transport and road research laboratoryfor the crossing beneath the river mersey, the loop utilises...
TRANSCRIPT
TRANSPORT and ROAD RESEARCH LABORATORY
Department of the Environment Department of Transport
TRRL LABORATORY REPORT 868
SITE INVESTIGATION AND CONSTRUCTION OF THE LIVERPOOL LOOP AND LINK TUNNELS
by
G West BA, FGS and A F Toombs
Any views expressed in this Report are not necessarily those of the Department of the Environment or of the Department of Transport
Tunnels Division Structures Department
Transport and Road Research Laboratory Crowthorne, Berkshire
1978 ISSN 0305-1293
Ownership of the Transport Research Laboratory was transferred from the Department of Transport to a subsidiary of the Transport Research Foundation on 1 st April 1996.
This report has been reproduced by permission of the Controller of HMSO. Extracts from the text may be reproduced, except for commercial purposes, provided the source is acknowledged.
CONTENTS
Abstract
1. Introduction
1.1 The Liverpool Loop and Link scheme
1.1.1 Description of the Loop
1.1.2 Description of the Link
2. Site investigation
2.1 Preliminary information
2.2 Geology of the site
2.3 Main ground investigation
2.4 Field work
2.5 Laboratory tests
2.5.1 Tests on soil samples
2.5.2 Tests on rock cores
2.6 Report
3. Observations during construction
3.1 Excavation methods
3.2 Ground conditions during construction of the Loop
3.3 Ground conditions during construction of the Link
3.4 Progress on the Loop and Link
3.5 Wells and caverns encountered during construction
3.6 Completion of the Loop and Link
3.7 Costs
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4. Discussion
4.1 Comparison of predicted and as-found conditions
4.2 Other comments on the site investigation
5. Acknowledgements
6. References
7. Appendix: Principal participants in the Liverpool Loop and Link scheme
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© CROWN COPYRIGHT 1978 Extracts from the text may be reproduced, except for
commercial purposes, provided the source is acknowledged
SITE INVESTIGATION AND CONSTRUCTION OF THE LIVERPOOL LOOP AND LINK TUNNELS
ABSTRACT
The site investigation and the construction records for the Liverpool Loop and Link underground railway tunnels have been compared to see what lessons of good practice in site investigation emerge. The tunnels were con- structed almost entirely within Triassic sandstones generally ranging in strength from weak to moderately strong, enabling roadheaders to be used effectively for most of the excavation.
The Report discusses the site investigation and the ground conditions as encountered during construction in some detail. The site investigation was generally satisfactory, giving an accurate account of the geology of the site, accurately predicting the location of major faults and providing most of the ground information needed to drive the tunnels. However, during construction the rock was found to have more joints than had been sugg- ested in the site investigation, no doubt because of the difficulty of inter- cepting steeply inclined joints with a vertical borehole. Trial shafts and headings are suggested to overcome this and other problems.
1. ~INTRODUCTION
The Laboratory is making a series of case history studies of tunnel projects 1 -5 with special emphasis on the
role played by the site investigation; the tunnels chosen for study were of varying complexity and were
constructed in different ground conditions. The aims of the studies are to bring out the essentials of good
site investigation practice, show where improvements in technique are required, and put on record the
experience gained in constructing tunnels in the particular conditions of each project.
1.1 The Liverpool Loop and Link scheme
This Report describes the site investigation and construction beneath Liverpool city centre of a new
underground railway system known as the Loop and Link. It has been prepared with the generous co-operation
of the promoters, British Railways and the Merseyside Passenger Transport Executive and their consultants,
Mott, Hay and Anderson. The scheme involved the construction of two tunnels at different levels, new
stations, escalator halls and construction access shafts. Both tunnel routes, shown in Figure 1, were planned
so as to utilise existing railway tunnels wherever possible.
The project is the result of an enquiry into the transport needs of the area in the late 1960s. The Loop
and Link lines were promoted separately, their respective Parliamentary Bills seeking powers for construction
being passed in July t 968 and July 1971. Together, they give travellers from Merseyside's main commuter
areas - Southport, Ormskirk and Kirkby in the north, Garston in the south, and the Wirral Peninsula in the
west - fast rail access to the city's business and shopping centres and to British Railway's inter-city station at
Lime Street. 1
1.1.1 Description of the Loop: The Loop, a single line tunnel 3.1 km long and 4.7 m internal diameter,
was constructed at a lower level than the Link. It commences from a new junction with the Mersey railway
tunnel from Birkenhead at Mann Island near James Street Station, and links new stations constructed at
James Street, Moorfields, Lime Street and Central. The line returns to James Street Station via another new
junction with the Mersey railway at Derby Square. Tunnel axis at James Street Station is at a depth of
-21 m OD and rises to -3 .7 m OD at Lime Street Station.
In addition to the work carried out on tile Liverpool side of the River Mersey, a new junction to the
Loop and a short length of tunnel have been constructed at Birkenhead. These additional works however are
not considered in this Report. For the crossing beneath the River Mersey, the Loop utilises the Liverpool to
Birkenhead railway tunnel built in 1886.
All trains proceed round the Loop in a clockwise direction and the line gives direct connection with
the main line services and improved distribution throughout the city centre. The Contract for constructing
the Loop was let in December 1971 and work commenced in January 1972.
1 .1 .2 Description of the Link: The Link is a double line constructed in two parallel tunnels and runs
under the city centre between Exchange and Central Stations. The tunnels commence from portals to the
north of the now closed Exchange Station and run to the upper level of the new station at Moorfields. Here
there is a passenger interchange with the deeper Loop line. From Moorfields, the twin tunnels continue to
Paradise Junction where they join the existing Mersey underground railway from James Street Station.
The Link continues thereafter by utilising the existing double-line tunnel to Central Station. At Central
Station another passenger interchange with the Loop line has been constructed. The Link line therefore
provides a direct rail connection between the lines from the north suburbs previously terminating at
Exchange Station and the lines from the south previously terminating at Central Station.
The length of the tunnels constructed between Exchange Station and Paradise Junction is 0.9 km and
each bore is 4.7 m internal diameter. Tunnel level is from +12 m OD at the portal descending to -7 .3 m OD
at Paradise Junction. The Link contract was let on 9 February 1973 and construction started later that month.
The principal participants in the Loop and Link scheme are given in the Appendix.
