QQ群聊

• CN 62-1112/TF
• ISSN 1005-2518
• 创刊于1988年

## Experimental Study on Effect of Water Content on Repeated Shear Strength of Remolded Red Clay

LIN Bin,, TIAN Zhuhua, CHEN Yuman

School of Civil Engineering and Architecture，Anhui University of Science and Technology，Huainan 232001，Anhui，China

 基金资助: 安徽省自然科学基金项目“高速铁路X形桩桩网复合地基动态荷载传递机制和变形特性研究”.  2008085ME143安徽理工大学研究生创新基金项目“红黏土地基力学特性及数值分析研究”.  2020cx2027

Received: 2020-12-06   Revised: 2021-05-12

Abstract

As a special soil，red clay is widely distributed in some areas of China.Due to the engineering problems such as slope cracking，subgrade subsidence and insufficient foundation bearing capacity，the strength characteristics of red clay is one of the main problems concerned by engineers，and the stability of red clay is closely related to safety production.In order to explore the influence of water content on the repeated shear strength of red clay，the repeated direct shear test was carried out on remolded red clay in Changzhi area of Shanxi Province.Because the water content of undisturbed soil in this area is between 17.0% and 23.7%，and the plastic limit water content is 20.34%，considering the law of test results and the influence of plastic limit，soil samples with five water contents of 15.0%，18.0%，19.5%，21.0% and 24.0% were prepared for test.The test results show that when no vertical pressure is applied， repeated direct shear is conducted on the sample，and the test is stopped when the sample’s strength gradually stabilizes.When there is no vertical pressure，the shear strength decreases and tends to be stable with the increase of shear times.The displacement of secondary shear to shear strength is less than that of the first shear，and the stable shear displacement is greater than that of the first shear.In the process of shearing，the shear strength has a negative linear relationship with the water content.When there is no vertical pressure，the shear strength is equal to the cohesion，and the shear strength is defined as the true cohesion.The vertical pressure of 50，100，150 and 200 kPa was applied to carry out the direct shear test.When there is vertical pressure，the shear strength decreases with the increase of water content.The internal friction angle and cohesion of shear strength index were obtained by physical expression fitting，and compared with the internal friction angle obtained by true cohesion and no vertical pressure.The internal friction angle obtained by the two kinds of analysis is basically close，and decreases with the increase of water content，and the decreasing range gradually increases.Due to the dilatancy effect in the test process，the true cohesion first increases and then decreases with the increase of water content，and the cohesion first decreases and then increases with the increase of water content.The contrast analysis shows that the difference of cohesion is obvious，so the difference index ratio of cohesion and true cohesion with vertical pressure is 0.75~9.96，and the change rule of the difference index ratio of cohesion is the same as the fitting cohesion，and the difference index ratio reaches the minimum when the water content is 19.5%.The empirical formula can provide some reference for the reasonable selection of soil parameters in the process of geotechnical investigation，design and excavation of mines in red clay area.

Keywords： remolded red clay ; water content ; repeated shear test ; shear strength ; true cohesion ; difference index ratio of cohesion

LIN Bin, TIAN Zhuhua, CHEN Yuman. Experimental Study on Effect of Water Content on Repeated Shear Strength of Remolded Red Clay[J]. Gold Science and Technology, 2021, 29(5): 680-689 doi:10.11872/j.issn.1005-2518.2021.05.210

### 1.2 反复直剪试验方法及过程

（1）本次试验采用0.8 mm/min的剪切速率，试样每产生0.2 mm剪切位移时读取位移数据，待测力计读数达到峰值或者峰值后下降至某一固定值时，停止试验。

（2）调整为反推剪切，速率为2.4 mm/min，使其回到上下剪切盒重合的位置，重复第一步的操作。

（3）反复重复第一步操作，待测力计读数达到峰值时，位移读数与前一次试验位移读数基本保持一致时，停止试验。

（4）加上50 kPa砝码进行直剪试验，待测力计读数达到峰值或者峰值后下降至某一固定值时，停止试验，第一组试验完成。

（5）分别对每组含水率下的环刀试样进行相同试验，在加载状态下，分别增加50，100，150，200 kPa的砝码，每组进行多次平行试验，取误差在允许范围之内的多次试验结果的平均值作为试验结果进行分析。

