Foundation Pit Leakage Detection Method Based on Electrical Resistivity Imaging Technology

Transcription

1 Erth Sciences 218; 7(4): doi: /j.erth ISS: (Print); ISS: (Online) Foundtion Pit Lekge Detection Method Bsed on Electricl Resistivity Imging Technology Ci Kejin 1, Yin Ybin 2, Liozhi 3, Ding Yueshung 4, Wu Yuho 2 1 Fculty of Geophysics nd Informtion Technology, Chin University of Geosciences, Beijing, Chin 2 Fculty of Erth Sciences, Jilin University, Chngchun, Chin 3 Fculty of Architecturl Engineering, Tinjin University, Tinjin Chin 4 Fculty of Environmentl nd Civil Engineering, Chengdu University of Technology, Chengdu, Chin Emil ddress: To cite this rticle: Ci Kejin, Yin Ybin, Liozhi, Ding Yueshung, Wu Yuho. Foundtion Pit Lekge Detection Method Bsed on Electricl Resistivity Imging Technology. Erth Sciences. Vol. 7, o. 4, 218, pp doi: /j.erth Received: My 21, 218; Accepted: June 7, 218; Published: June 29, 218 Abstrct: With the continuous development of city construction nd development of underground engineering, incresingly lrge scle, ll kinds of super lrge nd deep foundtion pit pper constntly, especilly in soft soil nd high groundwter level re. Due to the lekge problem of the foundtion pit engineering, ccidents hppen. Pit lekge is common construction defect of construction wterproof curtin, therefore, it is necessry to detect the lekge sitution of the foundtion pit supporting system in dvnce, so tht the lekge re is treted nd prevented in time. In the foundtion pit engineering, it is often used to observe the wter level chnges inside nd outside the foundtion pit to deduce the pproximte re of the foundtion pit lekge. Therefore, the use of geophysicl methods for lekge detection reserch hs become brekthrough in solving the problem of lekge of foundtion pit. In this pper, vrition of groundwter flow round the foundtion pit line is discussed when there is lekge of foundtion pit system, for instnce, leking wter ggregtion, intensive regionl seepge resistnce, resulting in low resistivity of lekge re round the foundtion pit of low resistivity. Bsed on the resistivity imging technique, the method for detecting the lekge of the foundtion pit by detecting the chnge of resistivity of soil round foundtion pit before nd fter dewtering is presented. It is through two engineering exmples show tht this method cn detect the pit lekge detection results of three-dimensionl position, detection results objectively nd ccurtely. Keywords: Foundtion Pit Lekge, Electricl Resistivity Imging, Groundwter, Stremline 1. Introduction With the development of urbn construction nd the continuous improvement of construction technology, vrious types of construction nd municipl engineering in Chin hve been developing rpidly in recent yers, especilly the emergence of urbn underground works, high-rise buildings nd super high-rise buildings represented by underground rilwys nd lrge trnsport hubs. The excvtion depth of the deep foundtion pit is lso getting lrger nd lrger, ccompnied by lrge number of deep foundtion pit projects, especilly in soft soil nd res with high underground wter level. Becuse of the lekge problem of foundtion pit engineering, ccidents often hppen. The retining wll, support, impervious curtin nd other structurl systems used in foundtion pit engineering re clled supporting structures [1]. Foundtion pit enclosure system is widely used in underground structure construction of high-rise buildings. The form of enclosure system is vried, nd the purpose is to form closed structure to fcilitte the construction of underground works. The min function is to chieve the purpose of retining nd supporting. Most of the foundtion pit ccidents (domestic nd foreign) re relted to the lekge of foundtion pit retining system. Due to the lekge of foundtion pit, the foundtion pit is unstble nd the rod surfce collpses, resulting in huge loss of life nd property. In order to prevent the occurrence of foundtion pit ccidents, mstering the seling effect of foundtion pit works. The lekge condition of foundtion pit enclosure system is

