Experimental study on dynamic parameter testing method of geological body engineering reinforcement in Three Gorges reservoir area

(Institute of Hydrogeology Engineering Geology Technology and Method, China Geological Survey, Baoding, Hebei, 07 105 1)

Acoustic wave test of grouting reinforcement test of loose geological body in Three Gorges reservoir area can obtain the main geophysical and dynamic parameters of loose rock mass, provide scientific basis for foundation treatment and rational development of resettlement area in the reservoir area, and quantitatively and comprehensively evaluate the stability of loose rock mass in Three Gorges reservoir area. This paper discusses the effect of acoustic wave testing method on grouting reinforcement of loose engineering in reservoir area by using acoustic wave testing technology and method.

Keywords acoustic wave test; Loose geological body in three Gorges reservoir area

1 preface

In the resettlement of the Three Gorges reservoir area of the Yangtze River, many new sites, such as Fengjie and Wushan, have encountered the problems of land resources development and utilization, which are composed of Quaternary loose accumulation layers and have complicated causes such as landslides, collapses and karst. These areas are basically the main part of the new site of the county. Because of its complex causes and special engineering geological conditions, it has not been fully utilized in the county relocation planning, which has seriously hindered the construction and development of the city. Although the geological origin of Quaternary loose accumulation body is complex and special, as a building foundation, its engineering geological conditions are not very bad. As long as it can be fully demonstrated, supplemented by necessary geological reconstruction projects, it can increase the land resources of the relocated towns and produce huge economic and social benefits. In recent years, the study of this complex Quaternary deposit has become a hot spot in engineering geology. This paper introduces the acoustic wave testing technology and its experimental study in grouting reinforcement of engineering geological bodies in the Three Gorges reservoir area. Combined with some previous experiments and research work on the relationship between elastic parameters and mechanical properties of rock and soil in the reservoir area, the mechanical indexes of engineering geological bodies are given through acoustic wave test results, which can reflect the dynamic properties of the test site to some extent and can quantitatively and comprehensively evaluate the reinforcement effect.

2 Geological conditions and geophysical characteristics of the test site

2. 1 Geological conditions of the test site

The experimental sites are selected in places where resettlement is urgently needed and geological conditions are typical, namely Zhaojialiangzi area in Baotaping planning community in Fengjie and Sidagou area in Wushan. Due to different locations, the geological conditions of the test site are quite different, which reflects the difference of loose accumulation structure. The lithologic characteristics of each test site are briefly described as follows:

The upper part of the first group in Fengjie, about 3m, is Quaternary eluvial clay, containing fragments and dense. The lower part is dark gray thin-medium thick marl, with developed cracks and broken rock strata, and the core is short columnar, pie-shaped and broken.

The upper part of the second group in Fengjie is silty clay with gravel and breccia, which is slightly dense and has weak water permeability, and the lower part is filled with gravel and clay. After excavation, it is confirmed that the loam on the gravel slope over 2m is dense; Below 2m, it is yellowish brown-gray marl. Rock fissures are developed and strongly weathered. Fractures above 6m are tightly filled with argillaceous materials, while those below 6m are less filled.

The upper part of Wushan Formation 13m is green-gray marl, moderately weathered, with developed vertical fractures, mostly filled with argillaceous rocks. The core is broken, and the 3 ~ 12m section is easy to collapse when drilling, and generally does not leak. Below 13m is calcareous silty mudstone, dark purplish red, with developed fractures, and the core is still relatively broken.

According to the design requirements, each test group consists of 7 boreholes, with/kloc-0 boreholes in the middle and 6 boreholes in the periphery, which are distributed in plum blossom shape, among which 3 boreholes are grouting test boreholes and 4 boreholes are experimental observation wells. The hole depth of Fengjie test point is 20m, and that of Wushan test point is 18m. The slurry ratio and grouting quantity of each hole are different.

2.2 Geophysical characteristics of the test site

According to the measured data of Huangtupo landslide in Badong and Guantangkou landslide in Wanzhou in the past, the sound wave velocity of the intact rock mass in the test site is generally above 3000 m/s. Due to the poor geological conditions in most parts of the reservoir area, the strata above the bedrock are broken, the fractures are developed and the integrity is poor. The velocity of sound wave varies greatly, mostly between 700 and 2600 m/s. When sound wave propagates in rock mass, the change of its parameters directly reflects the geological structure and physical and mechanical properties of rock mass.

Under the condition of rapid instantaneous loading, it is called dynamic method to measure the elastic mechanical parameters of rock mass (stone) with sound waves. The measured parameters are called dynamic elastic parameters, such as dynamic elastic modulus ed, dynamic Poisson's ratio μd and dynamic shear modulus Gd. As long as the longitudinal wave velocity, shear wave velocity and density of rock mass are measured, the dynamic elastic parameters of rock mass (rock) can be calculated according to the following engineering formula.

