# Study on Detection of the Bearing Stratum Integrity of Bored Pile Based on Transient Electromagnetic Method

Authors: Chunwei Pan; Guocai Zhang; Sheng Chen; Fangkun Zheng; Xue Liu
Journal: Geofluids
 (2022)

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## Abstract

To ensure the safe construction of bridge pile foundation in karst area, it is necessary to accurately detect the unfavorable geological conditions such as karst cave and soft layer before excavation. In this paper, the transient electromagnetic method is used to probe the geological conditions near the pile Y1 of an interworking main bridge in China’s Hunan Province. By arranging 5 measuring lines at the pile Y1, the voltage variation laws of 21 monitoring points on each measuring line are obtained, and the resistivity variation cloud maps at the measuring line positions are inversed. The results show that the voltage signals at line 1 and line 5 are obviously abnormal, and the resistivity varies greatly within 50 m underground depth, which indicates that there may be adverse geological conditions such as karst cave and carbonaceous limestone interlayer. Further, through on-site hole digging and sampling, it is found that the slag sample in the area above 50 m of the stratum of pile Y1 is wet, and the slag sample below 50 m is dry without argillaceous interlayer, which verifies the effectiveness of transient electromagnetic detection method. Because the transient electromagnetic detection technology does not need drilling and the detection process is convenient, it may provide a new means for quickly detecting the integrity of the bearing layer of bored cast-in-place pile in bridge construction.


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## Body

## 1. Introduction

Underground weak layer, water-bearing fracture zone, karst, and other unfavorable geological bodies may cause some safety accidents such as the hole collapse, the ground subsidence, the buried drilling, and the structural cracking of surrounding buildings in many engineering fields, such as the oil and gas exploration, coal mining, and tunnel engineering [1, 2]. To ensure safe construction in these engineering fields, it is essential to carry out the forward geological prediction [3]. There are many geophysical methods for the forward geological prediction [4], such as the drilling and coring method, the ground penetrating radar method, the seismic exploration method, and the transient electromagnetic method (TEM). Among them, being of low cost, convenience, and large detection ranges, the TEM has a broad application prospect.Initially, the theory of TEM was established in the former Soviet Union in the 1950s [5]. Subsequently, the TEM ushered in a stage of rapid development in the 1970s [6]. In principle, because TEM is sensitive to groundwater and geological structure fracture zone, it can realize the forward geological prediction by monitoring the abnormal evolution law of voltage or resistivity of geological bodies [7]. At present, TEM has achieved remarkable results in some engineering applications. For example, Jørgensen et al. [8] applied TEM to carry out geological exploration in western Denmark and successfully monitored a large number of buried valleys filled by various glacial sedimentary clay, moraine, and molten sand. Further, by inverting the apparent resistivity of geological bodies, they mapped the valley. Zeng et al. [9] developed the TEM with multiple radiation sources to realize the fine detection of hidden ore bodies in deep mineral resources. Based on the theory of TEM, Yuan et al. [10] calculated the transient electromagnetic wave logging response of layered media and analyzed its propagation characteristics and far detection ability to the formation boundary. Bobrov et al. [11] used TEM to evaluate the salt-dome tectonics under considering two geoelectric arrays. The physical and mathematical modeling analyses show that the TEM may provide key information on the relief of salt-dome top. Based on the TEM, Su et al. [12] developed a new method to probe the water-bearing geological body of coal mine, called the mining TEM (MTEM). Through numerical simulation and the field test in the coal mine in Linfen city in Shanxi Province, it is shown that the MTEM is effective in the water disaster detection of coal mine. In addition, in the field of tunnel engineering, Qian et al. [13] explored the influence of fixed interference source on the tunnel defect monitoring by TEM. Meng et al. [14] numerically studied the electromagnetic response of unfavorable structure in tunnel. Also, Su et al. [15] proposed the small fixed-loop TEM for the forward geological prediction in tunnel.The above review shows that TEM has good sensitivity in monitoring poor geological structures, especially the unfavorable water-bearing geological bodies. Although TEM has good applications in many engineering fields, there are few reports on the applications of TEM to the field of bridge engineering. In particular, in the area with obvious karst cave development, the forward geological monitoring of bearing stratum of bridge foundation cast-in-place pile is very important for the safe construction. As a potentially effective method, TEM may provide an effective means to detect the bearing stratum integrity of bored pile.This paper mainly focuses on the detection of the bearing stratum integrity of bored pile based on TEM in bridge engineering. The remainders of this work are as follows. In Section2, the basic principle of TEM is briefly introduced. In Section 3, TEM is applied to probe the unfavorable geological body near a bored pile at Caojiaben interchange main line bridge. Section 4 summarizes the conclusions.