2 145 Ci Kejin et l.: Foundtion Pit Lekge Detection Method Bsed on Electricl Resistivity Imging Technology detected in dvnce. If there is lekge re, it is prticulrly necessry to del with nd prevent it before excvtion. At present, the foundtion pit lekge detection problem in Chin is usully deduced from the observtion of wter level inside nd outside the foundtion pit. Rre geophysicl methods for lekge detection nd no relible detection system hve been pplied to the lekge detection of foundtion pit. Figure 1. Stremline chrcteristics of groundwter seepge when retining system of foundtion pit is leking. () The pln of groundwter initil stte; (b) The initil stte of groundwter; (c) The pln of groundwter stremline in the dewtering; (d) View of groundwter flow in the dewtering. The lekge of foundtion pit enclosure system will led to lekge of groundwter round the enclosure system, nd the physicl properties of the soil will chnge ccordingly. Using suitble geophysicl methods to detect chnges in physicl properties of soil before nd fter lekge, the purpose of lekge detection cn be chieved. [2] 2. Chrcteristics of Lekge Underground Flow Line in Foundtion Pit Lekge Interstitil flow of foundtion soil round foundtion pit occurs when lekge of foundtion pit enclosure system occurs. Simultion of stremline chrcteristics of groundwter lekge in foundtion pit enclosure system by Modflow [3], [4], Retining lek in foundtion pit (Figure 1 ()). In the lekge process of foundtion pit retining system, the groundwter flow line round the lekge point is not directionl from the initil stte (Figure 1 (), 1 (b)), When the test becomes dewtering, the stremline flls to the lek point, nd the stremline is concentrted (Figure 1 (c), 1 (d)). 3. Solid Model of Lekge in Foundtion Pit Enclosure System Wter nd electricity re two different substnces. Their flow cn be described by the sme mthemticl model. An electric field is used to simulte the percoltion lw of formtion wter, the mechnism of which is the similrity between the differentil eqution of the flow of wter through the porous medium nd the differentil eqution of the chrge flow through the conductor mteril [5]. The flow of wter in porous medi follows Drcy's lw [6]: V (1) In the formul: v current velocity, m/s; q flow rte cm 3 /s; A lekge section re, cm 2 ; k penetrnce, µm 2 ; µ fluid viscosity, mp s; P pressure,.1mp.

3 Erth Sciences 218; 7(4): Ohm's lw is observed by the current of the conductor: δ (2) In the formul: conductivity, the reciprocl of resistivity,/cm; U voltge, v; electric current density, A/cm 2 ; I electric current, A; S conductor cross-section, cm 2. Stedy seepge continuity eqution for homogeneous formtion wter through porous medi: div (3) Voltge distribution eqution in uniform conductor: div (4) Compring formul (1), (2), (3), (4), it cn be seen: The electric field nd seepge field cn be described by the sme differentil eqution. So the stedy seepge problem of incompressible fluid cn be simulted by stedy electric field. The distribution of pressure in seepge field cn be described by potentil distribution, current or velocity is described by electric current, nd resistnce is described by resistnce. The dense re of flow line in seepge loction hs smll seepge resistnce nd low resistivity. The distribution of electric potentil in electric field is the sme s tht in seepge field; the distribution of the current line is the sme s the distribution of the percoltion line. In order to crry out experiments in the lbortory, PVC structure model ws developed. (Long 2.m; wide 2.m; high 1.5m, wll thickness 2cm), Simultion of soil nd wter spce round the foundtion pit enclosure system (Figure 2). Legend Simulted pit re Figure 2. PVC model digrm ( ) ( b ).6 () Closed; (b) Wter lekge. Figure 3. PVC model resistivity mp PVC model - profile, PVC model - section uses resistivity imging method to conduct lekge detection test. Figure 3 is resistivity mp of the - section when it is closed nd leking. Compring Figure 3 () nd Figure 3 (b), the - test profile corresponds to n obvious low resistivity re t the outlet of the PVC tp. The model test shows tht the seepge flow is dense nd the resistivity is low. It is proved tht the lekge loction nd the resistivity of the ffected re hve chnged during the lekge of foundtion pit enclosure system. According to the lekge chrcteristics of foundtion pit enclosure system, the resistivity clcultion model is shown in Figure 4. The inversion of the clcultion model by resistivity imging method is very cler. Resistivity imging is used to detect the soil round the foundtion pit retining system before nd fter the test.

