Two Baseline Reports prepared for tunnels in Toronto, A Case Study

Transcription

1 Two Baseline Reports prepared for tunnels in Toronto, A Case Study J. Nick Shirlaw Golder Associates (Singapore) Pte Ltd, Singapore J. Westland, S.Boone Golder Associates Ltd, Canada ABSTRACT: Geotechnical Baseline Reports were prepared for two tunnelling projects for the subway expansion program in Toronto, Canada. The reports were based on recommendations published by the ASCE in Specific issues covered included: the number size and strength of boulders, the continuity of beds or lenses of sand, providing a baseline for dewatering requirements and for the construction of a number of shafts in broadly similar ground conditions. The tunnels have been successfully completed, and the geotechnical claims arising from the work are discussed in the context of the Baseline Reports. 1 INTRODUCTION In 1992 the Toronto Transit Commission (TTC) embarked on an ambitious program of expansion of the subway system in Toronto. This was initially known as the Let s Move program, later retitled the Rapid Transit Expansion Program. The full program involved the construction of three new lines, the Spadina, Eglinton and Sheppard lines, and expansion works to the Wilson Yard depot. Later changes to the program meant that only the Sheppard line and a portion of the depot expansion were completed. However, some preliminary works were also carried out for the Eglinton line, including the tunnelling for the diversion of a 2m diameter main sewer line. The TTC appointed Delcan-Hatch as the Program Managers, and Golder Associates as the Program Geotechnical consultant. Part of Golder Associates task was the preparation of a Geotechnical Baseline Report (GBR) for each of the construction contracts that would be let under the program. The GBR was the culmination of a series of Geotechnical Reports prepared for each construction contract. The site investigation which formed the basis of the geotechnical reporting was planned by Golder Associates to consist of three main phases: Phase 1: Boreholes at approx 500m centres along the route Phase 2: Additional boreholes such that the combined Phases 1 and 2 gave a coverage of about one borehole per 250m Phase 3: Further boreholes, resulting in a final, planned borehole spacing of about 75m along the route At the completion of the site investigation work, a Geotechnical Investigation Report (GIR) was prepared for issue with the tender documents. The site investigation was supervised by various geotechnical consultants who prepared Geotechnical Investigation Reports, under the direction of the Program Geotechnical Consultant. A historical land use survey was carried out by Golder Associates to identify past land use which might have resulted in the release of contaminants. The information from the land-use review was used to plan the site investigation program, such that regularly spaced boreholes along the proposed align-

2 ment were situated close to locations with a higher risk of contaminant discharges to the environment, such has dry cleaners and gasoline stations. Based on the Phase 1 borehole data, a Preliminary Geotechnical Design Report (PGDR) was prepared to provide geotechnical information sufficient for functional design of each of the subway lines. The functional design process provided for an overall horizontal and vertical alignment for the each line, identified the key requirements for each station and identified the basic construction methodology (bored tunnels versus cut and cover construction) for the major construction elements of each line. Following functional design, the subway lines were subdivided into major contract sections. Each of these sections, for example a station or reach of twin tunnels, were assigned to individual design groups working to standards set by and with the oversight of the Program Manager and Program Geotechnical Consultant. Later, each of these design sections became separate construction contracts. A Preliminary Geotechnical Design Report (GDR) for each construction contract was prepared following completion of Phase 2 of the site investigation, and provided the geotechnical information to allow final design of the stations and tunnels to commence. The initial design phase of each of these separate underground design contracts included assessment by the designer of the amount of geotechnical data available, and the requirements for additional investigation to complete final design. Thus, each section designer had input into the Phase 3 of the site investigation program. A final Geotechnical Design Report was prepared at the completions of the Phase 3 investigation The PGDR and GDR were deliberately structured so that the writing of the GBR could be based on these earlier reports. The primary difference between the Design reports and the GBRs was that the Design reports were written to provide guidance to the civil/structural designers for each contract, while the GBR addressed issues relevant to construction of the works and were thus directed to bidders on the projects. This paper will discuss the basis of the writing of two GBRs, one for the tunnels of the Sheppard line and the other for the Allen Sewer diversion tunnel (Figure 1). The latter tunnel was built as preliminary works for the Eglinton line. Spadina Line to York University & Yonge Line Sheppard Subway Eglinton West Subway Allen Station Existing Subway Lines Allen sewer tunnel Bloor-Danforth Extension Downtown Toronto Harbourfront LRT Extension Lake Figure 1. The RTEP program included the Sheppard, Eglinton and Spadina lines. About half of the full, planned, extent of each line was originally to be built under the program (the solid line)

