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INTERNATIONAL SOCIETY FOR MECHANICS AND

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Validation of permeability testing in mudrocks La validation des essais de perméabilité dans les sondages dans la pélite

J.H.MARTIN, Sir Alexander Gibb & Partners, Consulting Engineers, UK M.C.STEVENSON, Sir Alexander Gibb & Partners, Consulting Engineers, UK

SYNOPSIS: This paper presents the findings of numer ical modelling and theoretical studies to establish the viability of borehole permeability testing in over-consolidated clays. Transient effects during a one hour test int erval and hydrogeological and mechanical disturbances caused by drilling are discussed. It is concluded that sho rt duration permeability tests are practical in stiff, heavily over-consolidated clays and mud-rocks but are unlik ely to be useful in less competent clays. This conclusion is supported by ov er 700 field tests on the Lower Lias formation in U .K.

1 INTRODUCTION

Site characterisation for hazardous waste burial in volves All of the constant head field tests were successfu lly the insitu measurement of permeabilities as low as lx lO'l^ completed and their accuracy was proven by very lon g- term m/s. In U.K., four potential low level radio active waste single packer tests, by special recirculating pump- in and repositories on over- consolidated clays and mud-roc ks were pump- out straddle packer tests and by permeability investigated in a 20 M US$ programme. measurements in permanent . A total of 7 10 Before starting field work, studies were made to es tablish insitu permeability tests of all types were carried out in 33 the viability of modified (Corke & Smith 1988), rap id to depths of 150 m, during approximately 200 packer testing in boreholes drilled into the Jurass ic, Lower working days which included a period of heavy snow- fall. Lias at Fulbeck, Lincolnshire, U.K. Much low The potential disturbances which could have invalid ated permeability testing has been carried out in crysta lline these insitu permeability tests in less competent c lays or rocks (e.g. Almen et alii 1980), but less - rigid, cohesive mud- rocks will now be discussed. strata introduce extra difficulties as shown in Tab le 1.

3 HYDROGEOLOGICAL DISTURBANCE Table 1. Disturbances to insitu permeability testing considered To predict the effects of hydrogeological disturban ce in the , a finite difference model was run at Birmingham University. The model simulates radial and vertical flow in Test Period - Transient flow a porous elastic medium. As a reference case, a "pe rfect" Hydrogeological - Drill flush Air Entry constant head test was modelled for a 3 m long, 150 mm Mechanical - Dilatency Smear diameter test section created instantaneously and w ithout disturbance in an isotropic aquifer having a permea bility of 1 x 10‘9 m/s and of 1 x 10‘° m '1. The result for the reference case is plotted in Figure 1 and shows that the flow at the end of a one hour test a t 5 m This paper describes only the findings on the tabulated constant excess head is some 80% greater than that effects which were part of a wider study including predicted by Hvorslev (1951) for steady-state conditions. anisotropy, end effects, fissure loss, non- Darcian flow, It is notable that even after a day, the transient flow is salinity, temperature, barometric and pore chemistr y little closer to the steady-state value. effects. The rate at which flow equilibrium is approached in a constant head test depends upon the aquifer permeab ility and stiffness. The studies included sensitivity ana lyses on 2 FIELD PROCEDURES these parameters. Figure 2 shows the relationship b etween stiffness expressed as specific storage and flow at the end As part of a wide programme of field and laboratory of a one hour reference case test in an aquifer of investigations, over 400 short duration, insitu, co nstant permeability 1 x 10"° m/s. Clearly, the Hvorslev (1 951) head permeability tests were to be carried out in 1 50 mm steady-state flow is most closely approached in the stiffer diameter, water or air-flush drilled boreholes. Sin gle aquifer. packer tests were to be integrated with the 3 m cor e runs as the hole progressed to yield a continuous profile of permeability. Thus, it was important to obtain valid results within approximately 1 hour to minimise standing co sts of drilling plant. Results were to be corrected for time effects using the method proposed by Gibson (1963).

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Various types of drill flush disturbance were modelled. Water- flush drilling introduces an excess head while boring if the natural phreatic surface is below ground. Th is is 4000 - because the flush fluid discharges at ground level, thereby causing a surcharge and flow out from the hole walls. Modelling results for a test performed after drilling with 1 m excess flush head are plotted on Figure 1. It can be 3000 • seen that the test flow at 5 m constant excess head more rapidly approaches the Hvorslev flow than the refer ence case because the drilling started an outflow before the test began. Air- flush drilling was contemplated for certain holes in 2000 - order to recover water samples for stable isotope t esting. This causes a seepage gradient towards the empty ho le during drilling. A test section at the base of a 30 m air- flush borehole, with the phreatic surface 1 m below ground level, was simulated. The time to drill the test se ction and therefore the time the borehole was empty was taken to be 1 hour. A similar period was modelled for removing the drill string and installing permeability testing eq uipment. As an extreme case, leaving the borehole empty over a 1000 weekend before testing was also analysed. Figure 1 shows 900 that despite the hydrogeological confusion caused b y radial flow towards the borehole during the air-flush drilling and 800 subsequent flow away from the borehole during wet cleaning and testing, the 5 m excess head test is s ufficient 700 to overwhelm the disturbances. Flows during the tes t are Key comparable to those of the reference case. 600 ------'Perfect'reference case Air- flush drilling can cause extra difficulties for ------Water-flush permeability testing (Barden et alii (1972)) by int roducing 500 ------Air-flush probable delay bubbles into previously saturated ground. During co re ------Air-flush extreme delay drilling, it is quite common for clay cuttings temp orarily to ------Hvorslev steady- state "block-off" their own exit. Full air compressor sup ply flow _9 of perhaps 1200 kPa is then applied to the ground Aquifer k = 10 m/s near to the cutting bit. If the compressor pressure -6 -i ss= 10 m exceeds the air entry value (AEV) of the soil, then bubbles will be forced into the pores. The AEV is proportional to the surface tension of w ater in contact with air and is inversely proportional t o the effective pore radius. Assuming a pore radius of sa y 1 x Test Duration (seconds) 10"'m, then the AEV of the Lower Lias is approximat ely Logarithmic scale 1400 kPa. In saline , as exists at depth , the 200 AEV will be less since the angle of meniscus contac t and 0.1 1 10 100 1000 10fc 103 10° hence the effective radius is greater. In practice, air will preferentially enter the coarser horizons which ine vitably Figure 1. Influence of duration upon permeability test flows exist in a sedimentary sequence. As a standard proc edure, shut-in tests were made before each permeability te st. 100 These involved monitoring the recovery to the natur al environmental pressure in the completely filled, 3 m long test sections when all connections were closed afte r the packer was inflated. Piezometric measured in shut-in tests at the base of water filled boreholes after air-flush drilling were up to 130 kPa above those m easured in long-term piezometers. Air bubbles were observed emerging from the water surcharged holes in these c ases. Air forced into the soil pores has two opposing effects on permeability measurements. Firstly, the gas in t he pores is more compressible than the water it replac es. This increases the specific storage and therefore e nlarges early transient flow causing the test to over estim ate permeability. Secondly, the bubbles obstruct the flowing pores causing the test to under estimate permeability. Some tests were carried out in air- flush holes for comparison with adjacent water-flush measurements b ut did not systematically prove which error effect dominat ed.

