|FHWA > Bridge > Tunnels > Technical Manual for Design and Construction of Road Tunnels - Civil Elements|
Technical Manual for Design and Construction of Road Tunnels - Civil Elements
Chapter 14 - Tunnel Construction Engineering
This chapter focuses mostly on mined/bored tunnel construction engineering; the engineering that must go into a road tunnel project to make it constructible. Each decision made during the planning (Chapter 1) and design of a road tunnel project has impacts on the constructability, cost and schedule of the Work. This chapter will look at these cost drivers and how they influence the project's final cost. Planning, design and finally construction operations should be guided by people experienced in the actual construction of these underground works so that the projects are constructible. The schedules must be realistic and reflect all the restrictions that are imposed on the project whether they are physical, political or third party. Cost estimates must reflect the actual schedule time needed to complete the work and account for all the restrictions imposed on the project.
Tunneling is unique when compared to other types of civil construction. In non tunnel projects like a large building or treatment plant there are usually many places to work at the same time, so the work can continue even if there is a problem holding up work at one location. Tunnels are long linear undertakings with few opportunities to perform the work at more than one location. Tunnels are also a series of repetitive operations each of which usually must be finished before the next can be started.
This uniqueness and the linear, repetitive nature of the work must be understood by the planners and builders of tunnel projects to control and manage the project to a successful conclusion.
Perhaps the most significant factor impacting tunnel cost and schedule is the type of geologic material that the tunnel will be mined through and the amount of ground and surface water that will be encountered or crossed. Tunnels are mined through rock, soil or a combination of both. The geology encountered determines the tunneling methods that will be used, the speed that the tunnel can be constructed and the types of specialized equipment that are required.
The geologic material can also present some unique health and safety concerns that must be accounted for in the planning and construction of underground projects. Gas, petroleum, contamination, voids in the ground, hot water or large quantities of groundwater all pose safety concerns that must be addresses so that the workers building the tunnels are provided an environment free of hazards.
Of similar importance to the tunneling methods and hours of operation are the communities that the tunnel will pass under, the locations of the major work shafts or portals from which the work will be serviced and the streets through which the equipment, personnel and material will get to and from the worksite as well as how the muck removed from the tunnel is disposed of.
All of these factors will have impacts on the cost and schedule of underground projects and in fact represent risks to the project. These risks must be acknowledged, allocated and mitigated. Dealing with these risks can be accomplished through the contractual language between the parties to a tunnel project, or if not dealt with or if dealt with inappropriately, contractor claims or lawsuits.
The design for an underground project must be constructible. Too often road tunnels are designed by competent engineers who have never actually built anything. Their designs minimize the volume of excavation and concrete but are difficult to build. Underground construction is expensive due to the large proportion of labor used during the construction, the high wages paid to these workers and the linear nature of the work. In order for our tunnels to be less expensive to build, designers must also be schooled in how tunnels are built so they can recognize that their decisions on size, shape, location and esthetics all have cost impacts.
A brief discussion of the labor portion of the cost of underground construction is in order so that designers can start to understand how their decisions impact these costs. Most underground civil construction is performed in a union environment. The union provides skilled labor that performs specific job functions. Typically there is a crew actually performing the work. This crew will consist of miners, miner foremen, operators to run and maintain the equipment, electricians to maintain the power that runs the equipment and provides the necessary lighting levels as well as supervisory people. These folks actually performing the repetitive operations are called the heading or direct labor crews. These crews are supported by an entire separate group of people that supply the project with needed power, material, transportation, maintenance and overall project management. These are called the service crews. The service crew can be as big as the direct labor crews. If you have 25+/- direct labor doing the work you also have 25+/- people supporting the work. These two or more crews are being paid whether the work is going forward or not.
One typical example of where the design of a tunnel project can impact the cost is in a location where a tunnel must be widened out to accommodate an exit or entrance or even an emergency pull-off. In most designs you will see a constantly changing cross section going from the road tunnel and widening out to accommodate the exit, entrance or emergency parking area. This looks nice, is visually pleasing and minimizes both the excavated volume and the amount of concrete that is required in the lining, but is it easy to build and what does it add to the cost?
