Study on the Application of Flat Dilatometer Test in Subway Survey Engineering

Flat dilatometer test (DMT) is a new kind of technology in situ test. This study, through sorting out and analyzing the flat dilatometer test data in subway engineering, obtains the experience of the application of the flat dilatometer test in engineering, and solves practical problems such as soil classification. The results of this study provide an effective basis for the application of flat dilatometer test technology in subway survey and provide sufficient and reliable data for the compilation of regional specifications.


Introduction
In recent years, flat dilatometer test (DMT) has been paid much attention in geotechnical engineering. In Western Europe and North America, flat dilatometer test gradually replaced the standard penetration test, and became the two main means of in-situ test together with static contact test, and the American society of civil engineering recommended the use of this in-situ test method [1]. The flat dilatometer test is a new in-situ measurement technique, that is simple, fast, economical, repeatable, and low disturbance [2]. At present, this technology has been included in the technical standards of China's ministry of railways [3] and geotechnical engineering investigation specifications [4]. Because the geotechnical engineering characteristics have a strong regional, so it is necessary to study the engineering application of flat dilatometer test, put forward the regional experience correction, and provide an effective basis for the flat dilatometer test technology in engineering application.
With the construction of large area of shenyang subway project, the technique of flat lift test is applied in the survey, which provides a lot of test data for the research of this paper. The results show that the flatbed expansion test is an effective in situ test technique and has high accuracy.

Principle of flat dilatometer test
Flat dilatometer test is to use static force (or hammer force) to insert the flat shovel probe into a predetermined depth of the soil. The diaphragm on the side of the flat shovel probe expands laterally through the pressure system, causing lateral displacement and deformation of the soil. According to the parameters obtained in the flat dilatometer test, it can be calculated as follows:material index ID, Lateral expansion modulus ED, horizontal stress index KD, side bulge pore pressure index UD [5].
At present, the DMT-W1 flat dilatometer test produced by nanguang geological instrument factory is widely used in China. The instruments and equipment for the flat dilatometer test are shown in  Figure 1. 1-flat shovel probe; 2-sheath maerial; 3-gas-electric pipeline; 4-measurement and control box; 5-air pressure pipeline; 6-pressure source; 7-circular stainless steel diaphragm.

Conditions of the test site
In this experiment, the flat dilatometer test was carried out on the subway project. The strata along the line were relatively simple. The lower strata are tertiary strata with good engineering geological conditions. The Flat dilatometer test (DMT) was carried out in the upper clay, silt, and fine sand. The author mainly analyzed the collected silty clay and clay data, and at the same time conducted in situ test, geotechnical test and other test methods [6] at the test site, so as to make a comparative analysis of parameters.

Basic parameters of the flat dilatometer test
The readings A, B and C can be converted to 0 In the formula: 0 p -initial pressure; 1 p -expansion side pressure at 1.1mm displacement; 2 p -the termination pressure when the diaphragm returns to 0.05mm; m Z -initial reading of pressure gauge before zero setting. According to 0 p , 1 p and 2 p , the following four parameters can be calculated: horizontal stress index: Lateral expansion modulus: In the formula, 0 U -the static pore water pressure at the test depth;

3.3.Collation and analysis of test data
According to

3.4.Application of horizontal coefficient of subgrade reaction
Horizontal coefficient of subgrade reaction is an important parameter of displacement and internal force of foundation pit engineering and tunnel support design under horizontal load. Flat dilatometer test can reflect the stress state of the soil more truly, and has a unique advantage in solving the parameters related to the lateral basal bed reaction coefficient. At present, it is mainly applied in solving the reaction coefficient of lateral basal bed by flat dilatometer test in shenyang area. The test formula is as follows [8]: In the formula: Professor Tang shidong of Tongji university [9] proposed that the calculation of lateral bed reaction coefficient should take into account the properties of the soil, the width of the foundation, the loading rate and other factors, and proposed the following solution: Combined with the flat dilatometer test data of shenyang metro line 1 project, the calculation was carried out according to formula by using the Tang shidong recommended method, and the results were compared with the empirical formula for the same soil layer, as shown in table 1. recommendation method is relatively small, but this method is obtained under the condition that the square rigid foundation with a width of 1m is assumed. The calculation formula takes into account the correction of various factors, and the result is more reasonable.

Application of lateral expansion modulus
The lateral expansion modulus ED of soil can be directly determined by flat dilatometer test. While the vertical compression modulus is used to calculate the settlement of foundation soil. At present, it is very difficult to obtain the accurate ES value of the compression modulus in actual engineering. Considering that the vertical compression modulus MDMT has a good relationship with the lateral expansion modulus ED, the relationship can be established and the compression modulus can be solved by using the results of flat dilatometer test.
Marchetti proposed to calculate the vertical compression modulus by using the empirical formula of flat dilatometer test [10]. According to the data of Subway engineering test (DMT), the vertical compression modulus of soil of each soil layer was selected for comparative analysis with the average value of compression modulus of the earth test. It was found that the average value of flat dilatometer test was 14.2% larger than that of the earth test. This is because the soil samples in the laboratory geotechnical test are susceptible to disturbance, resulting in a relatively small measurement value. Since the flat dilatometer test can more truly reflect the stress state of the soil, it is suggested to apply the lateral swelling modulus ED of the flat dilatometer test to solve the vertical compression modulus MDMT of the soil.

Application of material index
Through mathematical statistical analysis of the 613 group ID values collected in the flat dilatometer test, we obtained the following distribution pattern, as shown in table 2. It can be seen that the ID value of material index in shenyang is basically within the classification range of soil layer proposed by  16.6 18.6 1.12 As can be seen from the table, the heavy component value obtained by the flat dilatometer test in shenyang area is 11.1% smaller than the average result of the geotechnical test, so it needs to be multiplied by 1.11 to make regional empirical correction.

Conclusion
This paper sorted out and analyzed the data of the flat dilatometer test of subway engineering survey, and studied the engineering application of parameters, and reached the following conclusions: The material index ID and Lateral expansion modulus ED curve are similar in shape, and the soil layer can be divided according to the shape of the curve. The material index ID in shenyang area is basically within the classification range of soil layer proposed by Marchettti, which can be used as one of the indicators to classify soil. The heavy index obtained by the flat dilatometer test needs to be multiplied by 1.11 for regional empirical correction.
Horizontal coefficient of subgrade reaction is an important soil parameter. The horizontal coefficient of subgrade reaction Kh of soil layer can be obtained accurately by using the results of the flat dilatometer test. The applicability of Tang shidong's recommendation method in shenyang area is verified by combining with the data of the flat dilatometer test.
The vertical compression modulus (MDMT) obtained in the flat dilatometer test was slightly larger than that in the geotechnical test (ES). Because the flatbed lateral expansion test can reflect the stress state of soil more truly, it can be used to solve the flat dilatometer test.