4 147 Ci Kejin et l.: Foundtion Pit Lekge Detection Method Bsed on Electricl Resistivity Imging Technology () (b) () Lekge resistivity model; (b) Inversion result Figure 4. Lekge resistivity model nd inversion result. By compring nd nlyzing the pprent resistivity mp, the lekge loction of foundtion pit retining system cn be detected. h ol e 1 ho l e 2 Instu Lekge hole to observe the current nd voltge dt between the two holes. Resistivity imging of soil between two holes is obtined by inversion. The resistivity chnges of the surrounding soil round the foundtion pit before nd fter the test re compred nd nlyzed, so s to chieve the purpose of foundtion pit lekge detection. [7], [8], [9], [1] The rry of electrodes is divided into even nd odd rry groups in dt cquisition. [11] Combintion of odd nd even pirs of power supply electrodes, rbitrry combintion of mesuring electrodes. The AB electric field dt is recorded nd mesure the voltge nd current vlue of the residul electrode nd M should be mesured. Ech rrngement hs 64 electrodes, odd rrys, 32 electrodes nd even groups of 32 electrodes. To mtch ech other (the whole rrngement) s power supply electrode, tht is to sy, row rrngement hs 32 x 32 = 124 times power filure process. Ech power supply cn collect 61 potentil difference dt t the sme time, so the totl dt volume should be 32 x 32 x 61 = 464. A lrge number of dt re retrieved to obtin resistivity profiles nd reflect the resistivity distribution between two holes. Smll chnges in soil resistivity round foundtion pit retining system cn be detected. [12] 5. Appliction Exmples 5.1. Deep Foundtion Pit Engineering of Hihe Est Rod Project Profile The foundtion pit dopts the underground continuous wll wter stop structure, with 8 mm thick diphrgm wll, 4m long plin concrete section, the wll top elevtion -2.3 m, the wll bottom elevtion m, C4 concrete design grde. The groundwter condition of this project is complex, nd the soil lyer chnges gretly, nd there re two micro confined wter ffecting the foundtion pit Anlysis of Detection Results B M A Figure 5. Mesuring hole lyout nd working principle of resistivity imging method. 4. The Principle of Resistivity Imging Technology For the lekge detection of foundtion pit retining system, electricl holes re spced t distnce from the outside of the foundtion pit enclosure system (Figure 5). A number of electrodes re plced in the electricl method 61 Figure 6. Electricl mesurement of hole plne position in the Hihe Rod Project.

5 Erth Sciences 218; 7(4): hole hole61 hole hole () ( b) () Before dewtering; (b) Dewtering Figure 7. Resistivity mp of profile-61. As shown in Figure 6, nd 61 electricl holes in the foundtion pit re used for resistivity imging detection. The horizontl section is horizontl, the ordinte is depth, the horizontl spcing is 16 m, nd the hole depth is 35 m. A set of blue - Green - Green - yellow - red - pink color mrks is used to indicte the difference in pprent resistivity from low resistnce to high resistnce. The resistivity of the region with lekge is low nd shows blue. The detection results of two times before nd fter the tril dewtering of foundtion pit re shown in Figure 7. Figure 7 () when the project hs not strted dewtering; the resistivity of the -61 section hs not reflected the obvious lekge position. After the foundtion pit strts dewtering, Figure 7 (b), the resistivity of the -61 section trnsverse 14m depth 11m re is obviously low, showing the lekge chrcteristics. When the foundtion pit is excvted, there is visible lekge current in the loction, nd timely diversion nd plugging mesures re tken during construction Foundtion Pit Engineering of Dongxing Rod Subwy Sttion Project Profile The retining structure of the min foundtion pit of the sttion is composed of reinforced concrete continuous wll nd steel support. The thickness of the diphrgm wll is 8 mm, nd the joint of concve nd convex joint is dopted t the joint of the diphrgm wll. The length of the diphrgm wll is 35.1m (bottom of the crown bem to the bottom of the diphrgm wll), the pit depth of the sttion is bout 17.39m, nd the covering soil is bout 3.15m. There re two wter bering groups tht hve gret influence on this project, nmely, diving (15m shllow depth) nd confined wter (2m 35m depth) Anlysis of Detection Results As shown in Figure 8, resistivity imging is used to detect electricl holes 1 nd 2 t the edge of the foundtion pit. The horizontl section is horizontl, the ordinte is depth, the horizontl spcing is 2m, nd the hole depth is 45m. Using set of blue-cyn - Green - yellow - red - pink color lbels to show the difference of pprent resistivity from low resistivity to high resistivity. Where wter lekge occurs, the resistivity is low, showing blue color. The detection results of two times before nd fter the tril dewtering of foundtion pit re shown in Figure Position nd number of electric hole Figure 8. Electricl mesurement of hole plne position in the Dongxing rod Project.

