RISK MANAGEMENT FOR MICROTUNNELED SEWERS

Steven W. Hunt, P.E. MWH

115 S. 84th Street, Suite 350 Milwaukee, WI 53150

ABSTRACT

An increased desire to specify trenchless construction methods to minimize public and utility impacts along with more contractor capability and competition have resulted in more use of microtunneling for new sewer construction. In some localities with favorable ground and site conditions, microtunnel construction has become fairly routine with few significant problems and claims. However in many ground conditions, a routine approach to subsurface investigation and preparation of a geotechnical report and specifications can result in significant risks and costly consequences. Owners and consultants can significantly reduce the risks and consequences of problems on microtunnel projects by following a more careful, focused subsurface risk management approach.

KEYWORDS

Risk management, subsurface investigation, geotechnical reports, microtunneling

INTRODUCTION

Managing risk for projects involving microtunneling of sewers has received considerable attention resulting in many papers and some guidelines. This paper is not a complete overview of risk management approaches for microtunneling, but instead focuses on several key elements involving characterization of ground conditions – desk studies, phased subsurface investigations and geotechnical report preparation.

Inherent Ground Risk

All parties to sewer construction by microtunneling, including owners, designers, geotechnical firms, construction managers, contractors and insurers, should never forget that the ground is inherently variable, uncertain and thus risky. Natural ground was formed by variable and potentially complicated processes that may not be easily interpreted. Activities of man tend to further complicate underground conditions. Ground conditions for microtunneling may vary from reasonably predictable to very complicated and unpredictable. Legget (1979) in his Terzaghi lecture reminded us that:

“There can never be any certainty about geological conditions between adjacent boreholes, even 5 ft apart, until the excavation has actually opened up the ground.”

In order to reduce uncertainties, designers and geotechnical firms strive to complete thorough subsurface investigation programs. Owners are tempted to believe that once they have paid to have a reputable firm complete a reasonable subsurface investigation program, the risk of encountering differening site conditions (DSCs) and receiving DSC claims is essentially eliminated. Designers and geotechnical firms often do not explain that significant risks of encountering DSCs remain even after completing an appropriate subsurface investigation program. Gould, 1995 in his Terzaghi lecture emphasized that the risk of encountering a DSC cannot be eliminated:

“Surprises are inevitable - there will always be unexpected ground conditions and neither the owner nor the design team can completely eliminate surprises from complex underground projects.”

Ron Heuer, a well-known tunneling specialist, responded to Gould’s Terzaghi lecture with the following comments (Heuer, 1997):

“Next, we must understand that it will never be possible to eliminate all encounters of unexpected conditions when we make underground excavations. We can send an individual to the moon and back, but we cannot reliably predict all details of the ground beneath or feet – we have never been able to do so and we never will, since geologic conditions are infinitely variable, with too many features that are unknowable prior to construction. Engineers must accept this basic truth, and owners must be made to understand it. Engineers should not suggest otherwise to owners.”

Ownership of the Ground

Owners should also recognize that they ultimately own or are responsible for the ground that microtunnels are constructed in. Attempts to transfer all the risk of unexpected ground conditions to the contractor have often been unacceptable and not cost effective (USNCTT, 1984; Hafer, 2000). Managing subsurface uncertainties for microtunneling projects becomes more equitable and less divisive once owners accept that ground risk mostly belongs to them.

While uncertainty is an unavoidable risk when microtunneling, most subsurface uncertainties and risks can be assessed and mitigated to minimize potential consequences. A focused, thorough geotechnical desk study, thorough subsurface investigation program and experienced data interpretation are key elements of subsurface risk minimization.