Most contractors will come back to the project's owner and propose to accommodate the same structure in a stepped fashion instead of a smooth transition. Why? It is relatively easy to excavate the transition cross sections in a rock tunnel (more difficult in a soft ground tunnel operation) and certainly a smoothly transitioning excavation does minimize the volume of material that is taken out. However the lining operation becomes real tricky and costly.
The smooth transition requires different custom built forms for each foot of the structure. There is no, or limited, reuse of forms and most importantly each of these custom forms must be built in place, used and removed thus slowing down the lining operation. Each use of a custom form requires both the direct crew and the service crew to be used for a longer duration driving up the cost and increasing the schedule for the whole project.
Now look at what the use a larger cross section or a stepped transition can do for the cost and schedule. If we simply go from the typical tunnel size to the full size required for the exit, entrance or parking lane we pay for some extra excavation and concrete but we only now have two forms (one extra) to build use and remove. If using just two different cross sections is not possible, then a multi-stepped transition can help to minimize the time and money spent building, using and removing all the specialized forms. An evaluation must be made whether it is faster and less costly to remove extra material and place extra concrete or to install, use and remove all the specialized forms.
So how do we make our designs more constructible? One way is to include a construction expert on the design team. This construction expert would then sit with the designers reviewing what approach they want to utilize, make suggestions on how the design could be more easily built, make sure that all the site constraints have been addressed and providing insight into how a contractor would price the designs so that modifications of the design can be made to control cost and schedule.
14.3 Construction Staging and Sequencing
14.3.1 Construction Staging
Each underground project is unique; however, there are certain requirements and functions common to most or all tunnels. Each project requires one or several places from which the work can be prosecuted. All projects require large quantities of labor, material and equipment be brought underground to excavate and support the tunnel and large quantities of muck and ground water must be removed from the tunnel. All projects therefore require land area to set up the contractor's offices, shops storage yards, muck storage piles, electrical substations and many other space needs. It therefore is logical that the more space that can be made available to the contractor to locate needed structures, store needed materials and allow for the movement of materials and equipment into and out of the worksite, the more efficient and less costly the operation will be. On the other hand the smaller the available worksite the more expensive and less efficient the operation will be (Figure 14-1).
Figure 14-1 Confined Worksite and Staging Area
Underground projects serviced by shaft(s) require room to excavate the shaft. There should be room all around the shaft to allow equipment access and easy flow of the work around the shaft location. Typically owners, who must acquire property to locate the shaft will minimize the size of the property and thereby minimize their expenditure for property acquisition. This can be shortsighted. Paying more for more room can actually provide for a more efficient operation, lowering the overall cost for the work and providing the owner the opportunity to sell off the extra property after the project is completed at a higher price thereby further lowering the total cost of the construction.
Portal projects benefit from not having the expense and schedule impact of excavating and supporting the shaft(s) but also require property on one or both sides of the project to enable the contractor to efficiently prosecute the work (Figure 14-2). Portal areas for a road tunnel may be limited by existing geotechnical hazards.
Figure 14-2 Tunnel Portal
14.3.2 Construction Sequencing
Underground construction is a series of individual activities that must be completed before the subsequent activities can start. This series of unique activities is then repeated and repeated until the operation is complete. For tunnels that employ drilling and blasting to create the tunnel opening the series is, "drill, load, shoot, muck and support." Each round is drilled a certain length or depth using a pre-engineered drill pattern. Once the drilling is done the explosives are loaded into the drill holes and "wired up". The equipment and crews are then pulled back a safe distance from the loaded face and the blast is "shot". Exhaust gasses produced by the explosives are removed from the face and fresh air is sent to the heading area. After around 30 minutes the crew is brought back into the area to scale or knock down any loose rock and remove the excavated material or "muck". Once the muck is removed, the initial tunnel support is installed to make the excavated opening stable and safe for the crew to work under. The cycle is complete and the tunnel has been advanced some distance. The next round can be started when all of these activities have been completed.
In TBM excavated tunnels there is also a defined sequence of activities needed to advance the heading. The TBM usually completes this series much faster that in drill & blast tunnels but the elements remain similar. The TBM cuts into the rock or earth a certain distance at the same time the muck is removed by conveyor to either waiting muck cars or to a continuous horizontal conveyor, so the TBM is able to combine these two operations thereby saving time and speeding up the tunnel progress. After the end of the TBM's stroke (the hydraulic pistons used to push the TBM cutting head into the rock have a defined length) the excavation is stopped and the TBM readied to start the next excavation cycle. While this is happening the length of tunnel that has just been exposed most be supported to provide a stable and safe opening. The TBM can sometimes be configured to perform this support function concurrently with the excavation sequence depending on the size of the tunnel opening, the type of ground being excavated and the design of the machine. This can be another advantage of using a TBM but does not change the fact that this operation must be done before the next excavation cycle can begin.
