A SIMPLIFIED ESTIMATION OF INFILTRATION CAPACITY FOR INFILTRATION FACILITIES

Transcription

1 A SIMPLIFIED ESTIMATION OF INFILTRATION CAPACITY FOR INFILTRATION FACILITIES MASAHIRO IMBE Association for Rainwater Storage and Infiltration Technology Koujimachi, Chiyoda-ku, Tokyo , Japan KATUMI MUSIAKE Department of Administration & Social Science, Fukushima University 1 Kanayagawa, Fukushima , Japan This paper presents the application of simplified formula to estimate the infiltration capacity for infiltration facilities. The simplified formula was applied for the infiltration trench which was planned to be introduced to the large-scale land development. The design value of the infiltration capacity was estimated using the formula which depends on the facility shape and the saturated hydraulic conductivity (K 0 ) of surrounded soil. Especially, the saturated hydraulic conductivity (K 0 ) was estimated carefully since it depends on the soil condition which is very sensitive and various from place to place. The infiltration capacity of the infiltration trench which is set up under the L-shape ditch along the roads was measured by water recharge test. The observed capacities were compared with the designed value which is derived from the simplified formula. Although the saturated hydraulic conductivity (K 0 ) is sensitive and various, it has been concluded that the simplified formula is applicable for the actual design work. INTRODUCTION Recently, permeable connection boxes and trenches are being widely used as urban drainage facilities in Japan and as a result, it becomes necessary to evaluate their infiltration capacities with sufficient accuracy for design works. According to Herath and Musiake(1987) [1], small scale in-situ permeability test and numerical simulation based on the Richard s equation are adequate to evaluate the capacity of infiltration facilities. The infiltration rate is expressed as the specific infiltration, Q/K 0 since the governing equation for the infiltration rate (Q) is primary scaled down with respect to the saturated hydraulic conductivity (K 0 ). Although this method is excellent to estimate the accurate capacity, alternative approaches without numerical simulation are much more preferable for practical design purpose. In this point Imbe and Saito (1995) [2] presented simplified formula which have been prepared based on the numerical simulation and, therefore, can precisely calculate the infiltration capacity. The simplified formulas for various types of facilities were adopted in the national engineering guidelines [3]. 1

2 This paper will present the case study of the design work introducing infiltration trenches in the newly developed residential area in order to verify the applicability of the simplified formula for infiltration capacity. OUTLINE OF THE INFILTRATION FACILITIES The study area is about 60 ha located at north-eastern 50 km from Central Tokyo and is newly developed by the Urban Development Corporation as a residential area as shown in the following Figure 1. 2 Figure 1. The newly developing area as of January 1998 In this development, infiltration facilities were planed to be introduced so that the impact against aqua-environment due to the development could be reduced to maintain the safety factor of flooding, the ordinary river discharge, the groundwater condition and so on. Since the area is mainly covered with the soil layer of Kanto loam which is a volcanic ash of Mt. Fuji and the permeability of Kanto loam is good enough, the order of 10-3 cm/sec in a saturated hydraulic conductivity (K 0 ), infiltration facilities were adopted to be one of the most effective measures for this development. The main facility is a infiltration trench which is set up under the L-shape ditch along the roads at the residential area. As shown in Figure 2, there are two types of the experimental facilities Type A and Type B. Type A is sealed at the roadway side by the impervious sheat to prevent the water infiltrated into the base course of the road. Type B is partially sealed at the upper roadway side so that the lower roadway side can permit the water infiltrated into the base course of the road. The design water depth is set up at 930 mm for Type A and 490 mm for Type B.

