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Analysis of clay layer compression

Charlie Fitts
,
University of Southern Maine, Geosciences Dept
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Summary

Students analyze time series data for ground subsidence in response to dewatering, to derive estimates of the compressibility and vertical hydraulic conductivity of a clay lens.

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Context

Audience

This is an upper level undergraduate hydrogeology course.

Skills and concepts that students must have mastered

Students need algebra skills, and must understand the processes of compression and time-dependent consolidation.

How the activity is situated in the course

This is an end-of-chapter exercise in Chapter 5 of "Groundwater Science" (2002). This chapter deals with deformation and storage concepts. In my course, this follows sections on hydrology, physical properties, flow, and groundwater geology.

Goals

Content/concepts goals for this activity

To better understand the mechanism of compression in response to pumping, and to understand how that compression is slow and limited in low-conductivity materials (time-dependent consolidation).

Higher order thinking skills goals for this activity

The hardest concept here is to understand what time-dependent consolidation is, and then understanding how to apply Terzaghi's theory of 1-D consolidation to this problem to estimate the vertical hydraulic conductivity of the compressing clay layer. Understanding the compressibility of the clay layer and analyzing for it is easier, but still good critical thinking.

Other skills goals for this activity

Students can download the data for the subsidence graph from the book's web site, and then import this into a spreadsheet for analysis.

Description of the activity/assignment

Students analyze time series data for long-term ground subsidence in response to dewatering. From this analysis, the students can back-figure the compressibility of a clay lens that is compressing, and the vertical hydraulic conductivity of the clay. The data may be downloaded from the book web site and imported into a spreadsheet for analysis.

Determining whether students have met the goals

I check their answers and also their explanation and write-up. I go over these with the class and address their questions.

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Other Materials

Supporting references/URLs

The book's web site with a comma-delimited file of the data to analyze is at: http://www.elsevierdirect.com/product.jsp?isbn=9780122578557