StormCon August 2017

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1 IMPROVED METHODS FOR STORMWATER INFILTRATION TESTING: BOREHOLE PERMEAMETER METHOD J. Scott Kindred, P.E. Kindred Hydro, Inc. Mercer Island, Washington Introduction The 2014 Western Washington Stormwater Manual (Ecology, 2014) recommends use of the Pilot Infiltration Test (PIT) for estimating infiltration rates to be used in design of Green Stormwater Infrastructure (GSI) facilities. Other states across the country have adopted the PIT as the preferred method for estimating infiltration rates. Unfortunately, the PIT approach ignores important physical processes and overestimates porous medium hydraulic conductivity by 20 to 60 percent, depending on test geometry (area and water depth) and porous medium hydraulic properties. Although the non-conservative nature of this approach is partially mitigated as size of the test excavation increases, the cost and logistical challenges (e.g., sufficient water supply) can become prohibitive. This paper provides an alternative approach, the borehole permeameter (BP) method, that better represents the three components (pressure, gravity, and capillarity) of infiltrative flow or recharge from an excavation or borehole completed within the unsaturated zone. The method is well suited for estimating infiltration rates for use in design of green stormwater infrastructure (GSI). The BP approach has been used in open excavations (similar to the PIT approach), in temporary wells installed using a vactor (vacuum extraction) truck, and in deeper drilled wells completed within the unsaturated zone. The BP procedure involves pumping water into the borehole/excavation at a steady rate and monitoring head rise until the head becomes constant. The test results provide estimates of bulk field saturated hydraulic conductivity (K) using approximate analytical solutions that were originally developed in the 1950's and have been refined in recent decades. Bulk K is a composite parameter that incorporates the effects of groundwater mounding, as well as fine-scale porous medium layering and larger-scale porous medium variability within the tested interval. The term field saturated refers to the fact that ponded infiltration into unsaturated porous media may entrap some amount of air, causing the effective porous medium K to be a much as a factor of two lower than if the porous media were completely saturated (i.e., no entrapped air). The BP approach has been tested and refined during numerous stormwater infiltration assessments, and examples of the approach are provided in this paper. Borehole Permeameter (BP) Equation Considerable research has been conducted regarding analytical methods for estimating K from borehole infiltration tests. The method, sometimes referred to as the Constant Head Well Permeameter approach, was originally developed by Glover (Zanger, 1953) and further refined by others, such as Elrick et al. (1989) and Reynolds (2008). It uses the equation: 1

2 CQ K= 2πH! + πr! C + 2πH Equation 1 α Where: K = Field-saturated hydraulic conductivity (feet/day) Q = Steady state flow into the borehole (feet 3 /day) H = Steady state ponded water depth in the borehole (feet) r = Radius of borehole (feet) α * = Porous medium sorptive number (feet -1 ), which related to the medium s capillarity for unsaturated conditions. C = Borehole shape factor (dimensionless) =!!!!!!!!! Z! Zhang et al., 1998 This equation assumes that the borehole is uncased within the saturated portion of the borehole test. Other formulas have been developed for cased boreholes (e.g., Reynolds, 2010). In theory, α * can be determined by back-to-back running of two borehole infiltration tests at different ponding depths (H). In practice, however, α * has been shown to be relatively constant for a broad range of porous materials. Elrick et al. (1989) developed a lookup table for α * (provided in Table 1) based on porous medium texture and structure. Table 1: Estimates of porous medium sorptive number (α * ) and shape factor (C) parameters for a range of porous medium textures and structures (adapted from Elrick et al., 1989 and Zhang et al., 1998). Porous Medium Texture α * (feet -1 ) Z 1 Z 2 Z 3 Compacted clays Unstructured fine-grained porous media Structured fine grained porous media or unstructured fine-medium sandy media Structured fine-medium sandy porous media and coarse-grained gravelly media Structured porous media refers to porous media with cracks or macropores (e.g., root holes) that can increase bulk K. For stormwater infiltration testing, the target horizon is usually below the root zone where the porous media are considered unstructured. Most porous media suitable for infiltration are predominately sands and gravels, and all the examples provided in this paper use α * = 3.7 feet -1. Using α* = 11 feet -1 for gravelly media will increase calculated estimates of bulk K by up to 30 percent, depending on the geometry of the test. Use of α* = 3.7 feet -1 for estimating the performance of facilities in gravelly media will provide more conservative design assumptions. Table 1 also provides shape factor (Z) parameters developed by Zhang, et.al (1998). These Z parameters were obtained by matching shape factor values obtained from numerical modeling results. The Z parameters are the same for the bottom two textures, which cover the K range of interest for stormwater infiltration facilities. 2

