1 Grounc 'reezinc a:8:ircaey by J. S. HARRIS+ and R. J. REEDII MEASURES 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 the problem and its solution with particular reference to the cryogenic refrigeration method pioneered and developed by Foraky Ltd. Ground freezing allowed excavation work to continue on a tunnelled section of the Stirchley and Cotteridge Sewerage Scheme being constructed for the Severn- Trent Water Authority (for whom Birming- ham District Council's City Engineer's De- partment acted as agents). History During the construction of a timbered heading to accommodate a 525mm dia. sewer beneath Charlotte Road, a collapse occurred at the face. This was due to the previously undetected presence of a wet alluvial deposit not shown by the original boreholes, which had indicated stiff boulder clay with sand inclusions along the line that the heading was to be driven. A 2.74m dia. segmental shaft was sunk by kentledge on to the collapsed end of the heading and an abortive attempt was made * Foraky, Ltd. r JF. J. C Lilley (Midlands), Ltd. to recommence driving from this shaft. Additional site investigation borings were then taken to determine the extent of the alluvial deposit which indicated that this lay in a comparatively short channel or depression in the boulder clay and was situated close to the shaft. At this point the original contractor went into liquidation and it was necessary to invite tenders for a completion contract. The completion contract allowed for driving a heading some 100m long in the reverse direction towards this alluvial filled channel which in the meantime had been allowed to drain into the shaft. This head- ing proceeded satisfactorily to 35m from the shaft at which point conditions sud- denly deteriorated to the extent that it was considered unsafe to proceed any further by conventional heading methods. The completed section was then piped out and packed. Adverse conditions A supplementary but very detailed site investigation was then commissioned and this revealed that the clay cover was in- adequate — less than $ m — throughout the remaining 35m section. It was considered necessary that the quality of the ground immediately above the intended tunnel be improved before resumption of further underground working. The 25m section from the face comprised non-cohesive soil amenable to grout injec- tion; it was therefore recommended that bentonite-cement grout be injected over the tunnel line immediately above the sand/ clay interface in order to reduce the per- meability and create cohesive strength, thus affording the required additional safety over that section. The soil above the clay over the remaining 10m length adjacent to the manhole shaft was not sufficiently porous to permit grout injection and ground freezing was recommended and implemen- ted. Ground freezing Two ground freezing schemes were con- sidered, each with the aim of creating an arch of frozen soil over the intended ex- cavation and linking on each flank with the basal clay. Minimal refrigeration is de- manded if horizontal freeze tubes can be installed around the eventual tunnel. How- ever, space limitations within the existing shaft were severe due to the relatively small shaft diameter coupled with the phy- sical presence of a bulkhead that had been constructed in the shaft invert to ensure stability following an abortive attempt to First collapse occurred here 25m. Length treated by Bentonite-cement grout ing A (under which pi pe thrusting was carried out from manhole 6A) X ,0,0 .4. h.,f! ~0:. 'I .X 0 X '0 XX X I I XX 0 0 0 0 0 0 0 0 4 ~ 0 0 0 0 I X I 0 '0 0 X. 0 I 0 00 0 j 0 'X T.: '4 . '00 ~4r Section showing the Charlotte Road turrrtel works and strata, with relevarrt ground treatment 46 Ground Engineering ! I J I