Transient Seepage Spreadsheet – Notes on Input Parameters

Introduction

These notes are intended to assist users of Excel spreadsheet:

Transient Response LJ

Specific reference is made to Version 1.01 of the spreadsheet.

The spreadsheet is intended for calculation of the transient response of groundwater in a leaky confined aquifer which extends under a flood defence dike. In particular the transient head at the toe of the dike can be determined and factors of safety against uplift and seepage failure can be assessed. The method used to calculate transient response of the aquifer follows Li H & Jiao JJ (2001), “Tide-induced groundwater fluctuations in a coastal leaky confined aquifer system extending under the sea”, Water Resources Research, Vol 37(5) p 1165-1171.

For input hydrographs of very long duration the solution tends to that of steady state analyses such as the USACE method given in EM-1110-2-1913 “Design and Construction of Levees”.

Warning

The spreadsheet is intended to calculate the head response to a transient hydrograph such as a tide, storm surge or flood. A suitably conservative approach must be adopted for the duration of the input hydrograph and other controlling parameters. Failure to consider these factors adequately could lead to a catastrophic collapse of the dike.

Aquifer Parameters

1. Coefficient of volume compressibility, mv.This parameter has a major influence in the storativity (S) of the aquifer. The larger the value of mv, the larger the value of S. Larger values of S generate slower responses in the aquifer and lower maximum aquifer heads. Hence small values of mv are more conservative with respect to aquifer heads than large values.

Parameter mv is the inverse of the drained bulk modulus of the soil K’. The bulk modulus may be estimated from the drained Young’s Modulus E’ with an assumed Poisson’s Ratio. For assumed isotropic elastic conditions:

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E’ may determined from correlation with SPT or CPT data. Note that the effective stress changes generated by the input hydrograph are likely to be small and hence the stiffness values will be relatively high.

Hydrograph Parameters

1. Amplitude of hydrograph, AThe hydrograph is treated as a cosine function in this spreadsheet. A cosine function is fitted to the actual hydrograph on the first worksheet of the spreadsheet.

The value of A would normally be the height from ground level at the toe of the dike to the peak level of the actual hydrograph.

2. Time to reach peak, Tp

The value input for time to peak Tp controls the period of the cosine function. It is conservative to envelope the rising and peak sections of the actual hydrograph. It may be necessary to check the sensitivity of stability results to longer hydrographs. for example the value of Tp may be increased by 50% or 100% to check sensitivity to the hydrograph duration.

3. Tidal efficiency. Te

This parameter relates to the response of the aquifer head upstream of the dike to an instantaneous change of loading from the water above the foreshore. This parameter is introduced in Li and Jiao (2001) but not explained in detail. A conservative approach is to assume a value of 1.00 for this parameter unless better data is available.

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