Use of piezometers to find the depth to impermeable layer in the design of drainage systems

A piezometric head method is proposed to estimate the impermeable layer depth used in drain spacing formulae. The method was tested and compared with soil texture changes and the US Bureau of Reclamation (USBR, 1978) criterion of impermeable layer being one with hydraulic conductivity one tenth to one fifth that of the weighted hydraulic conductivity of the upper layers. The proposed method, which works for below the water table conditions and does not require hydraulic conductivity measurements, gave approximately the same results as the USBR method. The soil texture changes method was erratic and not reliable. Utilization de piézomètres pour déterminer la profondeur de la couche imperméable dans la conception de systèmes de drainage Résumé Nous proposons une méthode piézométrique pour l'estimation de la profondeur de la couche imperméable, applicable dans les formules de calcul d'espacement des drains. Cette méthode a été testée et comparée à la méthode de changement de texture du sol et à celle de FUS Bureau of Reclamation (USBR, 1978) qui considère que la conductivité hydraulique de la couche imperméable est de 5 à 10 fois plus faible que la conductivité hydraulique moyenne des couches qui la surmontent. La méthode proposée, qui s'applique en conditions saturées et qui n'exige pas de mesures de conductivité hydraulique, donne approximativement les mêmes résultats que la méthode de l'USBR. La méthode de changement de texture du sol donne des résultats erratiques et n'apparaît pas fiable. INTRODUCTION For the purpose of subsurface drainage design, an impermeable layer or barrier is a stratum or layer that prevents or restricts the saturated movement of water in the soil. Geology is often a key in finding the layer, which is also known as the barrier stratum, barrier layer, barrier zone, or impermeable boundary. These terms are often used in drainage engineering and are related to relative hydraulic characteristics of various strata. Since soil strata in irrigated areas are found in a generally horizontal orientation parallel to the ground surface, an impermeable layer is usually considered a barrier to vertical movement of water. This is not exclusively the condition, however, because, in areas of nonconformity or folding of geologic strata, a vertical barrier may also restrict the horizontal movement of water (USBR, 1978). When soil water percolating downward under the force of gravity reaches an impermeable layer, a saturated condition develops on the top of the layer and a perched water table occurs. Some of the water then begins to move laterally in the soil above the barrier zone because of differential pressures. Open for discussion until I August 1999 26 Mirkhalegh Z. Ahmadi By definition, as used by the US Bureau of Reclamation (USBR, 1978), a barrier zone is a layer that has a saturated hydraulic conductivity less than or equal to one fifth of the weighted average hydraulic conductivity of the strata above it. An accurate appraisal of the depth to the barrier zone is important in drain spacing calculations, but identification of such barriers is not always easy or clear cut. Impermeable layers can be regarded as representing streamlines, because there is no flow across them. In practice, a soil layer is considered impermeable if its hydraulic conductivity is very small (one fifth or less) compared with the hydraulic conductivity of adjacent layers (Dieleman & De Ridder, 1979). Slowly permeable and impermeable layers are found where the soil is poorly aggregated or exhibits a massive type of structure. The soil of such layers typically belongs to one of the following textural classes: sandy clay loam, silty clay loam, clay loam, sandy clay, silty clay, and clay (Farr & Henderson, 1986). The depth to the impermeable layer is important in the design of subsurface drainage systems. Most of the drainage formulae for determining the depth and spacing of the drains require information about the depth to the impermeable layer (Luthin, 1973). The term "impermeable" is relative. All soils are permeable to some extent. Deep seepage occurs to a limited extent even in very slowly permeable subsoils. However, if the saturated hydraulic conductivity of the subsoil is about one tenth that of the surface soil, the subsoil is considered impermeable from a drainage design standpoint (Luthin, 1973). There will be waterlogging above this layer if the rainfall rate, or the rate at which water is added to the soil, exceeds the permeability of this layer. The flow patterns of the water moving toward the drains will be altered drastically by any layer of lower permeability. If the impermeable layer is shallow, the drains must be placed close together above or on the layer to achieve the same drainage effect as widely spaced drains in a deep permeable soil (Luthin, 1973). Smedema & Rycroft (1983) pointed out that if a distinct barrier cannot be found in the soil, the USBR criterion is to be applied. That is, if there is no known impermeable layer in the 1-2 m of depth below the drain, then the barrier is assumed to be at a depth equal to one quarter of the drain spacing. Drainage engineers usually use the USBR criterion for estimating the barrier depth. The purpose of this research was to evaluate an alternative method, i.e. a piezometric head difference criterion (a greater relative piezometric head in the lower piezometer while the direction of flow is downward) and compare the results with those from the soil texture changes and the USBR method. MATERIALS AND METHODS Three sites (A, B, and C) were chosen randomly on the area of the College of Agricultural Sciences, Sari, Iran to test and compare the three methods of estimating the depth to impermeable layers, i.e. soil texture changes, one fifth to one tenth of the weighted mean hydraulic conductivity of the upper layer and piezometric head differences of successive layers. The tests were repeated three times at each site, Use of piezometers to find the depth to impermeable layer 27 during different wet seasons of 1995 and 1996. The repeated test sites were about 50 cm apart. Sites A, B, and C were independent test locations. The soil texture of each site was determined by the hydrometer method on samples taken from each layer to a depth of 3.25 m. The horizontal hydraulic conductivities of the soil layers below the water table were measured using the piezometer method (Luthin & Kirkham, 1949). The piezometric head (pressure head + elevation head) differences were found by installing two 25 mm diameter piezometers in successive layers. The piezometric water depths were measured by an electronic depth-to-water meter, 24 h after each depth increment. Following each reading, the lower piezometer remains in its position, but the upper piezometer is driven to a new depth. The procedure is repeated as long as the piezometric head difference is positive, i.e. the piezometric head in the upper layer is greater than that of the lower layer. When the difference is positive, drainage is occurring, i.e. there is vertical downward water movement. As this positive difference becomes small and finally negative, it implies an occurrence of a relatively impermeable layer between the two piezometer depths. RESULTS AND DISCUSSION Figure 1 indicates the soil layers and water surfaces inside the piezometers at the test site A (test no. 1). In Fig. 1, Al, A2, A3, A4, and A5 are the repeated vertical positions of the piezometers at the same test site A, and the water levels shown in the piezometers indicate the piezometric head measured from test site A and test no. 1. Figures 2 and 3 show the soil layers and water surfaces inside the piezometers at test sites B and C (tests nos 2 and 3, respectively). The soil textural classes, hydraulic conductivities and piezometric head differences measured at sites A, B and C are shown in Tables 1-3, respectively. As can be observed from Table 1, except for the soil texture, the other data do not exist Table 1 The soil texture, hydraulic conductivity and piezometric head differences found at test site A (test no. 1). Soil depth (m) 0-0.25 0.25-0.50 0.50-1.00 1.00-1.50 1.50-2.00 2.00-2.25 2.25-2.50 2.50-2.75 2.75-3.00 3.00-3.25 Soil texture C C CL SCL SL LS SL SL SL LS Hydraulic conductivity (m day) 0.36 0.94 1.45 0.13 Hydraulic weighted ; (m day) 0.36 0.65 0.81 conductivity, average Piezometric head differences (mm) 45 56.5 59.5 10.0 -40 C: clay; CL: clay loam; SCL: sandy clay loam; SL: sandy loam; LS: loamy sand. 28 Mirkhalegh Z. Ahmadi Texture Depth,m •m/mt/i C C