effect of longwall mining on ground permeability ans ... · trcugh above a longwall mining...

SUMJV'i.ARY

SECTION I Investigation and Evaluation of Surface and Subsurface Drainage

7 Effect of Longwall Mining on

Ground Permeability and Subsurface Drainage

by B. N. Whittaker, R. N. Singh, and C. J. Neate, Department of Mining Engineering,

University of Nottingham, Nottingham, United Kingdom

The paper briefly reviews mining subsidence chc.racteris tici:: associated with longwal l mining and discusses the implications of subsidence on surface and subsurface drainage pattern changes. Both surface subsidence and subsurface sub~>idence aspects are discussed. Investigations are desert bed into grour.d permeability changes betwe.en the surface and the mining horizon. Instrument-ation and invest iga ti on techniques to study grc-und permeability ch c;nges are des er ibed and the results of United Kingdom studies presented and discussed.

LONGWALL MINING SGBSIDENCE

Longwal l extraction involving caving of the roof strata is the prevalent underground method in European Coal fields. The caving of the roof str·ata behind the longwall extraction pro due es controlled subsidence of the ground between the mining horizon and the surface. The amount of subsidence occurring at the surface can be predicted from knowledge of the principal mining

LONGWALL MINING/PERMEABILITY & DRAINAGE 161

dimensions namely depth below surface, width of longwall and extracted seam height together with knowledge of the geological conditions. The prediction method used in British Ccalfields is based on an empirical design procedure established over several years from precise levelling observations in different mining conditions covering a depth range of 100 to 1000 metres below the surface ( 1). It applies entirely to longwc..11 mining type of extractions anc'. allows accurate predicticrs to be made of anticipated subsiderce both in extent and amplitude in adc.ition to the calculation of surface grour,cl strain and tilt. An example of the general characteristics of a surface sub siderce trcugh above a longwall mining extraction is shown in Figure 1.

The cre2tion of a subsider.ce trough at the surface can itself introduce a change in surface drEinage pattern especially for thick extractions at relatively shallow def.'ths. Rib pillars are frequently left between successive longwall extractions in order to re due e surface subsidence and the magni tucle of surface ground strains. The method of using pre-designed ratios of width of longwall to widtr. of rib pillar between faces has been employed over many years with considerable success in European Co&lfields as a me ans of car.trolling subsidence in areas where surface drainage is critical, for example under low-lying agricultural land close to a major tidal river, and uncler major inland water courses such as rivers and canals.

LONGWALL SUBSIDENCE AND SUBSURFACE DRAINAGE ASFECTS

If tre surface ground strains, especially in the tensile zor.e, are sufficiently high and the surface rocks brittle then cracking and opening of fissures can occur which can affect surface and subsurface drainage patterns. It is generally thought that the depth below surface of such subsidence cracks which have a direct connection with the surface, is limited in extent and does not affect major surface water bodies such as the sea. or large lakes but small ponds have been known to be drained by such subsidence cracks opening at the

162 INVESTIGATION & EVALUATION OF SURFACE & SUBSURFACE DRAINAGE

Figure 1. General character~stics of subsidence trough and surface ground strain due to longw2ll mining of coal seam in shallow conditions, (The subsidence parHrneters have been calculated using the National Coal Board Subsidence Engineer's Handbook met~od).

s (.)

"' 0 .j.)

c G.J

~10 (.) ro

.-1

~o ·.-! '"O

ro""'O .!-) .....

Cli

Str·ata displacement datum r -~-,~~~~~~~~~~~~~

Strata I ~~~"-. ...._ Depth plug v \\~ \'.\ -...

