A Test Method for Soil−Water Characteristics Curve of Unsaturated Loess in the Full Suction Range
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摘要:
土−水特征曲线是反映非饱和土中水分运移现象以及力学行为的重要参数。笔者采用等温吸附分析法（VSA）和传统的瞬态脱湿吸湿法（TRIM）相结合，提出了一种测定黄土全吸力范围的土−水特征曲线的方法。通过VSA持续测定样品的相对湿度和重量，计算获得高吸力范围内（7.1×103 ～4.8×105 kPa）脱湿与吸湿路径下的基质势与体积含水率的系列离散数据点。依据VSA测试结果确定样品的残余含水率以限定TRIM反演过程，修正了TRIM试验结果的准确性。选用F&X数学模型光滑连接了TRIM在低吸力范围内（0～300 kPa）与VSA的实测结果。该方法具有测量的吸力范围大（1～106 kPa），测量时间短（5～8天），数据准确度高（R2=0.999）的优点。将此方法应用于甘肃董志塬剖面黄土−古土壤（L5-S5）的土−水特征曲线的测试，结果表明，预测模型得出的非饱和黄土土−水特征曲线和TRIM−VSA的实测值具有很好匹配性，证实了该方法的可靠性。该测定非饱和土土−水特征曲线的方式快速、有效，在工程实际中有重要的应用价值。
Abstract:The soil−water characteristic curve is an important parameter which reflects the moisture movement characteristic and mechanical behavior in unsaturated soil. A method is proposed based on vapor sorption analyzer combined with transient water release and imbibition method to determine soil−water characteristic curve in the full suction range. First, the relative humidity and the mass of the sample are continuously measured by VSA, series of discrete data points of matrix suction and volume water content are calculated under drying and wetting paths in the high suction range (7.1×103 ～ 4.8×105 kPa). Secord, the TRIM inversion process is limited through the residual water content of VSA test results. The accuracy of inversion results of TRIM is increased. Finally, the F&X model is used to smoothly connect the test data from TRIM and VSA. The measurement of suction range of this method is wide (1～106 kPa), the measurement time of this method is short (5～8 days), and the measurement accuracy of the method is high (R2=0.999). In this paper, the method is applied to test soil−water characteristic curve of the loess (L5) and paleosol (S5) in Dongzhiyuan, Gansu Province. The results show that the predicted results of this method are consistent with the measured results in the full suction range. The method of determining soil-water characteristic curve of unsaturated soil is fast and effective, and important value in engineering application.
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表 1 基本物理参数与饱和渗透系数表
Table 1. Basic physical parameters and saturated permeability coefficient
土层 土样基本物理参数 深度
（m）含水率
（%）密度
ρ（g/cm3）干密度
ρd（g/cm3）比重
（Gs）孔隙比
（e）饱和渗透系数
ks（10−5 m/s）塑限
PL（%）液限
LL（%）塑性指数
IP（%）L5 39.2 18.8 1.727 1.454 2.71 0.689 0.652 22.9 30.1 11.4 S5 42.1 19.60 1.975 1.651 2.73 0.653 0.218 16.7 31.8 15.0 
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表 2 碎屑矿物成分含量表
Table 2. Detrital mineral compositions
样品编号 石英 斜长石 钾长石 方解石 白云石 角闪石 赤铁矿 黄铁矿 TCCM L5 40.6 11.3 3.9 6.5 1.3 0.4 36.0 S5 43.1 12.5 1.7 5.7 0.7 1.2 35.1
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表 3 黏土矿物成分含量表
Table 3. Clay mineral compositions
样品编号 黏土矿物相对含量（%） 混层比（%） 蒙脱石（S） I/S 伊利石（It） 高岭石（Kao） 绿泥石（C） C/S I/S C/S L5 47 38 6 9 55 S5 46 40 6 8 55
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表 4 化学成分含量表
Table 4. Chemical compositions
样品编号 主要化学成分含量平均值 （%） SiO2 Al2O3 Fe2O3 FeO CaO MgO K2O Na2O TiO2 P2O5 MnO L5 57.35 11.91 3.59 1.01 9.05 2.16 2.39 1.52 0.64 0.14 0.08 S5 65.43 14.51 5.17 0.61 1.73 2.1 2.87 1.21 0.74 0.11 0.11
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表 5 脱湿与吸湿路径下的模型参数表
Table 5. The model parameters of the wetting and drying paths
土 层 脱湿路径 吸湿路径 
（k/Pa）
（cm/sec）
（k/Pa）
（cm/sec）L5 0.0099 1.134 0.063 0.408 1.73E-5 0.0208 1.149 0.06 0.322 4.37E-6 S5 0.0083 1.074 0.083 0.395 1.63E-5 0.0135 1.085 0.08 0.373 9.43E-7
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表 6 高吸力段土–水特征曲线脱/吸湿起始与结束点表
Table 6. The start and end points of wetting/ drying path of high suction range soil–water characteristic curve
土层 试验用时（Min） 吸湿起始点 吸湿结束点（脱湿起始点） 脱湿结束点 RH 
（%）
（kPa）RH 
（%）
（kPa）RH 
（%）
（kPa）L5 2384 0.0283 0.97 490929 0.9496 5.37 7122 0.0280 0.74 492397 S5 2238 0.0315 0.76 476177 0.9464 6.34 7587 0.0289 0.62 488040
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表 7 脱湿与吸湿路径下的拟合参数表
Table 7. The fitting parameters of the wetting and drying paths
土 层 脱湿路径 吸湿路径 
（kPa） 
L5 0.408 228.078 18247 0.662 1.425 0.322 101.914 14923 0.571 1.295 S5 0.395 370.008 22381 0.836 1.075 0.373 275.134 17542 0.716 1.148
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