Tunnels are usually stabilized for long term use by placing an internal final concrete liner. The concrete lining operation also contains a series of individual steps that must be completed in sequence before the next length of tunnel can be lined.
14.4 Mucking and Disposal
"Muck" is the industry term for excavated material produced during the advancement of the tunnel. All tunnel mining produces muck. This excavated material must be removed from the working face of the tunnel so that the next advance can be made. Tunneling is a series of individual steps, each of which must be completed before the next can start. Once the muck is produced it must be removed from the tunnel and finally disposed of in a legal manner or used as fill for some portion of the tunnel project or other project where it could have a beneficial use.
Muck is actually a broken down state of the insitu material through which the tunnel is driven. Because the natural material is disturbed by either blasting, cutting with a TBM, roadheader or cut out with a bucket excavator the volume of muck removed actually is larger than the natural bank material. This swell is usually approximated as 70% to 100% more in rock and 25% to 40% for soil.
The material that is excavated must be removed from the tunnel. The method chosen to remove this material depends on many factors such as the diameter or size of the excavation, the length of the tunnel excavated from any given heading, the material being moved, the grade of the tunnel being driven and whether the material is going to a shaft for removal or a portal. Horizontal conveyor belts are commonly used for large excavated tunnels that are longer than a few thousand feet and are excavated by a TBM (Figure 14-3). Conveyors can move a large quantity of material quickly. Conveyors require that the excavated material be of relatively uniform small size so that it will sit in the belt during the transfer to the shaft or portal. Conveyors can sometimes be used with a drill and blast excavation method if the contractor employs a crusher to make the drill and blast rock a more even and smaller consistency. This crushing is necessary to ensure that the material sits nicely on the belt, is small enough that when it is loaded onto the belt it does not damage or rip the belt material. Conveyors are usually limited to a grade (or slope) less than 18 degrees to successfully transport muck, but this is never an issue in road tunnels. Conveyors can transport rock or soil. The soil must not be too wet or it will not transport well. Conveyors can also be used in tunnels where there are curves in the alignment but this requires some special care and equipment.
Figure 14-3 Horizontal Muck Conveyor
Material that is too wet to carry on a conveyor belt can sometimes be pumped out of the tunnel through a pipeline from the TBM to the shaft or portal. This method is successfully used on soft ground tunnels where the material is clay like or where sufficient water (and often, conditioners) is mixed with the excavated material to make it slurry like.
For smaller tunnels excavated by a TBM, contractors often choose to load the excavated muck into rail cars and haul it out of the tunnel using locomotives. Rail haulage also has some limitations such as the grades are usually limited to less than 4%, a great amount of rolling stock is required and great care must be paid to maintaining the track.
Once the muck arrives at the shaft or portal it must be off loaded and then disposed of. Figure 14-4 shows a muck train dumping at a tunnel portal. A shaft is a vertical hole through which all excavated material must be lifted and removed and through which all material required for the tunneling operations must be lowered to the tunnel level. In addition all personnel working on or inspecting the tunnel must come in and out of this shaft. In other words it is a busy place. There are many ways to transport the muck up the shaft. Muck cars can be lifted one by one up the shaft, dumped in a pile on the surface and lowered back down to the tunnel. Muck cars can be dumped into a hopper at the bottom of the shaft and then loaded into a bucket that is hoisted to the top and dumped or the muck from the hopper could be loaded onto a vertical conveyor and conveyed to the top of the shaft and dumped onto a pile or hopper. Similarly the muck can be pumped to the surface and deposited on a horizontal conveyor, a stockpile or run through a processing plant to remove the water and the residual dumped on a pile or into hoppers.
Figure 14-4 Muck Train Dumping at Portal
Portals provide easier access to a tunnel since they eliminate the bottleneck that the shaft imposes. Muck is easier to remove at a portal since track can be paced on the ground or on an elevated trestle so that muck cars can be pulled outside to dump their loads onto a muck pile.
