软黏土取样扰动机理、评估方法及控制措施研究进展

才昊; 叶冠林; 兰立信; 张琪; 朱文轩

doi:10.11779/CJGE20231161

软黏土取样扰动机理、评估方法及控制措施研究进展 English Version

上海交通大学船舶海洋与建筑工程学院, 上海 200240

基金项目:

国家自然科学基金项目 42072317

上海市社会发展科技攻关项目 21DZ1204300

详细信息

作者简介:
才昊(1996—)，男，博士，主要从事取土扰动影响与土体性质关系方面的研究工作。E-mail：609559109@qq.com

通讯作者:
叶冠林, E-mail: ygl@sjtu.edu.cn

中图分类号: TU43
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出版历程
- 收稿日期: 2023-11-26
- 网络出版日期: 2024-07-23
- 刊出日期: 2025-01-31

Research progress on disturbance mechanisms, evaluation methods and control measures for sampling of soft clay

School of Naval Architecture, Ocean and Civil Engineering, Shanghai JiaoTong University, Shanghai 200240, China

摘要

摘要: 软黏土由于具有含水率高、压缩性强、灵敏度高等特点使其在取样过程中易受到扰动，导致土体的强度及变形特性显著改变。了解取样扰动的力学机理、减少取样扰动影响并对土样质量进行合理评估对于确定工程设计中有代表性的土体参数具有重要意义。为此，总结了国内外软黏土取样扰动相关的研究进展，具体包括软黏土取样扰动因素及力学机理、取样扰动影响的宏微观表现、扰动程度的评估方法以及减少扰动的措施。研究表明：目前针对取样扰动宏观力学机理的研究成果颇丰，但缺乏相应的微观试验证据及微观机理分析对宏观结果进行佐证；现存土样质量评估指标大都只针对纯黏土，对粉质黏土、粉土等低塑性中间土体的适用性存疑，目前仍没有针对不同性质土体的统一土样质量评价体系；室内再固结方法可以有效减少取土过程中的应力释放影响，但无法恢复附加扰动破坏的土体结构。基于此，对今后亟待开展的研究提出了4点建议与展望。
- 软黏土 /
- 取样扰动 /
- 土样质量评估 /
- 再固结方法
Abstract: The soft clay, characterized by high moisture content, strong compressibility and high sensitivity, is prone to disturbance during the sampling process, resulting in significant changes in the strength and deformation characteristics of soils. Understanding the mechanical mechanism behind sampling disturbance, reducing its impact and accurately evaluating the sample quality are crucial for determining the representative soil parameters in engineering design. To address these issues, the current researches on sampling disturbance in soft clay are summarized, including the mechanical mechanisms of sampling disturbance, the macroscopic and microscopic effects, the methods for the sample quality evaluation and the strategies to reduce disturbance. The researches show that there are abundant results on the macroscopic mechanical mechanisms of sampling disturbance. However, there is a lack of corresponding microscopic experimental evidence and analysis to support these macroscopic results. Furthermore, the most existing quality evaluation indices are designed for the clay and may not applicable to the low-plasticity intermediate soils such as silty clay and silt. Currently, there is still no unified quality evaluation system for the soil samples with varying properties. The reconsolidation methods can effectively reduce the stress release during the soil sampling process, but they cannot restore the soil structure damaged by sampling disturbance. Based on this, four suggestions and prospects for future researches are proposed.
- soft clay /
- sampling disturbance /
- sample quality evaluation /
- reconsolidation method

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0. 引言

各向异性是黏土的基本性质之一,分为原生各向异性和次生各向异性。针对原生各向异性对黏土力学性状的影响,许多学者对与沉积平面呈不同夹角试样进行压缩、无侧限压缩和三轴压缩等试验,发现原生各向异性对黏土变形以及强度特性的影响不容忽视。

小应变剪切模量特性作为土的重要力学性质之一,也同样受到原生各向异性的影响。Simpson等^[1]的研究表明,小应变剪切模量的原生各向异性对隧道及基坑周围土体变形的预测结果影响很大；Jovičić等^[2]和吴宏伟等^[3]分别针对伦敦黏土和上海软黏土进行研究,利用弯曲元测得两种土在低围压下水平和竖直方向上的最大剪切模量比值分别为1.5和1.21,说明对于不同种类黏土,原生各向异性对其小应变剪切模量的影响不尽相同。

结构性黏土在我国东南沿海地区分布广泛,许多工程建设涉及到此类黏土,迄今已对其小应变剪切模量进行了诸多研究,但以往的研究主要考虑孔隙比、应力水平和结构损伤等对小应变剪切模量的影响^[4],而考虑原生各向异性对小应变剪切模量影响的研究较少,有必要进行系统探究。

