Keywords

1 Introduction

Soft soil geology is the most common geological environment in China’s civil engineering construction. With the increasing improvement of China’s people’s livelihood project construction, it is especially important to solve the soft soil geology problem in the project construction. Liuxi River is located in the northwestern part of Conghua District, Guangzhou, which is the mother river of Guangzhou, and the deep soft soil is widely distributed. In recent years, Song et al. [1] quantitatively analyzed the mineral composition and binding water characteristics of marine soft soils in Shenzhen area. Gu et al. [2] determined the HSS model parameters of typical marine soft soil in Yangjiang area, Guangdong. Cai et al. [3] studied the spatial characteristics of soft soil parameters in Nansha area, Guangzhou, China, using mathematical and statistical methods and random field theory. Liu et al. [4] carried out a series of undrained variable circumferential pressure cyclic triaxial tests on saturated soft soils at the mouth of the Pearl River using the GDS dynamic triaxial test system. Liu et al. [5] studied the sub-consolidation characteristics and the change rule of sub-consolidation coefficient of soft soil with high water content in the sea phase of Zhuhai. Although the research on soft soil in China has made great progress, Song et al. [6], Kang [7], Li [8], Meng and Zhou [9], Zhou and Li [10], Ahad [11], Tawseef and Bashir [12], Kim et al. [13] so on have studied the soft soil in many places in Guangdong, but they only focus on the soft soil in the Pearl River Basin and other areas, and the research on the nature of the soft soil in the Liuxi River Basin has not been carried out. In this project, the soft soil in Liuxi River Basin was drilled and sampled, and the particle analysis, water content, consistency index, straight shear, consolidation and other tests were carried out on the soil samples to understand the special particle composition, water content and other coefficients, and on the basis of which the problems that are likely to occur in the construction were analyzed, so that it can be used as a reference for the construction on the soft soils in Liuxi River Basin.

2 Overview of the Test

2.1 Sampling Site Selection

In this test, the soft soil in different locations and depths of the experimental teaching building of Guangdong Institute of Water Conservancy and Electric Power Vocational and Technical College Conghua Campus in Liuxi River Basin of Guangzhou was selected for drilling and sampling, and the number of boreholes was 15 in total, with a total footage of 450.88 m. The total number of boreholes is 15, and the total footage is 450.88 m.

2.2 Test Program

The test can be divided into sieve analysis method and density meter method according to the particle size. Calculation formula:

$$ X = \frac{{m_{A} }}{{m_{B} }} \times 100\% . $$

Natural Index Test

  1. (1)

    Moisture content test. Adopt drying method: Select 20 g soil sample into the box, weigh the total mass, put it into the oven to dry until constant weight and then cool it down, weigh the box and dry soil mass. Calculation formula:

    $$ \omega { = }\frac{{m_{\omega } }}{{m_{s} }} \times 100\% = \frac{{m_{1} - m_{2} }}{{m_{2} - m_{0} }} \times 100\% $$
  2. (2)

    Density test. Adopt ring knife method: take soil samples with ring knife, the ratio of mass and volume of soil in the ring knife is the wet density of soil; dry the soil, the ratio of mass and volume of soil in the ring knife after drying is the dry density of soil. The ratio of mass and volume of the soil in the ring knife after drying is the dry density of the soil. Calculation formula:

    $$ \rho { = }\frac{m}{V} = \frac{{m_{1} - m_{2} }}{V},\rho_{d} = \frac{\rho }{1 + \omega } $$
  3. (3)

    Specific gravity test. The specific gravity bottle method is used: the volume of the soil particles is calculated according to the difference between the mass of a certain amount of dry soil before and after it is put into a water-filled specific gravity bottle, and the specific gravity of the soil particles is then calculated. Calculation formula:

    $$ G_{S} { = }\frac{{m_{s} }}{{m_{1} + m_{s} - m_{2} }} \times G_{\omega t} $$

Limit Water Content Test

Adopting liquid limit and plastic limit combined determination method: using liquid-plastic limit combined tester, take about 200 g of soil samples through 0.5 mm sieve, and prepare different consistency samples in three portions, read the depth of sinking of the cone and determine the water content of the samples after 5 s; the water content of the soil samples corresponding to the sinking depth of 17 mm is the liquid limit of the soil samples, and the water content of the soil samples corresponding to the sinking depth of 2 mm is the plastic limit of the soil samples.

Consolidation Test

Adopt the ring knife method: use the ring knife to cut the original moving soil sample, and take the residual soil to determine the water content. The specimen is placed in the protective ring of the compression container, and the bottom plate, moist filter paper and permeable stone are put on in turn.

Then put in the pressurized guide ring and pressure plate. Pressurized observation: Load level is 50, 100, 200, 400 kPa. Initial porosity ratio calculation formula; after pressurized porosity ratio calculation formula:

$$ e_{0} = \frac{{G_{S} \rho_{\omega } (1 + \omega_{0} )}}{{\rho_{0} }} - 1. $$

Calculation formula of initial pore space ratio: calculation formula of pore space ratio after pressurization:

$$ e_{i} = e_{0} - (1 + e_{0} )\frac{{\sum {\Delta h_{i} } }}{{h_{0} }}. $$

Direct Shear Test

A group of four specimens are cut with a ring cutter according to engineering needs and sheared under vertical pressure (50, 100, 150, 200 kPa). Calculation formula: \(\tau_{f} = CR\).