2. SITE INVESTIGATION
2.1 Preliminary information
The site investigation contractor was provided by British Railways with a site plan showing the Solid
geology and the approximate locations of the major faults. This plan was later modified as a result of the
site investigation, especially in regard to the position of the Castle Street Fault, but the rest of the data
was found to be fairly accurate. British Railways also supplied another site plan, on which all the known
data about the subsurface conditions at the site had been plotted. This information supplemented the site
investigation, although some of the soil descriptions were too vague to permit an accurate interpretation of the geology.
Other sources of information referred to by the site investigation contractor were the Department of
Geology, University of Liverpool, and papers on the geology 6 and construction 7 of the Mersey Queensway
2
road tunnel, driven in the 1930s.
2.2 Geology of the site
From the preliminary information an outline of the geology of the site was prepared. It was seen that
the Liverpool area is underlain by a thick series of sandstones of Triassic age which are covered, in places,
by glacial and post-glacial deposits. These include Boulder Clay, and various alluvial sediments located near
the River Mersey. Examination of records and exposures of rock in Liverpool indicate that the strike of the
sandstones is approximately north-south and that their dip is towards the east.
The site is crossed by two major north-south trending faults (Figure 1). The faults have thrown sand-
stones of different geological ages into contact with each other and, since the downthrows are to the east,
the older rocks are on the west side of the site. The Castle Street Fault, which crosses Derby Square and
passes beneath Exchange Terminus, brings the Middle Bunter Sandstone into contact with the Upper Bunter
Sandstone. The Kingsway Fault, which crosses the entrance to the Mersey Kingsway road tunnel and
passes to the west of Williamson Square, brings the Upper Bunter Sandstone into contact with the Lower
Keuper Sandstone. The throws of the Castle Street Fault and the Kingsway Fault have been estimated to
be 75 m and 300 m respectively.
The Upper Bunter Sandstone, which is the weakest and most friable sandstone of the three, thus
occupies a strip of ground 330 to 520 m wide near the centre of the site. It was estimated that about one-
third of the Loop line and two-thirds of the Link line would be located in this formation.
The processes of erosion, principally glaciation, have produced a V-shaped depression or channel in
the rock surface with the bottom of the depression in the Upper Bunter Sandstone (Figure 2). To the east,
the elevation of rockhead rises fairly quickly. To the west of the depression, in the vicinity of James Street
Station, the rock surface dips .towards the River Mersey. Boulder Clay occupies the lower part of the
depression in the Upper Bunter Sandstone and increases in thickness rapidly to the north of Dale Street.
At the most northerly point of the Loop line, the Boulder Clay is more than 15 m thick.
The most recent geological feature is a buried river channel which trends northeast-southwest across
the centre of the site parallel to, and probably just west of, Whitechapel. The buried channel is infilled
with soft alluvial clay and fine sand and follows the course of the glacially eroded depression in the Upper
Bunter Sandstone, but it was not clear from the available data whether the river channel cut completely
through the Boulder Clay to bedrock or not.
2.3 Main ground investigation
The main ground investigation was carried out by the site investigation contractor, and the objectives
were defined as being to determine the subsurface conditions along the proposed alignments of the two
tunnels and to briefly discuss the probable effect of the geological conditions on tunnel design and
construction. The following points were given special attention. Determination of:
(a) thickness of the weathered zone of sandstone
(b) nature and spacing of fractures and joints in the rock
(c) condition of soil and rock near faults
(d) nature and condition of deposits overlying rock
(e) ground-water conditions and their effect on tunnelling works. 3
The main ground investigation consisted of a programme of field work, a programme of laboratory tests and
the preparation of a report giving the results of the work carried out and an assessment of the ground con-
ditions.
2.4 Field work
The field work consisted of the sinking of twenty-four boreholes in the area being considered. Three
further boreholes were sunk in Birkenhead, on the other side of the River Mersey, but are not considered in
this Report. The holes, which ranged in depth from 7 to 43m, were sunk by boring through the overburden
soils to rockhead, and then drilling into the bedrock to a depth of about 1.5m below invert level of the
proposed tunnels. The number, location and depth of the holes were selected by British Railways, although
two additional boreholes were put down at the request of the site investigation contractor to provide
additional geological information in two specific areas. The borehole positions are shown in Figure 1. A
field inspector, representing British Railways, made daily visits during the site work, which was carried out
between April and June 1970.
The boreholes were sunk by two different methods:
(1) One set of boreholes was advanced by a Pilcon Wayfarer one-ton portable drilling rig, using 200 mm
diameter casing to support the sides of the borehole in non-cohesive strata. Samples of soil were recovered
at frequent intervals in all boreholes. In clay soils, 100 mm diameter tube samples (U 100s) were taken; in
sandy soils, standard penetration tests were carried out by driving a split-tube sampler into the soil using
the standard technique. Numerous jar samples of soil also were recovered. At the start and end of each
shift, measurements were taken of the water level, if any, in the borehole.
When rockhead had been reached, a trailer-mounted Craelius XC-90H diamond drilling rig was moved
over the hole. A 76 mm diameter casing was lowered into the hole and drilled into firm rock. Cores of 62 mm
diameter were then recovered from the underlying bedrock using double-tube swivel-type core barrels,
incorporating face-discharge bits. Water for drilling was supplied to the bit by a Simplex 80 pump.
(2) A Craelius-Mobile B-61 Pacemaker rig was used for both boring and drilling the other set of boreholes.
The Pacemaker is a heavy duty, truck-mounted, rotary drilling rig capable of applying a high torque to the
rotating head. It can auger through overburden soils using low speed and high torque, and then by means of
a torque converter, adapt to high speed and low torque drilling for coring into rocks. The Pacemaker rig had
the advantage over the other two rigs of allowing the work to progress continuously thus minimising
congestion in city streets.
The Pacemaker rig penetrated through the overburden soils using 125 mm internal diameter, 250 mm
external diameter continuous-flight hollow-shaft augers. A central plug, connected to the drilling head by
rods, is placed in the bot tom of the hollow shaft to prevent soil entering the shaft when the augers are being
advanced through the ground. As the borehole deepens, additional lengths of auger and rods are added to
those already in the ground. Samples of soil are recovered by removing the central plug and lowering a
sampling tube down the hollow shaft and driving or jacking it into the soil at the bottom of the borehole.