本文对不同削样方向的湛江黏土原状试样开展不同围压下的共振柱试验,研究原生各向异性对最大动剪切模量的影响以及考虑原生各向异性的最大动剪切模量随围压演化规律的表征方法。

1. 试验材料与试验方案

1.1 试验材料与试样制备

土样取自湛江市某基坑内地下10～11 m,尺寸为30 cm×30 cm×30 cm原状块状样。表1为其基本物理力学指标与颗粒组成。由表1可见,湛江黏土具有较差物理性质,与软黏土相似,但力学性质较优,呈现上述特性的原因为其具有的强结构性^[4]。

表 1 湛江黏土平均物理力学性质指标与颗粒组成

Table 1. Physical and mechanical indexes and particle composition of Zhanjiang clay

重度γ/(kN·m^-3)	含水率w/%	孔隙比e	渗透系数K/(cm·s^-1)	液限w_L/%	塑限w_P/%	塑性指数I_P	结构屈服应力σ_k/kPa	无侧限抗压强度/kPa	灵敏度S_t	颗粒组成/%
重度γ/(kN·m^-3)	含水率w/%	孔隙比e	渗透系数K/(cm·s^-1)	液限w_L/%	塑限w_P/%	塑性指数I_P	结构屈服应力σ_k/kPa	无侧限抗压强度/kPa	灵敏度S_t	>0.05/mm	0.005~0.05/mm	0.002~0.005/mm	<0.002/mm
17.1	52.98	1.44	2.73×10⁻⁸	59.6	28.1	31.5	400	143.5	7.2	8.2	39.5	20.7	31.6

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| 显示表格

图1（a）为不同方向圆柱试样示意图,定义试样轴线与土体沉积平面夹角为 $α$ ,即竖直方向试样为90°,水平方向试样为0°。针对 $α$ 为0°,22.5°,45°,67.5°,90°方向原状样进行研究,试样规格尺寸为直径50 mm,高度100 mm的圆柱体。

图 1 试样示意图与试验设备

Figure 1. Schematic diagram of specimens and test apparatus

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1.2 试验方法

试验所用设备为GDS共振柱仪,如图1（b）所示。试样的边界条件为一端固定,一端自由。通过电磁驱动系统对试样逐级施加扭矩,测得试样的共振频率和对应的剪应变,试样动剪切模量由下式得到：

$G = ρ {(2 π f H / β)}^{2}$ ,

(1)

式中,G为试样动剪切模量,ρ为试样密度,f为共振频率,H为试样高度,β为扭转振动频率方程特征值。

试样在抽气饱和后安装至共振柱仪上,随后进行反压饱和,当B值达0.98后,进行固结,围压分别设定为50,100,200,300,400,500,600,700,800 kPa。试样固结完成后,进行共振柱试验。

2. 试验结果与分析

2.1 不同方向试样G- $γ$ 曲线规律

如图2所示,不同方向试样动剪切模量G和剪应变 $γ$ 的关系曲线形态与规律类似。剪切模量在小剪应变下衰减速度较小；随剪应变发展,衰减速度增大。低围压下G- $γ$ 曲线随围压增大而上移,围压超过600～700 kPa,G- $γ$ 曲线随围压增长而下移,与通常软黏土G- $γ$ 曲线大多随围压增大而单调上移规律存在明显差异,说明结构性对湛江黏土G- $γ$ 曲线规律影响较大。

图 2 不同方向试样剪切模量G与剪应变

$γ$ 关系

Figure 2. Relationship between shear modulus G and shear strain

$γ$ for specimens in different directions

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2.2 原生各向异性对最大动剪切模量的影响

湛江黏土动应力-应变关系可用Hardin-Drnevich双曲线模型表征,如下式：

$τ = \frac{γ}{a + b γ}$ ,

(2)

式中,a,b为拟合参数。式（2）可以写为

$1 / G = a + b γ$ 。

(3)

式（3）中,当 $γ$ 趋近于0时,得到最大动剪切模量G_max=1/a,利用式（3）求得不同方向试样在各围压下的G_max。为了消除孔隙比对G_max的影响,引入孔隙比函数F(e)=1/(0.3+0.7e²)将G_max进行归一化处理,图3为经孔隙比函数归一化的G_max/F(e)-围压 $σ_{3}$ 曲线。随围压增大,不同方向试样G_max/F(e)- $σ_{3}$ 曲线均呈现先上升后下降的规律,在围压为400～500 kPa即在 $σ_{k}$ 左右时,曲线出现转折。

图 3 不同方向试样G_max/F(e)与围压σ₃的关系

Figure 3. Relationship between G_max /F(e) and confining pressure σ₃ for specimens in different directions

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为了更好描述原生各向异性对最大动剪切模量的影响,定义G_max/F(e)的原生各向异性系数：

$K_{α} = D_{α} / D_{90 °}$ ,

(4)