## 2. Principle of TEM

TEM belongs to the time-domain electromagnetic induction method. The basic principle of TEM is as follows. Firstly, as shown in Figure1(a), an electromagnetic emission loop is laid on the ground surface and a pulse current signal is provided. The signal can generate an induced magnetic field in the transmission gap, called primary induced magnetic field. Secondly, under the excitation of primary induced magnetic field, the induced eddy current is generated in geological body (see Figure 1(a)), and its intensity is related to the conductivity of geological body. Thirdly, when the excitation source is suddenly disconnected at a certain time, the primary induced magnetic field would disappear, but the eddy current does not disappear immediately, showing a gradual attenuation process (see ① marked in Figure 1(a)). Correspondingly, a secondary induced magnetic field is generated in geological body (see Figure 1(b)). Finally, by receiving the secondary induced magnetic field information, the geological conditions would be obtained. Schematically, Figure 2 illustrates the transient process of electromagnetic signal. The above process continues until the magnetic field energy is completely dissipated as the ohmic loss of geological body.Figure 1
Sketch map of the basic principle of TEM.h: formation depth; ①: the induced eddy current with larger and larger diffusion range and smaller and smaller intensity.
(a)(b)Figure 2
Schematic diagram of transient electromagnetic signal.VT is induced voltage; Ht is magnetic field intensity.Generally, for the secondary induced magnetic field shown in Figure1(b), its propagation depth can be calculated as [16, 17]
(1)h=4tπσμ0,where t is propagation time, σ is the conductivity of geological body, and μ0 is the permeability in vacuum.Taking the derivative of Equation (1) with respect to t, one can obtain the propagation velocity of secondary induced magnetic field as
(2)v=∂h∂t=2πσμ0t.From Equation (2), it shows that the diffusion velocity of induced eddy current or secondary induced magnetic field is related to formation conductivity. Obviously, the better the conductivity of geological body, the slower the diffusion speed, and the more conducive it is to detect the evolution law of formation transient electromagnetic field in longer delay time. In addition, it is worth noting that, due to the skin effect, in the process of transient electromagnetic propagation, the high-frequency part of the magnetic field is mainly concentrated in the local area near the formation surface, while the low-frequency part diffuses to the deep layer, and the distribution range is gradually expanded.Further, based on Equation (1), one can obtain
(3)t=2π×10−7h2ρ,where ρ is the overlying resistivity of geological body, which is equal to 1/σ.In addition, the propagation time of transient electromagnetic field can be also calculated as [16]
(4)t=μ0M/η2400πρ31/5,where η is the minimum resolvable voltage and M is the transmit magnetic moment.Here,M can be calculated as
(5)M=L2I,where L is the side length of transmitting coil of electromagnetic field and I is the emission current.Substituting Equations (4) and (5) into Equation (3) yields the detection depth as
(6)h=0.55L2Iρη1/5.Clearly, Equation (6) shows that the detection depth of TEM is related to the resolvable voltage and the resistivity of geological body. By measuring different voltage values, different detection depths can be obtained, and different voltage values mean different geological conditions. Therefore, by inverting the variation law of the voltage or resistivity of geological body, the geological conditions can be obtained.


## 3. Application of TEM to the Detection of the Bearing Stratum Integrity of Bored Pile

### 3.1. Engineering Background

Caojiaben interchange main line bridge is located in the southwest of Huangsha Town, Yizhang County, Hunan Province, China. Figure3(a) shows the specific location of the interchange main line bridge. The terrain near the bridge is relatively flat and locally steep with natural slope angles of 20~40°. The slope in the interchange area is Quaternary brown-yellow slope residual silty clay, and its overburden is thick, which is moderately weathered limestone. The unfavorable geology of the line section is mainly the soft soil layer. Through the previous drilling at Y1 pile, the geological conditions below the Y1 pile are shown in Figure 3(b). For example, at the stratum depth of 24.2~33.5 m, the moderately weathered carbonaceous limestone with broken joints is presented, while karst caves are developed at the stratum depth of 33.5~54.5 m. Because the detection range of borehole is only limited to the area where the borehole is located, it is difficult to accurately reveal the surrounding complex geological conditions. In addition, considering the tight construction period, other geophysical exploration methods such as CT or pipe wave methods are also not applicable. Therefore, in order to ensure the safe construction of bridge pile foundation, TEM is applied to further quickly detect the integrity of bearing stratum of bored cast-in-place pile on a larger scale.Figure 3
Location and geologic column of Caojiaben interchange main line bridge.