---1..,...-_ =--= =~l I 'ci \ \~~~ '---·-.-1.!2.1!1 ... ..i:L - --p5 .c.:; ~·,~~ - P6 ~ ,, " .... ______ ,_;,.m.._._._._

-----p4 IE \'\ \ ·---. 6m -·-p·5

543m

44m ---p3 I~ \"-..."\ \ Q) Lt-:~: ,1 ~ '-....~~\ ----l..2.W:,_,_l:i > '.J ·--• ..,._.__?3m. _ __!::i_ j50

1 I ~ . -:>;Om P2 CO I ...,._._,~•-"-•-·-•-

.-1 Face a~vo.nc e I 3 m Pl ~60--......i~--1~.....a.~-1.~ .... ~..L..~-'-~~·~-~·-~·==·~=t.:•:t:::::.::.=t:=Yll

'""' .j.) oot;,0

40 JO 20 J.O 0 10 20 30 40 50 60 70 80 Face advance relative to instrumented borehole, metres.

Figure 2. Development of inte:r-stratc; displacements in an instrumented borehole locc:ted in the path of an approac~ing longwall extraction. (The illustration has be en taken from King, Whittaker and Batchelor (1972)).


pond's base although this greatly depends upon t~ type of geological formc:ltions. The likelihood of cracks appearing at the surface greatly decreases with dee.r;er wa rkings since the grcund strain effffts are more widely spread with a significant reduction in their magnitude.

The caving process of the roof beds behind the longwall extraction creates a zone of brcken strata which in time becomes consolidated. Ccnse~ently the zone of broken strata immediately behind the longwall face is one which is likely to encourage flow of water towcirds the working horizon, providing an aquifer (or other source of water) is with-in the zone of influence. This is vitally im-portant to the safe working and success of all underground mining operations. The present in-vestigation has been directed towards examining the zone of influence of mining operations on ch2,nges in grcund permeability and pot en ti al subsurface drai n&ge pattern changes.

CHARACTERISTICS OF SUBSIDENCE IN PROXIMITY OF EXTRACTIO~ HORIZON

Figure 2 shows the development of inter-strata displacements in the irrunediate roof beds overlying a longwo.11 face 587 metres below the surface ( 2). The instrumented borehole was drilled vertically downwards from a horizon 44 metres above the longwall which was to subsequently undermine tr.e borehole and had seven strain wires Pl to P7 anchored at the depths sh own in Figure 2. The borehole was located cent rally within the pa th of the approaching 200 metres wide longwall extraction. The strata displacements are relative to a datum at the borehole mouth, that is a horizon 44 metres above the longwall extraction. The most important feature of the results shewn in Figure 2 is the zone of major strata movement recorded between 10 to 40 metres-: behind the longwall face. Also of importance is the progressive decrease in the amplitude of rel at iv e vertical strata displace-ments from the mining horizon. The results also show a progressive change in consolidation from the working horizon. It was viewed important to investigate the inf luer.ce of such subsidence on


rr ... r \'

,t ~ :

r '-,.. -(,.

..-C rt' I ( r,.

r ~~ ... ~ ,.. .. ---r r r r ~

, --

geological section

L =

N L = 45m

~ L = No,l I(

= 40m 4. i

test borehole No.5 L = 48m

test cavity

-_--__-.: Deep Soft Z2::z:2~?f:ZZZ2:ZZZZZ::Z. :zz,,,,a:_=_:::i_•lli ---- LongVJall extrc_ction pilot head

I I I I )Om

f;~,,t=ii'-~I

1¥-c:: =::ol g:_-::-·%1 l~;:-;:-F.::,..-;:.I

f_ - I

.JO 20 10 0

Di.st2.nce from gate CE::ntre-line

Sr.ndstone

MudEtone

Siltstone

Shale

Coal seam

50m

40

30

20

10

Geological legend Test borehole &ngles

Figure 3. Illustrating positions of test boreholes at ex_r-erimentiol site in Deep Soft Seam, East Midlands Coalfield and their location in relaticn tc the Longwc.11 extracUon.


ch::rnges in pe rme&bility of the strata within this zone close to the working longwall face.

RESEARCH OBJECTIVES

The main objective of this wcrk was to investigate the zones of increased permeability result':"" ing from undermining by a longwall extraction. Within this investigation also came the need to establish the base permeability of the rock types overlying the longwall extraction and to ascertain the ch&nge in permeability resulting from progressive undermining. It was firstly required to design a scheme of instrument at ion which permitted tt:ese changes to be investigated. Having established the investigation technique it was a major aim to study how the subsidence resulting from longwall mining affected the strata permeability,

Two major sites were selected for the: study. The first permitted the strata permeability change to be investigated in close proximity to the mining horizon, whilst the second concentrated on permeability changes arising close to the surface and well within the critical area of extraction.

