Designing Pumping Tests for Geotechnical Investigations

May 6, 2019

Recent ground investigation contracts for major infrastructure projects have featured increasing elements of groundwater testing, driven partly by the evolving environmental permitting and consent arrangements for temporary groundwater control and discharges from construction works.  BDA member companies are being asked, as part of the GI specification, to undertake more complex pumping tests and to construct more groundwater investigation boreholes.

It has long been recognised in the geotechnical community that pumping tests are the gold standard for determining permeability.  This particularly applies to rock aquifers, such as chalk or sandstone, where the hydraulic response is often governed by joints which cannot be usefully interpreted from the log, core samples or other in-situ testing. However, there is no one value of permeability in the ground waiting to be measured and blind application of standard procedures and specifications can result in unnecessary expense, missed opportunity and money wasted.

Field methods for pumping tests are defined in standards including; BS ISO 14686: 2003 Hydrometric determinations. Pumping tests for water wells. Considerations and guidelines for design, performance and use; and BS ISO 22282-4: 2012 Geotechnical investigation and testing. Geohydraulic testing. Pumping tests. These standards focus on conventional testing with a single, long screen pumping well and piezometer array using procedures and analytical methods developed by hydrogeologists to assess groundwater resources. The main results from such a test are the average horizontal permeability, the storage coefficient, the test well characteristics and, if the test duration and piezometer array permits, the far field boundary conditions. All useful for assessing water resources, but sometimes of limited relevance for the design of a groundwater control strategy for a construction project, or when seeking the necessary permissions and consents.

For deep foundations or excavations which extend to below the standing groundwater level, groundwater is often managed using a combination of physical cut-offs (sheetpiles, diaphragm walls or grouting) plus a pumping scheme. Specifiers are also increasingly interested in recharge of the pumped groundwater both as a means of disposal but also to limit derogation of water resources, mitigate against settlement, or to prevent the movement of legacy contamination. Under these circumstances the permeability profile (horizontal and vertical), pumping/recharge well performance and far field boundary conditions will be the main drivers for the groundwater control scheme. Knowledge of the ground profile and broad construction methodology should allow a pumping test procedure to be designed to address the specific parameters which underpin the groundwater control strategy. Furthermore, the marginal cost of the testing may be reduced by specifying and locating the test well(s) and piezometer array so that, as far as possible, they can form part of the main works scheme.

Alternatives and enhancements to conventional pumping test strategies include:

• Short screen wells: Specific geological horizons or levels can be targeted by short screen wells. These can be used to generate vertical flow with the resulting hydraulic gradient monitored with suitably targeted piezometers. This can allow an assessment of vertical permeability or variation of permeability with depth, which can be particularly important where there is a need to assess inflow under a partially penetrating physical cut-off.
• Multiple well testing: Designing pumping tests in marginal aquifers comprising silty sands, sandy silts or lower permeability rock requires particular care. The yield from a single well may not be sufficient to provide significant drawdown (hydraulic stress) to derive useful aquifer parameters or assess far field boundaries. Under these circumstances multiple well testing, comprising three or four wells typically in a square or triangular array may be appropriate. The test procedure may include initial pumping on a single well with pumping commencing on subsequent wells as the test progresses.
• Vacuum testing: The yield of wells in lower permeability soils can sometimes be significantly increased by application of a vacuum. This can readily be included in a test programme, enhancing the data and potentially providing information which may reduce the number of wells required for the main works scheme.
• Abstraction-recharge trials: Testing of recharge wells makes sense where recharge is planned but can also be a method for generating and enhancing vertical hydraulic gradients for vertical permeability measurement; as well as a means of disposing of the groundwater from a pumping test.
• Pilot testing: Pilot testing can be used for deep excavations which rely on partially penetrating cut-offs to curb groundwater inflows. Test pumping may be undertaken prior to the cut-off installation to provide information for review of the target cut-off depth, and subsequent to the cut-off installation to confirm inflows prior to excavation.
• Geophysical testing: This provides a relatively low cost enhancement to the data from a pumping test which can be particularly useful for unlined boreholes in rock such as chalk or sandstone. A wide range of geophysical equipment is available including tools to provide high resolution north orientated images of the borehole wall and pumped flow logs which can reveal the levels that groundwater is entering the well giving an indication of the variation in permeability with depth.

One reason for the historic focus on single long screen well testing is that the resulting radial flow is amenable to analysis using simple closed-form expressions. Most of the procedures outlined above can provide data for these methods of analysis but nowadays the availability of numerical modelling tools allows much more sophisticated interpretation of the results and assessment of the relevant parameters.

Of course, it is essential that any installations are carried out according to best practice such as the BDA Guidance for Safe Operation of Cable Percussion Rigs. Although there is some guidance on the operation of drilling rigs, due to the variable ground conditions within the UK, and differing pumping test objectives, there are  limited standards and guidance on how the abstraction well should be specified and installed. BS ISO 22475:2006 Geotechnical Investigation and Testing Section 9.3 states that piezometers should be installed when ground conditions and stratification are known. However, pumping tests often fall into the ground investigation phase of a project and are often bundled together in one contract along with the SI drilling and testing. General ground investigation contractors may not have the necessary plant and equipment, or the experience and expertise, to install abstraction wells, which are typically larger diameter and require different techniques to investigation boreholes. Specifying the well depths, diameters, filter pack, slot size, as well as understanding the objectives of the test, are key to delivering a pumping test of value.

Thankfully, very short term pump test periods of a few hours are no longer commonly prevalent. Given the significant cost of installing and equipping test wells, the short comings in the data verses the savings involved for short tests never made sense. Nevertheless careful consideration does need to be given to test duration with appropriate allowance for background monitoring, equipment/well testing, variable and constant pumping, and recovery monitoring. Test periods may be influenced by the prevalence of tidal conditions, seasonal or other variations, as well as individual well testing for multi-well set-ups.

For major projects, where there may be two or more phases of geotechnical investigation, careful consideration needs to be given to the timing of pumping tests. Pumping tests undertaken early on may be useful in developing the construction methodology but may not be designed to generate the most useful data to address a particular groundwater control strategy. Pumping tests undertaken later on can be designed to generate relevant data but the opportunity to exploit an alternative, perhaps lower cost, construction method may have been missed. There are no simple guidelines to resolving this issue other than to ensure that a clear assessment and review of the planned excavation and construction process is undertaken, and that the groundwater management risks and associated design data requirements are understood prior to each phase of geotechnical investigation.

In summary, there are limited standards and guidance available to help provide the specification of the pumped well, or wells; this generally relies on the experience and knowledge of a specialist contractor or consultant. There are available standards providing a basis for undertaking pumping tests, but to simply follow these may not provide the required information. A good understanding of the construction method and sequence, the ground conditions, and key groundwater risks is required to allow the objectives of the test to be identified. This should ensure that the pumping test is specified so that it provides the relevant information for the subsequent design, allows the risks to be understood and mitigated, and allows the works to be delivered in a cost effective way.

Author: Richard Fielden BDA Vice Chair