Tunnel Project Risk Management

Risk management is very important for controlling risks and maximizing chances for success of microtunneling projects. Abbott (2004) provides a well-written overview of this topic. Additional references that discuss risk management for microtunneling and tunneling include: Anderson (1998), Hinze & McClelland (1997), Richards (1999), Salem & Hegab (2001), Sangster (2003), and Westland et al (1998). Perhaps the most elaborate guideline on risk management for tunneling projects is a recently completed British code entitled The Joint Code of Practice for Risk Management of Tunnel Works in the UK (British Tunnelling Society, 2003). Both the British Tunnelling Society and Association of British Insurers prepared this guideline. Dix (2004) provides an overview of the code and discusses its implications. The code is intended to make thorough, comprehensive risk management a requirement from planning through design and construction in order to control risks and result in more insurable projects. While this approach has not taken root in the United States yet, elements of it may be coming. In any case, the code shows what thorough risk management might entail.

PRACTICES AFFECTING SUBSURFACE RISK MANAGEMENT

Many microtunneling problems and claims result from poor desk study, subsurface investigation and geotechnical report practices during planning and final design. The following paragraphs describe some poor geotechnical practices and example case history consequences.

Lack of an environmental desk study.

A Phase I Environmental Site Assessment (ESA) should be completed for every tunnel project. A tunnel

project in the Midwest included a thorough geotechnical subsurface investigation, but no Phase I ESA. The geotechnical firm did not offer environmental services and had a “we don’t do it attitude.” During construction, the owner was contacted by the owner of a landfill located near the end of the tunnel alignment and told that the contractor’s shaft and tunnel dewatering wells were causing leachate that had leaked from the landfill to migrate further from the spill zone and towards the tunnel. Tunneling was promptly stopped and the wells were deactivated. Completion of the tunnel without dewatering required the use of ground freezing for the remaining shafts and compressed air for the tunnel. The consequences were over six months in delay and $1 million in additional costs.

Poor communication of desk study findings.

Sometimes valuable information obtained during a planning-preliminary engineering phase desk study is not properly communicated to the final design team allowing it to “fall through the cracks.” This happened on a major sewer project near downtown of an urban area. The preliminary engineering team completed a thorough desk study that obtained records on previous site use and early 1960’s construction of a 20-story building along the alignment. These records showed that the street was previously narrower and that the new building was built over a 1930’s vintage concrete pile supported building that had been abandoned except for its basement. The records also showed that a soldier pile and lagging cofferdam was used to support the 1960’s construction excavation.

The preliminary engineering firm sent this information to the finally designer but failed to list the information in a risk register and warn the final designer in the preliminary engineering report why they placed the preliminary sewer alignment on the other side of the street. The final designer moved the alignment and neither the preliminary engineering firm nor owner’s staff that performed design reviews recognized this change as a potential problem. During construction, the microtunneling contractor began encountering difficult obstructions. The resident engineering staff investigated and found the information from preliminary engineering showing that the obstructions were steel soldier piles and that a basement was also in the tunnel path. The contractor was immediately notified and microtunneling was stopped just 3 m short of mining into the basement wall of the 1930’s vintage building (Fig. 1). The additional costs for delays, a recovery shaft, a realignment shaft and a different TBM were over $800,000. The consequences would have been much worse had they mined into the basement of the occupied building.

Inadequate subsurface investigation scope.

Budget concerns and lack of understanding of subsurface risk sometimes result in a grossly inadequate subsurface investigation scope.

Figure 1 – H Pile Obstructions

One example of an inadequate subsurface investigation involved a pipeline project where only two borings that extended to depths of 6.1 and 9.1 m (20 and 30 feet) for 490m (1600 feet) of pipeline to be constructed at a depth of 9.1 (30 feet). Neither boring was converted to a piezometer. This pipeline segment included a pump station, a levee undercrossing and a river intake. The two borings happened to terminate in clay allowing the tunnel subcontractor to optimistically assume low permeability soil and minimal shallow dewatering measures would be needed throughout most of the tunnel zone.

During construction, a thick stratum of high permeability sand that was directly connected to the river was found just below the clay. A major dewatering well operation was needed to control water resulting in over $500,000 in extra shaft and microtunnel construction costs. Upon questioning the manager of the geotechnical firm that completed the investigation, he admitted that his firm did not know or ask specifically why the borings were being drilled to the depths specified. Neither the geotechnical firm, the construction manger, nor the microtunneling subcontractor had questioned the adequacy of the subsurface investigation until construction problems developed.