3 Test Results and Analysis

3.1 Particle Composition

The specific surface area of the soil varies with the size of the particles that make up the soil. Therefore, the degree of interaction with the outside world, the type of water content, the nature and the quantity of the particles are different for different sizes of particles. The specific surface of a soil is expressed as the total surface area of all the particles per unit volume. The large variations in specific surface values due to the different sizes of the particles inevitably lead to a large variation in the specific surface area. The sudden change in the properties of the soil is caused by the composition of minerals in natural soils with different sized particles. The different sizes of particles in natural soil are composed of different types of minerals, which will directly affect the engineering properties of the soil. The particle contents of the soil samples are shown in Table 1.

Table 1 Particle composition of test soil samples
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From Table 1, the curvature coefficient Cc of the test soil samples ranges from about 13, and the coefficient of inhomogeneity Cu ≥ 5, which is a well-graded soil. In geotechnical engineering, the well-graded soil is easy to get high dry density and good mechanical properties after compaction, which is suitable for filling projects. In addition, well-graded soils are more porous and have better permeability and can be used in drainage structures and filter back layers.

3.2 Natural Indicators (Table 2)

Table 2 Statistical values of the main natural state indexes of the test soil samples
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  1. (1)

    Water content test. Changes in water content will change the physical and mechanical properties of the soil, and this effect is manifested in various aspects, such as the consistency, saturation degree, mechanical properties and structural strength of the soil. From Table 2, after taking the average value of each group of tests, the water content is 31.48, and the water content is low.

  1. (2)

    Density test (ring knife method). Determination of dry and wet density of soil, understanding of soil sparsity and wet and dry state, used to convert the other physical properties of soil index.

  2. (3)

    Specific gravity test (specific gravity bottle method). To determine the specific gravity of the soil and to provide the necessary data for calculating other physical and mechanical properties of the soil. The specific gravity bottle method is used for soils with a grain size of less than 5 mm.

3.3 Indicators of Consistency

The classification of the soil is determined by determining the liquid limit and plastic limit of the soil, which can be determined from the water content of the soil in its natural state (Table 3). The liquid limit of the specimen is given in Table 3.

Table 3 Statistical values of main consistency indexes of test soil samples
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Plastic limit, the plasticity index \(I_{p} = \omega_{L} - \omega_{p}\) and the liquidity index \(I_{L} = \frac{{\omega - \omega_{p} }}{{\omega_{L} - \omega_{p} }}\) were calculated. In order to determine the softness and hardness of the remodeled soil, the following criteria were used to determine the softness and hardness of the remodeled soil. The soil sample IL is 0.4%, and the soil sample IL is 0.4%. The IL of this soil sample is 0.4, which is a plastic state.

3.4 Consolidation Index

The relationship between compressive deformation and loading of the specimen under lateral and axial drainage conditions is determined. The test was analyzed by the consolidation test (rapid method) (see Table 4 for test results). The compressibility of the soil directly affects the deformation values of the foundation. As shown in Table 4, this test sample soil is high Compressive soil with large foundation deformation.

Table 4 Statistical values of main consolidation indexes of test soil
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3.5 Shear Index

Through the direct shear experiment, the horizontal shear stress was applied under different vertical pressures σ to obtain the shear stress τ at the time of damage, and finally the internal friction angle φ and cohesive force C were determined.

(The test results are shown in Table 5). As can be seen from Table 5, the test sample soil is a soil with low shear strength. When this soil is used as building foundation, it has a weak potential ability to resist shear stress and shear deformation under external load, and it is easy to be in the limit state of shear damage, and the shear stress reaches the limit value.

Table 5 Statistical values of main shear indexes of test soil samples
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4 Conclusion

According to the comparison of specific parameters (Table 6), it can be seen that the soft soil in Liuxi River Basin is sandy clay, while the soft soil in other areas of Guangdong is mostly silt and silty clay. Compared with the soft soils in other areas, the soft soils in Liuxi River Basin are mostly silt and silty clay. The domain soft soil has the following characteristics:

Table 6 Comparison of specific parameters
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  1. (1)

    The water content is low, with an average of 28.92%. Due to the thin water-binding film on the surface of the soil particles, the spacing is small, and the inter-granular electric force is mainly gravitational, so the relative displacement resistance of the soil particles is large, and it is difficult to overcome the resistance under the effect of compaction energy. Therefore, the compaction effect is poor, the pore ratio and compressibility are relatively small, and the soil is dense. Compared with silt, the sandy clayey soils in the Liuxi River basin have relatively good engineering properties, are generally less prone to uneven settlement, and are economical and easy to construct.

  2. (2)

    Compared with other areas, the soft soils in the Liuxi River Basin have higher cohesion and internal friction angles, higher shear strength, and higher bearing capacity. Since most of the damages to building foundations and geotechnical structures are shear damages, the soil can resist shear stress under external loads when used for building foundations. Therefore, when this soil is used for building foundation, its potential ability to resist shear stress and shear deformation under external load is relatively strong, and it is not easy to reach the state of shear damage.

  3. (3)

    The liquidity index of the soil sample is small at 0.36, and the soil sample is hard and plastic. Compared with silt, clayey soil in this state has the ability to mold various shapes under external force and keep the original shape unchanged after the external force is removed, which is of great significance to ensure the quality of the project.

In summary, the engineering properties of sandy clayey soil in Liuxi River Basin in Guangzhou are better than those of soft soil in other areas of Guangdong.