| Karst is a typical adverse geological phenomenon that endangers engineering construction.Ground subsidence in karst area presents strong sudden and destructive characteristics,which brings potential threats to residents' lives,engineering construction and surrounding environment.Karst disaster has become an urgent problem to be solved in many urban construction.There are several nearly East-West limestone strips in the built area of Wuhan,and the scale of rail transit construction in this area is increasing and will pass through these limestone strips,which seriously affects the safety of metro construction and operation.Therefore,it is of great significance to find out the law of karst distribution for the prediction and control of karst disasters.Geophysical exploration provides an effective method for Karst exploration.As a new geophysical technology,SPAC(Spatial autocorrelation method)can be applied to the detection of urban underground karst structure.In this paper,SPAC method is used to detect the karst distribution between Hongshan Government Station and Provincial Academy of Agricultural Sciences Station of Wuhan Metro Line 8.According to the results of drilling holes,the results are compared with those of electromagnetic wave CT method.The main research results are as follows:(1)consulting the engineering geological background information of the survey area,sorting out the karst geological hazards in Wuhan,and analyzing the basic situation of site strata.(2)The theoretical basis of fretting detection,the principle of SPAC method and the matters needing attention in field data acquisition are introduced in order to eliminate interference and ensure the accuracy of data.The fretting array is affected by the site condition in the built area.The fretting single-point sounding and two-dimensional profile detection are selected.After extracting Rayleigh wave dispersion curve from fretting signal by SPAC method,the underground S-wave velocity structure is inverted by using neighborhood algorithm.(3)According to the drilling results,the SPAC method and electromagnetic wave CT method are compared and analyzed to detect urban underground karst structure.The advantages and disadvantages of the two methods in practical field work are summarized,and the reliability and accuracy of the two methods are verified.SPAC detection results show that the underground medium structure of the site can be divided into four layers,the first layer,the bottom boundary of the stratum is about 3M and the velocity is 159m/s;the second layer,the bottom boundary of the stratum is 15 m and the velocity is 249m/s;the third layer,the bottom boundary of the stratum is 20 m and the velocity range is 497m/s;the remaining layer is the fourth layer,and the fretting detection velocity range is 1108-1379m/s.The result of electromagnetic wave CT detection is that the depth of the boundary between bedrock and cover is about 25 meters,the depth of the boundary between the complete bedrock roof is about 33.5 meters,and the number is 4052,which is supposed to be karst caves.In fact,siliceous rocks cause lithological changes and interfere with the detection to a certain extent.SPAC method can better distinguish the interface between soil and rock stratum,including the interface between soil stratum and stratum stratum,according to the difference of rock and soil properties and shear wave velocity.Electromagnetic wave CT method can distinguish stratum according to apparent absorption coefficient.It can better distinguish the interface between soil and rock stratum,and the interface between relatively complete and weathered rock.At the same time,its detection effect is greatly affected by lithology.The phenomenon of low velocity zone was found in the underlying bedrock of SPAC observation point,which was interpreted as a karst cave with a total karst cave.The depth of the cave roof is 30 m,and the corresponding vertical height is 10 m.Electromagnetic CT detection shows that there are two caves in YRK4107 borehole with buried depth of 29 m and 46 m,corresponding vertical heights of 3.6 m and 4.1 m,respectively.Both methods can detect the location of karst,and the size and range of karst caves are different.SPAC method can detect karst caves in low-speed areas,such as karst caves,fractured zones or filling conditions,and can observe without drilling or logging.Electromagnetic wave CT method can detect the location and scale of karst more accurately,and detect karst strips more accurately.In addition,it relies on drilling or logging for observation.(4)Obtain the S-wave velocity structure of single-point sounding,and obtain the velocity change map of the whole two-dimensional profile,and determine the depth of each interface and the location of the low-velocity layer to find the low-velocity zone,i.e.the location and scale of the cave.The conclusions are as follows: 1)The velocity of karst cave is less than 500 m/s,the karst developed section is 600-1050 m/s,and the karst undeveloped section is 1050-1379 m/s.The velocity of S wave in limestone area is less than 500 m/s as the basis for judging karst cave or karst cave.2)The overburden of the line is 8-25 m,and the upper stratum is Quaternary overburden.It is mainly composed of plain fill,miscellaneous fill,silty clay and red clay.The wave velocity anomaly varies greatly and the properties of the soil layer are not uniform.The underlying layer is bedrock,namely limestone.3)At the first observation point of the survey line,there is an abnormal area of wave velocity,which is presumed to be a karst cave.The depth of the cave roof is 28 m and the vertical height is about 5m.At the third observation point,there is an abnormal area of wave velocity,which is supposed to be a karst cave.The depth of the cave roof is 33 m and the vertical height is about 8 M.At the fourth observation point,the low-speed area is exposed,presumed to be a karst cave with a roof depth of 44 m and a vertical height of about 2 m.4)The whole section is buried below 50 m,and karst is not developed.5)The S-wave velocity of two-dimensional micro-motion profile is not continuous transversely,and the main development form is karst pipeline type. |
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