The really important thing to remember is that tunneling is a series of steps that must be done and complete before the cycle can start again. This means that any disruption in the muck removal operation will delay the start of the next round or the next advance. If you cannot get rid of the muck you can not produce more! This is also true once the muck reaches the surface. There should be a place to store the muck that is brought out of the tunnel until it can be loaded into trucks or rail cars and hauled away. Without this storage capability on the surface (Figure 14-5), all muck brought out of the tunnel must immediately be loaded into surface trucks or rail cars for disposal. If there is a holdup in the surface trucking or rail cars then no more muck can be brought out and the tunnel advance must stop. This situation is called being "muck bound" and must be avoided at all costs. The more muck storage that is available the more unlikely it will be for a project to become muck bound. Work sites must be large enough to provide this storage cushion, the larger a worksite the bigger the cushion. It is increasingly more difficult to find available land in and around cities to provide a suitably large worksite. Typically urban sites are small and therefore special care must be taken to ensure a steady stream of vehicles to remove the muck as it is produced, and to deliver workers and materials as needed. Thought must also be given to the hours of operation allowed in urban tunnel projects. If the hours of operation for surface work are restricted, i.e., surface work is not allowed after 10 PM at night, then in order to operate the tunnel 24 hours per day, there must be some place to store muck underground that is produced on the shift where no surface work is allowed, and construction noises must be kept below a threshold based on local ordinances and/or certain realistic decibel levels.
Figure 14-5 Surface Muck Storage Area
14.5 Health & Safety
Construction Engineering and safety go hand in hand. Underground construction is inherently a dangerous undertaking. Work goes on in a noisy environment, in close quarters often with moving heavy machinery. Careful attention must be paid to the layout of the worksites; workers must be protected at all times. The overriding philosophy must be that, "everyone goes home safely at the end of their shift".
Every step of the operation should be planned with safety in mind. The normal surface safety concerns are also appropriate for underground construction. Workers must be safeguarded from falling off of the work platforms used in the mining process. Workers must be protected from being struck by the moving equipment used throughout the mining process. Workers most be protected from being electrocuted. However there are also many additional hazards that workers must be protected from and guarded against.
Work underground involves mining through rock or soil or a combination of both. In order to excavate the opening required for the tunnel the natural properties of the ground are disturbed. The ground is usually not a homogeneous mass but has been subjected to massive forces of nature and has been altered. Once the opening has been excavated it must be supported in order for the workers to be protected from falling material, collapse or other deterioration of the tunnel roof or crown. So it is the job of the Construction Engineer to plan on making the tunnel opening stable to allow workers to move freely and without concern of falling material.
Because tunnels are by definition below the surface, lighting of the workspace is an important part of underground safety. OSHA has regulations governing all elements of working underground and the Construction Engineer must be familiar with them all. There are required levels of lighting for the actual work locations as well as the previously excavated openings. It is important to remember that the tunnels are long linear work places. As the tunnels are advanced more and more safety plant must be added along with the productive support elements.
One of the more challenging aspects of tunnel safety is the fact that workers must be constantly supplied with high quality breathable air. Again OSHA is very specific in its requirements. Each person underground must be supplied with 200 cubic feet per minute (cfm) of air. In addition much of the equipment underground is powered by internal combustion engines. Diesel fuel is the only fuel allowed underground. OSHA again has specific requirements for the equipment and for the amount of air that must be delivered to the underground for each and every piece of diesel equipment working underground. This diesel air requirement is in addition to the requirement for each and every person underground. The quality of the tunnel atmosphere must be tested on a regular schedule to ensure that sufficient quantities of oxygen are present and that concentrations of undesirable gasses and byproducts of the internal combustion engines are controlled to acceptable levels.
Also tests must be performed on a regular basin to ensure that the air movement across the excavated cross section is no less that 30 foot per minute.
If this were not enough, as discussed in Chapter 8, Mother Nature can often provide challenges to the safety of workers underground. There can be gasses underground that can seep into the tunnel opening after the excavation operation. These gasses can be poisonous like hydrogen sulfide or explosive like methane. Whenever these gasses are present or suspected to be present the Construction Engineer has additional OSHA requirements to be aware of and to follow. Extra ventilation will be required, in addition to the air needed for both people and diesel equipment and the required quantity can be substantial. Whenever these gasses are suspected there are extra requirements for continuous monitoring of the atmosphere with automatic shutdown of equipment should the gasses be detected in concentrations higher than allowed.
