1

Grounc 'reezinc a:8:ircaeyby J. S. HARRIS+ and R. J. REEDIIMEASURES TO overcome a collapse situa-tion in a heading at Stirchley, near Birming-ham, involved teamwork between four con-tractors each specialising in his own geo-technical field. This account describes theproblem and its solution with particularreference to the cryogenic refrigerationmethod pioneered and developed by ForakyLtd. Ground freezing allowed excavationwork to continue on a tunnelled section ofthe Stirchley and Cotteridge SewerageScheme being constructed for the Severn-Trent Water Authority (for whom Birming-ham District Council's City Engineer's De-partment acted as agents).

HistoryDuring the construction of a timbered

heading to accommodate a 525mm dia.sewer beneath Charlotte Road, a collapseoccurred at the face. This was due to thepreviously undetected presence of a wetalluvial deposit not shown by the originalboreholes, which had indicated stiffboulder clay with sand inclusions alongthe line that the heading was to be driven.A 2.74m dia. segmental shaft was sunk bykentledge on to the collapsed end of theheading and an abortive attempt was made* Foraky, Ltd.rJF. J. C Lilley (Midlands), Ltd.

to recommence driving from this shaft.Additional site investigation borings

were then taken to determine the extentof the alluvial deposit which indicated thatthis lay in a comparatively short channelor depression in the boulder clay and wassituated close to the shaft. At this pointthe original contractor went into liquidationand it was necessary to invite tenders for acompletion contract.

The completion contract allowed fordriving a heading some 100m long in thereverse direction towards this alluvial filledchannel which in the meantime had beenallowed to drain into the shaft. This head-ing proceeded satisfactorily to 35m fromthe shaft at which point conditions sud-denly deteriorated to the extent that it wasconsidered unsafe to proceed any furtherby conventional heading methods. Thecompleted section was then piped out andpacked.

Adverse conditionsA supplementary but very detailed site

investigation was then commissioned andthis revealed that the clay cover was in-adequate —less than $m—throughout theremaining 35m section. It was considerednecessary that the quality of the groundimmediately above the intended tunnel be

improved before resumption of furtherunderground working.

The 25m section from the face comprisednon-cohesive soil amenable to grout injec-tion; it was therefore recommended thatbentonite-cement grout be injected overthe tunnel line immediately above the sand/clay interface in order to reduce the per-meability and create cohesive strength,thus affording the required additional safetyover that section. The soil above the clayover the remaining 10m length adjacent tothe manhole shaft was not sufficientlyporous to permit grout injection and groundfreezing was recommended and implemen-ted.

Ground freezingTwo ground freezing schemes were con-

sidered, each with the aim of creating anarch of frozen soil over the intended ex-cavation and linking on each flank with thebasal clay. Minimal refrigeration is de-manded if horizontal freeze tubes can beinstalled around the eventual tunnel. How-ever, space limitations within the existingshaft were severe due to the relativelysmall shaft diameter coupled with the phy-sical presence of a bulkhead that had beenconstructed in the shaft invert to ensurestability following an abortive attempt to

First collapseoccurred here

25m. Length treated by Bentonite-cement grout ingA

(under which pi pe thrusting wascarried out from manhole 6A)

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September, 1975 47

Week No 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Preliminary Discussion gSite Preparation

Ground Consolidation

Installation jFreezing

Maintenance'unnelling

Pipe Jack

Clear Away

Programme for the specialist works

Tunnel lers excavating through the frozen ground

48 Ground Engineering

continue the first heading. This scheme wastherefore abandoned in favour of freezingvia a surface installed freeze tube array.

Again the requirement was to create aiiice arch. As a result a row of freeze tubeswas installed either side of the 10m tunnellength, and penetrating 1m into the claybase, supplemented by further freeze tubesover the tunnel zone terminating 0.3mabove the tunnel crown. The site laybeneath the carriageway of CharlotteRoad, in a surburban area comprising ter-race houses with short front gardens.Environmental considerations —quietoperations, no demand for electric poweror cooling water services —together witha need for rapid refrigeration led to thechoice of the liquid nitrogen refrigerationmethod in preference to the "conven-tional" ammonia-brine system.

The freeze tubes were installed by shelland auger boring methods following a pro-gramme of PFA injection to ensure thatany voids resulting from the collapse werefilled. Liquid nitrogen was then fed from aninsulated container, placed close to thesite, via suitable circuitry to the freezetubes and the required frozen archcreated. Tunnelling was resumed five daysfrom commencement of refrigeration andall excavation, ring erection and groutingoperations were undertaken in a further 15days during which refrigeration was main-tained on a reduced scale.

It was decided at the outset, followingconsideration of the proximity of residen-tial properties, that the tunnelling opera-tions be conducted on two eight-hourshifts daily. No particular difficulties wereexperienced with the freezing or miningmachinery as antifreeze lubricants wereused; cement grouting was undertakenwithout delay and the miners found theworking conditions quite bearable. Oncompletion of the 10m tunnelled length, ashort pipe was cast in the face to form aneye for the pipe-jacking contractor, to-gether with a corresponding invert overthe full 10m length.

Completion of operationThe 10m length from the shaft, construc-

ted under the protection of the frozenground, had been designed of suNcientdiameter to allow the employment ofpipe-jacking equipment for the remaining25m length. Double-reinforced spun con-crete pipes of 0.912m were then jackedwith the aid of a shield from the shaft bot-tom through the tunnel and tunnel eye, tolink up with the existing heading under thepre-grouted section. This operation occu-pied a further ten working days on asingle-shift basis. The annulus between thejacked pipe and the segmental tunnel wasthen pressure-grouted, marking the com-pletion of the specialist works.

Acknowledge mentsThe work was carried out for Birming-

ham District Council (Agent Authority tothe Severn-Trent Water Authority). Maincontractor (Completion Contract) wasHurley (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 worksincluded: Highways and Foundations Sur-veys Ltd., Wigan, Lanes., who undertooksite investigation and grouting; Foraky,Ltd., Nottingham, ground freezing; F. J. C.Lilley (Midlands) Ltd., tunnel construction;and C. W. Rees, Ltd., Eccleshall, Staffs.,pipe jacking.