3 The three terms in the denominator of Equation 1 account, respectively, for hydrostatic pressure flow out the sides and base of the borehole, gravity flow out the base of the borehole, and capillarity flow due to the surrounding unsaturated porous material. When H is much greater than r, hydrostatic pressure flow usually dominates vertical gravity flow and capillarity flow, while gravity flow often dominates when H is less than r. As a result, Equation 1 can be used for analysis of low-head infiltration tests in excavated pits, similar to the PIT approach (Ecology, 2014). As discussed by Archer et al. (2014) and others, Equation 1 and similar equations do not account for the following factors: Incomplete saturation within the flow zone due to air entrapment. These air pockets may dissipate over time, resulting in a gradual increase in effective K. Smearing and/or compaction along the borehole wall and base. Potential effects if the borehole intersects or terminates just above ground water or perching layers. The presence of ground water or perching layers would inhibit the flow of water from the borehole and reduce the estimated K value. Variability in the porous medium characteristics along the length of the borehole. The presence of such variability would provide an estimated K value that averages the permeability of the intersected porous medium horizons. Porous medium layering (anisotropy) where vertical K differs from horizontal K. This condition provides a bulk K that falls between vertical K and horizontal K. Comparison with numerical simulations indicates that tests with H/r > 1 are dominated by horizontal K and tests with H/r < 0.5 are dominated by vertical K. One or more of these factors are likely present in all borehole infiltration tests. Given these factors, the K estimates from the borehole infiltration tests are referred to as bulk K estimates, reflecting variability and layering within the tested porous media. Since production-scale stormwater infiltration facilities will be subject to similar limiting factors, bulk K estimates are appropriate (or even preferred) for feasibility assessments and preliminary design purposes. Infiltration Testing Configurations Figure 1 shows a variety of test configurations that are suitable for analysis using the BP approach. The permanent and temporary well configurations are appropriate for supporting design of drilled or dug vertical infiltration facilities (commonly referred to as underground injection control wells, UICs. or drywells). Permanent wells include a surface seal and are generally suitable for deeper tests where traditional well installation techniques are required or desired. These wells have been installed to depths of 120 feet with up to 40 feet of slotted screen. Temporary wells may be used in shallow holes excavated using an excavator or vactor truck. The well screen is placed in the temporary well excavation and the annular space filled with pea 3

4 gravel or coarse sand to maintain side-wall stability. The well screen can be pulled from the temporary well after testing. The open borehole or excavation approach is suitable for shallow tests that will be used to support design of shallow horizontal infiltration facilities. For test facilities that are not round, equivalent borehole radius (r e ) can be used instead of r and estimated using the following equation: r e = A π Equation 2 where: r e = equivalent radius A = ponded area of the test facility If the test facility is significantly longer than it is wide, this equation for re will result in an overestimate of K. Therefore, the test facility should be as close to square as possible. Tests are conducted in all three configurations by adding water to the facility until steady state conditions are achieved (i.e., steady Q and H). In some cases, steady state conditions may not be achieved in a reasonable time-frame and it may not be practicable to continue the test. If this occurs, the test results will provide an estimate of bulk K that is greater than actual conditions. Figure 1: Typical infiltration testing configurations 4

5 Comparison of the PIT Approach with the BP Approach As detailed in Ecology (2014), the PIT approach provides an estimate for the porous medium K (equivalent to infiltration rate) by dividing the steady-state flow rate (Q) by the ponded area of the excavation (πr!! ). This method assumes plug flow out the base of the excavation and unit hydraulic head gradient at steady state. When hydraulic gradient is unity and ponding depth (H) is negligible, the infiltration rate equals K and the PIT approach can be represented by the following equation: K = Q πr!! (Equation 3) Comparison of Equation 3 with Equation 1 indicates that the PIT approach accounts for gravity flow out of the bottom of the excavation (the second term in the denominator of Equation 1) but does not include hydrostatic pressure flow or capillarity flow. As a result, estimates of K using the PIT approach can be significantly higher than K estimates using the BP approach. As illustrated on Figure 2, K estimates from the PIT approach can be 60 percent higher than K estimates using the BP approach when the radius of the excavation is 3 feet (typical size of a small-scale PIT) and H = 0.5 feet. At a radius of 6 feet (typical size of a large-scale PIT), K estimates using the PIT approach are approximately 25 percent higher than K estimates using the BP approach when H = 0.5 feet. The overestimate of porous medium bulk K by PIT will increase as H increases. 5