In normal practice, Samples are taken at 0.75 or 1.5 m intervals. At this site, however, it was felt that
the overburden soils should be logged as accurately as possible, and two basic techniques were adopted to
record the strata between samples. One technique, used where the overburden soils were shallow in depth,
4
was to pull out the whole string of augers at frequent intervals and record the soils adhering to the flights.
The second technique, preferred during the later stages of the investigation, was to drive continuous samples.
Some of these were extruded for immediate identification and the others were sealed and labelled for
laboratory studies.
With the Pacemaker rig, rock cores were recovered in an identical manner to that o f a conventional
diamond drilling rig. The same type and size of core barrel and bits were used as those described previously.
Standpipe piezometers were installed in all but one of the boreholes. They consisted o f a rigid PVC
standpipe of 15 mm internal diameter fitted with a ceramic porous tip 50 mm in diameter and 250 mm
long at the lower end. The porous tips were placed about 0.6 to 1.0 m above the bo t tom of the hole, and
were surrounded by a filter of clean medium to coarse sand. The hole was filled with sand to a depth o f
approximately 7 m, corresponding to the crown of the proposed tunnel excavation, and sealed at this level
by a 1 m thick layer of small bentonite balls which were dropped down the hole. The remainder o f the hole
was backfilled with a weak sand-cement grout to a point just below the made ground, where a second plug
of bentonite was placed. Random fill taken from the boreholes was used to backfill the length o f hole in
made ground. Details of the piezometer installations were entered on most of the borehole logs. Water level
rea~Jings were taken throughout the field work.
At all boreholes, starter pits were excavated to locate the position of underground cables and other
services. The pits were up to 2.4 m deep, and in some cases a considerable width o f carriageway had to be
dug open before a gap of sufficient width could be found to insert casing between the services. The usual
practice was to install a length of casing in the pit and backfill around it, using a mechanical tamper to re-
compact the soil. On completion o f the boreholes located in the city streets, a temporary surfacing of
concrete and tarmacadam was placed over the pits.
The rock cores recovered during the site investigation were described at the site by engineering geologists.
The method used was that recommended by the Geological Society Engineering Group Working Party 8
for the logging o f rock cores for engineering purposes. The soil samples were forwarded to the site investigation
contractor's laboratory where they were described by a soils engineer. Borehole logs were prepared from
these descriptions together with any other relevant information recorded on the daily reports o f the field
c r e w s .
The rock cores were available for inspection by contractors tendering for the works and some did so.
2.5 Laboratory tests
A programme of laboratory tests was carried out on selected soil samples and rock cores to provide
quantitative data for the engineering studies. The soil tests were carried out by the site investigation
contractor, but the rock tests were subcontracted to another firm.
2.5.1 Tests o n soil samples. The following tests were carried out:
Liquid limit and plastic limit determinations were made on selected samples of Boulder Clay from
boreholes in the vicinity o f proposed shafts.
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Particle size distribution analyses were made on samples o f sand from within the Boulder Clay
formation and from the highly weathered zone of the Upper Bunter Sandstone.
Undrained triaxial shear tests were carried out on undisturbed samples o f Boulder Clay and clay
alluvium. In general, three undisturbed specimens 38 mm in diameter and 76 mm long were taken
from each U 100 sample tube and tested at three different confining pressures. However, some
samples of Boulder Clay contained too many stones for this procedure to be practicable and a
larger specimen 100 mm in diameter and 200 mm long was prepared and tested in a multi-stage
undrained triaxial test.
Consolidated-drained triaxial compression tests were carried out on sets of three specimens taken
from samples at the top and bo t tom of the Boulder Clay stratum in a borehole near the tunnel
portals.
A consolidation test, in the oedometer apparatus, was performed on a sample of clay alluvium.
2 . 5 . 2 Tes t s o n r o c k co res . The following tests were carried out:
Compressive strength tests were performed on the first length of rock core recovered from each
borehole, and also from the rock at the crown of the tunnel. Because many rock cores were
broken along the bedding planes forming a set of thick discs, some of the rock core specimens
had lengths which were less than twice the diameter. A correction factor was therefore applied
to the compressive strength measured on the short cores. The test specimens were saturated
with water by a vacuum technique.
The following tests were carried out on rock cores taken near the crown level of the proposed
tunnels:
Modulus o f elasticity and Poisson's ratio were determined from tests carried out on oven-dried
38 mm diameter triaxial specimens at a confining cell pressure of 14 MN/m 2. Each test consisted
of three cycles of loading and unloading. After the modulus test, the specimen was loaded to
failure.
Diametrical tensile tests (Brazilian tests) were carried out on some cores. In this test, the rock
core was laid with its axis horizontal and a vertical line load was applied to split the core across
the diameter.
Consolidated-drained triaxial compression tests were carried out to obtain the Mohr envelope
representing failure. Cell pressures of 3.5, 7 and 14 MN/m 2 were used and the 38 mm diameter
specimens were tested in a fully saturated condition.
The following tests were carded out on rock cores taken near the mid-height of the proposed
tunnels:
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Modulus o f elasticity and Poisson's ratio were determined from tests carried out as described
previously except that the 38 mm diameter test specimens were cored at right-angles to the
larger size cores recovered from the boreholes.
Consolidated-drained triaxial compression tests were carried out as described previously except
that the 38 mm diameter test specimens were cored at right-angles to the larger size cores recovered
from the boreholes.
Laboratory tests on rock could only be carried out on cores of sufficient length to prepare specimens.
The highly weathered sandstone zones, encountered near the surface in the Upper Bunter Sandstone and
occasionally at greater depths, could not be tested.
2.6 Report
A three-volume report was prepared for British Railways by the site investigation contractor.
Volume 1 comprised a description of the field work and the laboratory tests, a full description of the site
geology, a brief discussion of geological conditions likely to affect the design and construction of the
tunnels, a summary, conclusions and recommendations section, and summary sheets of field and laboratory
data. Volume 2 contained site plans showing Solid geology (Figure 1), surface levels, rockhead elevation,
Boulder Clay thickness and ground water levels. Some of the data on these plans were obtained from the
preliminary information and some were obtained from the main ground investigation. The geological
section (Figure' 2) does not come from the site investigation report, but was prepared for this Report to
illustrate the geological structure of the site. Volume 2 also contained the borehole logs, longitudinal
sections along the tunnel lines as shown in Figures 3 and 5, and charts and diagrams relating to the laboratory
soil tests. Volume 3 comprised a description of the rock testing programme, summary sheets giving the
results, and charts and diagrams relating to the laboratory rock tests.