Poor subsurface investigation monitoring.

Even if the subsurface investigation scope and budget are adequate for the phase being completed, poor monitoring of the work and documentation of conditions observable in the field can result in an investigation failure. A geotechnical firm that specialized in the competitive local foundation design marketplace completed the borings for one sewer project. Due to habit and a low price bid for the work, drillers were assigned to monitor the drilling, prepare field logs and select samples to be retained for laboratory testing. No desk study or anticipated conditions memorandum was completed the firms responsible geotechnical engineer to guide the drillers.

The ground at one tunnel shaft location was characterized in the geotechnical report by two nearby boring logs as being a few feet of clean fill overlying native alluvial and glacial soils. One shallow fill sample was described as having a “slight chemical odor.” During construction, the soil at the shaft and boring location site was found to consist of deep (over 6m [20 feet]), biologically hazardous (anaerobic preserved meat, bones and hides) and chemically hazardous (heavy metals, oil and solvents) fill overlying less contaminated native soil. The fill was situated within an abandoned timber pile and timber sheeted cofferdam that was previously a boat slip as indicated by readily available, but overlooked, municipal outfall sewer records. A Phase I ESA had not been completed but available records showed that the surrounding industries included tanneries and metal plating. Sufficient clues existed to allow discover of the contamination during design had a properly managed and monitored subsurface investigation been completed. Over $1 million in extra costs and six months in delay were incurred to complete the shaft and tunnel work in the contaminated ground with abandoned dockwall obstructions.

Lack of geologic interpretation.

Some geotechnical engineers have not had sufficient training in geology and lack sufficient underground construction experience to properly interpret subsurface data and predict ground conditions such as boulders.

Figure 2 – Boulder Obstruction of MTBM

Ground conditions for a sewer project along a ridge adjacent to a river valley within an upper midwestern city was described as stiff glacial silty clay and dense silty sand and sand outwash. During design, the geotechnical firm was asked to address the risk of encountering boulders. They responded that no boulders were encountered during drilling and that the contractor might encounter a scattered cobble or boulder during microtunneling.

During construction the ground was found to be cobbly and bouldery till, outwash and ice margin deposits along a glacial end-moraine. The geotechnical firm had not assessed local geologic publications indicating that cobbles and boulders were common and often large for the geologic units encountered. The microtunnel boring machine (MTBM) selected by the contractor was not equipped to handle frequent or large boulders. The cuttingwheel was badly damaged and the MTBM became stuck twice on large boulders before it was finally removed from the project (Fig. 2). A different TBM and dewatering method had to be mobilized to finish the project resulting in a delay of nearly 10 months and over $3 million in additional costs.

Consequences not considered.

Recognizing a risk and determining chances of its occurrence is often not enough. The potential consequences of the risk occurring should also be carefully considered.

A pipeline project in the Pacific Northwest required a river undercrossing. The location was an environmentally sensitive area due to its trout and salmon fishery and adjacent eagle nesting. Borings for the project indicated that cobbles and boulders were present. The geotechnical firm that prepared the microtunnel specification and geotechnical baseline report expected cobbles and small boulders based on frequent standard penetration test refusal blow counts and observations of drill rig chatter. However, they did not think that large boulders were present because none of the borings required rock coring for advancement.

The firm did not complete a desk study of available geological and tunnel case history papers. Had they done so, they would have recognized that large boulders were common in similar geologic units in this region and that a significant risk of encountering large boulders existed.

Furthermore, they did not consider that a recovery shaft would not be permitted if the MTBM became stuck below the river on a large boulder or nest of cobbles and boulders. The specifications did not require the MTBM to be capable of handling large boulders nor have face access or other backup systems to help ensure successful completion of the drive. The MTBM became stuck below the river (Fig. 3). The environmental permit did not allow a recovery shaft and the machine was too far into the drive for a rescue tunnel. The MTBM and approximately 150 m (500 feet) of steel casing had to be abandoned. The alignment was moved to avoid the abandoned tunnel and a new launch shaft had to be sunk. The contractor purchased a new, more rugged MTBM and mobilized it to the site.