Dynamic elastic modulus calculation formula:

Essays on Geological Disaster Investigation and Monitoring Techniques and Methods

Dynamic shear modulus calculation formula:

Essays on Geological Disaster Investigation and Monitoring Techniques and Methods

Dynamic Poisson's ratio calculation formula:

Essays on Geological Disaster Investigation and Monitoring Techniques and Methods

Where: VP-longitudinal wave velocity (km/s);

Vs—— shear wave velocity (km/s);

ρ-rock density (g/cm);

Ed-dynamic elastic modulus;

GD- dynamic shear modulus;

μd—— dynamic Poisson's ratio.

Therefore, the parameters such as P-wave velocity, S-wave velocity, amplitude and frequency can be used as quantitative basis for evaluating engineering rock mass and checking the grouting reinforcement effect of engineering geological body. Acoustic wave test is mainly to evaluate grouting quality, and grouting quality is mainly evaluated according to acoustic wave speed. According to the wave velocity data and geological data obtained from acoustic wave test, the grouting effect can be accurately and quantitatively evaluated, which provides scientific basis for the stability evaluation of the test site.

3 test methods and techniques

In order to understand the grouting reinforcement effect of Quaternary loose accumulation body, it is required that the method is fast and economical, and acoustic wave testing technology is the first choice to meet the above conditions. After repeated comparative study, core test, single-hole acoustic wave test and cross-hole acoustic wave test are the main detection methods for grouting reinforcement test of loose accumulation.

Sound waves propagating in solids are mechanical waves. The deformation caused by the magnitude of its acting force is in a linear range, which conforms to Hooke's law and can also be called elastic wave. Acoustic wave testing, shallow earthquake and surface wave exploration all belong to elastic wave testing technology. The fluctuation frequency used in acoustic wave detection ranges from tens of Hz to 50kHz (in-situ detection) and 50kHz to 500kHz (detection of rock and concrete samples), covering audio frequency to ultrasonic frequency, and it is still called "acoustic wave detection" in the field of detection acoustics. Because the frequency of the signal used is higher than seismic wave and surface wave, it has high resolution and is suitable for detailed study of geological targets such as rock mass. Measuring dynamic parameters has the advantages of light equipment, simple test, economy and rapidity, and many large-scale projects should consider the dynamic characteristics of rock and soil, so measuring dynamic elastic parameters of rock mass has practical significance.

3. 1 core sample test

Firstly, the core of the selected columnar rock is cut and ground to prepare for the test, and then the P-wave velocity is tested by coupling P-wave transducer, vaseline and core. Shear wave velocity is measured by coupling shear wave transducer, tin platinum paper and core.

The instrument used is CYC-4 ultrasonic rock tester, and the frequencies of BPFT and WT longitudinal wave probes are100 kHz, 25kHz and 25 kHz respectively. The frequency of HT shear wave probe is 460kHz. Table 1 lists the measured data of sound velocity and related dynamic parameters of rock samples drilled and cored before grouting.

Table 1 Core Test Results Table

3.2 Single hole acoustic wave test

Single-hole acoustic wave test adopts long source distance, double receiving probes, and the transmitting and receiving distance is 50cm and 30cm. Acoustic information is transmitted and received along the borehole wall in a borehole (a bare hole with well fluid). During logging, the probe is lowered to the bottom of the well and tested upward according to the logging point distance (0.5m point distance is selected for this test). The notebook computer completes the collection and storage, picks up the P wave through playback and data processing indoors, and determines the first arrival and departure time of the P wave according to the waveform interference point, amplitude and spectrum analysis in the collected waveform, and calculates the P wave speed.

The instrument used in the test is a super jet -4D full-wave acoustic logging tool, and the downhole probes are: source spacing of 0.5m, spacing of 0.3m and diameter of 78mm；; The cable length is 300 meters. Table 2 lists the measured data of single hole wave velocity in different periods of grouting reinforcement test in this test site.

Table 2 Wave Velocity Table of Fengjie and Wushan Single Hole

3.3 Cross-hole acoustic wave test

Cross-hole acoustic wave testing method adopts synchronous lifting method: it is excited in one borehole (bare hole), received in another borehole (bare hole), rises synchronously from the bottom of the hole to the upper part, and tests upwards according to the test requirements. In a borehole, the signal is sent by electric spark (or shear hammer), and the sound wave information is received by the transducer. The instrument completes acquisition and storage, and picks up waveforms through playback and data processing indoors.

SWS- 1 multi-function instrument (developed by Beijing Institute of Hydroelectric Geophysical Exploration) is used as the instrument, and the test excitation source generally adopts two excitation methods: electric spark (produced by Xiangtan Wireless Power Plant) or shear hammer. Wall-mounted three-component detector receiving. Table 3 lists the measured data of cross-hole wave velocity in different periods of grouting reinforcement test in this test site.

