1 Grounc 'reezinc a:8:ircaey by J. S. HARRIS+ and R. J. REEDII

MEASURES TO overcome a collapse situa- to recommence driving from this shaft. improved before resumption of further tion in a heading at Stirchley, near Birming- Additional site investigation borings underground working. ham, involved teamwork between four con- were then taken to determine the extent The 25m section from the face comprised tractors each specialising in his own geo- of the alluvial deposit which indicated that non-cohesive amenable to grout injec- technical field. This account describes the this lay in a comparatively short channel tion; it was therefore recommended that problem and its solution with particular or depression in the boulder and was bentonite-cement grout be injected over reference to the cryogenic refrigeration situated close to the shaft. At this point the line immediately above the / method pioneered and developed by Foraky the original contractor went into liquidation clay interface in order to reduce the per- Ltd. Ground freezing allowed excavation and it was necessary to invite tenders for a meability and create cohesive strength, work to continue on a tunnelled section of completion contract. thus affording the required additional safety the Stirchley and Cotteridge Sewerage The completion contract allowed for over that section. The soil above the clay Scheme being constructed for the Severn- driving a heading some 100m long in the over the remaining 10m length adjacent to Trent Water Authority (for whom Birming- reverse direction towards this alluvial filled the manhole shaft was not sufficiently ham District Council's City Engineer's De- channel which in the meantime had been porous to permit grout injection and ground partment acted as agents). allowed to drain into the shaft. This head- freezing was recommended and implemen- ing proceeded satisfactorily to 35m from ted. History the shaft at which point conditions sud- During the construction of a timbered denly deteriorated to the extent that it was Ground freezing heading to accommodate a 525mm dia. considered unsafe to proceed any further Two ground freezing schemes were con- conventional sewer beneath Charlotte , a collapse by heading methods. The sidered, each with the aim of creating an occurred at the face. This was due to the completed section was then piped out and arch of frozen soil over the intended ex- packed. previously undetected presence of a wet cavation and linking on each flank with the alluvial deposit not shown by the original basal clay. Minimal refrigeration is de- , which had indicated stiff Adverse conditions manded if horizontal freeze tubes can be boulder clay with sand inclusions along A supplementary but very detailed site installed around the eventual tunnel. How- the line that the heading was to be driven. investigation was then commissioned and ever, space limitations within the existing A 2.74m dia. segmental shaft was sunk by this revealed that the clay cover was in- shaft were severe due to the relatively on to the collapsed end of the adequate —less than $m —throughout the small shaft diameter coupled with the phy- heading and an abortive attempt was made remaining 35m section. It was considered sical presence of a bulkhead that had been * Foraky, Ltd. necessary that the quality of the ground constructed in the shaft invert to ensure JF.r J. C Lilley (Midlands), Ltd. immediately above the intended tunnel be stability following an abortive attempt to

25m. Length treated by Bentonite-cement grout ing A

(under which pi pe thrusting was carried out from manhole 6A) First collapse occurred here

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J Section showing the Charlotte Road turrrtel works and strata, with relevarrt ground treatment

46 Ground Engineering I 1

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.10m. Length treated by ground freezing A

(bolted concrete segmental MANHOLE 6A tunnel lining) "Reverse tunnelling 'rom here unsuccesdul MANHOLE 6

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September, 1975 47 continue the first heading. This scheme was Week No 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 therefore abandoned in favour of freezing via a surface installed freeze tube array. Again the requirement was to create aii Preliminary Discussion ice arch. As a result a row of freeze tubes g was installed either side of the 10m tunnel length, and penetrating 1m into the clay base, supplemented by further freeze tubes Site Preparation over the tunnel zone terminating 0.3m above the tunnel crown. The site lay beneath the carriageway of Charlotte Ground Consolidation Road, in a surburban area comprising ter- race houses with short front gardens. Environmental considerations —quiet Installation operations, no demand for electric power j or cooling water services —together with Freezing a need for rapid refrigeration led to the choice of the liquid nitrogen refrigeration method in preference to the "conven-

Maintenance'unnelling tional" ammonia-brine system. The freeze tubes were installed by shell and auger boring methods following a pro- gramme of PFA injection to ensure that any voids resulting from the collapse were filled. Liquid then fed from an Pipe Jack nitrogen was insulated container, placed close to the site, via suitable circuitry to the freeze tubes and the required frozen arch Clear Away created. Tunnelling was resumed five days from commencement of refrigeration and Programme for the specialist works all excavation, ring erection and grouting operations were undertaken in a further 15 days during which refrigeration was main- tained on a reduced scale. It was decided at the outset, following consideration of the proximity of residen- tial properties, that the tunnelling opera- tions be conducted on two eight-hour shifts daily. No particular difficulties were experienced with the freezing or mining machinery as antifreeze lubricants were used; cement grouting was undertaken without delay and the miners found the working conditions quite bearable. On completion of the 10m tunnelled length, a short pipe was cast in the face to form an eye for the pipe-jacking contractor, to- gether with a corresponding invert over the full 10m length. Completion of operation The 10m length from the shaft, construc- ted under the protection of the frozen ground, had been designed of suNcient diameter to allow the employment of pipe-jacking equipment for the remaining 25m length. Double-reinforced spun con- crete pipes of 0.912m were then jacked with the aid of a shield from the shaft bot- tom through the tunnel and tunnel eye, to link up with the existing heading under the pre-grouted section. This operation occu- pied a further ten working days on a single-shift basis. The annulus between the jacked pipe and the segmental tunnel was then pressure-grouted, marking the com- pletion of the specialist works.

Acknowledge ments The work was carried out for Birming- ham District Council (Agent Authority to the Severn-Trent Water Authority). Main contractor (Completion Contract) was Hurley (Civil Engineering), Ltd., Bury, Lanes., and the Co-ordinating Contractor (Special Works) was F. J. C. Lilley (Mid- lands) Ltd., Nuneaton, Warwicks. Special- ist sub-contractors involved in the works included: Highways and Foundations Sur- veys Ltd., Wigan, Lanes., who undertook site investigation and grouting; Foraky, Ltd., Nottingham, ground freezing; F. J. C. Lilley (Midlands) Ltd., tunnel construction; and C. W. Rees, Ltd., Eccleshall, Staffs., Tunnel lers excavating through the frozen ground pipe jacking. 48 Ground Engineering