Centrifuge permeameter facility

Posted 14 August 2013

The centrifuge permeameter.

A state-of-the-science, world class centrifuge permeameter facility has been approved as part of the NCGRT Program 1B. Funded by the ARC and NWC, the facility will be commissioned in late 2010. A contract was signed between UNSW and Broadbent Centrifuges in March 2010 to construct, test and commission the new centrifuge facility at the UNSW Water Research Laboratory.

The NCGRT centrifuge permeability facility would be unique in the following aspects.

The NCGRT centrifuge will be the only core permeameter centrifuge in the world capable of direct testing of minimally disturbed drill core of 100 mm diameter (Figure 1).

This type of drill core can be retrieved without rotation, distortion or compression of geological material using standard coring equipment. Other permeameters around the world are designed to test 33 diameter mm core or 151 mm diameter core. The 100 mm core is considered to be the optimum diameter to achieve representative flow in a relatively large sample, while minimising drilling costs and transport/handling issues of geological material. It will be possible to sample from the core while the centrifuge is in operation. This 'in-flight' operational capability is a major benefit for the collection of representative samples.

NCGRT Centrifuge - Core Permeameter Modeling
Figure 1. NCGRT Centrifuge - Core Permeameter Modeling. A) Base unit with permeameters, B) Small (R= 0.47 m, ~0.5 L) and large permeameter schematic (R = 0.3 m, ~5 L). Minimally disturbed core of 100-150 mm diameter is fitted in both types of permeameters that operate in duplicate (ie. one type of permeameter at a time).

The NCGRT centrifuge will be capable of higher g-levels than typical geotechnical centrifuges (ie. 300 gmax at 0.7 m radius as compared with 80 gmax typical). The high g-level is essential to drive fluid through low permeability aquitard material that is to be tested. Calculations show that the design centrifuge would be capable of testing material with hydraulic conductivity of 10-12 m/s, which is four orders of magnitude smaller than material that is considered by the EPA to be of low permeability.

The modular centrifuge base of the NCGRT facility would be the only one in the world operating with both core permeameter and tank testing systems. The swing-tank rotating on a small beam would enable testing of heterogeneity of groundwater systems (see Figure 2). The tank could be set up to study layered groundwater systems, heterogeneous sediments, discontinuous aquitards, and study the potential impact of fault lines and leaky bores at a suitable physical scale. A 0.3 m thick groundwater model at a scale of 300g would represent a 90 m thick groundwater system in the real world. The ability to speed up time using this approach provides an unparalleled opportunity for innovative research.

NCGRT Centrifuge - Tank Modelling of Hetereogeneity
Figure 2. NCGRT Centrifuge - Tank Modelling of Hetereogeneity - A) Small beam (R = 0.65m attaches to same centrifuge base as in Fig. 1. B-C) Swing tank and PIV Camera Mount (170 mm circumference, 180 mm radius, 300 mm high, ~10 L capacity).

Advancing science

The novelty of a 'time-machine' for scientific studies can generate both publicity and internationally relevant science. A popular article on 'time-machine' research was recently posted on this web site. Research opportunities arising from a centrifuge permeameter provide an outstanding example not only of a state-of-the-science piece of equipment, but leading edge research focused on fluid flow processes over spatial and time scales that cannot be readily studied.

Scientific advances occur typically at the interfaces of disciplines where gaps in knowledge are bridged. Centrifugation is an accepted technique in geotechnical engineering, yet remains largely unexplored in hydrogeology. Whilst geotechnical applications of centrifuge modelling reached a degree of maturity with the publication of the first text book in the mid-1990's (Taylor, 1995), the power and utility of centrifugation is not recognised or appreciated within the water resources and hydrogeological profession.

It is also recognised that there is a critical gap in knowledge between soil science and hydrogeology. Soil studies of deep drainage and infiltration focus on the unsaturated zone to a maximum depth of 6 m, while hydrogeological studies focus on the saturated zone where the first high yield aquifer may occur at 20-30 m depth. Centrifugation could study geological media in saturated, unsaturated and variable saturation state and help bridge these gaps.

Centrifugation can directly address questions of sub-surface flow at scales that are not otherwise possible:

  • Time scales - Centrifuge models can simulate flow over thousands of years within a reasonable experimental time frame of weeks or months. Centrifugation can quantify flow processes such as hydraulic diffusivity that occur over decades and hundreds of years in the environment. Centrifugation can provide realistic measurements and observations to support numerical modelling of very long term processes that occur in the sub-surface (eg. hydraulic responses in sedimentary basins over millennia).
  • Spatial scales and heterogeneity - Small centrifuge models can simulate thick geological media or replicate numerous permeability type tests to assess spatial heterogeneity. Studies of leakage through aquitard windows has for example identified pathways on a scale of metres and tens of metres as being critical, yet unquantified in groundwater systems. Scale modelling of complex groundwater systems in a centrifuge tank would provide this capability.
  • In-situ stresses - Centrifugation enables testing of geological media at in-situ stresses that occur at depth of burial. This capability is particularly important for compressible media such as clay aquitards. For example, a typical triaxial cell can replicate in situ effective stresses but is not capable of achieving large total stresses (i.e., overburden pressure) for cores from 10 m depth or more (Wright et al. 2002).
  • Direct observation of physical processes - Centrifugation can obtain realistic input data for numerical modelling by direct observation and measurement performed in flight. Development of techniques such as centrifugation has lagged considerably behind complex numerical modelling and computing power.