6 149 Ci Kejin et l.: Foundtion Pit Lekge Detection Method Bsed on Electricl Resistivity Imging Technology hole1 hole2 hole1 hole () () Before dewtering; (b) Dewtering Figure 9. Resistivity mp of profile1-2. Figure 9 () when the project hs not strted dewtering; the resistivity of the 1-2 section hs not reflected the obvious lekge position. After the foundtion pit begins dewtering, s shown in Figure 9 (b), the 1-2 section is 16 meters cross, the resistivity in the depth 6m re is obviously low, nd disply lekge chrcteristics. When the foundtion pit is excvted, there is visible lekge current in the loction, nd timely diversion nd plugging re crried out during construction. 6. Conclusions The method of detecting the seepge of foundtion pit is the chrcteristic of the chnge of the flow line of groundwter round the foundtion pit nd the chnge of the soil resistivity round the foundtion pit. Bsed on the principle of hydropower nlogy nd engineering ppliction, n effective method for detecting the lekge of foundtion pit by electricl resistivity imging is introduced. The nlysis principles of foundtion pit lekge detection re: 1) According to the vrition rule of low resistivity before nd fter dewtering, it is judged whether there is lekge nd lekge position of foundtion pit enclosure structure. 2) The inversion results of resistivity imging before nd fter the foundtion pit dewtering re compred nd nlyzed. After continuous occurrence of low resistivity nomlies nd constnt or low resistivity vlues in the sme loction, it is speculted tht the nomly is cused by lekge of foundtion pit enclosure structure. Compred with the trditionl wter level observtion method, the method hs the following chrcteristics: (b) (1) The method is simple on site nd hs no influence on the construction nd surrounding environment. It cn not only detect the lekge before the excvtion of the foundtion pit, but lso monitor the excvtion nd the lter opertion of the foundtion pit; (2) The results of this method cn provide three-dimensionl positioning for lekge of foundtion pit enclosure system, nd cn effectively guide the lekge prevention nd reinforcement of foundtion pit; (3) This method hs lrge mount of dt cquisition, nd the inversion clcultion is not n intervention. The inversion results truly reflect the objective chnges of the soil resistivity round the foundtion pit, nd the detection results reflect the rel time stte of the foundtion pit; (4) The method cn lso be used for sfety monitoring of underground soil in importnt rods, subwy opertion res nd importnt pipelines. [13] The rticle minly introduces the method nd ppliction of resistivity imging technology for foundtion pit lekge detection. The further ppliction of the method nd the nlysis nd interprettion of the detection results, nd the chrcteristic resistivity imge of lekge, should be perfected nd summrized in lrge number of engineering pplictions. References [1] GAO D Z. Deep Foundtion Pit Engineering [M]. Chin Mchine Industry Press (In Chinese). [2] LI X J, HUAG Y J, et l. Stndrd for Urbn Engineering Geophysicl Explortion [M]. Chin Architecture & Building Press (In Chinese). [3] XUE Y Q, XIE C H. umericl Simultion of Groundwter. Science Press (In Chinese). [4] Li G X, Higher Soil Mechnics (Second Edition) [M]. Tsinghu University Press (In Chinese). [5] CHEG L S. Higher Seepge Mechnics [M]. Petroleum Industry Press, 211. (In Chinese). [6] LI G X. Advnced Soil Mechnics [M]. Tsinghu University Press (In Chinese). [7] Pn H P, M F L, Ci B L et l. Geophysicl Logging nd Well Logging Geophysicl [M]. Science Press (6-8). [8] Li J M, Luo Y Z, editor. ew Progress in Electricl Prospecting [M] Beijing: Geologicl Publishing House, [9] Zhng S Y, Pn Y L. Applied Geophysics [M]. Chin University of Geosciences Press (15-17). [1] MAO H J. To study the Heiniudong Deep Orebody Resistivity Profile Method to Determine the Well Logging [J]. Sichun: Computing Techniques for Geophysicl nd Geochemicl Explortion, 211, 33 (3), (In Chinese). [11] Guo X J. ew Method nd Appliction of 3-D Resistivity Tomogrphy [J]. Journl of Jilin University (Erth Science Edition), 25, 35 (1), (In Chinese).

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