### 3.2. Monitoring Scheme and Result Analyses

In this paper, the transient electromagnetic field is generated by transmitting bipolar trapezoidal pulse signal. The length of the transmitting and receiving coils is 1.8 m. The emission frequency of the pulse signal is 25 Hz, and the stacking times are 128. The sampling frequency is 1.25 MHz. Figure4 shows the scheme for monitoring the bearing stratum integrity of bored pile by TEM. Five survey lines were arranged around the Y1 pile. The distance between two survey lines is 1.5 m, and the monitoring range of each survey line is 20 m with a total of 21 monitoring points.Figure 4
Monitoring scheme.Figure5 shows the inverted resolvable voltage at different measuring points on different survey lines. It can be seen that there are abnormally identifiable voltage response signals on all of five measuring lines. Clearly, single peak or multipeak discernible voltage signals were monitored at some monitoring points, such as the monitoring points 13 and 20 on survey line 1; the monitoring points 4, 9-13, 17, and 20 on survey line 2; the monitoring points 7, 13, and 18 on survey line 3; and the monitoring points 11, 16, and 18 on survey line 4, as well as the monitoring points 5-9 and 19 on survey line 5. In addition, the ranges of identifiable voltages on survey lines 1 and 5 are generally higher than those on survey lines 2-4. Specifically, the voltage signal values on survey lines 1 and 5 are from 0 to 300 μV, while those on survey lines 2-4 are only within 0~60 μV.Figure 5
The change rule of resolvable voltage on different survey lines: (a) survey line 1; (b) survey line 2; (c) survey line 3; (d) survey line 4; (e) survey line 5.
(a)(b)(c)(d)(e)Generally, for an ideal homogeneous geological body, its conductivity is relatively uniform in the transverse direction of the stratum. However, the stable formation conductivity may be broken due to the developed geological anomalies such as the karst caves and the weak layers. Accordingly, it would cause abnormal responses for the discernible voltage in geological body. Therefore, the monitored abnormal voltage signals on all of five survey lines indicate that there may be abnormal geological body near the cast-in-place pile Y1. In addition, abnormal geological bodies may be obviously developed in the areas of survey lines 1 and 5 due to the larger ranges of discernible voltage.Figure6 shows the variations of apparent resistivity of geological body with the formation depth at different survey lines. It can be seen that, at all of survey lines, the geological bodies with low apparent resistivity are mainly distributed within 0~20 m underground. When the stratum depth is greater than 20 m, the distribution characteristics of apparent resistivity of geological body monitored at different survey lines are different. For the survey lines 2 and 4, the apparent resistivities of geological body are relatively stable (see Figures 6(b)–6(d)). For example, they are about 45~75 Ω⋅mwithin 20-80 m for the survey lines 2 and 4. For the survey lines 3, the apparent resistivity of geological body is 45~75 Ω⋅m and 100~105 Ω⋅m within 20~70 m and 70~80 m, respectively. However, for the survey lines 1 and 5, the abnormal apparent resistivities of geological body can be monitored, especially within 20-50 m underground. For example, as shown in Figure 6(a), the apparent resistivities of geological body at the formation depth of 30 m are unstable, which vary from 10 Ω⋅m to 85 Ω⋅m abnormally. Similarly, the abnormal distribution of apparent resistivities of geological body can be found on survey line 5 at the same formation depth (see Figure 6(e)).Figure 6
The apparent resistivity profile of different survey lines: (a) survey line 1; (b) survey line 2; (c) survey line 3; (d) survey line 4; (e) survey line 5.
(a)(b)(c)(d)(e)Usually, when there are the structural fracture zones in formation, the geological body would be heterogeneous, resulting in the abnormal distribution of its apparent resistivity. The above analyses for Figure6 indicate that the abnormal geological bodies near the cast-in-place pile Y1 are mainly developed within the formation depth of 20~50 m. Among them, the geological bodies with low apparent resistivities may be the karst cave and weak interlayer, such as the locations of monitoring points 13 and 20 in Figure 6(a), as well as the monitoring points 5-9 and 19 in Figure 6(e). This is because the abnormal geological bodies such as karst caves and weak intercalations contain water, which enhances their conductivity and leads to the low apparent resistivities. On the contrary, if the abnormal geological body does not contain water, its conductivity is poor and its apparent resistivity is high. Thus, the abnormal geological body may be dry crushing zone. When the formation depth is below 50 m, the apparent resistivities of geological body are stable, which indicates the bearing stratum of the cast-in-place pile Y1 is good without fracture zone or karst cave.Further, to verify the effectiveness of TEM detection results, the geological body residues obtained through borehole sampling would be analyzed. As shown in Figure7(a), the drilling Y1 is arranged at the monitoring point 13 of survey line 1. After punching later, it can be found that there are moist strongly weathered carbonaceous limestones near the formation depth of 24.6 m (see Figure 7(b)), wet moderately weathered limestones with earth near the formation depth of 36.5 m and 47.4 m (see Figures 7(c) and 7(d)), and dry limestone without argillaceous inclusion near the formation depth of 50.3 m (see Figure 7(e)). In the above weathering, dry or wet characteristics of slag samples from the drilling borehole are in agreement with the distribution law of apparent resistivity of geological body at the monitoring point 13 (see Figure 7(a)). Therefore, the monitoring results of TEM are valid.Figure 7
Slug samples at different formation depths.


