Ground movements associated with the near-surface construction operations of a mine drift in coal measures strata

Int. J. Rock Mech. Min. Sci. & Georaech.Abstr. Vol. 14, pp. 67-75. Pergamon Press 1977. Printed in Great Britain

Ground Movements Associated with the Near,Surface Construction Operations of a Mine Drift in Coal Measures Strata B. N. WHITTAKER* J. H. PYEt

An account is given of'a study of strata movements resulting from the nearsurface (17.4 m depth) construction operations of an arch-shaped mine drift (5.5 m excavated width). Precise surface levels and strain wire instrumentation located in boreholes ahead of the advancing drift face have allowed a study to be made of roof strata behaviour and surface subsidence development both before and after undermining. The results indicate a zone of discernible uplift ahead of the advancing drift face. The horizontal extent back from the face of the zone of rapid subsidence movements of the roof strata immediately following undermining was observed to be of the order of 9 m (corresponding to 0.5 x depth). The limiting angle of draw of the final surface subsidence profile was found to be 45 °, although the resulting profile shows similarities with that predicted by deep mining subsidence knowledge. Problems associated with prediction of the anticipated maximum surface subsidence are discussed in relation to the principal mining dimensions, namely width and depth of the excavation. There is a general discussion on the role played by the drift .formation method and the excavation support system in achieving effective roof strata control.

1. INTRODUCTION Surface drifts (sometimes referred to as inclined mining tunnels) are an attractive means of access to shallow (300 m depth) mineral reserves by virtue of ease of drivage and economy and continuity of subsequent mineral transport operations. Indeed, the arguments in favour of surface drifts have been strengthened in recent times by technological advances in belt conveyors for mineral transportation and in some cases such drifts may extend to 600 m depth below the surface. There has been a recent increase in the number of surface drifts, and more extensive ones are being planned for exploitation of new coal reserves. It is construction practice to form an open-cut at the surface location of the drift, and then construct a monolithic concrete portal structure down normally to a depth of about 10-12 m; after reinforcing the exposed rock face, and particularly the ground arching around the intended excavation, drift drivage operations begin using a depth of cover of usually not less than 6-8 m of rock. It is the stage when the depth of cover above the drift is relatively shallow (<30 m) that most roof control difficulties tend to arise at the drift face. The work presented here gives an account of a study of

ground movements encountered during the near-surface drift construction operations in order to relate such movements to the relative position of the drift face. The investigation involved the use of borehole strain wire extensometers and precise surface levels to study the response of the strata immediately above the drift so that the onset and termination, in addition to peak rates, of ground movements could be accurately detected. The Kiveton Park mine drift studied is located in the Yorkshire coalfield. 2. MINE DRIFT AND SITE FEATURES The surface drift dimensions corresponded to an excavated arch-shaped opening of height to the crown of 4.0 m and width at floor level of 5.5 rn, whilst the radius of the arch was 2.6 m. The splay angle froin the floor comers was 7 ° to the vertical. Lining of the excavation was mainly by standard National Coal Board colliery arched girders of specification 16/17ft x 12ft, 3-piece of section 5 x 4½in., which after setting on wood foot pads of thickness 15 cm gave a finished drift height of 3.8 m and maximum width of 5.2 m. Inclination of the drift was l in 5.8 (9.8° to the horizontal). The strata dip was 1° to the horizontal in the

* Department of Mining Engineering, Nottingham University, direction of the drift axis. Nottingham NG7 2RD, U.K. Preliminary operations included the construction of $ International Ground Support SystemsInc., Denver, Colorado, an open-cut, along which an arch-shaped monolithic U.S.A. 67

68

B. N. Whittaker and J. H. Pye TABLE 1. DEPTH. THICKNESS Sub-section No.

Depth (m)

Thickness (m)

1 2 3 4 5 6 7 8 9 10 --11 12 13 14 15 16

0.60 1.82 2.73 6.75 7.00 8.92 10.35 10.65 12.69 13.39 13.40 17.40 17.54 17.60 18.30 18.76 18.94 23.60

0.60 1.22 0.91 4.02 0.25 1.92 1.43 0.30 2.04 0.70 --4.15 0.06 0.70 0.46 0.18 4.66

Rock type

3.1. Instrumentation scheme The programme of measurements was based on inter-strata displacements by means of vertical borehole strain wires and surface subsidence precise levels to a layout of special observation ground pegs including the instrument standpipes. STRENGTH DATA OF DRIFT ROOF STRATA SPECIMENS

Uniaxial compressive strength b Sub-section* (MN/m 2) No. 4 No. 6

56.86 + 6.50 S.D. 44.75 + 4.03 S.D.