The following summary, conclusions and recommendations of the main ground investigation is taken,
slightly modified, from Volume 1 of the site investigation contractor's report:
1. The site investigation generally provided confirmation of the Solid geology as shown on existing
plans. Three sandstones of different ages are present in the area under investigation, namely:
(i) Middle Bunter Sandstone, the oldest sandstone, is reasonably competent and a considerable
amount of experience of tunnelling through it has been gained during the construction of
tunnels below the River Mersey; it occurs on the west side of the site. The rock is typically
moderately weak to moderately strong.
(ii) Upper Bunter Sandstone, which is generally softer and more friable than the other two sandstones,
occurs in the centre of the site. The rock is typically weak to moderately weak becoming very
weak at rockhead.
(iii) Lower Keuper Sandstone, the youngest of the three sandstones, is similar in competence to the
Middle Bunter Sandstone and occurs on the east side of the site. The rock is typically moderately
weak to moderately strong.
2. In the Upper Bunter Sandstone, there is a zone of highly weathered rock at rockhead in which the
sandstone has been reduced to an uncemented sand with some sandstone fragments or highly
fragmented sandstone with a matrix of sand. Cores could not be recovered in this zone, which was
0.6 to 4.5 m thick in the ten boreholes where it could be measured. There was usually no thickness
of highly weathered rock at rockhead in the other two sandstones.
.
4.
5.
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However, highly weathered zones occur elsewhere in the sandstones. They were noted on the
borehole logs, and were plotted on the longitudinal sections. Highly weathered zones occur more
frequently in the Upper Bunter Sandstone formation.
Weathering has occurred differentially along the bedding planes of the sandstone which are oriented
at 0 to 15 ° to the horizontal and are dipping towards the east. In the more weathered zones of the
Upper Bunter Sandstone, core was recovered as a series of thin discs ranging from 13 to 76 mm
long and typically 25 mm long due to fracturing along the bedding planes. These cores were too
short to permit laboratory strength tests to be carried out. It was anticipated that rock falls from the
roof of the tunnel would be the principal source of weakness in the rock.
Joints in the rock are usually high angle, typically 60 to 70 ° . The joints appear to be more frequent
in the harder sandstones (Middle Bunter, Lower Keuper) and are fairly widely spaced. Joints were
not expected to cause tunnelling problems, although localized falls of rock could occur in the tunnels
where the rock is blocky.
The positions of the two major faults, the Castle Street Fault and the Kingsway Fault, appear to be
essentially as shown on the existing geological maps except that the Castle Street Fault probably
passes directly below Exchange Terminus and not to the west of the station. The site investigation
did not provide any evidence that Upper Bunter Sandstone occurs adjacent to Lime Street Terminus
as a result of faulting.
Boreholes located close to the major faults indicated that the sandstone in these regions is more
broken and weathered than elsewhere.
There is a shallow depression in the rock surface, with its base in the Upper Bunter Sandstone. A
deposit of stiff Boulder Clay partly fills the depression. The thickness of the Boulder Clay formation
increases rapidly to the north and is about 15 m thick at the most northerly point of the Loop tunnel.
On the south side of the site, a shallow post-glacial river channel has cut into the glacial deposits.
However, on the basis of the borehole information, the tunnels should have an adequate cover of
rock and the overburden soils will only affect shafts and other ancillary structures.
Ground water is very deep below the surface near the River Mersey and rises towards the east,
corresponding with the general rise in the topography. An anomalous situation was recorded at
Exchange Terminus, where ground water was locally at a much higher elevation than elsewhere on
the site.
The most significant influence on the level of ground water is the extensive pumping from deep
wells. Water level readings taken during the site investigation indicated that rainfall may cause
significant fluctuations of the water levels in the sandstones. The tidal range of the River Mersey may
also affect the ground water levels, possibly with fluctuations of up to a metre on the west side of
the site.
The invert level of the Loop tunnel will be below the ground water table over a substantial
part of the total length. The Link tunnel also will be below the water levels at its north end, near
Exchange Terminus, if the piezometric data is reliable.
8. It was recommended that additional studies of the ground water conditions at the site be carried out
to determine seasonal fluctuations in water table level, in situ permeability of the rock formations
and the feasibility of lowering ground water levels temporarily during construction.
3. OBSERVATIONS DURING CONSTRUCTION
A continuous record of joints, bedding planes, faults, roof instability and ground water flow was made by
engineering geologists on the Resident Engineer's staff in both Loop and Link tunnel drives as construction
was carried out. The colour and general condition of the rock was assessed and any bands or layers of sand,
mica or clay were noted. The length and inclination of joints and bedding planes were measured together
with width of parting, and a note made of whether they were open, filled or closed. Similarly, all fault
characteristics were determined and any associated shattered zones noted. An assessment of the severity of
overbreak in crown or sides was made and its possible cause recorded. All the information was entered
on the longitudinal sections of the tunnel as shown in Figures 4 and 6.
In addition to the above, face sections as shown in Figures 7 and 8 were drawn for the tunnel face
every 30 to 40 m. These gave a detailed description of the geology of the face, the condition of the rock and
the results of Schmidt hammer tests. Colour photographs were taken at the same time. These records
were used for reference, particularly on the Loop line where the same type of ground was encountered more
than once, and for possible use in settling any claims arising after construction was complete. Black-and-
white photographs of the tunnel construction were also taken; Plates 1 -4 are examples.
3.1 Excavation methods
It was originally intended that a full-face tunnelling machine would be used to drive the Loop and
Link tunnels. In the event, however, roadheaders manufactured by Dosco Overseas Engineering Ltd
(Plate 3) were employed on both drives, these machines being considered to have several advantages in
flexibility over full-face machines in sandstones of this range of strength. These include the ability to
excavate the enlarged sta{ion sections quite easily by making an initial cut at platform level and then a
return cut to excavate the invert for the track. Access to the face is easily obtained together with room
for rapid erection of tunnel supports or maintenance of the machine.
The tunnels were designed with circular sections which proved a difficult shape for the roadheader
to cut. Originally this design was chosen with a view to using a full-face machine. The roadheader in fact
excavated the 5.1 m external diameter running tunnels in two sections because the cutting range of the
boom was insufficient to cut the full height in one pass. The first cut was made moving forwards leaving
approximately lm of rock in the invert which was then removed by a second pass of the machine (see
Figures 7 and 8). When the invert was being cut, the construction track was temporarily hung on the
wall of the tunnel (see Plate 2).
