2.1 Methodology
The soil used for this experiment was taken from a loess accumulation dam in Ansai Nangou, Yan 'an city, Shaanxi province, which is ML in the USCS classification of Loess. Bettersize2000 laser particle size distribution instrument was used for particle analysis of dam body soil. The particle grading curve of the sample was shown in Fig. 1. Part of the soil was dried to measure the natural moisture content, and the natural weight was calculated by retrieving the ring cutter sample. The basic physical properties of dam body soil are shown in Table 1.
Table 1 Physical properties of soil specimens
water content/%
|
dry density/(g•cm-3)
|
specific gravity of soil
|
Percentage of soil particles at different particle sizes
|
<0.002mm
|
0.002-0.075mm
|
0.075-0.25mm
|
0.25-0.5mm
|
0.5-2mm
|
14.97%
|
1.557
|
2.71
|
5.72
|
83.01
|
11.27
|
0
|
0
|
The triaxial test used GDS unsaturated soil triaxial apparatus. The range of confining pressure controller of the system is 0-1Mpa, and the axial pressure controller can provide the maximum axial pressure of 5kN. The sensor and data conversion device of the triaxial test system can automatically collect and output the data of various parameters to be tested. The sample required for this instrument is 39.1mm in diameter and 80mm in height. It is prepared by the layered compaction method. Equal mass soil is added four times. In this test, four moisture contents, namely 10%, 15%, 20% and 25%, were taken into account, and the dry densities were 1.4g/cm3, 1.5g/cm3, 1.6g/cm3and 1.7g/cm3, respectively. The samples were subjected to confining pressures of 50kPa, 100kPa, 150kPa and 200kPa respectively.
The remolded soil samples retrieved from the site were air-dried under natural conditions. After air-dried, the air-dried moisture content was measured. According to the formula, the water quality of the soil samples with 10% and 15% moisture content was calculated. The samples with 20% and 25% moisture content were prepared by the dripping method, that is, the soil material with 15% moisture content was pressed into the surface of the soil sample with a rubber head dropper after forming, and then sealed and marked. The samples were placed in a moisturizing tank for 48h to make water evenly distributed in the sample.
After the sample preparation is completed, the sample is installed in the GDS static triaxial apparatus, the shear rate and confining pressure are set, and the test is stopped when the shear strain variable reaches 20%.
2.2 Results and analyzation
The failure types of soil in the triaxial test are mainly divided into three types: brittle failure, plastic failure and approximate ideal plastic failure. Brittle failure and plastic failure can be divided into three types according to their stress-strain curves: strong strain softening, weak strain softening, strong strain hardening and weak strain hardening. As can be seen from Fig. 2, when the dry density is 1.7g/cm3, the relative displacement of soil samples on the shear plane gradually decreases with the increase of confining pressure, and the penetration degree of the fracture surface also decreases. When the confining pressure is 50kPa and 100kPa, the shear plane is more obvious, and the confining pressure increases to 150kPa and 200kPa, and the shear interface is more blurred when the sample is destroyed. When confining pressure remains unchanged and dry density increases, the failure type of samples changes from plastic failure to brittle failure (Fig. 3). The soil sample with lower dry density has more pores inside, and the soil particles are overlapped with each other. The contact between the particles is mainly point-point, point-line, and the strength is low. When subjected to a small external force, the structure is destroyed and the soil particles fall into the pores. Therefore, most of the deformation during the shearing process is caused by the constant compression of the pore volume, and shearing contraction occurs during the shearing process. The increase of dry density of soil samples internal granular arrangement more closely, sample internal contact form is given priority to with mosaic structure between particles, forms are mainly: line-line and line-face, and the particles are closely attached. Therefore, the pores between the particles are less, the pore compression amount is small, and the mutual displacement between the soil particles during the shearing process causes the volume of the soil sample to increase after shearing, that is, the shear dilatancy phenomenon occurs. As shown in Fig. 4, with the increase of water content, the shear outlet at the lower end of the shear plane moves up gradually, the included angle (acute angle) between the shear plane and the horizontal plane decreases gradually, and the length of the shear plane decreases, and the sample changes from brittle failure to plastic failure.
Figure 5 shows the stress-strain curves of remolded loess under different confining pressure and dry density conditions. As the confining pressure increases, the stress-strain curve moves up and the peak strength of the sample increases. The strain hardening degree of all the samples after increasing the confining pressure is more significant than the upper confining pressure and is a strong strain hardening type under high confining pressure.