Tensile strength' (MN/m 2)

Density (kg/m 3)

2.92 + 0.48 S.D. 7.61

2306 2260

* See Table 1 for location. b Cylindrical specimens, 38 mm diameter x 76 mm. ¢ Brazilian disc, 38 mm diameter x 9.5 mm.

(87 m A.S.L.)

Levelled ground (made ground) Brown and yellow clay with rock fragments Stiff silty clay Partly weathered Pale grey Partly weathered with thin layers of mudstone

Seatearth (18 cm) under the coal seam

Mudstone Coal Seatearth + sandstones Mudstone Coal Mudstone

3. INSTRUMENTATION AND RESEARCH PROCEDURE

OF EXPERIMENTAL SITE

Remarks

Unconsolidated Unconsolidated Unconsolidated Siltstone + mudstone Sandstone Siltstone Mudstone Coal Mudstone Siltstone

concrete structure was formed and thereafter the ground was restored using fill from the drivage. The instrumentation site selected was on levelled ground at the position where the depth from the surface to the projected floor of the drift was 17.4 m, and this was 133.2 m (plan distance) from the drift portal. A drill core was taken at this position and the main rock formations observed are described in Table 1. The horizons of the drift crown and floor for this cross-section of strata are also shown. Samples of rock taken from sub-sections Nos. 4 and 6 of Table 1 were subjected to strength testing, and a summary of the results is presented in Table 2, whilst strength data under triaxial stress conditions are given in Fig. 1. It should be borne in mind that although the rocks between the drift and the surface at the instrument site showed effects of weathering, particularly the weaker strata such as seatearths, the siltstones exhibited a marked degree of relative competence as indicated by the strength data in Fig. 1.

TABLE 2.

3

AND TYPES OF STRATA AT BOREHOLE N O .

Crown of drift horizon Floor of drift horizon Laminated mudstones

Light and dark grey bands

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Fig. 1. Triaxial strength properties of rock samples.

Figure 2 shows the type of borehole strain wire extensometer arrangement used. Modified expansion shell rock bolt anchor plugs were placed at various horizons within unlined boreholes. Strain wires attached to the plugs were brought out of each hole and passed round specially designed pulleys in order for a mass of 7.26 kg to be suspended from each wire. The distance between the upper machined face of the platen, on which of the pulleys were mounted, and the upper machined face of the cylindrical mass was measured by means of a depth micrometer to an accuracy of 0.01 mm. Since the mass attached to each wire was in free suspension, the tension in the wire was maintained constant irrespective of the amount of subsequent vertical displacement between the plug and the extensometer. The depth micrometer could allow + 60 mm of movement from the median position before any adjustment would be required of the mass relative to the wire. An electrical circuit was closed and a bulb illuminated when contact was established between the measuring faces and the depth micrometer. A further

Ground Movements of a Mine Drift description of this type of extensometer which was designed and manufactured at Nottingham University, Mining Department is given by McCaul et al. [1] who carried out similar investigations in chalk formations. Six extensometer stations were established ahead of the advancing drift face. All the instruments were inserted and observed from the surface. The standpipes on which the extensometers were mounted were 3.66 m long. Figure 3 shows a diagrammatic form of the instrumentation layout. Five of the instrumented boreholes were in a line transverse to the axis of the drift, whilst two of the holes were on the drift centre-line. Twenty-three wooden surface subsidence measuring pegs were driven into the ground to a depth not less than 40cm to allow the subsidence profile to be observed.