STRATA PERMEABILITY CHANGES IN PROXIMITY OF THE LONGWALL FACE

The site chosen for the investigation was in the East Midlands Coalfield, in the Deep Soft Seam. The retreating longwall face was 220 metres long and hc:td an extracted seam height of o. 81metre whilst the depth below surface was 628 metres,

Figure .3 shows the general location of the test boreholes in relation to S.34's face, The gate from which the boreholes were drilled had been formed previously for the extraction of S.32's lcngwall advancing face, immediately to the right of S .34's, see Figures .3 and 4. A small pillar of coal was left between the gate and the pilot heading as shown in Figure 4.

A section of the roof strata above S.34's face is shown in Figure .3 and. this consists mainly of


Cav~d goaf region

Direction Deep Soft

Figure 4. Showing location of test boreholes and instrument z.ti on/test panel

shale, siltstone, mucstone and sandstone within the 50 metres above the Deep Soft Coal Seam, Figure 3 also shows the positioning of the test bore~oles and their respective lengths,

The test boreholes were drilled 60 millimetres diameter. The first borehole was completed when the face was still 55 metres from the borehole plane, The five boreholes were drilled in the sane vertical plane, Each test cavity was formed by pumping cement grout into the borehole mouth; cement was pumped against an increasing head until cement began to return via a 19 millimetre diameter breather tube, A second tube had also been placed in position leading to 3 metres beyond the end of the breather tube and was to be used later for pressure testing of the test cavity, A flexible seal was positioned between the end of the breather tube and the test cavity to prevent any tendency for cemer.t grout to continue filling the borehole beyond the end of the breather tube,


.. (/J (fl

0 .-i

Q.) o. 1 f.. ;:::l (/J

rJl (j) ~ ~

Borehole No ..

30 60

Hose length, metr·es

Figure 5. Correction for pressure loss in connecting hosing (Deep Soft test station)

II.)

.;..)

25 ;:::l c:;

·.-I s

20 f.. Q.) 0..

(fJ

c: 0

1 5 .-i .-l ~I,)

bO .. '1J

.;..) CiJ

10 f..

90 5 .-i ~

Figure 4 shows a general layout of the test boreholes in relation to the face-end, Flexible armoured hosing connected each borehole mouth to the instrumentation panel which was conveniently located for access some distance outbye of the face line.

STRATA PERMEABILITY TESTING PROCEDURE

Each borehole was pressure tested using water when the face was at different positions in relation to test boreholes, The pressure testing equipment consisted of an assembly similar to that shown in Figure 12. The inlet water pressure was 6.3 megapascals and this was reduced by an in-line reducing valve. Due account was ta~n of the pressure loss resulting from hydraulic connections and Figure 5 shows the nonogram for this correction for a given flow rate and length


(l)

r... ::::J Ul (fl

w ~ 1 a. .j.J Ul J)

.j.J

~ ii)

.j.J (1,)

~

b ,_ tr\ + II

x

a

I a

I a +

+x 2

Water flow, litres per minute Figure 6. Borehole No,3 flow characteristics

Ul

0

... ~ 2 u Ul w 0. ,<J bO ii)

J /f ./

E

2 3 Water flow, litres per minute

Figure 7. Borehole No.5 flow characteristics,


of hose. Static head was also corrected for by using the vertical height to the central position of the test cavity.

The test procedure involved observing the flow rate for increments of testing pressure usually up to 2 - 3 megapascals. Two flow meters were incorporated in the circuitry, one reading up to 100 litres per minute and the other up to to 20 litres per minute. Testing firstly involved attaining equilibrium saturation within each test cavity by al lowing fl ow under maximum pressure for at least 15 minutes and thereafter flo\.\ as observed at up to about 12 different test pressure le.rels within the testing pressure range given above. The flow rate was observed when steady state flow was established during each test and usually took less than 2 - 3 minutes. Testing all five boreholes usually took about 3 hours.