Figure 3 – Abandoned MTBM Below River

River

Boulder Obstruction

The result was a one-year delay and over $3.5 million in additional costs.

Lack of Professional or Qualified Professional Interpretation of Subsurface and Site Data.

A thorough geotechnical subsurface investigation alone may not find readily discoverable ground or groundwater contamination. A Phase I Environmental Site Assessment (ESA) should be completed for every tunnel project. Where the Phase I ESA indicates that contaminated ground or groundwater is likely, a Phase II ESA should be completed and the resulting environmental data should be interpreted by a tunnel engineer that is experienced with management of hazardous gas and contaminated ground risks.

A tunnel project in the Midwest included a thorough geotechnical subsurface investigation, but no Phase I ESA. During tunneling, the construction manager was advised by an outside party that a convenience store along the alignment was formerly a gasoline service station that had leaked. A quick check of regulatory agency records revealed that it was a leaky underground storage tank (LUST) site that was only partly remediated and was now being monitored. The construction manager retained a local environmental investigation and remediation design firm to complete a subsurface investigation of the tunnel zone adjacent to the LUST site before the TBM arrived. The environmental firm approached the work in the manner they were accustomed for remediation projects. They completed six geo-probe borings and only those analytical laboratory tests for normally needed to characterize carcinogenic petroleum compound contamination for a remedial work plan. They did not recognize that analysis of toxic and explosion hazards for tunneling work required determination of petroleum compounds present and total petroleum hydrocarbon concentrations in the tunnel zone. The contractor was told to expect slight gasoline contamination within approximately 100m of tunnel zone and that special disposal of the muck and pumped water from this zone would be required. They indicated that only a slight modification of the health and safety plan to avoid skin contact was required and did not address the risk of toxic and explosion hazards to the miners.

Upon tunneling into the contamination zone, the miners immediately suffered ill effects of a toxic atmosphere. Eventually the TBM encountered a granite boulder that created a spark when struck by drag cutters that set off an explosion that burned four miners (Fig. 4). Completion of the tunnel was delayed over a year and incurred nearly $5 million in additional costs. In addition, the environmental firm, designer, owner and contractor were sued for negligence by some of the injured miners.

DESK STUDIES

A thorough geotechnical desk study is one of the most underutilized tools for reducing microtunneling risks. In many cases a desk study is not completed or is not sufficiently thorough. Desk study costs are generally very small compared to their potential benefit for reducing risks and construction costs. Owners should recognize that they have an obligation to not withhold pertinent subsurface information

Figure 4 – Burned TBM Equipment

that may be in their files from previous studies or construction. If discovered after bidding in conjunction with microtunneling impacts, a contractor may have merit to a claim of undisclosed “superior knowledge” (USNCTT, 1984).

Geotechnical desk studies involve a process of collecting and evaluating available previous information including but not limited to:

• Subsurface conditions (ground, groundwater, gas, contamination, utilities, foundations, wells, abandoned ground support systems, etc).

• Sewer facility conditions (structural conditions, sewer leaks, sediment contamination, original design and construction records).

• Site conditions (adjacent structure and building conditions, exposed ground conditions, wetland constraints, petroleum station locations, site access constraints, overhead interferences, etc).

• Previous shaft and tunnel construction experience in the region (methods used, problems encountered, productivity, cost, adjacent property impacts, etc.)

Desk study information may be found in the archived files, published documents, conference proceedings and websites of the following organizations:

• Owners (geotechnical reports, preliminary reports, basis of design reports, contract documents, and construction inspection records and claims files from previous projects often contain valuable information.

• Engineers (design, geotechnical and supporting specialty firms often have pertinent information from projects that may not have involved the owner but that can be shared).

