Research Of Deformation Mechanism Of Easily Disturbed Soil Based On Self-Boring In-situ Shear Pressuremeter

The soil laboratory test and in-situ test are the important contents in geotechnical and geological engineering. With the development of the geotechnical engineering test technology, it has been realized that obtaining undisturbed or less disturbed soil sample to conduct conventional laboratory testing is very difficult, which impels related researchers to develop new in-situ testing instruments to exclude the disturbance while sampling. A new in-situ test instrument, the self-boring in-situ shear pressuremeter (SBISP), has been developed in2007based on conventional in-situ test methods both at home and abroad. The SBISP test is a combination of the Pressuremeter test (determination of the deformation modulus) and the direct shear test (determination of the shear strength). The deformation and shear strength parameters can be directly determined simultaneously compared with previous traditional test methods. Moreover, it can reduce the disturbance of surrounding soil as much as possible by self-boring; and save plenty of time because the test results can be obtained by dispensing with sampling and sample preparation. Hence, the SBISP test is particularly suitable for soils that are difficult to sample, easily disturbed and with low strength such as sandy soil and silty soil. However, the deformation and destruction of easily disturbed soil is mainly due to the translation and rotation of each soil particles and their interaction, which is a macroscopic phenomenon caused by the change of micro-structure. Therefore, it is of great necessary to study the deformation response of soil particles during the SBISP test process. Due to the multi-level loading method of SBISP test, the deformation mechanism of the soil surrounding to the probe is complex, it is difficult to study it through conventional laboratory test.The particle flow code in three dimensions theory is introduced in this study, and the particle flow code numerical simulation technique is developed. The PFC numerical model of the SBISP test is established based the SBISP laboratory verification model test. Through a series of PFC3D numerical test, the deformation mechanism of the soil surrounding to the SBISP probe during the multi-level loading process is studied systemically. The main contents of this study are as follows:1. The study of the evolution of the displacement field in the SBISP numerical sample. In the numerical test, the influence of the application of stress on the particle displacement can be characterized by two zones:the fine particles in the central region are affected remarkable, while there is almost no influence on the large particles in the outer region. The displacement of particles in central area increased with the shear stress imposed stepwise, and the direction of displacement vector showed a clearly preferred direction. The shape of displacement vector in influence area was a reverse cone under the fifth grade shear stress. Additionally, with the increase of the consolidation pressure, the displacement of the particles surrounding the probe decreases, so does the influence area. Under the stress of same grade, the influence area is small while the sample radius is large.2. The study of particle motion trails of numerical sample. In the sample in which the radius is10times of the probe, the motion trails of the particles are step-like lines in the central area which can be divided into three characteristic zones as the distance to the probe increase. The vertical and horizontal displacement of particles descends in a negative exponential form, but the vertical displacement has a faster attenuation rate.3. The study of the stress field of the numerical sand sample. With the application of the multi-level stress, the radial-and vertical-stress of sand sample increases gradually. Several radial stress cores which are approximately symmetrical distribution have formed near both sides of the probe. At the same time, the vertical stress has formed flat stress zones, and it has produced some stress concentration areas in the shoulder of the probe. Under the stress of same grade, the radial-and vertical-stress of sand sample is relatively large while the sample radius is small.4. The study of the SBISP test with spherical particles of different grain size. The influence of the grain size on the shear strength, particle displacement and particle contact property are analyzed. The following conclusion can be drawn:(1) When the grain size are in the intervals of2-3.2mm and3-4.2mm, under the radial stress of same grade, the peak value of the shear strength increases with the increase of the grain size; and the internal friction angle cp of the two samples basically has no change----the two curves of the radial-and shear-stress are in parallel with each other. However, when the grain size are in the intervals of4-5.2mm and5-6mm, The shear strength is no longer conform to the tendercy above. This indicates that the larger the grain size, the more randomness of the sand strength; the strength of the sample need to be studied exclusively. In addition, under the radial stress of same grade, with the increase of the grain size the radius of the probe increases as well.