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Groundwater Investigations

John Poullain, P.E.


Course Outline

This three-hour online course discusses the guidelines and criteria employed in groundwater investigations. Basic concepts and methods used to determine subsurface conditions pertaining to groundwater levels, pore water pressures and the permeability of subsurface materials are considered. Installation methods for observation wells and devices commonly used for sensing and measuring water levels in boreholes and observation wells are discussed. Permeability is measured in the field by a variety of tests, which include seepage, pressure or packer, pumping, slug and the piezocone dissipation tests. Quality assurance for testing, obtaining measurements and logging subsurface data are considered. The AASHTO and ASTM designations for the commonly used tests are provided.

This course includes a multiple-choice quiz at the end, which is designed to enhance the understanding of the course materials.

Learning Objective

At the conclusion of this course, the student will:

Intended Audience

This course is intended for civil engineers and project engineers.


Course Introduction

A ground water evaluation is part of the geotechnical investigations for most projects. The design of building foundations, excavations, fills and slopes require knowledge of the present groundwater conditions and potentials for groundwater seepage as well as other soil parameters including strength; soil characteristics, problem soil considerations and soil behavior under imposed loads. Determination of groundwater elevations and flow are required for construction and maintenance of earth dams, building excavations, foundations, air fields, roadways and detection of contaminant migration from hazardous waste fills and land fills. Groundwater investigations involve gathering data on groundwater levels and pressures and the permeability of the subsurface materials. These criteria and guidelines must be considered in order to correctly perform the appropriate tests especially since groundwater and permeability tests can be expensive but not nearly as expensive in case of a project delay or failure. The limitations of the tests must be considered and planned for with an alternative or supplemental test.

Observation Wells and Monitoring Wells - These wells are used to determine groundwater levels and pressures. Monitoring wells are installed to sample groundwater for chemical quality and level of contaminants and to determine the watertable elevations and aquifer gradients. Observation wells perform similar functions by determining the water table elevations. The data gathered from the wells is helpful in determining contour maps to analyze hydraulic gradients and directions of groundwater flows. In case of underground contamination the areas and limits of contamination can be assessed and remedial repairs designed for. Observation wells are best used where groundwater conditions are relatively stable. They are easy to install and read and must be in accessible locations.

Observation and monitoring wells must be developed by removing any sands, silts, clays and other material from water bearing areas surrounding the well screens. Development will permit the well to provide representative and relatively clean water samples and allow accurate pumping test evaluation. The procedure consists of cyclic pumping and surging and should continue until cuttings and any materials are removed from the well. There are time restrictions for development after well construction is completed. For instance development should not begin sooner than 24 hours after a well is constructed if cement grout is used to fill the annular spaces.

Piezometers - Piezometers are instruments installed in a well used for measuring water elevations at the sensor levels. They are useful where groundwater conditions are not stable and thus are more sensitive to groundwater changes than are observation wells. There are several types of piezometers including the standpipe, pneumatic, vibrating wire, and multi-level vibrating wire. Some can be automated to remotely read the data. Standpipe piezometers are inexpensive, simple to operate but cannot be read remotely. Pneumatic types can be read remotely but are expensive and difficult to automate and will not function accurately when condensation blocks the pneumatic tubing.

Piezometers are used to:

Slug Tests Aquifers and wells may also be tested with slug tests. They are quick, easy, and can provide quick estimates of the aquifer properties in the near vicinity of the test well. They may be used to initially assess an aquifer. However, they are not reliable and the results must be verified and complemented with pumping tests to reliably quantify the aquifer properties.

Pumping Tests (aquifer testing) - Pumping tests are used to characterize an aquifer by pumping a test well and observing the aquifers response in an array of observation wells placed around the test well. The pumping test may use monitoring wells but piezometer types are preferred. An aquifer test differs from well testing because it quantifies the response of the aquifer and not just the behavior of the well being tested. The water levels will drawdown in the wells as water is pumped from the test well located in the well array. The pressure of an aquifer feeding the observation wells declines and the observation well drawdowns are measured. Changes in the hydraulic head results in lower observation well water levels. Drawdowns decrease with distance from the test well and increase as the time for pumping continues.

In order for the data to be reliable, water levels, time of measurements and pumping rates must be carefully measured and recorded. Any changes in data such as pumping rates not recorded would lead to very erroneous assessments of the aquifer properties.