3 2 GENERAL GEOLOGY Toronto is largely built on a till plain with a number of deeply incised river valleys. The tunnels were driven through the typical Quaternary deposits of the Toronto region: glacial till, glaciolacustrine sand, silt and clay deposits and glaciofluvial silt and sand. More recent deposits of alluvium were found in the Don river Valley, which was crossed by the Sheppard line tunnels. The till and glaciolacustrine deposits were laid down during a number of glaciations and glacial retreats. The Quaternary soils overlie the Ordovician age bedrock of the Georgian Bay Formation; rock was not encountered during the tunnelling. 3 THE PROJECTS The Allen Sewer Tunnel The planned Allen Road Station, part of the Eglinton West subway line, conflicted with an existing 1830mm diameter storm water sewer. As part of preliminary works to the Eglinton Line it was decided to build a replacement section at a lower level, which would pass just below the future station. The replacement involved constructing a 1.1 km long tunnel of 3m excavated diameter and seven shafts. The seven shafts were: The access shaft for the TBM The removal shaft for the TBM A maintenance access shaft and Four shafts for the installation of grout pipes; the grouting was a specified protection measure where the sewer tunnel passed under an operational section of the existing subway system. Thirteen boreholes provided information on the ground conditions along the alignment. The interpreted long section (Figure 2) showed that the tunnel would be driven through a thick bed of glaciolacustrine sand for its whole length. Overlying the glaciolacustrine sand were fill and Wisconsin Till; these deposits would be encountered during shaft construction TBM LAUNCH SHAFT EXISTING SUBWAY TBM REMOVAL SHAFT Standard Penetration Test Results Elevation (m) GROUND WATER LEVEL BEFORE TUNNELLING Unit SPT "N" Fill 22 Till 75 Sand/ Silt >100 Chainage (m) FILL GLACIAL TILL GLACIOLACUSTRINE SAND & SILT GLACIOLACUSTRINE SILT & CLAY Figure 2. The Allen sewer tunnel.. The stratigraphy and ground water level are as shown in the GBR that was issued as part of the tender documents.

4 BAYVIEW AVE Underground Singapore The tunnels for the Sheppard Subway line The twin running tunnels for the Sheppard subway are 5.9m internal diameter. They were driven using two EPB shields. The length of tunnelling was 3.8km for each bound. The tunnels were launched from a shaft to the west of the Don River and driven westwards to Yonge Station. The shields were recovered and transported to another shaft, just to the east of the Don River. From there they were driven eastwards to Don Mills Station. The 224 boreholes installed for the project, prior to tender, revealed a general stratigraphy consisting of: Fill Recent deposits and alluvium (concentrated in the valley of the Don River) Upper Till Upper Sand/Silt Upper Clay Middle Sand/Silt Lower Clay Lower Sand/Silt These units were not necessarily persistent throughout the alignment, and the boreholes were not always deep enough to identify the lower units. The ground surface drops by nearly 40m in the area of the Don River valley (Figure 3) 180 YONGE ST TBM REMOVAL SHAFT WILKET CREEK SEWER EEB#1 CP#1 EEB#2 CP#2 BAYVIEW STATION CP#3 BESSARION STATION EAST DON RIVER 180 Elevation (m) CP#4 LESLIE STATION TBM LAUNCH SHAFT LEGEND FILL PREDOMINANTLY COHESIONLESS DEPOSITS (GRAVELS/SANDS/SILTS) PREDOMINANTLY VERY STIFF TO HARD COHESIVE DEPOSITS (SILTY CLAY/CLAYEY SILT & TILLS) INTERPRETED PIEZOMETRIC WATER LEVELS AT TUNNEL LEVEL SUBWAY Figure 3. The western section of the Sheppard line tunnels. The stratigraphy and ground water level are as shown in the GBR that was issued as part of the tender documents. 4 STRUCTURE OF THE GBRs The general structure of the GBRs followed the outline suggested in early drafts of the document that would later become Geotechnical Baseline Reports for Underground Construction, Guidelines and Practices (ASCE 1997). The document followed was Anon (1991). The standard table of contents used for the GBRs prepared for the RTEP program was: Introduction Project description Sources of information