Water entering the tunnel opening is also a safety issue in tunnels. Most tunnels are excavated below the water table. The tunnel opening acts like a big drain and any water running through the rock or contained in the soil tends to collect in the tunnel. Water running through the tunnel bottom or invert can cause several potential safety issues. Tunnels can be accessed by one or more shafts, by a combination of shafts and portal or from a portal alone. It is desirable to drive tunnels up hill so that any water that seeps into the excavated opening flows away from the working face by gravity. This water is usually allowed to run in a ditch located at the side of the tunnel invert. Care must be taken that workers do not step into or fall into this ditch. The higher the inflow of water into the tunnel the greater the problem of safely conveying it back along the tunnel and finally out the shaft or portal.
Tunnels that are driven down hill have the problem that water flows to and accumulates at the working face. This collected water must be removed from the work area by pumping. The water is pumped through a pipe at the side of the excavation. This pipe must extend all the way to the shaft or portal where it can be removed from the tunnel. Water can also enter the tunnel in sudden large flows. These can be very dangerous occurrences and any tunnel where this is a possibility extra care must be taken in the planning for worker safety. Tunnels under bodies of water are of particular concern for this risk of sudden large inflows of water.
Fires in tunnels are especially dangerous and can lead to extensive damage and risk to worker's safety and life. The Tunnel Construction Engineer must be aware of this potential danger and plan to mitigate the risk at every stage of the project. Most tunnels are driven from one point to another from a single point of entry. This single point of entry is what makes tunnel fires so dangerous and concerning as shown in (Figure 14-6). The tunnel environment contains numerous potential sources of fire. Equipment can malfunction and catch fire. Workers using welding or burning torches can set off a fire. Leaking hydraulic fluid or fuel from equipment can be ignited by a stray spark or discarded cigarette. Conveyor belts used to transport muck can build up heat from rubbing on or over something and ignite. All these possible fire risks, and more must be addressed by the Construction Engineer to minimize the possibility of a fire or to minimize the potential damage and injuries resulting from a fire. Only retardant material and hydraulic fluid should be allowed on any underground equipment or material. Fire suppression systems should be required for all underground equipment, conveyor belt motors and storage magazines. Vertical muck removal belts should be equipped with deluge water systems to dump large quantities of water on any belt fire event. Fire and life safety issues during operation and maintenance of road tunnels are not included in the scope of this Manual.
Figure 14-6 Fire in Work Shaft
Of equal importance in dealing with tunnel fires is how to best provide for the safety of the workers underground. This can be accomplished in several ways. Rescue chambers, where workers can take refuge in a fire, are fully equipped and supplied with independent air supplies and insulation can be deployed along the tunnel as the tunnel is advanced. Equally important the tunnel can be planned with intermediate access points that can be fully equipped to be able to remove workers from the tunnel when the tunnel has been excavated past these locations.
The Tunnel Construction Engineer must also be certain to make sure that the job specifications require strict compliance with all safety measures and regulations local, state and national. The Engineer must stress to the designer and the owner that money spent on worker and job site safety is money well spent since the cost of accidents and replacing structures damaged or destroyed by a fire event is so high.
14.6 Cost Drivers and Elements
There are numerous cost drivers associated with underground construction. These can be grouped into physical, economic and political.
14.6.1 Physical Costs
The single most important driver of project cost is the ground through which the tunnel will be driven. The ground controls the methods and equipment used to drive the tunnel, the support elements that will be needed to ensure that the excavated cross section remains stable and safe for the personnel constructing the tunnel and the final lining needed for long term stability of the structure. In addition the ground through which the tunnel is driven will contain varying amounts of ground water that will dictate the pumping requirements, waterproofing needs and lining quality that will ensure a dry tunnel environment.
The use that the tunnel will serve also has a significant impact on the costs. Tunnels for roads and rail must be dry to safeguard the traveling public so a watertight structure is imperative. Road and rail tunnels are also grade restrictive and curvature restricted which also impact project cost. Tunnels that will service as road and rail infrastructure must be able to deliver large quantities of fresh air throughout the length of the tunnel and be able to remove smoke and heat developed during a fire incident anywhere in the tunnel. Large ventilation structures or in line fan systems are needed to supply this air and remove the smoke.
