetic types of Quaternary strata along the line are complex, mainly distributed in Holocene to Pleistocene alluvial and diluvial deposits (mainly clay, gravel, etc. ) and Pleistocene slope residues (mainly sandy clay). The distribution and thickness vary greatly. There is 2 ~10 meter of muddy soil in the valley between mountains or hills. The groundwater types along the line mainly include loose rock pore water, bedrock fissure water and karst water.
2 mountain high-speed railway measurement method
Engineering geological investigation of high-speed railway in mountainous area is a comprehensive work. Collect geological data along the line first, then carry out geological or engineering geological mapping, and then carry out exploration means such as geophysical exploration, drilling, geological test and indoor test. Finally, through comprehensive analysis and data collation, a qualified investigation report is compiled. The development trend of engineering geological survey of high-speed railway is to adopt comprehensive survey and comprehensive analysis methods. Therefore, in the investigation of high-speed railway in mountainous areas, we should not only consider the sequence of working procedures, but also consider the connection of various professions, and arrange the investigation work reasonably to ensure the investigation work to be carried out in a reasonable and orderly manner.
The research on engineering geological survey method of high-speed railway is carried out with the construction of high-speed railway. In recent years, in the process of high-speed railway construction in areas with complex geological conditions, railway engineers and technicians such as He Huawu have been exploring the exploration technology and methods of high-speed railway and achieved certain results. There are many geological and geomorphological units that often pass along the high-speed railway project in mountainous areas, and a large number of controlled projects such as tunnels and bridges are distributed (for example, Gan Long high-speed railway accounts for 77.8% of the line length). The topography, geological structure, engineering geology and hydrogeological conditions along the Fujian section of Gan Long high-speed railway are extremely complicated, and there are many adverse geological problems along the line, such as collapse, landslide, debris flow and karst collapse, which have the characteristics of many types, wide distribution and large scale, and their influence is mainly manifested in broken rock mass or weak lithology.
As a result, carbonate karst is widely distributed in Changting basin, but also sporadically distributed in Zhongfu, Xiaochi and Longyan, all of which are covered karst. The application of comprehensive survey method of Gan Long high-speed railway has laid a foundation for finding out the engineering geological background, the development characteristics of karst and karst water, the relationship with bridges and tunnels and the degree of harm, and played a vital role in effectively avoiding the existing geological risks.
3 Problems in the investigation of high-speed railway
Summarize the common problems in high-speed railway survey in mountainous areas, mainly in the following aspects.
(1) Surveyors in different industries have insufficient understanding of railway surveying and have deviations in understanding railway specifications. For example, a large number of housing construction, metallurgy, chemical industry, port traffic and other survey units carry out cross-industry railway survey, and all departments habitually copy their own industry survey methods, resulting in a wrong evaluation of engineering geological conditions along the line. For a long time, there has been a situation of multi-head management and compartmentalization in China's survey industry, and a complete and unified engineering survey system has not been formed, and there is no national standard that can be applied to the whole civil engineering. Due to the differences in current engineering survey codes and standards in different industries, there are also differences in specific survey requirements and contents, such as the naming of geotechnical materials, calculation parameters and formulas of geotechnical design, different methods of liquefaction of seismic sand, corrosiveness evaluation of water and soil on building materials, requirements for sampling and in-situ testing, and indoor test procedures and operations.
(2) The reconnaissance period is unreasonable due to the merger of reconnaissance stages. Railway engineering geological investigation should be carried out in stages from the surface to the inside, from shallow to deep, and accurately provide the investigation results needed in different investigation stages. However, due to the tight time and heavy tasks of individual lines, the investigation stages are often merged, and there is not enough time to carry out investigation work. The consequences are often serious. For example, the geological conditions are not clearly investigated, and the design is constantly revised after construction, resulting in rework and waste, which will lead to passive work in the later stage, prolonged construction period and increased investment. In the worst case, potential safety hazards may be left in the operation stage, resulting in major safety accidents.
(3) The drilling technology is backward, the drilling progress and core recovery rate are low, the drilling depth is insufficient or ultra-deep, and the drilling work cannot meet the requirements of survey and design. It is common for bridge foundation holes to have insufficient or ultra-deep drilling depth. The lithology of the structural fracture zone is relatively broken and loose, so the foundations of large and medium-sized bridges, extra-large bridges and Gao Qiao should avoid the structural fracture zone, especially the contact zone between soft and hard rocks, which brings difficulties to foundation construction or treatment. If the fault zone is wide, the minimum number of piers and abutments should be set on it. The fault zone should be exposed during the survey.