• Public agencies (library, city, county, state, federal, and regulatory agency historical information on previous site use, landform alteration, existing building and foundation design files, utilities, geotechnical reports, ground and groundwater contamination, water quality and water supply well records, etc.)

• Utilities (abandoned and existing utility locations; types and construction methods used; boring logs and geotechnical reports)

• Adjacent property owners (building and foundation design files, geotechnical reports, construction reports, building performance problems, vibration and settlement sensitivity, previous site use, etc).

• Design, construction and manufacturing organizations (articles, published papers, bulletins, conference proceedings, manuals, guidelines, etc).

• Universities (published research and study results involving ground, groundwater, geology, land use, facility performance, etc).

SUBSURFACE INVESTIGATION

Subsurface Investigation Cost

Some owners may question the value of money spent on a thorough subsurface investigation if a risk of differing site condition (DSC) claims remains. Studies have shown that an inadequate subsurface investigation generally results in a higher risk of microtunneling problems and claims, while an excessive subsurface investigation may not add much additional value. In order to provide a guideline on what amount of subsurface investigation should generally be cost effective and beneficial at reducing

risk, a committee of interested parties to the tunneling industry studied over 100 tunnel case histories (U.S. National Committee on Tunneling Technology, 1984). The Committee recommended that total expenditure for all phases of subsurface investigation and data reporting for tunneling projects should generally range from 1 to 3 percent of the estimated construction cost with the lowest expenditures where the geology is less erratic and complex or where consequences of poor behavior are least. Table 1 summarizes the subsurface expenditure recommendations made by various committees and authors.

Table 1 – Subsurface Investigation Cost Guideline

Source / Reference Recommended Percentage of Total Project Cost*

Golder Associates & James F. MacLaren, Ltd (1976) 1 to 3% (6% if complex) West et al. (1981) 0.5 to 3% USNCTT (1984) 1% min, 3% average Legget & Hatheway (1988) 0.3 to 2% Sauer (1997) 3 to 4% normal in Europe Essex (1993) 3 to 5% (8% if complex) *Not including cost for tunnel engineering and geotechnical baseline report preparation.

Allocating sufficient funds and retaining a geotechnical firm to a complete subsurface investigation is not adequate to satisfactory reduce tunneling risks. A geotechnical firm experienced with tunnel design and construction should be retained to direct and manage the subsurface investigation and then complete the data analyses and geotechnical report preparation. Unsatisfactory results may occur if a geotechnical firm that has inadequate tunnel experienced is retained.

Subsurface Investigation Phasing

In general, subsurface investigations for microtunnel projects should be phased rather than completed under a single investigation. While it may be contractually convenient to have only a single phase, more valuable information for the expenditure results from a phased approach. Phasing provides better results for three primary reasons:

1. Some subsurface data should be utilized during planning and alternatives assessments prior to the final design phase to help ensure that microtunnel constructability assessments, risks and preliminary cost estimates are based on reliable limited knowledge of ground and groundwater conditions.

2. Phasing allows each subsequent phase to be focused on reducing remaining uncertainties and risks from previous phases. With a single phase, boring locations, boring depths, piezometer locations, field and laboratory testing scope, and need for geophysical or alternative exploration methods must be made based on many ground, groundwater, sewer diameter, alignment and grade assumptions which may not be correct.

3. Phasing allows later phase borings to be completed at the most strategic locations after adjustments in alignment and shaft locations are made.

The initial phase subsurface investigation should not be scoped or initiated until after a geotechnical desk study has been completed. Once the available subsurface information is collected and evaluated, a viable prediction of anticipated ground conditions and likely microtunnel risks can be made based on sound geologic principles. The scope of the initial phase subsurface investigation can then be developed to verify the anticipated conditions and reduce critical uncertainties. The result should be more valuable

information. Without initially completing a desk study, an excessive amount of the exploration expenditure may be wasted. Glossop (1968) wisely advised site investigation planners:

“If you do not know what you should be looking for in a site investigation, you are not likely to find much of value.