3.2 Ground conditions during construction of the Loop
The Loop line was constructed in four sections. These were (i) Mann Island to Moorfields, (ii) Moor-
fields to Lime Street, (iii) Central to Lime Street and (iv) Central to Derby Square. Three roadheaders
were used.
9
The drive from Mann Island, on the east bank of the River Mersey, to Moorfields commenced in the
Middle Bunter Sandstone. This was one of the wettest areas on the Loop with water flows from a series of
vertical joints in the otherwise fairly competent and moderately strong rock. The Middle Bunter Sandstone
was characterised by numerous mica and clay bands, bedding planes and open and closed joints. These
features did not reduce the stability of the rock to the point where progress of the drive was seriously
affected, although temporary support in the form of steel arch ribs was necessary through much of this
section. Rock bolts and wire mesh were used at one point to support a disturbed area in the crown where
a minor fault crossed the tunnel line. The Castle Street Fault was encountered at about 500 m. This
major fault, which actually consisted of three distinct fault zones, caused little interference to tunnelling
despite its large throw. Steel arch ribs were used in the fault zone. The downthrow brings the Upper Bunter
Sandstone into the tunnel and the remainder of the drive to Moorfields was through this stratum. The
Upper Bunter Sandstone in this section, although less jointed than the Middle Bunter Sandstone, was
weaker and fssile along the near horizontal bedding planes causing some instability in the crown.
The drive from Moorfields to Lime Street remained in the Upper Bunter Sandstone until faulted
out after 600 m by the Kingsway Fault. This sandstone, a weak reddish brown fine to medium grained
material, contained a large number of small joints, shear planes and disturbed zones associated with minor
faulting. It was also weakly bedded allowing overbreak to occur in a few places. Despite the apparent
instabilit E of the rock, once loose material was removed from the crown and sides, the tunnel was found to
be sdf-supporting (Plate 2) and in fact no temporary support was provided for most of the length of tunnel
excavated in Upper Bunter Sandstone in this section of the Loop. The rest of the drive, in Lower Keuper
Sandstone, required temporary support.
The Kingsway Fault was a little more noticeable than the Castle Street Fault and the erection of
arch ribs was again necessary whereas the surrounding sections remained unsupported. The fault was
characterised by a belt of very hard altered sandstone which had to be removed by hand mining methods,
and also contained a thin infilled layer of brown mud. The neighbouring sandstones were highly weathered
but the cemplete width of the zone amounted to less than 10 m. The downthrow of the Kingsway Fault
brought the Lower Keuper Sandstone into the tunnel and the remainder of the drive eastwards to Lime
Street was through this stratum. The Lower Keuper Sandstone, a yellow moderately weak to moderately
strong sandstone contained numerous well defined joints and bedding planes which caused some instability
in the crown. At Lime Street the rock contained a number of very hard feldspathic bands which required
drilling and blasting to remove (Plate 1).
Lime Street Station is situated at the highest level of the Loop, and it was intended that most of the
drives would be uphill, where possible, so that water would drain away from the face. In practice_ failure to
channel water properly from the face caused it to become trapped within the crushed sandstone rubble and
the resultant slurry became difficult for the conveyors to manage. In addition, as also occurred on the drive
from Central Station to Lime Street Station, the machine tended to sink into the wet spoil that collected in
the invert. Progress had to be continuous to prevent this and when delays did occur during the week-ends,
boards were placed beneath the machine to prevent it from sinking.
Much of the rock between Central Station and Lime Street Station proved very weak and fragments
could be crushed by hand. Most of this section was supported by steel arch ribs, placed normally at one
metre centres (Plate 1). Numerous open and closed joints were present, some letting in water.
10
The Kingsway Fault was again passed through during the drive from Central Station to Derby Square.
Conditions at the fault were as encountered at the northern end with a hard ironstone band surrounded by
zones of shattered and weathered sandstone. A set of 16 ribbed arches was erected at this point. The
Upper Bunter Sandstone replaced the Keuper Sandstone west of the Kingsway Fault. The Upper Bunter
Sandstone was weak and weathered, especially below the water table. Rock bolts and steel mesh were
required at one point to control rock falls due to minor faulting. At Derby Square the Loop tunnel merges
with the existing Mersey railway tunnel.
Most of the invert of the Loop tunnel lies below the water table. Ground water flow into the tunnel
occurred frequently although rates of flow were not sufficient to cause delays. The most serious problems
occurred, again due to inadequate drainage provision at the face, when water mixed with loose or crushed
sandstone to form a slurry. One effect of this has already been described as causing machine instability on
the drive from Central Station to Lime Street. Another more widespread problem was due to the abrasive
nature of this slurry causing wear of moving parts of the tunnelling machines.
A summary of ground conditions encountered during construction of the Loop is given in Table 1.
The associations between open joints and ground water flow, and between weak horizontal bedding planes
and instability in the crown that had been observed during construction of the tunnel are confirmed by the
general qualitative correlation of these items in the Table.
3.3 Ground conditions during construction of the Link
The upline and downline Link tunnels were driven consecutively from the portal north of Exchange
Station keeping the leading face 200 to 300 m ahead of the other. A mixed face was encountered for the
first 140 m of drivage of both tunnels with the top section of the face in Boulder Clay and the rest in
Middle Bunter Sandstone. Bolted concrete segmental rings were used until there was at least 1 metre of
good rock cover, when ribs were used as support. The tunnels remained in the Middle Bunter Sandstone
until the'Castle Street Fault was reached near Moorfields Station. The sandstone was found to be
moderately strong with a few weaker or shattered zones associated with minor faulting. Weak horizontal
bedding caused some slight overbreak and slabbing in the crown but no delays to progress were incurred.
The Castle Street Fault zone continued over 40 m of the Link tunnels because of its acute intersection
angle with the tunnel centre line but its only effect on progress was a little extra packing behind the ribs.
However a roof fall in the Moorfields downline station tunnel next to the Castle Street Fault was probably
due to disturbance of the ground in the fault zone (see Figure 6). A face section in the fault zone in the
downline tunnel is shown in Figure 8. The fault brought the Upper Bunter Sandstone into the tunnels.