5 Geology Man-made features significant to construction Soil units and groundwater levels related to excavation Selection and design of temporary works Anticipated ground behaviour in relation to construction Instrumentation Soil and groundwater management Figures provided with the GBRs included Long sections along the alignment (Figures 1 and 2), Particle size distribution envelopes for each of the main beds, with the associated long sections and tables, which were contract specific. 5 PARTICULAR ISSUES IN THE PREPARATION OF THE GBRs Particular issues that arose during the preparation of the reports were: How to provide a baseline for the number, size and strength of the boulders that would be encountered in the glacial strata The level of detail that could be incorporated on the long sections, particularly with respect to the beds and lenses of granular soil that were present within or interbedded with the clay till How to provide a baseline for the dewatering required in the granular soils for shaft and for cut and cover construction How to provide the baseline conditions for a number of excavations which were in broadly similar, but not identical, conditions These items are discussed below, and are followed by a discussion on those areas of the GBRs that were an issue during construction. Baseline for boulders Based on discussions with contractors, the number and size of the boulders that would be encountered during construction was the biggest single geotechnical risk for tunneling using earth pressure balance methods in the glacial strata in Toronto. Further, it was common in underground construction projects in Toronto for there to be disputes about the number and size of boulders encountered, because of the significant effect such obstructions had on construction of tunnels, bored piles, tie-backs, etc. It was therefore decided that a baseline for the number, size and strength of boulders should be provided in the GBRs. The baseline for boulders was provided in the list of Potential Subsurface Hazards. It is very rare to encounter a boulder during site investigation in Toronto. It was a standing instruction that any boulders encountered should be cored. No boulders were cored during the site investigation for the Allen Sewer. The boreholes were drilled using hollow stem augers. If the auger flights strike a boulder or cobble the rods chatter, and it was another standing instruction that such behaviour should be recorded on the log of the hole. However, for the Allen Sewer, no such behaviour was recorded at tunnel level. There was therefore no direct information from the site investigation which would allow an estimate of boulders to be made. The tunnel was to be driven through glaciolucustrine sands. A relatively low concentration of boulders was anticipated in this deposit, which consisted of sands laid down in a lake fed by glaciers. The only source for boulders would be sections of glacier, containing boulders, which broke away and floated into the lake. Melting of the resulting ice rafts would add the boulders to the lake deposits. Relative to the glacial till deposits the proportion of boulders would be low. It was decided to provide a baseline of two boulders greater than 600mm in any dimension in the GBR. It was considered that the anticipated method of construction, EPB tunnelling, the primary concern would be boulders of this size and greater. For the Sheppard Tunnels, three boulders were recorded as being encountered in the 224 boreholes (total length of drilling 4,940m) drilled prior to tender. It was a standing instruction that any boulders encountered should be cored. However, none of the boulders was cored, so there was no information as

6 to the size of the boulders or the type of rock. There were a number of cases where the rods were observed chattering, probably because the flights of the auger had encountered a boulder or cobble. The first quantitative data on the boulders in the soils came during the early construction of a tailtrack section of cut-and-cover tunnel, at the western end of the Sheppard Line. The data was obtained during installation of 598 soldier piles with a diameter of up to 920mm, and a total drilled length of about 9,000m (Westland et al 1996). 107 boulders were encountered, which represented 0.14% of the volume of soil excavated. Eighty of the boulders were recovered whole, and were used to construct a histogram of the distribution of the boulders by size. This histogram was adjusted to allow for the fact that the method of sampling, in piles, was limited by the diameter of the piles. It was known from previous subway construction in Toronto that boulders of 1m to 3m in maximum dimension did occasionally occur in the glacial soils. The boulders were mainly formed out of igneous and metamorphic rocks typical of the Canadian Shield. The Canadian Shield is found in the north of Canada; weaker sedimentary rocks are found locally in the Toronto areas. The boulders had therefore been subject to glacial transportation over a long distance. Naturally only very strong rocks had survived this process: measured Uniaxial Compression Strength was typically in the range of 130 to 250 MPa, with tensile strengths (measured using the Brazilian Test) of 9MPa to 20MPa. Other tests carried out on the boulders included: Point Load Index, Bond abrasion and Modified Tabor abrasion. The results of these tests were reported in the GBR. To assess the number of boulders along the tunnel alignment, a stoniness index was derived, for each of the strata, based on all of the indications (mainly rods chattering) from the site investigation. For each stratum the ratio of this index to that from the area of the tailtrack was then used to assess the overall boulder content, as a percentage by volume. The assessment of the soils along the Sheppard Tunnels was that boulder content would range from 0.02% to 0.17%. Figure 4.Distribution of boulder size measured at the Sheppard tailtrack, and the mathematical relationship used to represent the distribution of boulder sizes The histogram of distribution of boulder sizes and the proportion of boulders, by volume, for each stratum were provided in the GBR. This provided a baseline for the number and size distribution of boulders that would be encountered during tunnelling. The baseline was established by measuring the equivalent diameter of the boulders (Figure 4). From the measured values, it was assessed that the volume of boulders in each size range was 90% of the volume in the preceding (lower) range.

