Reflections on Differing Site Conditions, or Why I Believe That Mother Nature Is a Bitch

Anyone who has been involved with the design and/or construction of tunnels for more than a few years becomes well aware of the so-called Differing Site Conditions (DSC) clause. This clause was introduced by the federal government in 1926 after the government was involved in lawsuits alleging the inaccurate description of ground conditions in contracts released for bidding purposes. In essence, the courts found that the bidding contractors had a right to rely on “indications” about ground conditions made in the bidding documents that would be encountered during construction.

Therefore, if the ground conditions encountered during construction were shown to have “differed materially” as compared to contract indications, then the contractor became eligible for additional compensation. As stated in the clause there are two types of differing site conditions:

Type I – A ground condition that can be shown to be materially different as compared to ground conditions as indicated by the contract, or

Type II – A ground condition that can be shown to be of an unusual nature that differs materially from what would ordinarily be encountered on similar projects.

In general, the DSC clause is a good idea and provides an appropriate level of risk distribution for both the project owner and the contractor. In addition, this clause has now been the subject of hundreds of state and federal court decisions over the past 90 years, which provides all parties to prospective contracts with a solid understanding of what to expect from the application of this clause in a specific situation.

It is important to note, however, how different the application of the clause is for a “vertical” building project as compared to a “horizontal” project, such as a tunnel, a dam, a highway or a major pipeline. For a typical vertical project, maybe 10% of the project comes into contact with the ground, which greatly limits the possibility for a DSC claim. However, for a horizontal project, literally 100% of the project is exposed to the vagaries of ground conditions, which is why the impact of Mother Nature on tunneling projects becomes so keenly foreboding.

Who is Mother Nature?

I have often wondered about the expression “Mother Nature.” The word “mother” can be defined as a person of tenderness and affection who is imbued with a sense of caring for other people, especially children. Hence, the expression Mother Nature seems to imply that the characteristics of motherhood would somehow apply to the natural world. When viewed from a cosmic perspective, that may be true. From an earthly perspective, however, we see that Mother Nature is not so benign. Hurricanes, tornadoes, earthquakes, landslides and tsunamis are a testament to her fickle nature – and that is just what we see on the surface. Below the surface, tunnelers face a plethora of challenges millions of years in the making by ever evolving geological processes. Interestingly, once you go below the surface of the ground the moniker Mother Earth is probably more appropriate as compared to Mother Nature, but the “mother” part still applies, i.e., a place of assumed tenderness and affection.

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Based on my 45 years of experience with tunneling, however, I do not view Mother Earth as benign and cooperative, but rather as an entity that does not appreciate the fact that human beings continually want to perforate it with holes of various sizes and for various purposes. Years ago I wrote a paper titled “Tunneling: A Battle Against the Ground” (TBM: Tunnel Business Magazine, February 2001) wherein I characterized the building of a tunnel as comparable to trying to win a battle. In that paper, I listed Mother Earth’s greatest potential for causing trouble for tunneling projects as follows:

  • Complexity – The ground can be extremely complex and difficult to describe based on the results of test borings.
  • Secrecy – The ground is fully capable of hiding adverse features such as faults, voids and natural gas.
  • Adverse Behavior – Even when you think you know what the ground is, it may not behave as you anticipated and adverse ground behavior is a huge problem for tunneling.

Any and all of the topics listed above can result in a “differing site condition” that can have massive negative consequences for a tunneling project. In general, and based on experience, I think the three biggest issues that Mother Earth brings to bear on the success of a tunneling project are:

  1. Groundwater
  2. Gravity
  3. Boulders


I seem to remember reading a paper by Karl Terzaghi about 30 years ago wherein he stated that something like 75% of all the problems he encountered on his projects had something to do with the groundwater. I agree with that statement. To begin, the potential for water to cause trouble for your tunneling project is even more difficult to evaluate than the impact associated with the ground itself.