6 Figure 2: Over-estimate of K using the PIT approach Infiltration Test in an Open Excavation Figure 3 illustrates the results of a shallow head infiltration test in fine sand. The test facility was an excavation with a ponded area of 4 feet by 6 feet (r e = 2.8 feet). Water was added to the pit and the flow rate adjusted to maintain H = 0.8 feet. The fixed head infiltration rate was determined by dividing the flow rate into the excavation (Q) by the ponded area (25 square feet) and is equivalent to the estimate of K provided by the PIT approach (i.e., Equation 3). At the end of the steady state portion of the test (233 minutes), the PIT approach provided a K estimate of 8.1 inches/hour. Measurements of hydraulic head continued after the water was shut off and the falling head infiltration rate is also shown on Figure 3. The falling head infiltration rate provides a K estimate of approximately 8.3 inches/hour at a head of 0.7 feet and declined to 5.3 inches/hour at a head of 0.36 feet, illustrating the influence of head on K estimates using the PIT approach. Bulk K calculations using the BP approach are also shown on Figure 3. The BP approach provided a K estimate of approximately 4.0 inches/hour during both the steady state and falling head portions of the test. A factor of 2 over-estimate of K by PIT could have substantial impacts on GSI design specifications. 6

7 Figure 3: Infiltration test results in an open excavation with r e = 2.8 feet and steady Q = 2.0 gallons/minute. Infiltration Test in a Temporary Well Figure 4 illustrates the results of an infiltration test in glacially consolidated slightly silty fine sand (advance outwash). The test facility was a temporary well excavated to a depth of 10 feet in a using a vactor truck. A 2-inch diameter PVC screen was inserted in the boring and the annular space filled with pea gravel. The radius of the boring was 0.5 feet. Water was added to the well until H exceeded 6 feet and the flow rate was gradually reduced to Q = 1.6 gallons/minute. At this flow rate, H stabilized at 5.0 feet. The water was turned off after 35 minutes and H declined to less than 0.5 feet after 90 minutes. At Q = 1.6 gallons/minute and H = 5 feet, the BP approach provided a bulk K estimate of 2.3 inches/hour. 7

8 Figure 4: Infiltration test in a temporary well with r = 0.5 feet and an estimated bulk K of 2.3 inches/hour using the BP Approach Infiltration Test in a Permanent Well Figure 5 illustrates the results of an infiltration test in a permanent well drilled to a depth of 90 feet using a Sonic drilling rig. The bottom 38 feet of the well was sealed with bentonite pellets, a 2-inch diameter PVC screen was placed from 20 to 50 feet below the ground surface, and the annular space around the screen was filled with washed sand from 17 to 52 feet below the ground surface. The remainder of the annular space above a depth of 17 feet was filled with bentonite pellets. The radius of the borehole was 0.25 feet. The test was conducted in advance outwash deposits that included trace to slightly silty sand and silty gravelly sand. Water was injected into the well at Q = 80 gallons/minute using a water truck equipped with a pump. Three truck-loads of water were added to the well with brief pauses of less than 15 minutes when the trucks were switched. The borehole head (H) was approximately 17.5 feet above the bottom of the well after 175 minutes, although it appeared to still be rising slowly at the end of the test. At Q = 80 gallons/minute and H = 17.5 feet, the BP approach provided a bulk K estimate of 15.6 inches/hour. 8

9 Figure 5: Infiltration test in a permanent well with r = 0.25 feet and an estimated bulk K of 15.6 inches/hour using the BP Approach Measured K Values for Vashon-Age Advance Outwash King County s Wastewater Treatment Division is currently evaluating the feasibility of using green stormwater infrastructure to reduce the frequency of combined sewer overflow (CSO) events within the City of Seattle. As part of this assessment, over 50 infiltration test have been conducted in both temporary wells and permanent wells. The temporary wells were excavated using a vactor truck and were limited to a maximum depth of 10 feet. The permanent wells were drilled using a Sonic drilling rig and ranged in depth from 21 to 92 feet. All the tests were evaluated using the BP approach to estimate bulk K. A total of 37 infiltration tests were conducted in glacially consolidated advance outwash, the likely infiltration receptor material in most locations. Advance outwash in the Puget Sound area was deposited by glacial melt-water streams in front of the advancing glaciers during the last ice age. Although predominately sandy deposits with varying amount of silt and gravel, advance outwash can range from sandy silt to sandy gravels. Figure 6 provides a cumulative distribution plot for estimated bulk K results for advance outwash. Separate plots are shown for the 23 tests conducted in vactor explorations and 14 tests conducted in permanent wells installed using the Sonic drilling rig. As indicated in the figure, the cumulative distribution plots are relatively consistent for the two types of tests, with bulk K ranging from 0.6 inches/hour to 42 inches/hour and a median value of 4.5 inches/hour. 9