式中,D_α定义为α方向试样的G_max/F(e),D_90°定义为90°（竖直）方向试样的G_max/F(e)。

G_max/F(e)的原生各向异性系数K_α与围压的关系如图4所示。相同围压下,K_α随方向角 $α$ 变化,K_α整体上随 $α$ 增大而减小,即试样的方向越靠近水平其刚度越大,说明原生各向异性对湛江黏土最大动剪切模量G_max的影响十分显著。湛江黏土基本单元为扁平状片堆、粒状碎屑矿物与单片颗粒,上述基本单元在沉积时,其长轴更倾向于水平方向,导致颗粒间水平方向的接触更紧密,结构更强^[3],进而更靠近水平方向试样的刚度更大。

图 4 不同方向试样K_α与围压σ₃的关系

Figure 4. Relationship between K_α and confining pressure σ₃ for specimens in different directions

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当围压低于400～600 kPa时,同一方向试样K_α随围压增长基本保持恒定,K_0°,K_22.5°,K_45°,K_67.5°,K_90°分别为1.314,1.279,1.148,1.045,1；当围压高于400～600 kPa时,同一方向试样K_α随围压增长呈明显减小趋势,不同方向试样的G_max/F(e)差异减小。说明围压低于 $σ_{k}$ 时,围压的增大几乎不影响原生各向异性对G_max的影响,但当围压超过 $σ_{k}$ 后,围压的增大减弱了原生各向异性对G_max的影响。文献[2]中伦敦黏土在围压超过屈服应力后,其水平与竖直方向试样的最大剪切模量的差异随围压增长也呈减小趋势,与本文试验结果一致。

2.3 考虑原生各向异性的最大动剪切模量表征方法

图3中出现G_max/F(e)随围压增大呈先上升后下降的特殊现象,文献[4]认为G_max同时受到平均有效应力、孔隙比和结构损伤的影响,采用该文的表征方法对试验结果进行分析,具体的表达形式如下所示：

$G_{\max} / F (e) = \frac{A (1 + {(\frac{{σ^{'}}_{m}}{p_{a}})}^{n})}{1 + B (1 + {(\frac{{σ^{'}}_{m}}{p_{a}})}^{n})} (k_{r} + \frac{1 - k_{r}}{1 + {(η \frac{{σ^{'}}_{m}}{p_{c}})}^{λ}})$ 。

(5)

式中 A,B,n,k_r,η和 $λ$ 为反映各种应力历史和土体性质的参数； ${σ^{'}}_{m}$ 为围压；p_a为标准大气压；p_c为表观前期固结压力即结构屈服应力 $σ_{k}$ ,不同方向试样压缩试验得到的 $σ_{k}$ 差异较小,均取400 kPa。

采用式（5）将不同方向试样G_max/F(e)与围压的关系进行定量表征。从图4可得,高应力下各向异性对试样的G_max/F(e)影响减弱,可假定不同方向试样G_max/F(e)极限值相同。最终将试验数据与拟合曲线一同绘制于图5,发现拟合效果很好,拟合参数见表2。

图 5 不同方向试样的G_max/F(e)与固结围压lgσ₃关系曲线

Figure 5. Curves of G_max/F(e) and confining pressure lgσ₃ of specimens in different directions

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表 2 不同方向试样拟合参数

Table 2. Fitting parameters of specimens in different directions

α	A/MPa	B	n	k_r	η	λ	R²
0^°	39.92489	0.16678	0.54309	0.35092	0.56433	6.42998	0.99251
22.5^°	37.89951	0.15999	0.58264	0.35462	0.56426	6.37147	0.99075
45^°	33.76328	0.15168	0.54642	0.37740	0.55402	6.38473	0.99432
67.5^°	31.15476	0.15761	0.56254	0.42499	0.60889	6.07737	0.99727
90^°	29.75422	0.15743	0.56067	0.44448	0.57750	6.05669	0.99835

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| 显示表格

分析表2中拟合参数与试样方向的关系,可得参数A,k_r, $λ$ 和试样轴线与土体沉积平面夹角 $α$ 呈线性关系（图6）,参数B,n,η随 $α$ 增大分别保持在0.1587,0.5591,0.5738上下,且波动范围较小（参数B,n,η的标准差S分别为0.005455,0.01570和0.02131）。

图 6 拟合参数A,k_r和λ与试样方向的关系

Figure 6. Relationship between fitting parameters A, k_r and λ with directions of specimens

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将图6中参数A,k_r, $λ$ 的拟合方程和参数B,n,η的平均值同时代入式（5）,得到考虑原生各向异性的最大动剪切模量的表征方法：