Not only does ground permeability vary by several orders of magnitude, the water comes at you from three completely different perspectives: quantity, pressure and quality. With respect to water quantity, trying to access the face of your tunnel in a row boat is not terribly productive. Under high groundwater pressure the total collapse of various portions of your project can also lead to lots of downtime. Finally, when the water brings hazardous chemicals into your tunnel, both natural and manmade, the negative impacts both inside the tunnel and based on water treatment requirements can be highly problematic. Hence, groundwater is a powerful weapon in Mother Earth’s arsenal against tunnel projects.


It is also amazing how something as simple as gravity can cause so much trouble. A couple of years ago I was playing golf with a total stranger when he asked me what I did. When I said tunneling, he asked me to describe a current project so I talked about an investigation I was performing about a major roof collapse in a large rock tunnel. After which, this fellow said (somewhat jokingly, I think) that he couldn’t believe that someone had hired me to discuss the concept of gravity. Rock joints and rock bolts notwithstanding, in his mind, this problem was caused by gravity.

Gravity is also the major cause of sinkholes. If you lose control of the ground during tunneling then gravity will cause the ground above the tunnel to fill the void and, thereby, result in a sinkhole. Bob Goodfellow, co-author of the Underground Construction Association’s “Guidelines for Improved Risk Management,” during his lectures on risk invariably shows pictures of buses that have fallen into sinkholes and recommends that if your city is building a tunnel, then you should not take the bus! Once again, something as simple as gravity causes lots of problems.


Finally, we have boulders, which are frequently associated with glacial soils and/or rock fragments such as occur in a weathered rock profile. Boulders, too, seem to cause a lot of trouble for tunneling projects. This is a little hard to understand because the possible occurrence of boulders is usually well discussed in the geotechnical reports and both tunneling contractors and TBM manufacturers have various methods for dealing with boulders. Having thought about this issue, however, I think the real problem is Mother Earth’s uncanny ability to place those boulders in the worst possible locations, such as half way outside the tunnel perimeter or in the middle of very busy highway interchanges. Once again, Mother Earth does not reveal the slightest sense of caring about the successful outcome of your tunneling project.


When I was a graduate student at the University of Illinois (many moons ago), there was a story going around that one of our professors, Dr. Don Deere, was asked to investigate a differing site condition for the Perini Corp. After considerable investigation, Dr. Deere informed “Old Man” Perini (everyone, apparently, called him Old Man) that he did not think this was a differing site condition since the test borings clearly revealed the ground in which the tunnel was being constructed. After a pregnant pause, Old Man Perini was reported to have said: “Dear Dr. Deere, yesterday I was, ah, makin’ da money but today I am, ah, not makin’ da money, and that according to my, ah, definition, is a differing site condition!”

Actually, there is a lot of truth in what Mr. Perini is saying. If anything goes wrong on a tunneling project, then it is easy to blame Mother Earth for causing the problem since Mother Earth has a long history of causing trouble for tunneling projects, and what’s even better, Mother Earth has no legal standing for defending herself.

Everyone knows that Mother Earth has a huge arsenal of potential problems ready to make trouble for tunneling projects. It is largely up to tunnel owners and designers to accurately anticipate and to describe those problems in the contract document. In that regard, a thorough and comprehensive subsurface investigation is a good start, but appropriate project specifications for anticipated ground behavior is also a step in the right direction.

Thoughtful and competent project implementation on the part of the contractor is also essential for project success, but the underlying basis for all claims of a differing site condition is what is said about that ground condition in the contract document. Hence, and in final analysis, the project owner and its design consultants must, to the best of their ability, try to accurately anticipate and accommodate all of the problems, road blocks, issues and concerns that a totally unsympathetic and uncaring Mother Earth can impose on their tunneling projects.

Gary S. Brierley is president of Dr. Mole Inc. He began his career in 1968 with the Bachelor’s Degree in Civil Engineering from Tufts University and the Masters and Doctoral Degrees from the University of Illinois in 1970 and 1975, respectively. During that time Dr. Brierley was fortunate to work on the instrumentation program for DuPont Circle Subway Station in Washington, D.C., a project that formed the basis for his doctoral dissertation. Since that time, Dr. Brierley has devoted his entire professional career to the design and construction management of underground openings.

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