4 MECHANICAL DISTURBANCE

When a borehole is drilled through a stiff material, elastic theory predicts that the loss of radial stress is AO 10'8 10" 10'° 10'" compensated by an equal gain in circumferential str ess. The sizes of these changes are inversely proportion al to Specific storage (m-1 ) the square of the radius. Major strains will occur within Logarithmic scale about one diameter of the borehole wall and yield m ay Figure 2. Variation of measured permeability with g round occur close to the wall. stiffness 288 2/31

Shear will dilate over- consolidated clays and mudro cks to REFERENCES cause pore suction which will dissipate at a rate c ontrolled by the soil’s stiffness, permeability and boundary Corke, D.J., Smith, A (1988) Developments in Insitu conditions. Table 2 shows representative properties of the Permeability Testing, Proceedings of Conference on Field Lower Lias. It was estimated that for the proposed Testing in Engineering , Sunderland, U.K. investigation, suctions would typically be 95% diss ipated Almen, K.E., Carlsson, L., Hansson, K. (1980) Some after 5 hours. Field shut-in tests usually conducte d 2 or 3 experiences from testing in Precambrian rocks of hours after drilling indicated suctions greater tha n 50 kPa Sweden Proc. 3rd International Symposium on Well in 2% of tests. Pore water suctions may also be pro duced Testing in Low Permeability, Environments, Lawrence by the local change in total stress when the consta nt head Berkeley Laboratory, U.S.A test pressure is applied (Gibson (1970) ), and when the Gibson, R.E., (1963) An analysis of system flexibility and packer is inflated against sides of the hole. its effect on time- lag in measurements. Geotechnique 13. No.l, 1- 11, London, U.K. Table 2. Representative properties of the lower lia s Gibson, R.E., (1970) An extension to the theory of the constant head insitu permeability test. Geotechniqu e 20, No.2, 193-197, London, U.K. Pressuremeter undrained shear Barden, L., Madedor, AD., Sides, G.R. (1972) The flow of strength (MPa) 3 air and water in partly saturated clay soil. Fundam entals Pressuremeter shear modulus (MPa) 300 of Transport Phenomena in Porous Media. Development s Liquid Limit (% ) 45 in 2, Elsevier, Amsterdam, Holland. Plastic Limit (% ) 20 Hvorslev, M.J. (1951) Time- lag and soil permeabilit y in Activity 0.5 groundwater observations. Bulletin No.36, U.S. Wate rways Experimental Station, Vicksburg, U.S.A. Wilkinson, W.B. (1968). Constant head permeability tests in clay strata. Geotechnique 18, No.2, 172-197, London , The additional water drawn in to satisfy pore suctions will U.K. cause higher permeability test flows. It was calculated that for the worst case when yield had occurred the permeability in a 5 m excess constant head test wou ld be over estimated by one order of magnitude. The action of a rotary drill locally remoulds the b orehole wall. This effect was initially investigated using a concentric layer, steady-state model based upon Hvo rslev (1951). A 25 mm thick smeared annulus having 1/10 o f the isotropic permeability of the surrounding mass was analysed. It was found that the measured permeability would be 41% less than the undisturbed insitu value . The transient behaviour of the same case was investigat ed using the Birmingham University Finite Difference Model. This showed a 49% under estimate of the permeability bas ed upon the instantaneous flow after one hour, confirm ing the conclusions of Wilkinson (1968).

5 CONCLUSIONS

1. Before undertaking borehole permeability tests in stiff, heavily over- consolidated clays and mud-rocks, it is necessary to establish whether adequate time can be allowed to account for transient effects. The main criterion is the ratio of permeability to specific storage. 2. Hydrogeological disturbance from drilling in the se materials does not seriously affect permeability measurements unless air is forced into the pores. T his is indicated by high shut- in pressures. 3. Mechanical disturbance is likely to invalidate s hort duration permeability test measurements in less competent clays.

6 ACKNOWLEDGEMENTS

This work was part of a programme undertaken by United Kingdom Nirex Limited aimed at identifying a site for a low level radioactive waste repository. The help an d work of Prof. K.R. Rushton of Birmingham University is acknowledged with thanks.

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