TEST RESULTS IN PROXIMITY TO MINING HORIZON

Figures 6 and 7 show typical test results for two of the boreholes tested at different positions of longwall face advance. The results presented in Figure 6 show the flow characteristics before the ground was undermined together with test data showing the effect of undermining. The flow characteristics indicate the formation of widening cracks along the test cavity as the longwall face gradually undermined the test section. Figure 7 shows a test section which was only slightly affected by undermining; these results indicate that the ground became more impermeable before moving to a phase of increased permeability some distance after undermining. Comparing the results given in Figures 6 and 7, it is inferred that the ground associated with borehole No.3 became significantly affected by caving whilst the test section of borehole No.5 was virtually intact during under-mining. The pressure-flow curve variation was due to opening and closing of minor fissures/cracks.

The degree of variation in flow in two of the test boreholes is clearly illustrated in Figures 8 and 9 which show flow rate plotted against longwall face position. In the case of Figure 8


o--.; 1~0 20

' I I

/

0

Test pressure = 2 MPa

Test horizon

"-~~8ml

-20 -40 -60 -80 -100

Distc.nce between face and borE:hole ,metres Figure 8 1 Relationship between flow characteristics of borehole No. 1 and face position

-20

pressure = 2 MPcl Test horizon

""~I I 41m

-40 -60 -80 -100

Distc.nce between face and borehole ,metres

Figure 9. Relaticnship between flow ch2racteristics of borehole No. 4 and face position


which shows the test horizon covering a band 2 to 8 metres above the seam, the results show that the strata were significantly affected immediately after undermining. In the case of Figure 9 which represents data from a test horizon covering 31 to 41 metres above the longwall extraction, the strata were not greatly affected until about 15 metres after undermining. In the latter case, a bed of sandstone and appreciable thickness of shale formed the strata test horizon and it appears that the sandstone behaviour during undermining may have accounted for the relatively high flow rate after undermining.

The results show that the maximum effect of undermining on change in ground permeability occurred between the face line and 40 metres behind, and there is a progressive upward movement of change in permeability behind the face line. The test results also generally indicate opening and closing of cracks and bed separation cavities during the undermining phase. After 40 metres behind the face line there is an indication of increasing consolidation of the strata taking place. The results and discussion here are in general agreement with the strata displEcement results given in Figure 2.

STRATA PERMEABILITY CHANGES CLOSE TO THE SURFACE AND ABOVE LONGWALL EXTRACTIO~

The site selected for this part of the investigation was located in the Yorkshire Coalfield in the Swallow Wood Seam at Wentworth where the coal seam is 2.1 metres thick and 54 metres below the surface,

Figure 10 shows the position of the instrumented borehole in relation to the approaching longwall extraction (190 metres long) at the time the borehole was drilled.

The instrumented borehole was 96 millimetres diameter and drilled 42.7 metres deep, This depth was judged to give an adequate thickness of strata between the base of the hole and the underlying Swallow Wood Coal Seam (at 54 metres) which


Depth below surface(h) = 54m Longwall face length(w) = 1~ Extracted seam height(m)=2.1m

Position of i rs t ru me nt e d borehole

Figure 10. Location of instrumented borehole in relation to long~~ll face position (Went~orth site)

was to be undermined by S12 1 s longwall face. A section of the strata at the borehole is given in Figure 13 and as can be seen the beds are mainly Coal Measures formation types which are well known for their relatively high impervious properties. The thickness of cover between the borehole base and the Swallow Wood Coal Seam was further increased by the 1.4 metre thickness of cement grout seal used at the base of the borehole to secure the first strain wire.

Figure 11 shows the instrumented borehole used at the Wentworth Test Site. Four permanent resin seals were located as shown. The seal was formed by firstly lowering a fairly tight-fitting multideck platform of fibre/fo&m discs secured to a steel framework, and thereafter Celtite-Selfix M100 resinous injection grout pumped to rest on the upper surface of this flexible temporary seal. The general procedure involved firmly securing the flexible temporary seal in position at the desired depth by clamping the 19 millimetre diameter pl.as -tic tubes (for subsequent testing) at the surface rig. Each temporary flexible seal was located in position by special insertion rods which were uncoupled from the seal prior to pumping the MlOO grout onto the temporary seal. The grout produced a seal which was about 3 metres in length and