This stratum was found to be of weak to medium strength, but very weak along numerous horizontal
bedding planes. Frequent overbreak and unstable crown conditions occurred requiring temporary support.
In fact temporary support in the form of steel arch ribs was used throughout the length of the Link running
tunnels not only to prevent local roof,falls but also because, for the most part, the cover of good rock above
the tunnel crown is less than 7 m.
Since the tunnels as excavated are 5 m in diameter all this rock cover is under the stress effects of the
excavation. Cast-iron bolted rings were used where the Link tunnels passed above the Queensway road
tunnel and below the Dale Street sewer (see Figure 5). Clearances at top and bottom were only 0.5 m and
the brick-lined Dale Street sewer was over 100 years old, but in fact there was no broken ground above the
road tunnel and no chemically weathered ground or foul water present below the sewer. There was no
indication during construction of the high water table recorded at Exchange Station during the site
investigation. 11
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A short section of upline tunnel was constructed on the Link just to the east of Central Station and off the map in Figure 1. This drive encountered better rock conditions than the rest of the Link and was
either self-supporting (Plate 4) or required only wire mesh and rock bolts (Plate 3).
A summary of ground conditions encountered during construction of the Link is given in Table 1.
Again there is a general qualitative correlation between bedding planes and instability in the crown, but not
in this case between open joints and ground water flow because the Link was above the water table. The
instability in the crown, as has been previously noted, was also due to the small thickness of good rock cover
on the Link.
3.4 Progress on the Loop and Link
Observations on the progress of excavation and the factors affecting it were made during construction
of the Loop and Link. It will be seen that ground conditions, ground water conditions and operational
arrangements all had an effect on progress rates.
Tunnelling progress when using a roadheader usually depends upon rock characteristics such as
bedding plane spacing and joint intensity as well as compressive strength. The type of pick used on the
cutting head is a drag pick which is designed to cut the rock. Although there is an actual limit of rock
strength through which a pick can cut, a harder rock may well be excavated more easily if it is bedded. It
was found in practice that the most important criterion governing progress was the skill of the roadheader
driver in being able to exploit the various weaknesses of the rock such as bedding planes or joints so that
slabs of rock were pulled out rather than allowing the picks to grind the rock with a milling action.
Rock cores were taken at intervals during construction and their compressive strength was measured.
When the hardest bands, which required blasting, were encountered the maximum strength observed was
74 MN/m 2 and it was found that the machine could not effectively tackle massive sandstone stronger than
about 45 MN/m 2. Fortunately the sandstones were mostly much weaker than this, and overall progress
rate did generally correlate with rock strength, in that the best rates were achieved in the Keuper and Upper
Bunter Sandstones which are of similar mean strength (13 and 11 MN/m 2 respectively) but which are
weaker than the Middle Bunter Sandstone (19 MN/m2).
Overall excavation rate for the Loop was 55 m 3 of rock per 12 hour shift and for the Link was 88 m 3
per shift. The difference in these rates may have been due to contractual factors, but it may have also been
due to the provision of ground support erection attachments on the Link's roadheader. The design of the
arch rib and the presence of attachments on the Link's roadheader for supporting the men who were setting
the ribs, enabled less interrupted cutting of the Link face in contrast to the Loop where there was a longer
delay in cutting while each arch was set.
The presence of ground water on the Loop also affected progress rate but because the Link was situated
above the water table this difficulty was not encountered there. In addition to these factors, the construction
of the Link benefitted from the experience gained during construction of the Loop.
3.5 Wells and caverns encountered during construction
Some comments will be made here on the old wells and caverns that were encountered during
construction of the Link tunnels because these features were well known from past experience to be a
potential hazard to tunnelling in Liverpool. 13
Local information suggested that nearly 900 wells exist in the city centre some of which have been
loosely backfilled but in most cases not filled at all. In order to reduce the possibility of unexpectedly
encountering any of these wells during construction of the tunnels, a comprehensive search was made of
data from the local history library and records of the local water undertaker. In addition, advertisements
were placed in the local newspapers requesting information from the public which resulted in many replies
giving details of hitherto unrecorded well shafts.
It was estimated that the only wells in use have a total capacity of less than 200,000 gallons per hour
and that the effect of this extraction on the water table is negligible.
Seven old wells were encountered during the driving of the Link tunnels and five of these were
positioned almost on tunnel centre line. The bases of these wells were all located at least 10 m above the
existing water table and at least 2 m above the highest level of the Loop line.
The well shafts were accompanied by interconnected underground caverns. During construction of
the upline Link tunnel at chainage 1422 m, a system of 6 m high caverns was broken into. Had the tunnel
been located directly in the path of these old caverns, considerable difficulty would have been involved
during construction, since the invert of the tunnel was at least one metre higher than that of the old
excavations. At some time in the past attempts at filling the shafts had been made, leaving piles of debris
at the base of each shaft which had trapped water to form a series of lagoons.
Delays during construction of the tunnels due to wells were small. The smaller shafts, which were
mostly dry, were plugged and cavity-grouted while the caverns adjacent to the Link line were boarded up
and partially filled.
Three wells were also encountered at tunnel level on the Loop; one of these, at Moorfields, involved
considerable work in cleaning and sealing.
3.6 Completioil of the Loop and Link
The enlarged station sections in both the Loop and Link tunnels were excavated by the roadheader
operating over the full face at about platform level and then reversing to cut the invert for the track.
Temporary support in the form of steel arch ribs with wooden lagging boards behind were used at all
station sites. At Moorfields, the Link station escalators and concourse were constructed in a large cofferdam.
There is a passenger interchange at this station between the Link and Loop lines and escalator shafts were
constructed to reach the Loop line where it passes beneath the Link line some 40 m below ground level.
Cast-iron bolted rings were used to line the sections where excavation was carried out within Boulder Clay.
Measurements of the settlement of the ground surface above the Loop and Link tunnels and associated
escalator shafts etc at Moorfields station were made by the Department of Mining Engineering, University
of Newcastle-upon-Tyne9; the maximum surface settlement observed was 45 mm.