$\begin{array}{l} G_{\max} / F (e) = \frac{(c_{1} α + c_{2}) (1 + {(\frac{{σ^{'}}_{m}}{p_{a}})}^{n})}{1 + B (1 + {(\frac{{σ^{'}}_{m}}{p_{a}})}^{n})} \cdot \\ ((d_{1} α + d_{2}) + \frac{1 - (d_{1} α + d_{2})}{1 + {(η \frac{{σ^{'}}_{m}}{p_{c}})}^{(e_{1} α + e_{2})}}) \end{array}$ 。

(6)

式中 ${σ^{'}}_{m}$ 为围压； $α$ 表示试样的方向,为试样轴线与土体沉积平面夹角；p_a为标准大气压,取101.325 kPa；p_c为 $σ_{k}$ ,取400 kPa；B=0.1587,n=0.5591,η=0.5738；c₁=−0.1204,c₂=39.9166；d₁=1.144×10⁻³,d₂=0.3390；e₁=−4.625×10⁻³,e₂=6.4722。

3. 结论

（1）在同一围压下,不同 $α$ 试样经孔隙比函数归一化的最大动剪切模量G_max/F(e)与90°方向试样G_max/F(e)的比值K_α随 $α$ 增大而减小。当围压低于和高于 $σ_{k}$ 时,同一 $α$ 试样K_α随围压增长分别呈基本保持恒定与明显减小趋势,说明当围压低于 $σ_{k}$ 时,围压几乎不影响原生各向异性对G_max影响,围压超过 $σ_{k}$ 后,不同方向的G_max/F(e)差异减小,围压的增大减弱了原生各向异性对G_max的影响。

（2）受固结压硬和结构损伤的影响,湛江黏土的G_max/F(e)变化规律与通常软黏土试验结果不同,不同方向试样的G_max/F(e)随围压增大均呈先增大后减小规律,当围压在 $σ_{k}$ 左右时出现转折。

（3）基于采用考虑结构损伤的公式可很好拟合湛江黏土不同方向试样G_max与围压关系曲线,提出了考虑原生各向异性影响的G_max演化规律表征方法。

图 1 ISA扰动原理图^[5]

Figure 1. Schematic diagram of ISA

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图 2 PSA与ISA应力路径示意图

Figure 2. Schematic diagram of stress paths for PSA and ISA

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图 3 不同取样方式下软黏土典型三轴不排水剪切试验结果^[7]

Figure 3. Typical triaxial undrained shear test results of soft clay by different sampling methods

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图 4 扰动作用下软黏土典型固结试验结果^[10]

Figure 4. Oedometer test results of soft clay under disturbance

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图 5 不同扰动形式对软黏土压缩行为影响示意图^[11]

Figure 5. Schematic of effects of disturbance on compression behaviour

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图 6 50 mm Shelby取样管土样试验结果^[18]

Figure 6. Test results of 50 mm-Shelby sample

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图 7 不同塑性土体受扰动后有效应力降低程度图^[36]

Figure 7. Diagram of degree of effective stress reduction in soils with different plasticities under disturbance

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图 8 基于孔隙指数的土样质量评价指标^[24]

Figure 8. Indices of sample quality evaluation based on void index

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图 9 修正体积压缩法扰动度定义^[25]

Figure 9. Definition of modified volume compression method

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图 10 M₀/M_L指标定义^[9]

Figure 10. Definition of M₀/M_L

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图 11 通过SCPTU和弯曲元在不同样品上测量的土体剪切波速剖面图^[15]

Figure 11. Measured shear wave velocity profiles of different samples using SCPTU and bending elements

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图 12 取土扰动影响示意图^[37]

Figure 12. Schematic diagram of sampling disturbance effects on compression curve of soils

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表 1 54 mm取土管样品与高质量块状样品的强度对比^[9]

Table 1 Strengths of 54 mm-piston sample as compared to block samples of high quality

试验类型	高于54 mm取土管样品强度/%
CAUC	10~50
CAUE	0~10
DSS	5~20

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表 2 5种合成土体的物理指标^[21]

Table 2 Physical parameters of five synthetic soils

土体	${w_{\text{L}}}$ / %	${w_{\text{P}}}$ / %	I_p/ %	细粒含量/ %	土体分类
0S100K	59	25	34	100	CH
50S50K	31	15	16	88	CL
70S30K	24	15	9	83	CL
85S15K	19	15	4	79	CL-ML
98S02K	18	NP	NP	75	ML
注： ${w_{\text{L}}}$ 为液限， ${w_{\text{P}}}$ 为塑限，Ip为塑性指数，细粒： < 0.075 mm。

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${\Delta }e/{e_0}$ 土样质量评价指标^[7]

Sample quality evaluation based on ${\Delta }e/{e_0}$

OCR=1~2	OCR=2~4	评级
＜0.04	＜0.03	very good to excellent
0.04~0.07	0.03~0.05	good to fair
0.07~0.14	0.05~0.10	poor
＞0.14	＞0.10	very poor

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参考文献(38)

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