A desk study, initial phase subsurface investigations and a Phase I ESA should all be completed before the scope of a final design phase subsurface investigation is developed. Contaminated ground and groundwater can have a significant impact on tunneling if not found prior to bidding and construction. The final design phase site investigation should focus on reducing uncertainties and providing information needed by tunneling contractors to determine means and methods and assess productivity and costs.

Phasing is a cost-effective method of maximizing the benefit of the exploration program while simultaneously helping to reduce microtunneling risks. Even after several phases of investigation and completion of the design, design team engineers should communicate the remaining risks and uncertainties to the owner and indicate the costs and potential risk mitigation that would likely be realized if additional focused subsurface investigation work is completed. If necessary, late phase subsurface information can be added to the contract documents being bid by addendum.

GEOTECHNICAL REPORTS

Owners need to understand that geotechnical reports vary considerably in scope and effectiveness at explaining and baselining anticipated ground conditions to bidders and contractors. Table 2 lists eight types of geotechnical reports that may be involved with a microtunneling project. Two categories of reports are most common: recommendations reports and data-baseline reports.

Geotechnical Recommendations Reports

So-called commodity geotechnical firms are accustomed to completing Geotechnical Recommendations Reports (GRRs). GRRs are typically prepared early during final design as a result of a single phase subsurface investigation based on a competitive proposal. The final pipeline design is often incomplete or unknown by the geotechnical firm. Budgets for these subsurface investigations are often lean. Commodity geotechnical firm experience with shaft and microtunnel construction is generally limited. As a result, the GRR is written as a recommendations report filled with uncertainties about the actual design and specified work. GRRs typically have the following shortcomings:

• A comprehensive summary of desk study and previous phase subsurface investigation results is generally missing. GRRs often do not show the locations of available previous borings by others and seldom include the boring logs and pertinent field and laboratory testing results.

• Geology is often ignored. The geologic setting is not given or is inadequately explained. Layers and deposits indicated on boring logs are generally not characterized as geologic units or formations.

• No interpretive ground and groundwater conditions profile along the microtunnel alignment is provided. Providing a profile is often considered too risky by these firms.

• Groundwater data often only consists of driller’s observations in boreholes during or immediately after drilling. Piezometers are not installed or rare. Piezometer permeability or pump tests are even more rare. Water level readings generally do not extend throughout the design period. Fluctuations from seasonal and weather events are seldom measured.

• Ground and groundwater properties and anticipated ground behavior during shaft and tunnel work are not baselined. If quantities are provided, wide ranges are given without consideration of how the low bidder is entitled to interpret the statements. Tunnelman’s ground classifications are not explained or provided.

• Numerous disclaimers are provided indicating that at best, the only ground conditions that can be relied upon are the specific locations of the borings. Some disclaimers suggest that recommendations are only valid if the firm is retained to monitor construction.

• Numerous specification-like and additional work recommendations are provided that may not have been included in the actual specifications or completed during design. As a result, significant inconsistencies with the contract documents often exist.

GRRs are commonly completed and often lead to claims. Tarkoy (2004) opined that 95 percent of all tunneling problems associated with geotechnical conditions are not from inadequate expenditure on subsurface exploration, but instead from:

• “A lack of professional interpretation,

• Non-existent critical thinking, and

• Monumental lapses in common sense.”

He considered commodity geotechnical approaches to the work such as overuse of inexperienced junior engineers for interpretation of data as a serious problem and concluded:

Table 2 – Geotechnical Report Types

Type of Geotechnical Report

When Completed Report Usage Typical Report Scope

GOR Geotechnical Overview Report

Typically completed during planning or pre-design and kept internal

Generally written for use by owners, planners and preliminary design engineers.

Results of a desk study of available geologic papers, subsurface data, geotechnical reports and underground construction case histories.

GFRR Geotechnical Field Reconnaissance Report

Typically completed during planning or pre-design and kept internal.

Generally written for use by owners, planners and preliminary design engineers.

Results of a site visit(s) by a geologist or geotechnical engineer to observe and map geologic features at sites or along a pipeline alignment.