LONGWALL MINING/PERMEP..BILITY & DRAINAGE 173

6. 1

s.~

11 • 6

18.8

21. 9

25.1

28.0

41.3

42.7m Figure

------strain wires contained in water tubing

Cement grouting

----Resin seal, No.4

I ..... I "" +-- I I -

~

~ .,.. ~

"" 'I- ~

Test section, No,4 (test cavity consists of unlined section of borehole between two permanent resin seals

Resin seal, No,3

Test section,No.3

--~ -Resin seal, No,2

:U[ji w-----Test section, No. 2

~ Resin seal, No.1

~Test section, No.1

,. Cement grout seal securing strain wire at base of hole

11. W£ntworth instrumented borehole


Stop valve (SV)

\ Inline

Reducing

Low range flow

High range flow meter

Four such connect.ims to borehole

'""

Figure 12. General hydraulic circuitry used at Wentworth instrumented borehole

proved effective in sealing different sections of a borehole which contained an increasing number of plastic tubes and was to be subsequently affected by undermining, Mechanical seals were judged to be inadequate for such geotechnical and mining conditions. Each seal was established as a separate operation and the resinous grout allowed to cure (about 2 - 3 hours) before the next seal operation was carried out, This method of providing a borehole with several sealed sections has been successfully applied in boreholes down to a depth of 70 metres and where six seals have been installed in a 100 millimetre diameter unlined hole. Strain wires were secured to the base of each resinous grout seal, and brought to the surface via the water pressure testing tube. Resin seal No,1 plastic testing tube contained two strain wires, one connecting to the base of the seal and the other to the base of the hole, The upper section of the borehole was grouted to a depth of 6.1 metres to test section cavity No.4, Each strain wire was tensioned and observations made of displacement with an extensometer,

The testing arrangement is shown in Figure 12. A rotameter was used to measure low flow rates up to 1 litre per minute. A constant test pressure of 2 bars (at the borehole mouth) was used throughou~ the testing prograrrune.


;;;!oce,,_0~0-.... =f..._~o~~~~~~~-~.5 .., ... •. I 'o-o-o•o-o- 1

I 1.5

I

•0-000-0-0.0--01>0

I 'o -ooo- 0 -o-o.......___ I \_

•o ~ o-o·o· -30 10,

I o-o-o-o·o-

1

I

-5om I---">-

- - - :li. + :::= Swc:i.l low Wood Seam

-- // 80 60 40 20 O -20 -40 -60 -80 metres: Position of face relative to borehole (x)

m:.:ft;i:-1 Sandstone(= ii:::i:::i;.i==.-i%fMudstone i=· .. -:::·,::iJ Sil ti:.tcne Coal

Figure 13. Surface subsidence and sub-surface subsidence ch2 racte ri sties, Wer:twor·th borehole


o~ w Surface C\I

Flow characteristics ~ 20 .

of test o"- c: ·.-1

section No.4 v ' 15 .. s ~"-'1' 'b-o-o

10 0 Q.) o-i )...

6. 1- ,,/0 ~ 5 t:.t.. .j.) ·.-1

0 o-i

s. 4--""-4A-A-.......-.eJ.--.. A

.. 0 (j,)

Subsidence ~ u

0.5 5 Q.)

11. 6- /curve at 11.6m A' 1 'O r... ·.-1 +>

below surface °'A-.A..A.A-+- {/} Q.)

(m) 1.5 ..0 s ;:l

E.o 30 0 30 60 rn

Distance from face line, metres

Figure 14. Flow characteristics of test section No.4 in relation to sub-surface subsidence aris.ing from undermining, Wentworth instrumented borehole

Surface levelling was carried out to determine the progressive subsidence of the borehole mouth and also the seal positions within the borehole since strain readings relative to ihe bor2hole mouth were observed also.

TEST RESULTS IN PROXH1ITY TO SURFACE

The surface subsidence curve for the borehole is show:i plotted in Fig11t~e 13 together with the subsurface subsidence curves as determined using the strain wires. Some u~lift of the strata ahead of the face line was recorded at the low~r horiz0ns. Subsidence showed a more marked change at the lower horizons as depicted by the more distinct step characteristic of the subsidence profile.