10 The vactor explorations can be conducted in a manner that minimizes smearing and compaction of the borehole walls. The similarity of the results for vactor explorations and drilled wells implies that the smearing, compaction, and siltation of the borehole walls were not significant for Sonic drilling in glacially-consolidated advance outwash materials. These results may not be applicable, however, for other drilling methods, porous media with higher silt content, and/or materials that are not glacially consolidated. Figure 6: Cumulative distribution plots for infiltration test results conducted in Vashon-Age advance outwash. Estimating Design Infiltration Capacity King County, the City of Seattle, and numerous other entities are building deep infiltration drains (also known as UICs or drywells) that penetrate glacial till (typically 10 to 40 feet thick) and infiltrate treated stormwater into deeper, more-permeable advance outwash materials. Figure 7 illustrate a typical configuration. Designing the number and configuration of deep infiltration drains requires knowledge of subsurface conditions, including the thickness of the glacial till, the depth to groundwater, and the bulk K of the advance outwash. Design capacities for a broad range of infiltration facility configurations can be estimated based on the dimensions of the proposed infiltration facility (radius and maximum ponded head, H m ), bulk K, and α* (presumed to be 3.7 ft -1 ). Re-arranging Equation 1 provides the following equation for estimating maximum flow capacity (Q m ) for an infiltration facility: Q! = K 2πH!! C + πr! + 2πH! Cα Equation 4 10

11 Figure 8 provides maximum flow capacities for 10-inch diameter infiltration drains given a range of H m and bulk K scenarios. Assuming a bulk K of 5 inches/hour (typical for advance outwash), Q m ranges from 10 gallons/minute for H m = 15 feet to Q m = 100 gallons/minute for H m = 45 feet. Figure 7: Illustration of how deep infiltration drains can route treated stormwater through low-permeability glacial till to more permeable advance outwash. Figure 8: Estimates of deep infiltration drain capacity for a range of maximum hydraulic head and bulk K (10-inch diameter wells). 11

12 Conclusions This paper describes the use of the BP method for estimating bulk K in porous media to support the design of stormwater infiltration facilities. The BP method represents the three components (pressure, gravity, and capillarity) of infiltrative flow from an excavation or borehole completed within the unsaturated zone and is applicable for tests in open excavations (similar to the PIT approach), in temporary wells installed using a vactor truck or excavator, and in deeper drilled wells completed within the unsaturated zone. Future work will focus on addressing the effects of groundwater mounding, anisotropy, and variability within the tested porous media. References Archer, N.A, M. Bonell, A.M. MacDonald, N. Coles, 2014, A Constant Head Well Permeameter Formula Comparison: Its Significance in the Estimation of Field-Saturated Hydraulic Conductivity in Heterogeneous Shallow Soils, Hydrology Research, 45(6), pg Ecology (Washington State Department of Ecology), 2014, Stormwater Management Manual for Western Washington, as Amended in December 2014, Publication Number Elrick, D.E., W.D. Reynolds, and K.A. Tan, 1989, Hydraulic Conductivity Measurements in the Unsaturated Zone Using Improved Well Analyses, Ground Water Monitoring Review, IX(3), pg Reynolds, W.D Saturated hydraulic properties: Well permeameter. p In M.R. Carter and E.G. Gregorich (ed.) Soil sampling and methods of analysis. 2nd ed. CRC Press, Boca Raton, FL. Reynolds, W.D Measuring Soil Hydraulic Properties Using a Cased Borehole Permeameter: Steady Flow Analyses., Vadose Zone Journal, Volume 9, p Zangar, C.N., 1953, Theory and Problems of Water Percolation, U.S. Department of the Interior, Bureau of Reclamation, Engineering Monogram No. 8, Denver, CO. Zhang, Z.F., P.H. Groenevelt, and G.W. Parkin, 1998, The Well-Shape Factor for the Measurement of Soil Hydraulic Properties using the Guelph Permeameter, Soil and Tillage Research, 49, Author s Bio: J. Scott Kindred is the president of Kindred Hydro, Inc., a hydrogeologic consulting firm specializing in stormwater infiltration. He is a registered professional engineer and hydrogeologist with over 25 years of consulting experience. He can be contacted at scottk@kindredhydro.com. Acknowledgements: Thanks to Dr. Dan Reynolds with the Science and Technology Branch of Agriculture and Agri-Food (Canada), and Peter Raunecker with Ing.-Büro Raunecker GmbH (Germany) for their review of this paper and helpful suggestions. To be clear, the author is to blame for any errors or omissions. 12