7 Figure 5. Extract from a Sheppard Line GBR, showing how the baseline distribution of boulders was established. The volume and size of boulders predicted for any part of the work could be established from: 1) The total volume of boulders. The proportion of boulders was given, as a percentage by volume, for each of the major strata. Combining this percentage with the volume to be excavated would give the total volume of boulders. 2) The size range of boulders. Having established the total volume of boulders, the number of boulders in each size range could be predicted based on the distribution histogram. Figure 5 is a figure from a GBR showing the distribution. As a result of the boulder analysis, a major change was made to the EPB shields. The shields had been ordered by the owner (TTC) to drive the running tunnels for the Eglinton Line. The order was placed and the shields built before most of the investigation for the Sheppard Line was carried out, and before the specific information on boulder content was obtained during soldier piling for the Sheppard tailtrack. When the construction of the Eglinton line was cancelled, the shields were stored for use on the Sheppard line. The shields provided for the Eglinton line had heads suitable for cutting soft ground, but no disc cutters for cutting hard rock. Analysis of the Sheppard data indicated that between 1,500 and 3,000 boulders, with a size greater than 300mm in any dimension, could be encountered during tunnelling. As a result of this assessment the heads of the machines were changed, with the new heads equipped with both picks (for soft ground) and discs (to cut rock). Baseline for beds of granular soil The identification of layers of granular soil was important for tunneling, shaft construction and cut and cover construction. The sand layers are generally uniform, with a low fines content, and are unstable

