By Raymond Sandiford, P.E., F. ASCE
The tunneling industry is witnessing an evolution in project delivery. Traditional design-bid-build procurement is giving way to faster, more streamlined design-build delivery and complex public-private partnerships. In response, owners have increasingly shifted more risk to the contracting community.
The business of assigning risk is understandable and inherent in these delivery methods; however, the amount transferred should be based on transparent and accurate data. Decision-making should not rely solely on an overly conservative geotechnical baseline report.
It goes without saying that Geotechnical Data Reports (GDRs) and Geotechnical Baseline Reports (GBRs) are essential tools in the underground construction industry. However, their application by owners is not universally accepted and in many instances improperly applied. In some instances, the GBR is used by owners as a purely risk-shedding instrument. This approach to the GBR is shortsighted and quite often counterproductive.
Shedding Old Beliefs – Not Risks
The GDR is an informational document, while the GBR is a risk defining document. The first has been the mainstay of the industry for nearly a century, while the second has evolved into practice in the past two decades. The GDR presents few issues for owners, while the GBR has much more subtle meaning and implications. It is this subtly that initially caused many owners to view the GBR with suspicion and has led to its frequent use as a vehicle for risk reduction. Owners must temper their risk aversion instincts and weigh the benefits of prudently selecting baseline data
Owners should recognize some basic facts inherent with construction involving geotechnical conditions. These are:
- The contractor has not chosen the project site, it is owner’s choice. Thus, the owner owns the site’s uncertainties.
- Risk allocation is not free. Excessive risk allocation involves considerable cost, more often than not, the public or rate payers are footing the bill.
In the simplest terms, a Geotechnical Baseline Report is the owner’s attempt at providing limitations as to what will constitute changed conditions. In most instances, there is no practical way to perform a thorough enough investigation to preclude the possibility of unknowns. The best one can achieve is a general understanding of the subsurface stratigraphy and the general bounds of the material properties.
Ideally, one would like to select baseline values that balance the level of uncertainty, or variance, in the data with the corresponding cost implications for the construction. However, this is not a trivial task for an owner or an engineer, and a soul-searching exercise for a contractor. This exercise becomes much more difficult and considerably more expensive if the owner provides overly conservative baseline values. If the contractor reads into the GBR that he is taking all the risk, then it will be in his bid whether these risks are materialized or not. If one looks at this situation on a national level, mega-dollars are involved and the public deserves better.
Taking a More Informed Approach
How should owners approach the presentation of the baseline values? They should recognize that they probably do not have the expertise to make this selection. If an overzealous risk manager is leading the effort, odds are the selection process will be unreasonably skewed. Owners should strive to leverage the knowledge of the tunneling industry with a prudent risk sharing strategy.
This process starts in the project planning. The site investigation should be budgeted to provide sufficient funds for a thorough subsurface investigation and a comprehensive material testing program that will provide sufficient data to ascertain reliable statistical variances for each of the geotechnical materials to be encountered. Without sufficient large data sets, the selection of baseline values is a precarious exercise. Also, limited data sets are unduly biased by statistical outliers.
One of the more common pitfalls of over conservatism relates to rock strength. If one overstates both the compressive (UCS) and the tensile (BTS) strength, this leads to a completely erroneous representation of rock brittleness. The rock brittleness is a parameter just as important to the TBM designer as the compressive or tensile strengths. This is only one example of many where over conservatism leads to misrepresentation of the site conditions.
With sufficiently robust data sets, the statistical variance of the material properties can then be weighed and worked in conjunction with the provisions in the changed conditions clause. As an upper bound, the selection of baseline values should be established so that they reflect the best judgment of where the character of the work would have to be markedly changed. This character change must be substantial enough to require the contractor to change his means and methods for that which he anticipated during bidding. To ease the process in determining what constitutes a substantial change, it is beneficial to have the contractor’s bid documents escrowed.
A GBR is a tool that should not be abused. Using the data to shift as much risk as possible onto the contractor doesn’t benefit anyone — owner, contractor or taxpayers — in the long run. When contractors are comfortable with balanced GBRs, bids will be lower and more in line with budgets. Keeping bids low and in line with budgets supports owners’ missions to be good stewards of precious taxpayer dollars.
Ray Sandiford is the geotechnical and foundation practice leader for HNTB’s tunneling and underground practice. Prior to joining HNTB, Sandiford was the Chief Geotechnical Engineer of the Port Authority of New York and New Jersey. He can be reached at (917) 438-0921 or rsandiford@HNTB.com.