There are many things to consider when designing a new project as risk is inherent when it comes to underground construction; this includes using Trenchless Technology methods. Here Dennis Doherty explains some of these risks and how to work with the contractor and owner to manage them.
There is risk of failure due to unknown conditions; including control of groundwater that has tidal influence, undocumented utilities, improper earth support, improper selection of tools, improper design methods and inexperience of key project members not familiar with the specific trenchless methods being deployed. The consequences of failure (or risk) are generally in the form of financial impacts to the project owner or contractor. The consequences of failure can be caused by contractor production rates slower than originally anticipated at the time of bid, failure of works such as jacking shafts, failure due to inappropriate rate of excavation for a specific geology with resulting earth movement affecting nearby structures, or unforeseen subsurface conditions.
There is also risk to surface activities due to improper work zone selection that can cause traffic delays or accidents, the inability to access off-street parking or loading zones, or disruption to “˜other’ utilities serving critical facilities such as hospitals.
The effects of the consequences of failure can be claims by the project owner against the contractor, claims by the contractor against the owner, or even third-party claims against one or the other. There are many who believe that it is the contractor who controls the construction method and is solely responsible for control of risk. When it comes to underground construction, it is often said the owner owns the ground and the contractor owns the method. However, it is the professional engineer’s duty to provide the owner with a design that protects the owner and assists the contractor in managing these risks.
The risk analysis and management process should answer the following questions:
- What are the risks?
- What is the probability of loss that results from them?
- How much are the losses likely to cost?
- What might the losses be if the worst happens?
- What are the alternatives?
- How can the losses be reduced or eliminated?
- Will the alternative produce other risk?
The following sections defines some, but not all, tools and methods for managing risk, including identification of risk, assessment of risk, risk response planning, and monitoring and controlling risk.
With respect to risk response, there are five basic responses:
- Accept – there is nothing that can be done to mitigate the risk. You have to accept it or hope it does not occur.
- Avoid – the project plan can be modified so as to avoid the situation that creates the risk
- Contingency planning – if the risk occurs, what will you do?
- Mitigate – what will you do to minimise the impact should the risk event occur?
- Transfer – pass the impact should the risk event occur (i.e. buy an insurance policy).
The engineer controls how to avoid the risk. Contingency planning and mitigation is in the realm of the contractor based on the contractor’s means and methods, but the engineer must identify the risk and put the contractor on notice via the contract documents.
With respect to engineering and construction, the strategy should be to assign the risk to the party that can best manage the risk. This requires designing projects with managing risk in mind. The engineer must understand the capabilities and limitations of the proposed trenchless method and match these to the existing conditions (both below and above ground), and understand the material science of the rehabilitation material or pipe being installed by the trenchless method. For example, when rehabilitating a pipe by either slip lining or cured-in-place pipe (CIPP), the chemicals within the pipe flow stream or in the surrounding ground may be detrimental to the long-term strength of the material. Another example is the cost associated with handling and
disposing of contaminated ground, groundwater or pipe sediments.
In addition to the engineer being cognisant of the capabilities and limitations of each of the trenchless methods and existing conditions, the owner must also understand and be willing to pay for the extra field investigation during the design process to identify the existing conditions that may affect the selection of the trenchless method.
For example, if groundwater levels are known to be high with a high recharge rate, then microtunnelling may be the method of choice for installing a new pipe. However, if existing pile-supported utilities that cross the proposed alignment are suspected, then microtunnelling most likely will not work. Instead, open face pipe jacking may be required to gain access to the face in order to cut the pile. However, this could require extensive groundwater control methods such as dewatering or soil modification. It may be prudent for the owner to have the engineer conduct additional field investigations using subsurface utility investigation techniques or test pitting to confirm the existence of piles. Without this, and if microtunnelling is specified, the contractor will most likely submit a claim for changed conditions. This will also cause a significant delay in the project schedule and could be even more costly if the ground or groundwater is contaminated or the resulting stoppage is under a busy highway or intersection.
There are many field investigation techniques that can be used during the preliminary and final design. These range from simple additional testing during subsurface geotechnical borings to ground penetrating radar (GPR), including the new in-pipe GPR, to side scan sonar to pipe sediment testing. Below is a list of some of these. It is recommended that the design engineer planning to use these techniques consult with the vender of the specific methods to determine the capabilities and limitations of the specific method.
- Ground penetrating radar (GPR)
- Magnetic abnormalities scan and side scan sonar
- Roto-sonic borings
- In-pipe GPR
- 3-D laser profiling
- Sediment testing
- Historic drawing review.
Choosing the right trenchless method and right tooling
Selecting the correct trenchless method, whether for rehabilitation or new installation, requires the designer to have a detailed understanding of the trenchless methods under consideration. The designer must also consider the existing site conditions such as site layout and geological conditions, as well as the intended use of the installed product, and environmental conditions including contaminated ground or pipe sediment.
The presence of boulders and cobbles will make it difficult to control line and grade and can also be detrimental to cutter heads on microtunnelling if the correct tooling for the head is not selected.
Another example is the potential for material degradation of plastic pipe or CIPP liners when the existing pipe being rehabilitated passes through a zone of highly contaminated soil and/or groundwater where the contamination is of a hydro-carbon nature. Both the new installation and rehabilitation methods with the above examples can be costly to the owner, the first due to rework of the drive and possibly change in method, the second due to reduction in the life of the liner thus effecting the life cycle cost.
Understandably, most designers leave it up to the contractor to select the right tooling on a specific trenchless method. However, there may be times when specifying specific requirements may assist in controlling risk. For example, specifying the allowable overcut in a specific geology with nearby sensitive structures for microtunnelling will assist in control of potential settlement that could cause unacceptable movement of the sensitive structures. Another example is specifying or requiring downhole pressure monitoring during a horizontal directional drill when the potential for frac-out exists due to design depth or geology.
The designer should still be careful of over-specifying the methods and tooling to be used. It is better to specify what will not be allowed or to specify specific tolerances that need to be met.
It is the responsibility of the professional engineer working in the trenchless industry to control risk and hence cost for the owner. It is also the duty of the professional engineer to provide the contractor with contract documents that are realistic and buildable based on experience and knowledge.
Conversely, the contractor should be responsible and work with the owner and engineer to construct a successful project that adheres to the risk mitigation plans put in place. Although it is the contractor’s means and methods that make the engineer’s design a reality, the engineer usually has more detail on the existing conditions and detailed knowledge of decisions made during the design process that identified risk and methods for avoiding them.
Ultimately, it is the responsibility of the engineer and the contractor to deliver a successful project, economically and on schedule, to the owner. Just as important, they have an obligation to the trenchless industry as a whole to recognise and manage risk in a responsible and professional manner.
This is an edited extract of an NASTT paper presented at the 2011 No-Dig held in Washington, DC.