求取现场黄土饱和渗透系数的双环入渗法

土体渗透性参数的测定是解决与水有关的岩土工程问题的关键所在。为使现场原状土体饱和渗透系数的测定更为准确，通过一种易于安装和固定的防蒸发型双环入渗仪，研究黄土的入渗规律与特点，试验结果表明:黄土的入渗分区可分为饱和区、湿润区与干土区，其中饱和区的入渗深度约占总入渗深度的1/2，而湿润区土体的饱和度在70%以上，这一特征与Green-Ampt入渗模型的假设较为接近，但在应用基于Green-Ampt模型的规范法（SL237-042-1999）求取黄土的饱和渗透系数时，发现该法会高估黄土的饱和渗透系数。因此，结合Green-Ampt入渗模型与土水特征曲线主要增湿路径的特点，提出了能合理测定现场黄土饱和渗透系数的双环入渗法，对Green-Ampt入渗模型参数加以修正，即直接采用干土区的初始基质吸力水头值，且该值由张力计实测或由主要增湿曲线求得；需采用入渗试验前期的平均入渗率；所对应的湿润锋发展深度需经由水分传感器实测而得。研究结果表明该法能合理估算现场黄土的饱和渗透系数。

A simple estimation approach for the saturated permeability of loess in field by a double-ring infiltrometer

The saturated permeability, k_s is a key parameter in solving geotechnical engineering problems such as stability analyses of slopes and earth-retaining structures during rainfall. Precisely determining this quantity for soils in the field is difficult. Hence, this paper suggests a simple approach for estimating the saturated permeability of loess in the field using a new, modified, double-ring infiltrometer. We use this device, which can perform infiltration tests accurately and effectively, to study the infiltration characteristics of loess. The results of these tests show that water infiltration into uniform, dry loess can be considered to divide the soil into a saturated, wetting, and dry-soil zone. Moreover, the depth of the saturated zone is approximately equal to that of the wetting zone, and the degree of saturation in the wetting zone is greater than 70%. The soil-moisture profile derived from the infiltration tests is similar to that given by the Green-Ampt model. The field tests also indicate the double-ring infiltrometer method based on the conventional Green-Ampt model (Specification of soil test, SL237-042-1999) provided larger k_s values of the loess. To determine the saturated permeability more reliably, we suggest a modified approach herein, which is based on the Green-Ampt model and employs the soil-water characteristic curves (SWCC) along with the main wetting path. The concept of the modified approach is to redefine the meanings of the key parameters of the Green-Ampt model (the matric suction head at the wetting front, the wetted depth, and the infiltration rate). We take the matric suction head at the wetting front to be that corresponding to the initial soil moisture content; its value can either be measured with tensiometers in the experimental field or else determined by the SWCC along with the main wetting path. The wetted depth during the infiltration test period is taken to be that observed by the soil moisture sensors. In the Green-Ampt model, we also adopt the average infiltration rate in the early infiltration stage rather than the steady-state infiltration rate. From the results of our infiltration tests and laboratory permeability tests, we find this approach to be a suitable method for determining the saturated permeability of loess in the field.