GDR Geotechnical Data Report

Typically completed during preliminary, intermediate and/or final phases of design.

Generally written for use by owners, planners and preliminary design engineers, design engineers, regulatory agencies and contractors.

A factual report summarizing a phase of subsurface investigation work and presenting field and laboratory results without ground behavior or constructability interpretations.

GRR Geotechnical Recommendations Report

Typically completed during preliminary or pre-design phases.

Generally written for use by an owner and designer and not by the contractor.

A report presenting field and laboratory results for a subsurface investigation program along with recommendations or suggestions for design and possibly construction methods.

GIR Geotechnical Interpretive Report

Typically completed during pre-design as part of preliminary engineering report and final design.

Generally written for use by owners, planners, preliminary design engineers and final design engineers.

A report summarizing results in the GDR and presenting interpretations of ground behavior and discussing constructability issues. The GIR also includes recommendations and parameters for design. Quantities and ground conditions are not rigorously baselined. The GIR is essentially a preliminary GBR with design recommendations.

GDSR Geotechnical Design Summary Report

Typically completed during final design.

Generally included in contract documents for use by the bidders and contractor.

A report summarizing results in the GDR and presenting interpretations of ground behavior and discussing constructability issues. Quantities and ground conditions are not rigorously baselined.

GBR Geotechnical Baseline Report

Typically completed during final design

Generally included in contract documents for use by the bidders and contractor.

A report summarizing results in the GDR and presenting interpretations of ground behavior and discussing constructability issues. Quantities and ground conditions are rigorously baselined and the bases for quantities are explained.

GCR Geotechnical Conditions Report

Typically completed during construction.

Used to evaluate DSCs, modify designs, and document actual conditions for reference.

Results of a site visit(s) by a geologist or geotechnical engineer of observed or mapped geologic features at sites or along a pipeline alignment.

“We have found it notable that some of the most prominent geotechnical firms consistently commit the same mistakes, repeatedly while relying on a dearth of disclaimers.”

Baker et al (2004), authors from a law firm that represents contractors in support of differing site condition claims, explain in their paper how poorly written geotechnical reports and specifications can readily be shown to be defective contract documents and used to win claims. To avoid claim problems they cite many of the recommendations contained in the 1984 report by the U.S. National Committee on Tunneling Technology (USNCTT, 1984). They conclude:

“If only these guidelines were followed! What appear to be simple concepts – brevity, clarity, specificity, and no discrepancies between documents – somehow rarely seem to make it into practice. Therefore, contractors should evaluate any contracts involving underground work in light of these standards. Where an owner’s specification [or geotechnical report] deviates from the standards, it may indicate some lack of information or research. In any event, such deviation should raise the proverbial red flag for the contractor.”

In summary, owners who desire to minimize claims and risks should avoid the use of GRRs as either reference documents or as part of the contract documents. In most cases the disclaimers contained in these reports will be found to be invalid (USNCTT, 1984; Parvin, 1994; Baker, 2004). The vagueness, inconsistency with final design and recommendations style of these reports will tend to increase claims rather than minimize them.

GDRs and GBRs

During the last 10 years, the tunneling industry has adopted geotechnical data reports (GDRs) and geotechnical baseline reports (GBRs) as better approach to communication of anticipated ground conditions and expected ground behavior for minimizing risks and excessive DSC claims. While no approach is without some problems and potential for DSC claims, this approach has generally been more successful at managing subsurface risks than other approaches. The GDR-GBR approach is explained in detail within an ASCE publication entitled Geotechnical Baseline Reports for Underground Construction – Guidelines and Practices (Technical Committee on Geotechnical Reports of the Underground Technology Research Council, 1997). Clarifications and recommended improvements on preparation of GBRs were subsequently made by several authors including: Edgerton (1998), Samuels (1998), Essex & Klein (2000), Hafer (2000) and Tarkoy (2004).