3 3 TYPE A Roadway TYPE B UNIT:mm Roadway 930 PVC Porous Pipe (φ100m m ) PVC Porous Pipe (φ100m m ) Sand Mat 50 Sand Mat : Pervious Sheet : Impervious Sheet : Infiltration Zone Figure 2. Experimental facilities to be introduced into the study area DESIGN METHOD FOR INFILTRATION CAPACITY Brief overview of theory Japanese design of infiltration facilities is based on the calculation of a unit design infiltration rate. The design infiltration rate is a function of the standard infiltration rate, which is unique to each type of facility being designed. It is deemed impractical to design each individual facility on a theoretical basis, so a term called specific infiltration has been devised. The specific infiltration, Q/K 0 is defined as the infiltration rate, Q divided by the saturated hydraulic conductivity, K 0. Since our research shows that the specific infiltration is not seriously influenced by in-situ soil properties, the simplified formula which separately related to the facility shape and the hydraulic head or ponding depth has been developed. This allows large scale facilities to be designed using simple permeability test for the in-situ saturated hydraulic conductivity and the final steady infiltration rate. Method of equations The design infiltration rate, Q is expressed as Q = C * Q f (1) where C is a safty factor which depends on the groundwater table and the likelihood of clogging and is generally taken to equal Q f is the standard infiltration rate for the facilities in question. The units of Q are m 3 /hour.

4 4 The standard infiltration rate, Q f is evaluated using the following expressions. Q f = (Q t / K t )*K f =K 0 * K f (2) where Q t is the final steady state infiltration rate obtained by an in-situ permeability test in m 3 /hour. The in-situ test has two types (Borehole and PWRI method) of water recharge methods under the constant hydraulic head as shown in Figure 3. In the guideline, the borehole method is recommended as the standard method because installing the test facility is easier than the PWRI method. Figure 4 shows their test methods. The infiltration rate generally reaches a steady state condition after about 2 to 4 hours and the constant rate is adopted as a final steady state infiltration rate, Q t for design purpose. K t is the specific infiltration for the device of the in-situ test in m 2. K f is the specific infiltration for the designed facility in m 2. They are calculated using the simple formulas in Table 1. K 0 is the saturated hydraulic conductivity of the soil in m/hr, which is expressed as K 0 = Q t /K t. Borehole Method PWRI Method Water Supply Pipe Ground Surface Water Supply Pipe PVC Pipe Ground Surface Borehole by Auger 50~150 cm 60~100 cm Backfilling Soil Sealing Material Gravel 75 ~80 10cm Sand φ20cm Gravel φ30cm Figure 3. Two types of in-situ permeability test. PWRI method was developed by the Public Works Research Institute (PWRI). H= Constant Q becomes constant. TEST METHOD: Recharge water into the cylinder to the specified water level, H. Adjust the recharge amount so that the water level, H can be maintained. Measure the amount of recharge, Q over time. Continue measurements until the amount of recharge, Q becomes constant. The guideline for recharge time is 2 to 4 hours with a simple facility test. Figure 4. Outline of the test method

5 Table 1. Calculation formulas for specific infiltration [values K t and K f (m 2 )] of various infiltration facilities 5 Facilities infiltration trench Cylinder inlet Point of infiltration Side and bottom Side and bottom Bottom Illustration Applicable range of calculation formulas Facility scale Basic formula Design head About 1.5 m About 1.5 m About 1.5 m Width about 1.5 m K = ah + b H: Design head (m) W: Facility width (m) 0.2 m diameter 1 m K = ah 2 + bh + c H: Design head (m) D: Facility diameter (m) 1 m < diameter < about 10 m 0.3 m diameter 1 m K = ah + b H: Design head (m) D: Facility diameter (m) 1 m < diameter < about 10 m a D D D D Coefficient b 1.34W D D D D D D D c D Remarks Value per unit length PROTOTYPE WATER RECHARGE TEST Rrototype water recharge tests were conducted for 6 facilities (Type A and B) as illustrated in Figure 5. PLANE VIEW Water Supply Tank Impervious Sheet (t=0.5) Flow Meter Sand Mat 500 (t=50) CROSS SECTION Pervious Sheet (t=0.6) PVC Porous Pipe 1,000 Crushed Stone PVC Porous Pipe 1,000 SIDE VIEW 500 Water Level Observation Hole Impervious Sheet (t=0.5) UNIT:mm Figure 5. The facility of prototype water recharge test