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69

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3.2. Measuring procedure Continuous recording was carried out on borehole No. 6, whilst the readings were taken at 3 hourly intervals in respect of the extensometers at the other five boreholes. The frequency of extensometer reading was decreased to twice daily after the ground displacements indicated settling down some distance after the stations had been undermined. Precise levels were carried out weekly for the period of 12 weeks which spanned the undermining of the instrumented site. 4. DISTURBANCE OF ROOF STRATA RELATIVE TO DRIVAGE POSITION

4.1. Development of transverse roof movements

of the rate of roof movement became established. These distances represent the length of the critical zone for immediate and effective roof control and expressed in terms of depth correspond to 1/20 x depth for onset of roof movement and ~ x depth for departure from major movement of the roof. An important implication here is the fact that the method of support used in the drift should be designed to take into account the need to control this movement. This aspect is discussed in more detail later in this report. It will be noted from Fig. 4 that the greatest relative movement occurred with plug No. 2 which was located over the drift shoulder. This is explained as being due to localised instability of the immediate roof in the drift and occurred during blasting operations; the small cavity required 1 m of back filling. The results of plug No. 2 should have been of the same order of those obtained for plug No. 4, by virtue of symmetry and similarity of strata formation for the respective plug positions. Consequently it can be argued that the difference in the results between plug Nos. 2 and 4,are indicative of the effect a roof cavity has on overall roof control. Under normal roof control conditions the strata displacements obtained for plug No. 3 (on the drift centre-line)could have been expected to yield the greatest movements for such a set of observations. Movement of the plugs Nos. 2, 3 and 4 relative to the surface had virtually ceased after 0.5 x depth (9 m) advance of the drift past the observation line. Closure of minor roof voids is indicated to have occurred mainly during 0.%1.0 x depth after undermining, although this persisted with plug No. 2 until 1.5 x depth of drift advance. The relative strata displacements for plugs Nos. 1 and 5 indicate only minor disturbances during undermining and the roof instability phase during the subsequent 6m of drift advance past the observation station. The results show the ground movements ceased

An assessment of the degree of disturbance of the "2°r ~__~__! roof strata has been made by considering the displace+10Ii.~ ~ S ment relative to the surface of the strain wire plug at the bottom of each instrumented borehole. Figure 4 shows the relative displacements in each hole plotted -I-0 -05 O~ +0"5 *I-0 +I'5 +2"5 _40 ~L :,C/h ad~,ance4'~h against drivage advance in terms of the depth of the drift at the instrumented borehole position (17.4 m). All ~-20 | \ % ' o - o ~ ° ~ 0 ~the results show an apparent uplift during the early stages since the values in Fig. 4 have been plotted assuming that the surface has remained fixed and unaf":?t'-!:; :",!"-" ~ "-.. so3 fected by the presence of the drift face. The results indi"' "-~'"';2,. cate that ground movement became discernible at a distance of 0.75 x depth ahead of the tunnel face, and Section apparent uplift continued until undermining took place. 5~ I 2345 After undermining of the instrumented site, it can be seen from Fig. 4 that the plugs, Nos. 2, 3 and 4 : : I -8C located in the roof immediately above the excavation, underwent rapid subsidence until the drift face was \ No2 17.4m -9 C around 0.5 x depth (9 m) past the observation plugs. Marked subsidence of the immediate roof began at about l m after undermining and this continued until Fig. 4. Relative displacements of plug 1 at instrument stations. Nos. I 5. the drift was 6m past before significant levelling off t

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Ground Movements of a Mine Drift

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after 1.0 x depth of drift face advance beyond the line of plugs.

12

4.2. Development of roof movements along axis of drift Figure 5 shows the development of relative displacements for three horizons in borehole No. 3 which was located on the axis of the drift. Apparent uplift began at a distance of 0.75 x depth ahead of the drift face and attained its peak value at about 2.5m (0.14 x depth) in front of the advancing drift face. Major movement of the strain wire plugs began in this case after the drift face was 1.7 m (1/10 x depth) past the station and there was substantial levelling off of the curves at the 6 m (½ x depth) position whilst relative movements practically ceased at 0.5 x depth beyond the line of plugs. Inter-strata separation began almost immediately after undermining of the instrumented borehole. The absolute vertical movements of the plugs and the surface of borehole No. 3 are plotted in Fig. 6. Discernible uplift of the ground began at about 0.75 x depth and attained a maximum displacement of + 4 mm. Even though relative separation between the plug horizons became extablished after about 0.5 x depth of drift advance, the ground continued to subside until the drift was 1.0 x depth past the observation station.