GBRs are difficult for commodity geotechnical firms and inexperienced geotechnical engineers to write. A good understanding of tunneling, ground behavior and ground conditions are essential in making good judgments on what to baseline, quantities to baseline and what associated work to cover with a separate pay item or include as incidental.

One of the most difficult parameters to baseline is boulders or boulder obstructions. Most commodity geotechnical firms will not attempt to baseline boulders due to concerns of being wrong. While risks and uncertainties of boulder baselining are significant, reasonable boulder baselining is needed for microtunneling and can be successfully completed (Hunt & Angulo, 1999; Hunt, 2000; Hunt & Mazhar, 2004).

Reluctant and inexperienced geotechnical firms and engineers need to learn that baselines are not necessarily predictions of actual conditions to be encountered, but are instead a contractual mechanism for obtaining reasonable allowances and compensation methods for select anticipated conditions. GBR writers and owners should recognize that risk management is more about equitable allocation of risk and

fair compensation than about preparation of contract documents that attempt to eliminate DSCs. In fact, experienced tunnel engineers believe DSCs should be expected as part of the risk sharing process. Heuer (1997) expressed this concept:

“We must constantly strive to improve our knowledge of the underground and our ability to predict it. But we will never be 100% successful. This means that evitable encounters of unexpected conditions are not automatically a reflection of the engineer’s professional competence Rather, such encounters are a basic fact of life that we must learn to deal with. …We should accept that a DSC is not a four letter word; it is not unethical or an inherently evil thing The contact DSC serves a useful and necessary function.”

If owners truly desire to manage microtunneling risks by adopting an risk sharing approach through the use of a DSC clause and by GDRs and GBRs that represent anticipated ground conditions and behavior, then care must be taken to ensure that retained GBR writers understand the purpose of baselines, have significant baselining experience and understand microtunneling as a result of involvement on many projects from design through construction.

CONCLUSIONS

Risk management is an important part of achieving a successful microtunneling project. Proper completion of desk studies, subsurface investigations and geotechnical reports are key elements of risk management for microtunneling.

Desk studies should be completed to help cost-effectively utilize available subsurface and tunneling information during planning and initial phase subsurface investigations.

Subsurface investigations should follow a desk study and then be phased to progressively reduce uncertainties and improve the geologic model or profile depiction of subsurface conditions. Experienced tunnel engineering firms should be retained to complete the scope of work and oversight of subsurface investigations for microtunneling. An experienced tunnel engineer or firm should guide geotechnical firms that that are retained for subsurface investigation and geotechnical data report work, but lack credible tunnel engineering experience.

Geotechnical data interpretation and preparation of geotechnical baseline reports should be completed by or under the close guidance of an experienced tunnel engineer. Geotechnical firms that lack tunnel design and construction management experience are unlikely to prepare proper geotechnical baseline reports. Such firms generally produce geotechnical recommendations reports that are filled with widely ranging values for parameters and numerous disclaimers about the reliability of interpretations. Geotechnical recommendations reports are not the best choice for management of microtunneling risk.

Owners need to take ownership of the ground and recognize that even thorough subsurface investigations by experienced geotechnical firms or engineers will not eliminate DSC claims. Some DSC claims should be expected and resolved with contractors in a cooperative and equitable manner. While the ground is inherently variable and risky, ground related microtunnel risks can be managed and controlled by good risk management practices starting with geotechnical desk studies, phased subsurface investigations and geotechnical baseline reports and specifications prepared by geotechnical engineers having credible tunnel design and construction experience.

REFERENCES

Abbott, D.G. (2004). Risk Management in Microtunneling – A 20 Year Personal and Industry Perspective. Proceedings: NO-DIG 2004, Paper D-2-01-1, 10p. North American Society for Trenchless Technology, Arlington, Virginia.

Anderson, J. (1998) Evaluating Risks from Tunnelling, World Tunnelling, August 1998, 315-316. ASCE, (2001) Standard Construction Guidelines for Microtunneling, CI/ASCE Standard 36-01, 41p. British Tunnelling Society (2003) The Joint Code of Practice for Risk Management of Tunnel Works in

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