Access and mucking-out shafts, 3.6 m diameter, were sunk in March 1972 at Mann Island, Moorfields,
William Brown Street and Central Station. Two of these shafts, those at Mann Island and Moorfields, were
lined in their upper sections with pre-cast bolted concrete segments. The Mann Island shaft was sunk through
water-bearing alluvium which caused problems and delays due to instability and excess water. Although a
drawing of the Mersey railway tunnel was available that gave an indication of the nature of the ground, no
14
borehole was put down in the vicinity of this shaft during the site investigation and therefore the exact
nature of the ground was not determined beforehand.
In those sections of tunnel where arch ribs were erected as temporary support, the final lining was
formed in situ with concrete to a minimum thickness of 200 mm using wooden or steel shutters. Where
lengths of running tunnel on the Loop were left unsupported, a final lining of shotcrete was sprayed-on to
a minimum thickness of 70 ram. In both cases more concrete was in fact used because of the need to fill
the deep grooves cut by the roadheaders during excavation.
Some design changes to the Loop and Link were made after the construction contracts were let, but
none of these were as a consequence of ground conditions.
Further details of the construction of the Loop and Link are given by Smith 10.
Passenger services commenced on the Link line on 2 May 1977 and on the Loop line on 9 May 1977.
3.7 Costs
The tender price for the construction of the Loop was £6,550,666 (October 1971) and that for the
construction of the Link was £4,742,754 (November 1972); the final contract prices are not yet available.
The contract price for the site investigation was £16,000 (March 1970). Without making any adjustment
for inflation between 1970 and 1972, the site investigation contract price is 0.14 per cent of the tender
price for construction of the works.
More,recent estimates of the costs of construction have been quoted 11,12 as being £24.2 million and
£15.6 million for the Loop and Link respectively.
4. DISCUSSION
Taken overall, the consultants considered the site investigation to be one of the better ones in their
experience, and commented that the logging, an important matter, had been particularly well done; the
following discussion should be read with this in mind.
The site investigation contractor considered that one reason that the site investigation was so
satisfactory was the close liaison with British Railways.
4.1 Comparison of predicted and as-found conditions
The geology of the site as found during construction generally agreed closely with the geology
established during the site investigation as described in Section 2.2. In particular, the major faults were
found very close to the positions predicted by the site investigation. The faults had little effect on tunnel
driving, rib arches were put up every time a fault was crossed and sheeting was'installed behind the ribs in
badly disturbed ground.
The bedding of the rock was adequately determined by the site investigation, but during construction
the rock was found to have more joints than had been suggested from the site investigation. This was
inevitable, because the vertical boreholes would readily intercept the more-or-less horizontal bedding planes,
15
but would be likely to miss many of the vertical or steeply inclined joints.
Rockfalls from the roof had been predicted from the site investigation. In general this was not a
problem during construction except that some trouble was experienced in cutting a circular crown section.
A flat crown tended to form in the tunnel roof because of the detachment of rock along bedding planes.
The contractor for the Loop considered that the hard zones of rock and the amount of water were
not evident from the site investigation, and the consultants said that these factors had resulted in claims.
Although the site investigation did generally record weak to moderately strong rock, borehole B 10 near
Lime Street Station encountered feldspathic sandstone of which One sample, from 7 m above tunnel crown
level, had a compressive strength of 58 MN/m 2, thus indicating the presence of strong rock on site if not at
tunnel level. As regards the water, the site investigation report recognised that ground water might be a
potential problem to tunnelling on the Loop, and recommended additional studies of ground water
conditions at the site (see recommendation 8 in Section 2.6).
The abrasive nature of the slurry formed from crushed sandstone and water has been described in
Section 3.2. This was a considerable problem on the Loop where the abrasive slurry got into machinery
and produced wear in hydraulic pumps, motors etc. This was only briefly mentioned in the site investigation
report but could have been foreseen because it was one of the more serious problems that had been
encountered during the driving of the Mersey Kingsway tunnel 13,14 through similar sandstone.
The presence of wells and caverns was foreseen by the consultants and some were encountered during
construction as described in Section 3.5. In the event they caused little delay to tunnelling works.
4.2 Other comments on the site investigation
No borehole was put down at the site of the Mann Island shaft, and in the event the shaft was sunk
through water-bearing alluvium with problems and delays due to instability and excess water (see Section 3.6).
However the Geological Survey 1:63 360 geological map of Liverpool (Sheet 96, Drift) of 1967 shows
alluvium at the site of the shaft, as does a geological sketch map in the site investigation report. Also the
closeness of the shaft location to the bank of the River Mersey should have indicated that water might be a
problem.
The site investigation included a large programme of laboratory rock testing (see Section 2.5.2). The
compressive strength tests provided data on the range of compressive strength of the rock and this together
with the data on bedding and jointing allowed the decision to be made to use roadheaders for the excavation
of the tunnels. The other rock tests were required for lining design, but the consultants said that were the
site investigation to be done again the amount of rock testing called for would be less.
One consideration to emerge from this study is that trial shafts sunk during the site investigation,
preferably at locations where shafts would be needed for the works, would have been of great value. They
could have been used to make in situ observations of the rock condition - particularly of the jointing which
was difficult to detect in the boreholes, observations could have been made on the rate of water inflow, and
trial headings could have been driven from them to study excavation and support and to allow prospective
contractors to examine the ground directly.
16
In conclusion it is considered that the site investigation effectively met the objective of providing the
ground information necessary for the safe and practicable construction of underground railway tunnels in
sandstone rocks beneath a major city.
5. ACKNOWLEDGEMENTS
This Report was prepared in the Tunnels Division (Head of Division: Mr M P O'Reilly) of the Structures
Department of TRRL. Two sources of information are acknowledged. The first is the unpublished site
investigation report, prepared by Dr D H Cornforth of Nuttall Geotechnical Services, which supplied
information for Section 2. The second is an unpublished geotechnical report, prepared by Mr M D de Figueiredo
of Mott, Hay and Anderson, which supplied information for Section 3. Figures 1,3 and 5 are adapted from
the site investigation report and Figures 4, 6, 7 and 8 are adapted from the construction records. The site
investigation report, geotechnical report, construction records and Plates 1 -4 were kindly supplied by
Mott, Hay and Anderson.
The following are thanked for making useful comments on the draft report: Mr M C Henn and
Mr D R E Holland of Mort, Hay and Anderson, Mr F M Jardine of Nuttall Geotechnical Services and
Mr R J Coon of British Railways. Dr L M Lake of Mott, Hay and Anderson was helpful in putting the
authors in touch with the main participants.