When the sample has a water content of 10%, the dry density of 1.4g/cm3and 1.5g/cm3are strain hardened under different confining pressure conditions; when the sample with a dry density of 1.6g/cm3 was shear under confining pressure of 50kPa, the deviator stress remained almost unchanged with the increase of axial strain, showing an approximate ideal plastic failure characteristic.
Samples with a dry density of 1.7g/cm3 showed different degrees of strain softening under different confining pressures. During the formation of loess, the combination and arrangement of spatial stacking of scattered particles directly affect the strength and deformation characteristics of the soil. Under the condition of low confining pressure, the confining pressure will compress the pores between the soil particles but does not change the contact mode between the skeleton elements, and the microstructure does not change significantly, so the shear strength is low. When the confining pressure gradually increases, the pores between the soil particles are compressed, and the pores of the supports formed by lapping between the coarse particles are destroyed by external forces, forming a more stable skeleton structure, so the threshold of resistance to external forces is higher, the shear strength index also rises. In the triaxial shearing process, the change of the contact mode between the particles, the change of the mutual position of the particles and the secondary structure formation process of re-engagement between the particles, the process from unstable to stable to unstable, and finally the destruction.
It can be seen from Fig. 6(a) that the peak strength increases significantly with the increase of dry density. At low confining pressure, samples with a dry density of 1.7g/cm3 showed strong strain softening, while other dry densities all showed strain hardening under high and low confining pressure conditions, and when confining pressure remained unchanged, the strain hardening degree gradually weakened with the increase of dry density (Fig. 6(b)). The arrangement of the skeleton element parts is tighter as the dry density is increased, the inter-particle bite force and the joint force are increased, and the number of overhead pores is reduced compared with the low density. The shape variable caused by the pores during the shearing process is small, and the deformation mainly comes from the damage that occurs after the particles are bitten to each other, so the strength is increased.
It can be seen from Fig 7 that the stress-strain curve changes significantly with the moisture content, and the peak stress of the sample decreases with the increase of the moisture content. When dry density was 1.4g/cm3 and 1.5g/cm3, the change of moisture content did not significantly change the failure type of the sample. When the dry density of the sample is increased to 1.6g/cm3and 1.7g/cm3, the sample changes from brittle failure to plastic failure as the water content increases, and the strain-softening type changes to the strain hardening type. When the moisture content increases, water fills the intergranular pores in the loess framework, dissolves intergranular suffusion, thickens the water film on the surface of soil particles, decreases the intergranular bonding force and the occlusal friction force, and decreases the strength.
Fig. 8-11 shows the change rule of shear strength index and moisture content and dry density. The relationship curve between the shear strength index and moisture content and dry density is drawn by using cohesion and internal friction angle as the vertical coordinate, dry density and moisture content as the horizontal coordinate respectively. The Soviet scholars divided the cohesion into two parts: the original cohesion and the solidified cohesion (Li 2013). The original cohesive force comes from electrostatic force and van der Waals force between particles. The closer the distance between particles is, the more contact points of soil particles per unit area, the greater the original cohesive force will be. When the particles are separated from each other by a certain distance, the original cohesion is completely lost. Solidification cohesion depends on the suffusion substances present between particles, such as free chlorides, iron salts, carbonates, and organic matter in the soil. When water is filled in the intergranular pores of the soil, the suffusion substances between the soil particles are dissolved, and the strength of the intergranular connection is partially lost. When the moisture content is increasing, the salt substances between the particles are continuously dissolved, until the solidification cohesion is completely lost. Therefore, when the moisture content increases to 25%, the cohesion decreases significantly compared with 10%.
The strength of the internal friction angle mainly comes from two parts: sliding friction and occlusal friction, and the former is mainly due to the rough contact surface of minerals. The occlusal friction comes from the mutual restraint between soil particles. When the moisture content increases, the water film is formed between soil particles, the particles between the movement to promote the role, when the dry density is small, within the more pore, under the effect of external force, the particles falling into a big pore after the relative sliding, although the pore filling effect, but due to the internal porous sample, cause the strength of the growth is very limited, so the overall is still showed a trend of decline. When the dry density increases, under the condition of low moisture content, although water is favorable for sliding between particles, the internal particles are closely arranged with fewer pores. After the relative displacement between particles, it falls into the pores and occludes closely with other soil particles again, so the angle of internal friction rises. When the moisture content increases to 20%, the water film between particles thickens and the lubrication effect is obvious, so the angle of internal friction decreases. When the water content is 25%, the water film dissolves a large number of suffusion substances, and the properties change, the lubrication effect decreases, and the internal friction angle increases.