8 unless dewatered or a support pressure is provided. Shaft construction using soldier piles and lagging is common in Toronto. For this construction method it is necessary to carry out advanced dewatering for any beds of sands present in the walls of the excavation. Dewatering below the base of the excavation is also commonly necessary to prevent hydraulic rupture of the base of the excavation. In tunnelling there is a tremendous contrast between the behaviour of the beds of hard glacial clay, which are stable even without a support pressure, and the behaviour of the beds of sand which are highly unstable when exposed below the water table. It was a standard requirement for the site investigation that a piezometer tip should be placed in each of the beds of sand encountered. Almost every borehole had one or two piezometer tips installed. The major patterns of drainage were horizontal, into the river valleys (the Don River for the Sheppard Line) and vertically, with each successive bed of sand having a lower elevation of piezometric pressure than the bed above. Where more than two beds of sand were encountered a second hole was drilled to install additional piezometers. It can be seen that the beds of sand shown as continuous in Figure 2 were quite convoluted, with each bed being of variable thickness and elevation along the alignment. The continuity of the beds was confirmed by the measurements of piezometric pressure. Due to the particular hydraulic conditions, plotting the piezometric elevation along the alignment showed clearly how the various major beds of sand identified in the boreholes were connected. The continuity of the beds was an important factor in assessing dewatering requirements for shafts. While the major beds of sand/silt could be clearly identified, smaller lenses of sand/silt were found in some boreholes within the Upper Till and the clay layers. Such lenses were highly localized, and their extent could not be assessed from the borehole information. It was likely that similar lenses occurred between boreholes which, because they had not been encountered in the drilling, were not identified in the site investigation. As no realistic assessment could be made of the number or extent of such lenses, it was decided not to show those local lenses of sand/silt that had been identified in the boreholes in the general stratigraphy, but to include in the text a statement that such lenses were an inherent part of the fabric of the till deposits, and that the tunnelling methods should allow for this inherent risk. Baseline for dewatering for shafts As discussed above, the major beds of sand/silt posed problems with respect to potential instability, both in the walls and base of the shafts. In Toronto it was common to control this potential instability by dewatering; use of eductor wells is a common method of dewatering the sand/silt layers. The volume of water to be pumped during the dewatering of the launch shafts for both the Allen Sewer and the Sheppard Subway tunnels was considered an important issue for construction. In addition to affecting the cost of the dewatering, there was a planning issue: depending on the volume to be pumped, a permit to take water would need to be obtained from the relevant authority. It is recommended in the guidance notes on preparing GBRs that anticipated groundwater flow into tunnels be given in the GBR, as this is clearly an important consideration in assessing the cost of the project. By extension, the volume of groundwater to be pumped to control the groundwater for tunnel shaft construction should also be given. However, the volume of groundwater to be pumped would be dependent on choices made by the contractor, such as the choice of temporary works for the shaft. Another key issue in cost, the spacing of the wells, would also be highly dependent on the choices made by the contractor, including the type of temporary works, the well type and the sequence of work. With the GBR being prepared as part of the tender documents, it was not known how tenderers would actually carry out the works. The issue was therefore how to provide a reasonable baseline for dewatering which could be used despite these unknowns. Furthermore, with aquifer properties varying over such a large range (many orders of magnitude in the case of hydraulic conductivity) it is not appropriate to design dewatering systems on the basis of single value design parameters. Even small variations in the actual parameters from the baseline parameters would result in either the wells being too far apart (if actual permeability was less than the baseline), or the pumping rate being higher than anticipated (if actual permeability was greater than the baseline). Because an appropriate design approach is to consider a range of design parameters, it was decided to provide a range of baseline parameters for the design of dewatering systems: the Hydraulic conductivity, Transmissivity and Storativity of the beds that would require dewatering were each assigned a

9 range based on available data and contractors were expected to consider the range in their design of dewatering systems. In addition an outline dewatering design was carried out, based on certain assumptions, with the purpose of assessing the likely rate of pumping required. The assessed rate of pumping was given in the GBRs; it was stated that this was a preliminary assessment carried out for the purpose of assessing whether a permit was required, and the assumptions used in deriving the rate of pumping were given. A baseline for similar, but not identical, conditions along the alignment The conditions along the line of the Allen sewer were broadly similar. The tunnel was driven in a thick bed of glaciolacustrine sand, underlying a bed of glacial till. The four shafts were to be constructed in broadly similar conditions, but with local variations, such as the depth of fill. The anticipated geotechnical issues for construction of the shafts were also broadly similar, but with some variation related to the differences in the ground conditions. In these circumstances, one of the issues was how to describe this in the GBR. There were two options: 1. Give one generic description of the ground conditions and construction issues for all of the shafts, and then provide specific information of how each shaft varied from that generic decryption 2. Treat each shaft as a completely separate item, identifying the specific ground conditions and construction issues for each shaft in turn Because of the relative consistency of the ground conditions along the sewer tunnel alignment, the second option would result in a repetitive report. However, if the first option was chosen it was considered that there was a greater risk that the text could be read in a way not intended by the author. It was decided to write the report based on the second option, on the basis that clarity was more important than conciseness. 6.0 THE RESULT OF THE WORK Following construction of the Sheppard Subway line, Poot et al (2000) provided a summary comparison between the predicted number of boulders and the actual number encountered. In practice it was not feasible to compare the anticipated and actual number of boulders encountered during the tunnelling, as the EPB machines used were designed to cut the larger boulders, and the smaller ones tended to be broken within the screw conveyor. It was therefore not possible to establish the number of boulders actually encountered during the tunnelling. However, the number of boulders encountered during the drilling of soldier piles for shaft and station construction could be accurately measured, both in terms of number and volume. The Sheppard tunnels passed through three of the stations, so this gave some indication of how accurate the predictions for the number of boulders had been. The highest predicted boulder content was at Yonge Station, where a boulder content of 1% by volume was predicted in the Upper Till (boulder clay). For the soldier pile construction at Yonge, the Baseline prediction was for 528 boulders in 986 piles. The actual number encountered was 666. Over 6 construction contracts (five stations and the tunnels) the number of boulders encountered was consistently higher than predicted, on average by 36%. The prediction for the volume of boulders was also generally slightly low, on average by about 20%, suggesting that a greater number of smaller boulders was encountered compared to the predictions. Given the uncertainties of predicting the number of boulders in a glacial soil, this level of accuracy must be considered at least reasonable. However, for future projects in Toronto, Poot et al recommend that the predicted boulder distribution shown in Figures 4 and 5 is changed such that the volume in each range is 80% of that in the preceding (lower) range. This would give a greater proportion of smaller boulders, closer to what was found in practice than the GBR predictions. During tunnelling, many boulders of unknown size were encountered and broken by the cutting heads based on visual evidence of chunks of igneous and metamorphic rock in the muck. The screw conveyor was jammed on two occasions by such chunks. A boulder, estimated to be more than 3 m in diameter, was encountered when installing piles in the launch shaft. Other large boulders on the order of 1 to 2 m in diameter were encountered during construction of Bessarion and Bayview Stations. By