## 3.1. Engineering Background

Caojiaben interchange main line bridge is located in the southwest of Huangsha Town, Yizhang County, Hunan Province, China. Figure3(a) shows the specific location of the interchange main line bridge. The terrain near the bridge is relatively flat and locally steep with natural slope angles of 20~40°. The slope in the interchange area is Quaternary brown-yellow slope residual silty clay, and its overburden is thick, which is moderately weathered limestone. The unfavorable geology of the line section is mainly the soft soil layer. Through the previous drilling at Y1 pile, the geological conditions below the Y1 pile are shown in Figure 3(b). For example, at the stratum depth of 24.2~33.5 m, the moderately weathered carbonaceous limestone with broken joints is presented, while karst caves are developed at the stratum depth of 33.5~54.5 m. Because the detection range of borehole is only limited to the area where the borehole is located, it is difficult to accurately reveal the surrounding complex geological conditions. In addition, considering the tight construction period, other geophysical exploration methods such as CT or pipe wave methods are also not applicable. Therefore, in order to ensure the safe construction of bridge pile foundation, TEM is applied to further quickly detect the integrity of bearing stratum of bored cast-in-place pile on a larger scale.Figure 3
Location and geologic column of Caojiaben interchange main line bridge.


## 3.2. Monitoring Scheme and Result Analyses

In this paper, the transient electromagnetic field is generated by transmitting bipolar trapezoidal pulse signal. The length of the transmitting and receiving coils is 1.8 m. The emission frequency of the pulse signal is 25 Hz, and the stacking times are 128. The sampling frequency is 1.25 MHz. Figure4 shows the scheme for monitoring the bearing stratum integrity of bored pile by TEM. Five survey lines were arranged around the Y1 pile. The distance between two survey lines is 1.5 m, and the monitoring range of each survey line is 20 m with a total of 21 monitoring points.Figure 4
Monitoring scheme.Figure5 shows the inverted resolvable voltage at different measuring points on different survey lines. It can be seen that there are abnormally identifiable voltage response signals on all of five measuring lines. Clearly, single peak or multipeak discernible voltage signals were monitored at some monitoring points, such as the monitoring points 13 and 20 on survey line 1; the monitoring points 4, 9-13, 17, and 20 on survey line 2; the monitoring points 7, 13, and 18 on survey line 3; and the monitoring points 11, 16, and 18 on survey line 4, as well as the monitoring points 5-9 and 19 on survey line 5. In addition, the ranges of identifiable voltages on survey lines 1 and 5 are generally higher than those on survey lines 2-4. Specifically, the voltage signal values on survey lines 1 and 5 are from 0 to 300 μV, while those on survey lines 2-4 are only within 0~60 μV.Figure 5
The change rule of resolvable voltage on different survey lines: (a) survey line 1; (b) survey line 2; (c) survey line 3; (d) survey line 4; (e) survey line 5.
(a)(b)(c)(d)(e)Generally, for an ideal homogeneous geological body, its conductivity is relatively uniform in the transverse direction of the stratum. However, the stable formation conductivity may be broken due to the developed geological anomalies such as the karst caves and the weak layers. Accordingly, it would cause abnormal responses for the discernible voltage in geological body. Therefore, the monitored abnormal voltage signals on all of five survey lines indicate that there may be abnormal geological body near the cast-in-place pile Y1. In addition, abnormal geological bodies may be obviously developed in the areas of survey lines 1 and 5 due to the larger ranges of discernible voltage.Figure6 shows the variations of apparent resistivity of geological body with the formation depth at different survey lines. It can be seen that, at all of survey lines, the geological bodies with low apparent resistivity are mainly distributed within 0~20 m underground. When the stratum depth is greater than 20 m, the distribution characteristics of apparent resistivity of geological body monitored at different survey lines are different. For the survey lines 2 and 4, the apparent resistivities of geological body are relatively stable (see Figures 6(b)–6(d)). For example, they are about 45~75 Ω⋅mwithin 20-80 m for the survey lines 2 and 4. For the survey lines 3, the apparent resistivity of geological body is 45~75 Ω⋅m and 100~105 Ω⋅m within 20~70 m and 70~80 m, respectively. However, for the survey lines 1 and 5, the abnormal apparent resistivities of geological body can be monitored, especially within 20-50 m underground. For example, as shown in Figure 6(a), the apparent resistivities of geological body at the formation depth of 30 m are unstable, which vary from 10 Ω⋅m to 85 Ω⋅m abnormally. Similarly, the abnormal distribution of apparent resistivities of geological body can be found on survey line 5 at the same formation depth (see Figure 6(e)).Figure 6