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4.3. General pattern of roof strata movements Figure 7 shows the general pattern of relative vertical strata movements. The effect of local instability of the roof within the drift is clearly evident in terms of promoting concentration of relative vertical displacements.

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It should be borne in mind that the formation of the subsidence trough may have influenced the positive values of displacement observed above the sides of the drift cross-section. The results are indicative of the tendency for vertical displacements to be concentrated essentially above and confined within the drift excavation area.

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5. SURFACE SUBSIDENCE OBSERVATIONS ASSOCIATED WITH D R I F r CONSTRUCTION OPERATION 5.1. Dynamic subsidence development curve Five independent sets of results were obtained for the dynamic subsidence development curve (also referred to as the travelling subsidence curve). The depth of the drift ranged from 17.4m to 20.2m for the observation positions. The results in Fig. 8 are

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maximum subsidence (Sm~x) and distance from the drift centre-line in terms of depth below surface for presentation in Fig. 12. The limiting angle of draw is 45 °. There are occasions when the 5% Sin,, position is used as a reference to define the subsidence limit in view of difficulties arising in specifying exactly when subsidence begins since this depends greatly upon the accuracy of the levelling instrument and procedure. In this case the angle of the 5% Smax position is 41 ° to the vertical. The transition point is 1/20 × depth from the side of the drift and lies over the solid ground. There is a close resemblance between the measured subsidence profile for the drift and that which is predicted using the deep mining subsidence prediction method for an excavation of this particular width/depth ratio (see N.C.B. Subsidence Engineers' Handbook [2]). It should be noted, however, that the limiting angle of draw results in a wider spread of the subsidence trough than that anticipated by prediction. A further important point is that it is not possible to use the N.C.B. Subsidence Engineers' Handbook to predict the magnitude of the maximum subsidence value for such a drift, but this will be examined further in the next section. 6. PREDICTION OF MAXIMUM SUBSIDENCE ABOVE D R I F r Deep mining subsidence knowledge has allowed a reliable method to be devised whereby the anticipated D~tan<:¢

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shown as surface subsidence plotted against advance of the drift face relative to the surface observation station. The results show close consistency and this is most probably due to the small differences in depth. A more meaningful comparison is presented in Fig. 9 which shows subsidence expressed as a percentage of the maximum observed (Sin,,) plotted against drift face advance expressed in terms of depth. Only two sets of subsidence results show discernible uplift ahead of the advancing drift face. It should be borne in mind however that the subsidence observation ground peg may have influenced this since very small movements ( < 4 m m ) were involved. All the results show that ground movement became discernible when the drift distance was about 1.0 x depth from the observation point. The surface was already subsiding before being undermined. Surface subsidence was complete after a drift advance of 1.0-1.4 x depth past the observation peg. Figure 10 shows the mean of the five sets of subsidence data for the dynamic development curve compared with the subsidence curve established from deep mining surface subsidence observations (N.C.B. Handbook [2]). Although there is a similarity between the two curves, the results for the drift show that surface subsidence started appreciably earlier and continued over a greater distance expressed in terms of depth. This signifies a greater angle of draw and using the beginning of the development curve gives an advance/ depth ratio of 1.0 which corresponds to a draw angle of 45 °. 5.2. Transverse subsidence curve The relevant subsidence data used to determine the transverse half-profile are shown plotted in Fig. 11, and these results have been expressed as percentage of

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Fig. 11. Half-profile transverse subsidence trough results.