10 giving a reasonable estimate, the contractors could select and include in their costs the appropriate methods for removing the boulders. For the tunnelling, the number of boulders fully justified the change made to the cutting heads of the TBMs. Prior to the construction of the Sheppard Line, it was very common for tunnelling contracts in the Toronto region to end in a dispute between the contractor and the client over whether the number of boulders encountered could have been foreseen. This was because no means of defining what was reasonably foreseeable was given in thee contract. The GBR prepared for the Sheppard tunnels provided a clear baseline of what was anticipated, and the contract documents provided a means of paying for the variance from that baseline. The only major dispute related to geotechnical issues that arose from the Sheppard tunnels related to the consumption of the foam conditioning agent. The foam was injected into the head and chamber of the TBMs to condition the spoil and reduce wear. No baseline of foam consumption had been given in the GBR, as the consumption of foam was not related only to the soil encountered. Although the soil is a factor in the consumption, other factors that were completely under the contractor s control included the type of foaming agent used and the amount of foam that the contractor decided to inject to minimize machine and tool wear and to enhance tunneling productivity. Indeed, it is considered that when preparing baselines for tunneling projects, the authors of GBR s should seek to quantify the fundamental characteristics of the soil and rock deposits, including the geometry and distribution of the deposits, but avoid attempting to provide a baseline for the interaction between the ground and a contractors equipment, means and methods. In the case of tunneling additives, it is considered appropriate to provide a baseline for the soil water content, plasticity characteristics, strength and particle size distribution. All of these influence the selection and quantity of conditioning agents, but the variety of available agents and the potential combinations of such agents make it impossible to provide a baseline for the quantities that might be consumed on a project. For the Allen sewer tunnel, one boulder exceeding 600mm in any dimension was encountered, compared with the baseline of two. This confirmed the relatively low boulder content of the glaciolacustrine sand compared with the glacial till encountered on the Sheppard Line tunnels. It also confirmed the value of the qualitative assessment, based on depositional processes, made as an adjunct to the quantitative assessment based on the site investigation program. There were two claims related to geotechnical issues for the sewer tunnel. One was related to the difficulty of jacking grouting pipes into the glacial Till, and was quickly settled. The other claim was because the contractor had underpriced the cost of dewatering for the TBM access shaft. The GBR was clear on the need for this dewatering, which was described in some detail, and the conditions were as assessed in the GBR. This claim was rejected. REFERENCES Anon Avoiding and Resolving Disputes during construction: Successful practices and guidelines. Publ. ASCE 1989, updated and revised ASCE (1997). Geotechnical Baseline Reports for Underground Construction, Guidelines and Practices. American Society of Civil Engineers. Westland, J., Shirlaw, J.N., and Busbridge, J.R Geotechnical investigations and assessment of boulder frequency for Toronto s subway project. Canadian Tunnelling 1996 Poot, S., Boone, S.J., Westland, J., and Pennington, B Predicted boulder frequency compared to field observations during construction of Toronto s Sheppard Subway. Canadian Tunnelling 2000