Figure 14 shows test results for the uppermost test section (No.4) and it demonstrates that discernible change in the fl ow characteristic was taking place at some 50-60 metres ahead of the fa~ line and this increased in marked steps implyi~g


opening and closing of near-surface cracks/fissures. The flmv curve settled to a consistent value at about 35 - 40 metres behind the face line.

The testing condition has been assumed to be equivalent to a constant head test and the following equations have been used for determination of in situ permeability of the strata. These equations represent Horslev's approach (3) and they have been discussed elsewhere (4) and (5) regarding application to estimation of in situ permeability of bedded and jointed rock structures.

k = q

••••••• ( 1 ) F. He

F = 2 1T f loge0m f/D)

• • • • . • • ( 2)

combining ( 1 ) and ( 2)

k = q. log (2 m 1/D) e • . . . • . • ( 3) 2 1T 1. He

Where, k = coefficient of permeability normal to hole kp = coefficient of permeability parallel to hole q = flow rate F = shape factor of test cavity ( 1 > 4D) He = constant pressure head of water applied dur

ing test (above any original ground water value)

l = length of test cavity D = borehole diameter in test cavity m = ( k/kp ) 0

"5

The authors have adopted a value of k/kp = 106,

this being consistent with an earlier paper by the present authors (5) and has been discussed in detail previously.

Values of in situ permeability have been determined using these equations and a detailed presen~ ation of the results is given in Figure 15 which shows the ratio of subsequent permeability/base permeability plotted against longwall face posit~ ionv These results indicate that the upper ten metres of the strata section (which contained a bed of sandstone, see Figure 13) experienced the


Subs~.en.ca~e--~A----A------A-~ O

~ 01 I"'" S 1 a"'A.tA-A A

8 /°""'<\~ t> 6 ,,f / 'i---0~-- 5 ·.-i 40 -r-i

:3 20 ' '~ ~ O __ .o ____ ~

E ~ Base permeability (!.)

0..

(!.) fJl (lj

~ e.o :>;

~ 60 r-i

:.a ~-0 Cl)

Q.) 20 Fi .....

lJ} Q.)

~ .;..)

I ~1---0------~ (!.) 0..

---o --- _,0 -0 ~--f/ 1 0 ...... -i..~~__..__. __ ~.._~~.,_~~...a...~~-1...~~ ......... ~---t

.;..)

c: (j.)

::l c:r (j) fJl .a ffi 80

tt-... 60 0

0 ~.o ·.-i .µ 20 ro

BP = 1.43 x 10-· 7 cm/s c.

.o,,~ -0

~ 01to.----~----.:)a.c,.m .. ~)~....__.__.....___._~.___._~ ....... .-..-1

60 40 20 -20 -40 -60 Face position (x), met res

Figure 15. Illustrating compc~rison of chcnge in ground perme2.bility of strata overlying a longw2ll extraction (Wentworth instrumentec borehole)


0 Surface

!Jj 0 (I) 1 s...

.j..)

v 20 s

50

Onset of pe rmeabili t o,/ change (Pc)

''o ' ..... ..... ___ -0 .. ,

\?

....... ' '• ' \ •

?~ '\ Position of peak value of permeability change, (Pc)

60 50 40 30 20 10 0 10 20 30 Distance from face line, metres

Figure 16. Position of onset and peek value of pe rme2bi l ity change of st rat a over lying a longw<dl extraction due to undermining

greatest change in permeability. It is considered that the main factor responsible for this feature is the close proximity with fhe surface even though sandstone was present which would have also contributed to this degree of change. All the test curves indicate a tendency towards decreased change in permeability after 35 - 40 metres behind the face line.

Figure 16 has been plotted using data from Figure 15 and it indicates that onset of permeability change occurs significantly ahead of the face line with the upper test horizon when compared with the lower test horizons. A similar trend is indicated with the position of the peak value of permeability change.