Ground Movements of a Mine Drift

73

drift/tunnel situations, then the graph shown in Fig. 13 could be used as a basis of back-analysis to determine the value of the "equivalent extracted height m" and this has been done for the data presented in Table 3. The "equivalent extracted height m" may be considered as representing the amount of vertical closure of the drift roof which occurs between initial excavation and complete arrest by the drift support system. Conse~, 4¢r?-'T'-~ quently this is greatly dependent upon the method of drift construction as borne out by the values for "derFig. 12. Half-profile transverse subsidence curve in terms of Sm=~ ived m". In the case of the Kiveton Park drift study plotted against distance/depth. reported here, the back-analysis gives an "equivalent ,t" of 355 mm which when compared with the other maximum subsidence above a longwall coalface can be values of m suggests that the use of deep mining subsidetermined (N.C.B. Subsidence Engineers' Handbook dence data to predict the anticipated value of S,,x is [2]). The maximum anticipated subsidence above a not valid. The results suggest that the drift/tunnel contrough-shaped extracted area is a function of the width struction method and associated support operations are to depth (below surface) ratio of the extraction; the the outstanding factors which influence any subsequent maximum subsidence is also directly related to the surface subsidence. For example, even though the resextracted seam height. Whilst it is appreciated that pective rock types and tunnel shapes were different, and deep mining subsidence knowledge is based on obser- the Chinnor Tunnel (case 2) was of comparable width vations made where the depth below surface has been and only half the depth of the Kiveton Park drift study greater than 100 m and the extraction height to width location, the greatly reduced surface subsidence in the ratio is rarely greater than 0.06, there are some similar case of the former is undoubtedly due mainly to the features worthy of consideration. However, in the case use of a full-face tunnelling machine, even though the of near-surface mining drifts and other tunnels the respective rock types and tunnel shapes were different. extraction height to width ratio is usually in the range Figure 13 does suggest however, that the critical depth 0.7-1.0, and it is usually only when the drift/tunnel is above which appreciable surface subsidence can be within 100 m of the surface that surface subsidence expected is 30--40m for tunnel widths up to 10m, becomes discernible from an engineering point of view. although as the results in Table 3 indicate the method Consequently, these latter points alone are sufficient of tunnelling and support are the over-riding factors to suggest that extrapolation of deep mining surface in this depth range. subsidence knowledge to near-surface drift/tunnel situations must be regarded only in a tentative manner. Figure 13 represents a tentative relationship between 7. TRANSVERSE SURFACE SUBSIDENCE the anticipated maximum subsidence expressed as a PROFILE OVER SHALLOW TUNNELS function of excavated height 'm' of a thin tabular extracPeck [3] and Schmidt [4,5] have reported that the tion plotted against depth below surface of the extraction horizon. This relationship has been extrapolated transverse surface settlement profile above a shallow using the deep mining subsidence data compiled in the tunnel in soft ground conditions conforms to a shape N.C.B. Subsidence Engineers' Handbook [2], and the similar to the Gaussian error function, see Fig. 14. extraction widths used are 5 m and 10 m, respectively. According to Schmidt, the trough width can be estiThe basic assumption here is that the extracted region mated using the equation i/a = (Zo/2a) °'s, where i is corresponds to a caved excavation in mining termino- the standard deviation of the error function and is logy, that is no support is offered to the excavation regarded as the width parameter; Z0 is the depth below other than that of natural arching. If a valid relation- surface to the tunnel centre-line and 2a is the tunnel ship exists between the deep mining and near-surface diameter, The Kiveton Park results have been plotted on Fig. 14 and these show consistency with the Gaussian error function. The calculated area of the transverse surface subsidence profile (complete) is 0.76 m2; 0.a ~ ~ using the value of i of 4.1 m as obtained directly from o+ i h Fig. 14 gives a subsidence profile area of 0.66 m 2, whilst using Schmidt's formula above gives a value of i of m 0.+ k---'--.l 6.26 m and a corresponding subsidence profile area of 0.2 1.00 m ~. In view of the results used by Schmidt to derive his formula having a fairly wide scatter, this can 0 I I I Io 20 30 4'0 ~ sb 7b 80 90 ~ explain why there is a larger difference between the Depth h actual and the predicted result with the Schmidt forFig. 13. Tentative extrapolated relationship between anticipated mula than by direct reading of the value of i from the maximum subsidence and depth below surface for narrow excavations. actual profile. 1.4

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Fig. 14. Error function related to settlement trough and comparison with Kiveton Park drift results.