6. REFERENCES
1. DUMBLETON, M J and A F TOOMBS. Site investigation aspects of the Sydenham Road Sewer Tunnel.
Department of the Environment, TRRL Report SR 235. Crowthorne, 1976 (Transport and Road
Research Laboratory).
2. DUMBLETON, M J and S D PRIEST. Site investigation aspects of the River Tyne Siphon Sewer Tunnel.
Department of the Environment Department of Transport, TRRL Report LR 831. Crowthorne, 1978
(Transport and Road Research Laboratory).
3. DUMBLETON, M J and A F TOOMBS. Site investigation aspects of the Empingham Reservoir Tunnels.
Department of the Environment Department of Transport, TRRL Report LR 845. Crowthorne, 1978
(Transport and Road Research Laboratory).
4. DUMBLETON, M J, J N COOPER, P P FOWLER and A F TOOMBS. Site investigation aspects of
the River Medway Cable Tunnels. Department of the Environment Department of Transport, TRRL Report SR 451. Crowthorne, 1978 (Transport and Road Research Laboratory).
5. DUMBLETON, M J, A R KIDDIE and S D PRIEST. Site investigation aspects of part of the Tyneside
South Bank Interceptor Sewer. Department of the Environment Department of Transport, TRRL Report SR 454. Crowthorne~ 1978 (Transport and Road Research Laboratory).
6. BOSWELL, P G H. The geology of the new Mersey Tunnel. Proc. Liverpooi Geol. Soc., 1937, 17, 100.
7. ANDERSON, D. The construction of the Mersey Tunnel. Jl Instn Ov. Engrs, 1936, 2, April, 473-516.
17
.
.
10.
GEOLOGICAL SOCIETY ENGINEERING GROUP WORKING PARTY. The logging of rock cores for
engineering purposes. Q. J1Engng GeoL, 1970, 3, (1), 1-24.
TOMLIN, N and T SKLUCKI. Ground deformation around a tunnel excavation in Bunter Sandstone.
Proc. Conf. on Rock Engineering, Newcastle-upon-Tyne, 1977, 623-640.
SMITH, W. Innovation and inflation on the Link/Loop lines. Tunnels and Tunnelling, 1977, 9, (5),
47-49 .
11. GOSNEY, J. Linking the loop on Merseyside. Contract Journal, 1977, May 5, 26-27.
12. ANON. Liverpool loop-link open at last. New Civil Engineer, 1977, April 28, 9-10.
13.
14.
McKENZIE, J C and G S DODDS. Mersey Kingsway Tunnel: construction. Proc. Instn Or. Engrs,
Part 1, 1972, 51, March, 503-533.
MEGAW, T M, C D BROWN, J C McKENZIE, G S DODDS, A A CAIRNCROSS and S T JONES.
Mersey Kingsway Tunnel: discussion. Proc. Instn Cir. Engrs, Part 1, 1973, 54, May, 313-335.
1 8
Promoters:
Consultants:
Site Investigation Contractor:
Rock testing sub-contractor:
Main Contractor for Loop:
Main Contractor for Link:
7. APPENDIX
PRINCIPAL PARTICIPANTS IN THE LIVERPOOL LOOP AND LINK SCHEME
British Railways (London Midland Region) and the Merseyside Passenger Transport Executive
Mott, Hay and Anderson
Nuttall Geotechnical Services
Cementation Ground Engineering Ltd
Edmund Nuttall Ltd
Leonard Fairclough, Tunnelling Division
19
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Neg. no. H89/77
Plate 1 LOOP LINE RUNNING TUNNEL, CENTRAL TO LIME STREET Excavation by drill and blast. Support by steel arches
and timber lagging
Neg. no. H92/77
Plate2 LOOP LINE RUNNING TUNNEL, MOORFIELDS TO LIME STREET Excavation by Dosco roadheader. Completed section after
invert excavation
Plate3 LINK LINE RUNNING TUNNEL, CENTRAL UPLINE Excavation by Dosco roadheader. Support by wire mesh
and rockbolts
Neg. no. H94 /77
Plate4 LINK LINE RUNNING TUNNEL, CENTRAL UPLINE Excavation by Dosco roadheader. Completed self
supporting section
Neg. no. H90 /77
{2354) Dd0536316 1,$00 12/78 HPL(dSo ' t on G1915 PRINTED IN ENGLAND
ABSTRACT
Site investigation and construction of the Liverpool Loop and Link tunnels: G WEST BA, FGS and A F TOOMBS: Department of the Environment Department of Transport, TRRL Labor- atory Report 868: Crowthorne, 1978 (Transport and Road Research Laboratory). The site investigation and the construction records for the Liverpool Loop and Link underground rail- way tunnels have been compared to see what lessons of good practice in site investigation emerge. The tunnels were constructed almost entirely within Triassic sandstones generally ranging in strength from weak to moderately strong, enabling roadheaders to be used effect- ively for most of the excavation.
The Report discusses the site investigation and the ground conditions as encountered during construction in some detail. The site investigation was generally satisfactory, giving an accurate account of the geology of the site, accurately predicting the location of major faults and providing most of the ground information needed to drive the tunnels. However, during construction the rock was found to have more joints than had been suggested in the site invest- igation, no doubt because of the difficulty of intercepting steeply inclined joints with a vertical borehole. Trial shafts and headings are suggested to overcome this and other problems.
ISSN 0305-1293
ABSTRACT
Site investigation and construction of the Liverpool Loop and Link tunnels: G WEST BA, FGS and A F TOOMBS: Department of the Environment Department of Transport, TRRL Labor- atory Report 868: Crowthorne, 1978 (Transport and Road Research Laboratory). The site investigation and the construction records for the Liverpool Loop and Link underground rail- way tunnels have been compared to see what lessons of good practice in site investigation emerge. The tunnels were constructed almost entirely within Triassic sandstones generally ranging in strength from weak to moderately strong, enabling roadheaders to be used effect- ively for most of the excavation.
The Report discusses the site investigation and the ground conditions as encountered during construction in some detail. The site investigation was generally satisfactory, giving an accurate account of the geology of the site, accurately predicting the location of major faults and providing most of the ground information needed to drive the tunnels. However, during construction the rock was found to have more joints than had been suggested in the site invest- igation, no doubt because of the difficulty of intercepting steeply inclined joints with a vertical borehole. Trial shafts and headings are suggested to overcome this and other problems.
ISSN 0305-1293