Chapter 19: Single Vertical Dikes

Analysis and Evaluation of Pumping Test Data

Second Edition

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Background• Two types:

1. Igneous2. Sedimentary

• Igneous dikes are formed when magma rises through the subsurface and intrudes, or cuts through, pre-existing strata.

• Igneous dikes are normally coarse-grained due to slow rate of cooling.

• Weathered material may form sedimentary dikes when sediment fills a pre-existing crack.

• Sedimentary dikes are not as common as igneous dikes. • Both types have high aspect ratios.

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Background

Dikes can be highly permeable.

If a single, permeable, vertical dike bisects an aquifer whose transmissivity is several times less than that of the dike, a specific flow pattern will be created when the dike is pumped.

“Trough of Depression” similar to cone of depression

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Assumptions and Conditions The dike is vertical and of infinite extent over the length influenced by the test. The width of the dike is uniform and does not exceed 10 m The flow through the fracture system in the dike is laminar (Darcy!) The uniformly fractured part of the dike can be replaced by a representative

continuum to which spatially defined hydraulic characteristics can be assigned The fractured part of the dike is bounded above and below by impermeable

weathered zone and solid rock, respectively. The well fully penetrates the fractured part of the dike and is represented by a

plane sink; flow through the dike towards the well is parallel The hydraulically connected country rock aquifer is confined, homogeneous,

isotropic, and of infinite areal extent. All water pumped from the well comes from storage within the composite system

comprised by the dike and aquifer. The ratio of hydraulic diffusivity of the dike to the aquifer is not less than 25. Well-losses and well-bore storage are negligibe.

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Methods for Observation Wells

• Partial solutions for the drawdown in observation wells:– Boonstra – Boehmer’s curve fitting method– Boehmer – Boonstra’s curve-fitting method

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Boonstra – Boehmer’s Curve Fitting Method

To analyze the drawdown behavior for early and medium times along the pumped dike:

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Boonstra – Boehmer’s Curve Fitting Method

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Example: Observation Well in the Dike, p. 284

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Example: Observation Well in the Dike, p. 284

WdTd = 2.6 x 104 m3/d

WdSd = 4.3 x 10-4 m

S T = 3.2 x 10-4 m2/d

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Boehmer – Boonstra’s Curve-Fitting Method

To analyze early and medium time drawdown behavior in observation wells drilled in the aquifer along a line perpendicular to the dike and even with the pumped well:

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Boehmer – Boonstra’s Curve-Fitting Method

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Example: Observation Well in the Aquifer, p. 285

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Example: Observation Well in the Aquifer, p. 285

T/S = 2.7 x 105 m2/d

Combining results from observation wells in the dike and aquifer, we get separate values for transmissivity and storativity of the aquifer:

T = 9.3 m2/dS = 3.4 x 10-5

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Methods for Pumped Wells

1. For early and medium pumping times2. For late pumping times

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Early and Medium Pumping Times

At early times, all the pumped water is from dike storage (no water from aquifer storage). At medium times, most of the drawdown is governed by parallel flow from the aquifer into the dike (no water from dike storage).

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Early and Medium Pumping Times

Early time:

Medium time:

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Early and Medium Pumping Times

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Late Pumping Times

At late times, the flow in the aquifer is no longer considered parallel, but rather pseudo-radial.Solution for the drawdown in the pumped well during late time:

For a pumping test of usual duration, this method can only be applied to fractures or thin dike structures.

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Late Pumping Times

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Example: pumped well, p. 285 - 286

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Example: pumped well, p. 285 - 286

(WdTd) √(ST) = 425 m4/d3/2

Substituting the values of dike width and transmissivity (WdTd) and aquifer storativity and transmissivity (ST) obtained with Boonstra – Boehmer’s method into (WdTd) √(ST), the value is 465.