Table 3 Wave Velocity Table of Fengjie and Wushan Cross-hole

4 Analysis of mechanical parameters and methods of test site

4. The mechanical parameters of1are obviously improved.

Through the detection of grouting effect by acoustic logging method, the mechanical parameters of engineering geological body are obviously improved after grouting.

(1) acoustic parameters

① Before grouting:

A loose rock and soil body containing clay (Wushan), P-wave velocity1320m/s ~1480m/s.

B broken bedrock and broken rock mass (Fengjie), with longitudinal wave velocity of 810m/s ~1100m/s.

② After grouting:

A. For loose rock and soil mass containing clay (Wushan), the average single-hole wave velocity is increased by 1 1%, and the average cross-hole wave velocity is increased by 25%.

B Broken bedrock fractured rock mass (Fengjie), the average single-hole wave velocity increased by 14.6%, and the average cross-hole wave velocity increased by 65%.

(2) Mechanical parameters of site

① Before grouting:

A. loose rock and soil mass containing clay (Wushan), foundation bearing capacity [R]=557(kPa), cohesion [c]= 15 1(kPa), compressibility [Es]=8.9(MPa), and friction angle [φ] = 36 (degrees).

B loose rock mass with broken bedrock (Fengjie), with foundation bearing capacity [R]=388-438(kPa), cohesion [c] = 92- 1 10 (kpa), compression [es] = 6.9-7.3 (MPa) and friction angle.

② After grouting:

A. loose rock and soil mass containing clay (Wushan), foundation bearing capacity [R]=636(kPa), cohesion [c]= 18 1(kPa), compressibility [Es]= 10.3(MPa), friction angle [φ

B loose rock mass with broken bedrock (Fengjie), foundation bearing capacity [r] = 504 ~ 568 (kpa), cohesion [c] = 134 ~ 157 (kpa), compressibility [ES] = 8.1~ 8.9.

4.2 Analysis of test methods

As can be seen from the above, the P-wave velocities of core samples, single hole and cross-hole have obvious changes, because the test results of core samples, single hole acoustic waves and cross-hole acoustic waves are comparable, and the relationship between wave velocity changes and rocks in various methods is corresponding and the trend is the same. Just because of the different test methods, the results also show different characteristics.

The testing of core samples is usually carried out on the specified size. Relatively speaking, it can be regarded as a test of a certain point of rock mass, and the test frequency range is UHF; The interval of single-hole acoustic wave test is 30cm, which only measures the acoustic characteristics of rock mass in a limited range near a wavelength of the borehole cylinder, which can be regarded as the test of one-dimensional rod rock mass with a high frequency range. Cross-hole method is carried out in the range of small hole spacing. Compared with the above two methods, the measuring range is much larger. In a wider range, the propagation of elastic waves is not only restricted by rock mass, but also controlled by rock mass structural plane. It can also be regarded as an experiment on two-dimensional plane rock mass, and the frequency range is relatively low. Due to the above differences, the relationship of wave velocity parameters is that the sound velocity measured by core specimen is greater than that of single hole and that of single hole is greater than that of cross hole (V-shaped core > V-shaped single hole > V-shaped cross hole). The above is in line with objective laws. Core test reflects the acoustic characteristics of rock mass points, single hole reflects the longitudinal acoustic characteristics of local rock mass, and cross-hole represents the lateral change of rock mass.

5 Conclusion and discussion

Acoustic wave testing technology has been used in grouting reinforcement test of loose accumulation body in Three Gorges reservoir area, and good results have been achieved. The test results of grouting reinforcement in Fengjie and Wushan show that the above methods are feasible and effective. Acoustic detection is not only fast, simple and accurate, but also a nondestructive detection method, which can evaluate the grouting quality as a whole and in all directions.

It should be pointed out that the dynamic method is tested under instantaneous load, and the stress exerted on the rock mass is small, so there are some differences between the dynamic elastic parameters and the static elastic parameters. In order to meet the requirement that the dynamic elastic parameters still need to be converted into static elastic parameters with similar load conditions, it is necessary to further study the relationship between them. However, this problem is more complicated, and the corresponding relationship generally varies with different lithology and regions. In practical work, it is often necessary to compare a certain number of dynamic and static elastic parameters to find out the corresponding laws.

refer to

Guo et al. Handbook of geophysical techniques for geological hazard exploration. Beijing: Geological Publishing House, 2003.

Lin Zongyuan. Geotechnical engineering test manual. Shenyang: Liaoning Science and Technology Press, 1994.

Course of engineering and environmental geophysical exploration. Beijing: Geological Publishing House, 1999.