The Tensile Strength of Loess in Northwest China by Unconfined Penetration Test and the Distinct Element Simulation

Xuyang Wu,Yun Bao,Mingyang Ren,Qingguo Liang,Peng Zhang

doi:10.1155/2021/1484078

Abstract

The unconfined penetration test (UP test) is one of the indirect methods to measure the tensile strength of soils. Through a series of UP tests of undisturbed and remolded loess, the split angle (α) of the wedge body which was shaped in the process of the experiment was discussed. And then, the particle movement, the force transfer, and the fracture development law of the sample were studied by the distinct element method (PFC2D). The experiment and numerical simulation results show the following: (1) the split angle (α) presents an exponential decrease with tensile strength (σt) and a linear decrease with internal friction angle (φ); (2) K that can be written as tan (2α + φ) is a coefficient to calculate tensile strength, which is equal to 2.00 for remolded loess and 1.50 for undisturbed loess; (3) the distinct element simulation shows that the resisting force by the UP test comes from both tensile stress and shear stress; (4) the tensile fractures and shear fractures appear at almost the same time when the tensile stress is approximately 70% of tensile strength.

Highlights

Loess is widely distributed in the world, accounting for 10% of the world’s land area
The tensile strength measured by the unconfined penetration test (UP test) has a very high correlation with some other methods, such as the bend beam method or the direct split tensile test [4], which confirmed that the UP test can be used to determine the tensile strength of geomaterials; in addition, that study points out that when the heightdiameter ratio of sample is close to 1, the value of coefficient K which is determined by the split angle (α) and the internal friction angle (φ) in the tensile strength formula is always set as 1
On the aspect of tensile strength, [10] collected the test results of Malan loess’s tension characteristics in China, Advances in Civil Engineering pointing out that tensile strengths have a strong correlation with water content and dry density, and the tensile strength of Malan loess in China ranges from 10 kPa to 160 kPa; [11] collected the Q2 undisturbed loess samples from Yanan city and Lanzhou city in China to test the tensile strength by UP test; the results show that the tensile strength of Q2 undisturbed loess in China ranges from 67.73 kPa to 282.55 kPa

Summary

Introduction

Loess is widely distributed in the world, accounting for 10% of the world’s land area. The factors influencing the UP test, including loading rate, the radius of loading disk, and specimen size, were studied [5] It is a special soil with structure strength [6, 7] and water sensitiveness [8], which decide the special performance in strength and deformation that may lead to the value of K which is different [9]. There are abundant research studies on the tensile strength of loess, there is uncertainty that the recommended value of K obtained by clay tests suits loess, and the relation between mechanical parameters is unclear for remolded loess and undisturbed loess. Based on the particle flow theory, the rules of fracture evolution, microparticles displacement evolution, and force evolution were studied by the discrete element method (PFC2D)

Experimental Material and Test Method

Results

Bottom load column 7 Bottom centring ring 8 Gasket 9 Bed plate

Discrete Element Simulation

Results and Analysis