8. GENERAL DISCUSSION ON ROOF STRATA BEHAVIOUR IMMEDIATELY ALTER UNDERMINING The major observation to emerge from these measurements of roof strata displacement is the response of the roof immediately after undermining. Figure 4, which has been referred to earlier, clearly shows that the first 9 m (representing 0.5 x depth) section of the drift behind the face is the critical zone requiring immediate and effective support. It is often the case for drifts, and tunnels, to be driven giving substantial emphasis to the nature and strength of the final lining, whilst too little emphasis is placed on early temporary support of the right quality and strength. Whilst it is appreciated that construction operations often dictate the form of temporary support provided, they should not preclude the due consideration effective temi~orary support undoubtedly deserves. The use of powered shields is becoming more popular and they are making a major contribution in respect of providing early effective temporary support in weak ground. However, where drilling and blasting formation methods are used appropriate temporary support assumes greater importance. The support technique also deserves detailed consideration. Standing supports, such as arched-girder sets, require a time-lapse before they generate significant supporting resistance. Conversely, shotcreting can provide early and effective supporting resistance in the critical roof supporting zone and this can be followed up by other traditional permanent support methods. Achieving more effective support in the early stages means that the loads subjected to permanent lining will be less, in view of the degree of disturbance and roof fracturing being more effectively controlled.

1. The near-surface construction operations of the mine drift studied resulted in a zone of discernible uplift ahead of the advancing drift face; this, however, was Observed at the measuring station where the drift depth was 17.4 m. 2. A critical zone of roof movement was observed immediately after undermining, and the horizontal extent back from the face of this zone was found to be 9 m (corresponding to 0.5 x depth). 3. The limiting angle of draw of the final subsidence profile was observed to be 45°; the resulting transverse profile was consistent with the Gaussian error function and showed similarities with the shape of curve predicted for deep mining situations. 4. The provision of early and effective temporary roof support deserves equal attention to that given to the final lining design, since the degree of loading experienced by the final lining is very often greatly influenced by temporary support methods. Acknowledgements---The authors gratefully acknowledge the valuable comments and advice given during the research by Professor H. J. King, Head of Department of Mining Engineering, University of Nottingham. Thanks are recorded to Mr. V. Cassapi and Mr. D. DeBarr of the Department's technical staff for valuable assistance given to the research. The authors also record thanks to Mr. C. D. Breeds, research student at the department for assistance given in the field. Special gratitude is recorded to Mr. G. Hayes, Director, South Yorkshire Area, National Coal Board, for permission and excellent assistance given during the research investigations` The authors also wish to record thanks to Mr. J. Pocock, Deputy Chief Mining Engineer, South Yorkshire area for excellent help given with the detailed planning of the instrumentation programme. Thanks are also recorded to Mr. C. J. Dorlin, Site Engineer, and the Survey Department, Kiveton Park Drift Mine for excellent help and cooperation given during the research. The research described in this report was sponsored by the National Coal Board. The authors express gratitude to the National Coal Board for financial and practical help given to this project, and for kind permission to publish the research findings. Any views expressed are those of the authors and not necessarily those of the National Coal Board. Received 1 November 1976.

REFERENCES I. McCaul C., Morgan J. M. & Boden J. B. Measurement of ground movement due to excavation of a shallow tunnel in lower chalk. TRRL Supplementary Report 199 UC, DepL of the Environment (1976). 2. Nation,l'Coal Board Sohsidenve En.qineers" Handbook. National Coal Board (London 1966, revised 1975). 3. Peck R. B. Deep excavation and tunnelling in soft ground. Seventh Int. Conf. Soil Mechanics and Foundation Engineering, Mexico City 0969). 4. Schmidt B. Settlements and ground movements associated with tunnelling in soil, Ph.D. Thesis, University of Illinois. (1969). 5. Prediction of settlements due to tunnelling in soil: three case histories. R.E.T.C. Proc. 2, 1179-1199 (1974). 6. Attewell P. B. & Farmer I. W. Ground disturbance cuased by shield tunnelling in a stiff, overconsohdated clay. Engng Geol. 8, 361-381 (1974).