For example, the hole T 14 in JZ-Ⅲ 097-B of Wenfang Bridge is over 40.0 m, which is a weathered granite layer. 40.0 ~ 55.0 m is a structural fracture zone, and the lithology is cataclastic strongly weathered granite with scratches; The hole is 5m deep, and enters the lower weakly weathered granite. For large and medium-sized bridges, extra-large bridges and Gao Qiao, the load on the bridge foundation is large, and large-diameter rock-socketed piles are often used. There is no doubt that the bearing stratum is weakly weathered rock. However, if the buried depth of weakly weathered rock is large, and there is a thick layer of strongly weathered rock on the upper part, and the pile side resistance can meet the bearing capacity requirements, the hole depth can be controlled at a certain thickness of strongly weathered rock. According to relevant literature, when the pile length and diameter of rock-socketed pile are large (L/D > 20) (that is, the pile length is longer than 50m) and the soil around the pile is good, the side resistance load sharing ratio is more than 70%, most of which is above 80%, and the resistance load sharing at the pile end is relatively small. For example, the JZ-Ⅲ 097-BG hole of Zhenghuashan Bridge is completely weathered granite with alluvial depth of more than 6.2m6.2 ~ 49.6; 49.6 ~ 67.0 m is strongly weathered granite, and the final hole of friction pile can be used.
(4) The pursuit of accuracy of geological parameters misleads the credibility of evaluation. There is a well-known uncertainty principle in physics, which is also applicable to some geological problems in geotechnical engineering or geological engineering: that is, geotechnical body is full of uniqueness and variability, and the design parameters or physical and mechanical properties of geotechnical engineering should not be determined by absolutely accurate fixed values, but only through comprehensive analysis, statistics and empirical judgment can a suggested interval value be put forward for designers to use.
Because it is difficult to predict quantitatively and accurately in geotechnical engineering, this is because rock and soil are discontinuous media, rocks are full of joints and cracks, and soil is a collection of dispersed particles, which are composed of multiple phases. For example, when measuring the occurrence of structural planes such as joints and cracks on rock slopes, it is more realistic to use an interval value to represent the occurrence of structural planes, but it is not realistic to use only a fixed value to represent the occurrence of structural planes. For another example, the degree of rock weathering is gradual from top to bottom, and the mechanical indexes are also gradual, and the geotechnical engineering design parameters can only be an interval value or statistical value. It is inappropriate to think that the geotechnical parameters accurate to a certain value are absolutely accurate, otherwise the accuracy of geotechnical engineering evaluation will be affected.
(5) The management of engineering geological test and geotechnical test is not in place, and the quantity and quality do not meet the requirements, which affects the certainty of geotechnical parameters. Engineering geological tests include field in-situ tests and indoor tests, and there are many kinds. Because of different testing methods, the same parameters get different data. Whether the test method is reasonable becomes an important link in the accuracy of geotechnical engineering design parameters.
There are many reasons for the inaccuracy of geotechnical test data. First, the disturbance of soil samples is mainly caused by improper sampling method, long-distance transportation, unqualified sample preparation and improper test operation. The second is to calculate the error, so that the test data are randomly distributed; Third, compared with building materials, geotechnical materials are complex and changeable. When the number of samples is insufficient, the test indicators are generally not representative. There should be a certain number of test indicators, and representative values can be obtained through mathematical statistics.
(6) Geological technicians are inexperienced, unable to do a good job of comprehensive analysis of geological background data, and their comprehensive analysis level is insufficient. Due to the difference of geological conditions and the inaccuracy of geotechnical design parameters, the survey results can not reflect the geological conditions well, which reduces the reliability of geotechnical design calculation. Therefore, it is necessary to emphasize the combination of qualitative analysis and quantitative analysis of engineering geological survey results and implement comprehensive evaluation.
Comprehensive judgment requires both solid theoretical knowledge and rich practical experience. Lack of experience is also reflected in misjudgment of strata or geological bodies. For buried karst, karst generally develops at the junction of soluble rock and insoluble rock, overlying sandstone is misjudged as coarse gravel soil, and karst filling is easily misjudged as alluvial material. For example, the hole JZ-Ⅲ 097-B18907 of subgrade is purplish red sandstone at 1.8 ~ 4.3m, and it is broken and misjudged as coarse gravel soil; 4.3 ~ 7.4m is dark gray clayey soil, containing breccia, with a gravel diameter of 5 ~ 10 mm, which is karst filling and misjudged as alluvial material (gravel sand).
4 conclusion
The investigation of high-speed railway in the hilly areas of middle and low mountains has the characteristics of complex topographic and geological conditions and developed unfavorable geological conditions, and its development trend is to adopt the method of comprehensive investigation and comprehensive analysis. In the process of investigation, it is necessary to thoroughly study the relevant norms and regulations of railway engineering and sum up all kinds of successful experiences and lessons of failure, which is the guarantee of high-quality investigation.
The above analysis on engineering geological investigation of high-speed railway is collected and sorted by Zhong Da Consulting.
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