6 The water is recharged from water supply tank and the recharge amount is adjusted so that the specified water head (930mm or 490mm) can be maintained. The recharged water amount is measured over time until it becomes constant. The constant recharge rate is adopted as a final steady state infiltration rate. The test results are described in Table 2. Table 2. The results of prototype water recharge tests Facility No. Type Water Head (mm) Observed (liter/hour) 1 A A A B B B DESIGN WORK OF INFILTRATION CAPACITY 6 Saturated hydraulic conductivity of the soil, K 0 The saturated hydraulic conductivity, K 0 (=Q t /K t ) was estimated using the final steady state infiltration rate, Q t obtained by the in-situ permeability test and the specific infiltration, K t for the device of the in-situ test given by the simple formula (Table 1). The in-situ permeability tests (PWRI method) were conducted at six places selected as a typical soil condition in this developing area. The estimated value, K 0 was averaged in six data to be 4.25x10-3 cm/s which is equivalent to m/hour. Estimation of infiltration rate for prototype facilities The infiltration capacity for prototype facilities (Type A and B) was estimated using Eq. (1) and (2). Since the prototype facilities are sealed at the roadway side, the specific infiltration, K f given by Table 1 should be modified according to the difference of hydrostatic pressure distribution along the permeable zone before substituting into Eq. (2). The difference of hydrostatic pressure distribution among the general type, Type A and Type B is illustrated in Figure 6. Permeable Trench (General) Permeable Trench (Type A) Permeable Trench (Type B) Pervious Pipe Pervious Pipe Sealing Pervious Pipe Sealing Hydrostatic Pressure Hydrostatic Pressure Hydrostatic Pressure Figure 6. Hydrostatic pressure distributions of the general type, Type A and Type B

7 Figure 7 is the results of the numerical analysis based on the Richard s equation for cylindrical infiltration facilities with various diameters and water heads. The hydrostatic total force, P (tf) acting on a permeable surface is calculated by the following Eq. (3). 7 P = p da (3) where p (tf/m 2 ) is a hydrostatic pressure acting on an infinitesimal area, da (m 2 ). Figure 7 shows that the specific infiltration, K f is proportional to the hydrostatic total force, P in the same plane shape of the facility (the diameter in this case). Therefore, the specific infiltration, K f for Type A and Type B were converted by this proportional relationship. Designed values were calculated as tabulated in Table 3. Figure 7. Relationship between the specific infiltration and the hydrostatic total force in the cylindrical facilities by the numerical simulation Table 3. Calculation results of the design infiltration rate for Type A and B Facility No. a w (m) b H (m) Kf (m2) K0 (m/hour) Qf (m3/hour) General P (tf) Modified P' (tf) Modified Qf (m3/hour) Designed Q (liter/hour)

8 8 CONCLUSIONS Figure 8 shows the comparison between the designed values based on the simple formula and the observed values by prototype water recharge tests for six experimental facilities. Infiltration Rate (liter/hour) Facility N o. Designed Observed Figure 8. Comparison of designed values with observed data It has a very good coincidence which approves that the infiltration rate of the actual facilities can be estimated applying the presented simple formula for design purpose without executing the prototype water recharge test. However, the infiltration rate depends on the saturated hydraulic conductivity, K 0 which is very sensitive and variable due to the soil condition and storongly recommended to be evaluated carefully. ACKNOWLEDGEMENT The authors would like to express our heartfelt appreciation to the personnel of the Urban Development Corporation who kindly provided the valuable data and materials. REFERENCES [1] S. Herath and K. Musiake, Analysis of infiltration facility performance based on insitu permeability tests, Proc. 4th Int. Conference on Urban Storm Drainage, Lausanne, (1987), pp [2] M. Imbe and M. Saito, A simplified estimation method of infiltration capacity from wells and trenches, Proc. 2nd Int. Conference on Innovative Technolgies in Urban Storm Drainage, Lyon, (1995), pp [3] Engineering guideline for rainwater infiltration facilities -Survey and Planning, (1995) published by Association for Rainwater Storage and Infiltration Technology