Strategic industry investment

While there are clearly applications of centrifuge modelling for industry, there will likely be an extensive period of promotion and demonstration that is required for industry to become aware of the prospects and powerful utility of centrifuge modelling. Examples of strategic industry involvement is outlined below:

  • Pore water extraction - Extraction of water for contaminant studies of estuarine sediments in major cities. Centrifuge extraction of pore water is a preferred method since the chemistry of pore water is not affected by dilution or hydraulic filtration. The UNSW WRL consulting team have given several indicative quotes and discussed manly of requests for pore fluid separation each year, demonstrating that there is a need for such methods. Current techniques using very small centrifuges (10-100 mL vials) and hydraulic press devices are too time-consuming and therefore prohibitively expensive.
  • Rapid measurement of soil-water-characteristic-curves and K-functions. This is currently the basis of a commercial UFA centrifuge business in the U.S.A. In Australia, the market would need to be developed, but could potentially be used by the agricultural sector in the medium term.
  • Fluid separation technology - The UNSW Water Research Laboratory has previously developed patented fluid separation technology based on a small radius, high velocity centrifuge device. Further development of this technology would be possible with the NCGRT centrifuge
  • Uranium mining and hazardous waste disposal. The potential for long-lived nuclear constituents to migrate and persist in the sub-surface environment can be assessed using centrifuge methods. In fact, much of the research and commercial market for mini-permeameter centrifuges (UFA type) in North America has been driven by uranium mining and waste disposal issues.
  • Seepage through engineered flow barriers. The mining and waste-disposal industries could over the medium to long-term horizon generate a range of project-specific design testing for the centrifuge. There is an immediate application in the coal mining industry where there is uncertainty of the integrity of swelling and cracking clays overlying areas of potential dewatering. Other immediate applications include potential seepage through clay-caps that overlie landfills or mined out pits, and the effectiveness of flow barriers where contaminants may alter the integrity and permeability of the flow barrier over the long term.

Worldwide Distribution of Geotechnical Centrifuges (Courtesy: CCORE, Canada, undated)
Figure 3: Worldwide Distribution of Geotechnical Centrifuges (Courtesy: CCORE, Canada, undated). Note that there is currently only one other centrifuge permeameter testing facility of the type to be commissioned by NCGRT (University of Texas, USA).

Links and further information

References

Acworth, I. and Timms, W (2009), Investigation of deep drainage through smectite-dominated clays at Breeza on the Liverpool Plains of New South Wales. Australian Journal of Earth Sciences 56 (71-86).
Conca, J.L. and J. Wright. (1998). The UFA method for rapid, direct measurements of unsaturated transport properties in soil, sediment and rock, Australian Journal of Soil Research 36, 1-25.
McCartney, J.S. (2007). Determination of the Hydraulic Characteristics of Unsaturated Soils using a Centrifuge Permeameter. PhD Thesis, The University of Texas at Austin, Faculty of the Graduate School.
Nimmo, J.R., and K.A. Mello. (1991). Centrifugal techniques for measuring saturated hydraulic conductivity. Water Resources Research 27, no. 6: 1263-1269.
Taylor, R.N. (1995). Geotechnical centrifuge technology. Taylor and Francis, London.
Simunek, J and Nimmo, J. (2005). Estimating soil hydraulic parameters from transient flow experiments in a centrifuge using parameter optimization technique. Water Resources Research 41(4).
Timms, W, Hendry, J., Muise J, and Kerrich, R. (2009). Coupling Centrifuge Modeling and Laser Ablation ICP-MS to determine contaminant retardation in clays. Environmental Science and Technology. 2009, 43, 1153-1159
Timms, W.A., Hendry, M.J. (2008). Long term reactive solute transport in an aquitard using a centrifuge model. Ground Water 46(4): 616-628
Timms, W.A., and M.J. Hendry. (2006). Quantifying the impact of cation exchange on long-term solute transport in a clay-rich aquitard. Journal of Hydrology 332: 110-122,
Timms, W.A. and Hendry, M.J., (2003). Application of centrifuge modelling to assess long term brine migration in thick clay till, Saskatchewan, Canada., Australian Institute of Mining and Metallurgy Conference "Water in Mining", Brisbane, October 13-15, 2003., pp. 363-372.
Timms, W. and Hendry (2004). Quantifying the impact of ion exchange on long-term solute transport in clay aquitards using centrifugation and geochemical modelling. Geological Society of America, 7-10th November 2004, Denver, USA.
Timms, W., J. Hendry. (2003). Verifying accelerated physical modeling of reactive solute transport: the long term development of pore water solute profiles in a thick clay till. European Geophysical Union Convention, Nice, France, April 2003.
Wright, M. D., P. Dillon, P. Pavelic, P. Peter, and A. Nefiodovas. (2002). Measurement of 3-D hydraulic conductivity in aquifer cores at in situ effective stresses. Ground Water 40, no. 5, 509-517

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