Abstract
The borehole permeameter (BP) method was developed in the 1950s by the United States Bureau of Reclamation to estimate saturated soil hydraulic conductivity (KS) in shallow boreholes completed above the water table. The approach has been improved over the years, and now accounts for flow due to pressure, gravity and soil capillarity. However, the BP method is calibrated only for normally consolidated soils and ponding depth (H) versus borehole radius (r) ratios (H/r) ≤ 22. The primary objective of this study was to recalibrate the BP method for use in glacially over-consolidated soils with H/r ranging from 0.05 to 200. Recalibration consisted of using numerically simulated steady BP flow for five representative glacially over-consolidated soils to update the BP shape function fitting parameters (Z1, Z2, Z3) for nine specified KS values and 15 test pit and borehole configurations. Four sets of fitting parameters were determined, which apply for H/r ≤ 20, H/r ≥ 20, soil with <12% silt content, and soil with >12% silt content. Relative to specified KS, the updated shape function parameters yielded BP estimates of KS with a maximum error of 13% and an average error of 3%, whereas the original shape function parameters (developed for normally consolidated soils and H/r ≤ 22) produced a maximum KS error of 94% and an average error of 23%. The numerical simulations were also used to develop criteria for estimating time required to achieve steady BP flow, and for correcting BP estimates of KS where steady flow was not achieved.
Highlights
Structures for infiltrating stormwater runoff are common in areas with declining groundwater resources or excessive surface erosion
The primary objective of this study was to develop revised C(H/r) shape function fitting parameters that would improve the accuracy of the borehole permeameter (BP) method for a broad range of test configurations (i.e. H/r ratios between 0.05 and 200) in glacially over-consolidated soils
The recalibrated BP analysis provides KS estimates with maximum error of 13% and average error of 3% for a selected range of representative soils (Table 4) and H/r ratios (Table 3)
Summary
Structures for infiltrating stormwater runoff are common in areas with declining groundwater resources or excessive surface erosion. Infiltration of stormwater is a key aspect of low impact development (LID) and green stormwater infrastructure (GSI), which are desired and/or required by many jurisdictions throughout the world. Structures for infiltrating stormwater include ponds or basins, gravel-filled trenches, bioretention swales, drywells or “soakaways”, subsurface “leach fields”, and infiltration wells or “underground injection control” (UIC) wells. In most cases, these facilities are installed above the water table, and thereby infiltrate into unsaturated soil, i.e. the soil “vadose” zone. Many jurisdictions require site investigations and formal sizing calculations to obtain more reliable estimates of design infiltration rate and capacity
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