The apparent resistivity profile of different survey lines: (a) survey line 1; (b) survey line 2; (c) survey line 3; (d) survey line 4; (e) survey line 5.
(a)(b)(c)(d)(e)Usually, when there are the structural fracture zones in formation, the geological body would be heterogeneous, resulting in the abnormal distribution of its apparent resistivity. The above analyses for Figure6 indicate that the abnormal geological bodies near the cast-in-place pile Y1 are mainly developed within the formation depth of 20~50 m. Among them, the geological bodies with low apparent resistivities may be the karst cave and weak interlayer, such as the locations of monitoring points 13 and 20 in Figure 6(a), as well as the monitoring points 5-9 and 19 in Figure 6(e). This is because the abnormal geological bodies such as karst caves and weak intercalations contain water, which enhances their conductivity and leads to the low apparent resistivities. On the contrary, if the abnormal geological body does not contain water, its conductivity is poor and its apparent resistivity is high. Thus, the abnormal geological body may be dry crushing zone. When the formation depth is below 50 m, the apparent resistivities of geological body are stable, which indicates the bearing stratum of the cast-in-place pile Y1 is good without fracture zone or karst cave.Further, to verify the effectiveness of TEM detection results, the geological body residues obtained through borehole sampling would be analyzed. As shown in Figure7(a), the drilling Y1 is arranged at the monitoring point 13 of survey line 1. After punching later, it can be found that there are moist strongly weathered carbonaceous limestones near the formation depth of 24.6 m (see Figure 7(b)), wet moderately weathered limestones with earth near the formation depth of 36.5 m and 47.4 m (see Figures 7(c) and 7(d)), and dry limestone without argillaceous inclusion near the formation depth of 50.3 m (see Figure 7(e)). In the above weathering, dry or wet characteristics of slag samples from the drilling borehole are in agreement with the distribution law of apparent resistivity of geological body at the monitoring point 13 (see Figure 7(a)). Therefore, the monitoring results of TEM are valid.Figure 7
Slug samples at different formation depths.


## 4. Conclusions

In this paper, the TEM is applied to probe the geological conditions and the bearing stratum integrity of bored pile. The main conclusions are drawn as follows:(1)
The TEM can effectively probe the abnormal geological body. The electromagnetic signals such as the identifiable voltages and apparent resistivities of geological body are sensitive to the weathering degree and dry or wet characteristics of geological body. The higher the weathering degree of geological body, the more obvious the abnormal response signal for identifiable voltage, accompanied by single peak or multipeak. At the same time, if the abnormal geological body contains water, the better its conductivity is, and the lower its apparent resistivity is(2)
The geological conditions near the Caojiaben interchange main line bridge in Hunan of China are complex. The TEM monitoring results show that there may be abnormal geological bodies such as karst caves and weak interlayer near the cast-in-place pile Y1 within 20-50 m of the stratum. Below the formation depth of 50 m, the bearing stratum of cast-in-place pile is good due to the stable apparent resistivities of geological body. Therefore, in the construction of main line bridge foundation, some necessary measures should be taken for abnormal geological bodies in the ranges of 50 m formation depth(3)
As a supplement to borehole geophysical prospecting, TEM can carry out noncontact monitoring of geological bodies in a wide range. It provides a new means for monitoring the integrity of bearing stratum of cast-in-place pile in bridge safety construction



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*Source: 2583113-2022-07-11.xml*