Software is used for graphical analysis, data managing and reporting and presentation of pumping test and slug test data. Software provides the means to predict hydraulic properties of an aquifer: conductivity, storativity and transmissivity and pump test data can be compared with multiple solution methods for greater accuracy. Presenting and reporting are made easier by importing site maps from several formats, on screen displays of water level vs. time graphs and base maps showing all wells and any new wells as the test progresses. Data can be analyzed for any aquifer type, different solutions for the same data and tells when data is missing.

Inclinometers - Inclinometers are devices used to measure any horizontal movement of soil over time. They are installed in boreholes to record changes from the vertical by measuring horizontal movements of borehole casing from the bottom of the casing to the top. Initial readings serve as the baseline and subsequent readings are compared to determine the direction and amount of movement. They are used to monitor slope failure, to find the depth of failure planes and for stability analysis. Installation of the casing, operation and data reduction can be complicated.

As with any scientific investigation certain procedures must be followed and documented to obtain accurate data and maintain quality control or the data generated will be valueless. Because there are a large variety of soils and a large variety of applied soil mechanics problems there is also a large variety of soils tests for determining the engineering properties of soils. Accurate measurements are of great importance and the test equipment must be properly maintained, otherwise the test results will be valueless and misleading. Poorly constructed and maintained equipment will produce erroneous test results.

Existing subgrades may have poor strength or instability due to excess clay, expansive clays, silts, fine sands, voids, collapsing soils or high watertables. The existing watertables, soil permeability as well as other soil properties must be known to achieve the design bearing capacities required to protect from settlement. There are problem soils such as loess, hydraulic fills and tailings, which have collapsing or low-density structures, and when saturated have large decreases in volume and loss of strength. Other soils, which contain clays such as bentonite, can expand and increase in volume when exposed to water. Expansive soils however can shrink or decrease in volume when water is not present. There are also dispersive clays so named because the soil particles are not structurally sound and can easily disperse or detach and erode in still water.


Course Content

This course is based on Chapter 6 of the US Dept of Transportation FHWA publication FHWA NHI-01-031, "Subsurface Investigation-Geotechnical Site Characterization", (2001 Edition, 18 pages), PDF file and the course paragraph "Course Introduction". The course is also based on Chapter 4 (partial) of the US Army Corps of Engineer manual EM 1110-2-1421, "Groundwater Hydrology", (1999 Edition, 11 pages, PDF FILE.

The links to both documents are as follows:

US Dept of Transportation FHWA publication "Subsurface Investigation-Geotechnical Site Characterization", Chapter 6

US Army Corps of Engineers manual EM 1110-2-1421 "Groundwater Hydrology", Chapter 4.

Please click on the above underlined hypertext to view, download or print the document for your study. Because of the large file size, we recommend that you first save the file to your computer by right clicking the mouse and choosing "Save Target As ...", and then open the file in Adobe Acrobat Reader. If you still experience any difficulty in downloading or opening this file, you may need to close some applications or reboot your computer to free up some memory.

Course Summary

This course should serve as a guide for groundwater investigations and to better understand the variety of tests and devices used to gather the required data along with their advantages and limitations. Basic concepts and methods used to determine groundwater elevations, pressures and to measure permeability of subsurface soils are presented. Devices and methods for measuring water elevations are also presented. Tests for permeability include seepage, packer, slug, and pumping tests. The importance of quality assurance for groundwater measurement and permeability testing is stressed. This course should serve as a guide for selecting the appropriate methods needed to determine water levels and tests for permeability to avoid potential settlement, liquefaction or ground movement problems.


Related Links

For additional technical information related to this subject, please refer to:

http://www.dnrec.state.de.us/water2000/
Information and applications describing the construction methods for observation wells, water wells and materials used. Provides in formation on project application.

http://water.usgs.gov/pubs/wsp/wsp1536-E/html/pdf.html
Discusses the theory of aquifers, test data and observation wells.

http://www.slopeindicator.com/instruments/piezo-intro.html
Discusses groundwater instruments, piezometers, types, wells, inclinometers, extensometers and data loggers.


Quiz

Once you finish studying the above course content, you need to take a quiz to obtain the PDH credits.

DISCLAIMER: The materials contained in the online course are not intended as a representation or warranty on the part of PDH Center or any other person/organization named herein. The materials are for general information only. They are not a substitute for competent professional advice. Application of this information to a specific project should be reviewed by a registered architect and/or professional engineer/surveyor. Anyone making use of the information set forth herein does so at their own risk and assumes any and all resulting liability arising therefrom.