### 图1

Fig.1   Failure mode of specimen

### 2.2 无竖向压力时反复直剪次数与含水率的关系

Table 1  Statistics of repeated direct shear times of remolded red clay samples

15.054573454
18.044446445
19.545445543
21.044453544
24.043364444

### 图2

Fig.2   Relationship between shear stress and shear displacement without vertical pressure

$τf=c+σtanφ$

$τf=c$

### 图3

Fig.3   Relationship between shear stress and shear times of remolded red clay

### 图4

Fig.4   Relationship between initial shear stress，stable shear stress and water content

$τf=aω+b$

Table 2  Fitting parameter values of the relationship bet-ween shear stress and water content at different shear times

1-10.5783270.15750.9704
2-5.5840137.60000.9729
3-2.909573.66140.9972
4-2.909573.66140.9972

### 图5

Fig.5   Relationship between shear stress and shear displacement under vertical pressure

Table 3  Shear strength with vertical pressure under different water content

15.0%18.0%19.5%21.0%24.0%
5087.0075.8959.2353.6846.28
100135.12115.69114.7678.6262.93
150177.70175.85155.4898.1070.34
200218.42198.06174.92122.1774.41

### 图6

Fig.6   Fitting curves of relationship between shear strength and vertical pressure

Table 4  Fitting values of cohesion（c）and friction angle （φ）parameters

15.045.3541.140.999
18.034.7140.480.974
19.529.1537.800.958
21.031.9124.200.998
24.040.5410.400.911

$τf-c'=σtanφ$

Table 5  Friction calculation values and fitting parameter values of internal friction angle

50 kPa100 kPa150 kPa200 kPa
15.04.6382.37130.49173.07213.7948.870.9887
18.010.1865.71105.51165.67187.8845.480.9917
19.516.6642.5798.10138.82158.2640.660.9923
21.012.9640.7265.6685.14109.2129.950.9903
24.03.7042.5859.2366.6470.7123.230.9359

$∆c=c-c'$
$kc=∆c/c'$

Table 6  Calculated values of $Δc$ and kc

15.040.728.79
18.024.532.41
19.512.490.75
21.018.951.46
24.036.879.96

## 3 结论

（1）对重塑红黏土在无竖向压力时进行反复直剪试验，不同含水率下重塑红黏土经反复直剪4次基本能达到稳定剪切状态。

（2）无竖向压力时重塑红黏土的抗剪强度随着剪切次数的增加而逐渐减小并趋于稳定，第2次剪切达到抗剪强度时对应的剪切位移小于第1次剪切时的位移，稳定时低含水率剪应力变化不明显，达到抗剪强度时对应的剪切位移大于第1次剪切时的位移，土体含水率对抗剪强度的影响可用线性减小关系来反映。

（3）无竖向压力时，真黏聚力随着含水率的增加呈现出先增大后减小的变化趋势；当施加竖向压力后，黏聚力随着含水率的增加先减小后增大，内摩擦角随着含水率的增大而减小。有无竖向压力时黏聚力差异明显，建立土体黏聚力差异指数比这一概念来反映含水率对黏聚力的影响。黏聚力差异指数比在含水率为19.5%时最小，在低含水率和高含水率时差异明显。

（4）在有无竖向压力的条件下，重塑红黏土的抗剪强度和黏聚力存在差异，可以通过建立相关经验公式，为工程勘察、设计和施工可靠性提供一定的参考。

http://www.goldsci.ac.cn/article/2021/1005-2518/1005-2518-2021-29-5-680.shtml

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