(2) The maximum displacement of the numerical sample is recorded after the application of the shear stress of each level. It shows that, in the numerical sample of same grain size, the maximum displacement increases with the shear stress imposed stepwise. Moreover, under the shear stress of same grade, the maximum displacement increases with the increase of the grain size. In addition, the displacement change in the vertical direction of the particles with different distance to the probe is monitored. When the distance is less than50mm, the vertical displacement tends to reduce more rapidly with increase of the grain size. However, when the distance is greater than50mm, the change curves tend to steady.(3) The contact property in the central region is extracted after the application of stresses of each grade under consolidation pressure of100kPa. The results indicate that, in the sand sample of the same grain size, the total number of contact tends to decrease gradually with the stress imposed stepwise. Contrast the contact data of the radial-stress and shear stress of the same grade; it can be found that the number of valid contact after the application of the radial-stress is greater than that of the shear stress, while the number of the invalid contact after the application of the radial stress is less than that of the shear stress. It suggests that the spherical particles are compacted under the radial stress, while particles are driven to move upward with the probe after the application of shear stress and separated from each other. In the sand sample of same grain size, the average contact force increases with the application of stresses of each level, and the maximum contact force shows a rising tendency.The number of the contacts deceases with the increase of the grain size under the stress of same grade, due to the decrease of the number of the particles. This reflects that the more the number of the contacts, the more homogeneous the distribution of stresses become in the sample.5. The study of the SBISP numerical test with particles of different shape. The conclusions are as follows:(1) The shear strength increases as the content of the cylinder-like particles increases. The increase range of the internal friction angle increase with the increase of the radial stress. However, the internal friction angle decreases while the radial stress increases as in the case of the same kind of sample.(2) With the shear stress imposed stepwise, in the sample of which the particles are cylinder-like, the number of particle orientation inclined to vertical direction increases gradually. It indicates that both translation and rotation are happened during the loading process of the SBISP test.(3) The shear strength of the three samples of angular particles is greater than that of the spherical particle. However, the maximum displacement of the samples with angular particles is less than that of the spherical particles; and the maximum displacement of the cylinder-like particles is greater than that of the hexahedron-like particles. It indicates that the angular particles can provide interlocking and thus reduce particle displacement.(4) Within the three numerical samples established with the three kind of angular particles, the number of contacts increases with the increase of the angularity of particles. And with the increase of the contacts, the average contact force and the maximum contact force show a decreasing tendency.6. The study of the SBISP numerical test with probes of different length-to-diameter ratio. It has been found that, with the increase of the L/D, the slope of the exponential fitting curve of the peak value of the shear-stress and the radial-stress decreases. When the L/D is greater than a certain value, the fitting curve basically has no change. It demonstrates that the shear strength index φ is overestimated when the L/D is small.7. The study of Self-boring Pressuremeter test with probes of different length-to-diameter ratio. The distribution of the displacement and radial-stress field can be divided into two forms according to the size of the L/D. When L/D equals6, the distribution shape of the displacement and stress field shows an arc-shaped lanterns appearance; the deformation of the surrounding soil along the height of the probe dose not satisfy with the plane strain and axial symmetry conditions. And the soil stress of L/D=6is smaller than the later three. However, when L/D has a value of10,15and20, the distribution shape shows a straight wall lanterns appearance; the deformation of the surrounding soil along the height of the probe basically satisfies with the plane strain and axial symmetry conditions. The stress distribution along the height of the probe is relatively uniform than that of the L/D=6, and the soil stress decreases with the increase of the L/D.There are some innovations as follows in the study:1. The multi-level loading process of the SBISP is simulated using the PFC3D program. The evolution of the displacement field, stress field and particle motion trails of the sand sample surrounding to the probe is analyzed and the deformation mechanism of it is revealed.2. The influence of the geometry property of particle on the strength, displacement field and contact property of the sand sample is discussed.3. The influence of the L/D on the shear strength, displacement field and stress field of the SBISP and SBPM test is studied.