GENERAL DISCUSSION OF RESULTS

The testing procedure and equipment described in the paper pr av ed sat is factory for investigat.irg ground permeability changes resulting from longwall mining operations. The instrumentation scheme was sufficiently sensitive to monitor small changes in permeability. The results permit the onset of perneability change arising from mining


5 z G)

~ r r s::: z z G)

;J m :IJ s::: ~ ~ r

~

S.ite Test

section

Table No. I

In situ strata permeability data

Test section position, metres

Strata type

In situ permeability (k) centimetres per second

Base value Maximurn value ·------------------------------------------------

Deep Soft 1 2-8>1i' M Slt Sh 1 • 5 x 1 0- i. 1.4 x 10- 3

Deep Soft 2 8-11• Slt M 1 • 1 x 10-·5 1.5 x 10- 3

Deep Soft 3 13-29* M C Sh 1.4 x 10-i. 4.5 x 10-i.

Deep Soft 4 30-41. Sh M Sd 5.7 x 10-1> 1.3 x 10- 3

Deep Soft 5 25-45* M S~ Sd 5.2 x 10- 0 4.9 x 10- 5

Wentworth 1 28-41 ** C M Sd Slt 1. 0 x 10- 9 NA

Wentworth 2 22-25** C M Sd Slt 1. 7 x 10-7 2. 1 x 10-"

Wentworth 3 12-19 ** Slt M C 1.4 x 10 -7 6.6 x 10- 6

Wentworth 4 6-81h** Sd 4.5 x 10-7 2.3 x 10- 5

----------

I«' =f$ measured vertically above coal seam; ** measured below surface ~ C = coal, M = mudstone, Sh = shale, Slt = siltstone, Sd = sandstone z NA = not available }> G) m ... CD ...

proximity to be read i.ly evaluated. Values of in situ permeability have been calculated for the experimental sites described in the paper, and Table I presents the base and maximum values of coefficient of permeability (k) for each of the test horizons studied.

In the case of the Deep Soft Coal Seam Experimental Site the ground was found to have a discernible degree of permeability before being disturb-ed by current longwall mining, see Table I. Thls is probably due to previous mining. The effect of current mining operations was to pro&1ce appreciable change in ground permeability especially in the test zones near to the mining hoc izon.

The strata tested can be d·2scrib·2d as virtually imperme ab.le before undermining in the case of the Wentworth Site, but after undermining change in ground permeability was sufficient to promote minor flow in the case of the upper horizon but the lower horizons were not so affected. The presence of impervious beds within the strata sequence plays a major role in such mining si tuations.

CONCLUSIONS

1. The instrumentation and testing procedure proved successful for studying ground permeability change resu 1 ting from undermining by a .longwal l extraction.

2. The main zone of appreciable change~ in in situ permeability was found to lie between the face line and 40 metres behind the face.

J. Appreciable in situ permeabi .lity change was observed to occur up to 40 met res above the extraction horizon.

4, Changes in ground fl ow properties of the strata were found to be of a stepped characteristic and this is thought to be due to opening and closing of cracks and separations,

5. Significant change in ground permeability


was observed close to the surface above a longwall extraction in shallow mining conditions.

ACKNOWL EDGEME NTS

The authors acknrn.vle dge the generous financial and practical support given by the National Coal Board to this Project. Special thanks are due to several mining engineers, surveyors and geologists within the National Coal Board for excellent co-operation and help given to the au th ors during the field studies. Thanks are also due to the technical staff of the Mining Department, Not t.ingham University for valuable design contribution to the instru-nentation used in this project.

REFERENCES

1. Sub si denc e Engineers' Handbook, Production Department, National Coal Board, London, 1975.

2. King, H. J., Whittaker, B. N, and Batchelor, Ao s. The Effects of Interaction in Mine Layouts. Fifth International Strata Control Conference 197Z Paper No. 17, 11 r>P·

3. Hor-slev, M. s. Time lag and soil p12rmeabili ty in ground water measurementso u.s. Corps of Engineers Waterways Exp.Stn,Bulletin 36, 50pp, 1951.

4o Hoek, E, and Bray, J, w. Rock slope engin-eering, The Institution of Mining and Metallurgy, London, 309 PP1 1974.

5. Whittaker, B. N. and Singh, R. N, Design aspects of barrier pillars against water-logged workings in coal mining operations. Sym~osium on Water in Mining and Underground Works, SIAMOS 1978, Granada, Spain, Vol,1, pp 675 - 692.


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