In rail or road tunnels refuge areas or rest areas are often needed along with on and off ramps or connections to outside rail or road systems.
14.6.2 Economic Costs
All tunnels require personnel, equipment, materials incorporated into the physical structure, materials that are consumed during the construction of the tunnel along with insurances, bonds, offices, shops and other indirect elements. These all impact the cost of the project. The largest portion of these costs is the actual cost of labor. Labor is broken down into the labor actually driving the tunnel or the direct or heading crews; the support crews that provide all the needed supplies of the tunnel, maintain the equipment used during the tunnel driving operations and provide access to and from the tunneling operation and the supervision needed to ensure that all the components work together in the required sequence.
Material is another major cost component of tunnel operations, Materials like cement, steel, copper wiring are all very price volatile now due to strong worldwide demand. Currently the price escalation of key materials is a significant cost driver and one that is often not addresses in the contract specifications as a separate cost. Tunnels require large quantities of both permanent and consumable materials in a constant stream.
We have also the continual cost of disposing of the muck or excavated material that is produced during the tunnel operations. Muck can sometimes be sold off by the contractor or owner to help reduce the cost of tunnel construction. However the market for this material is not guaranteed and often the contractor must pay to haul this muck away and also pay to dispose of it at approved dump locations. More and more regulations governing the disposition of materials are driving up the cost of tunnel construction.
Bonds and Insurance are smaller components of tunnel costs that are becoming cost drivers due to the increased scrutiny being imposed by the insurance and surety industry. Since most owners require both bonds and insurance on their projects by law and as risk management tools any contractor that cannot qualify for bonds and insurance cannot bid the project. After the terrorists attacks of September 11 and some high profile corporate failures, the marketplace for both bonds and insurance has tightened up and many providers have actually stopped writing bonds and certain types of insurances.
14.6.3 Political Costs
Significant costs are placed on projects by either the communities through which the tunnels will be mined or by the owner agencies by the requirements and restrictions incorporated into the specifications. Tunnels are expensive undertakings even without these restrictions but when concessions to various groups are added to the requirements the costs can skyrocket. Tunnels built in rural areas experience few of these political costs but those driven through urban settings can experience significant costs due to these restrictions. Typical restrictions are, mandating certain types of construction to minimize community disruptions, i.e., mining an underground cavern instead of digging down from the surface or not having a work shaft at a certain location because it is too close to neighbors. Restrictions on the hours worked is commonly employed when the tunnel is in a urban location. Tunnels are a cyclical series of operations where one cannot start till the predecessor is complete. With restrictions on the hours of operation fewer steps can be completed in the reduced time so the job takes longer. In one case an owner agency allowed 24 hour tunneling (recognizing that this is a typical mode of operation) but limited the hours that could be worked at the surface where the muck is brought out to be trucked away. In order to compensate for this reduced time the underground opening had to be made larger, so that the muck that was produced during the time where no surface work could be done, could be stored underground awaiting that time of day when it could be brought to the surface and trucked away, the political cost of being a good neighbor.
Owners might drive up the cost of doing underground work by restricting what costs are recoverable by the contractor in a change order or claim situation and by preventing the contractor from recovering delay costs if the delay is caused by the acts or inaction by the owner. These "No damage for delay" clauses might suggest to the contractors to incorporate into their bids these potential costs and the owner pays for them whether they occur or not.
The importance of the development and use of a realistic schedule and cost estimate for all phases of a project cannot be overemphasized. It is critical to understand the relationships among all the activities and costs that go into a project as well as the needs and interests of all those who are affected by the planning, design, construction, testing and commissioning of the work. With this understanding, projects can go forth in an orderly, predictable manner, which in the end benefits everyone.
The schedule is the road map of how the project progresses through all the necessary steps. It is advised that a comprehensive schedule be developed during the early stages of the conception of a project. During this early stage the project may be too immature to support realistic time durations but some time must be assigned to each and every component; such as planning, siteing, environmental process, permitting, right of way acquisition, preliminary and final design, bidding, contract award, construction, testing, commissioning start up and any activity or phase that is important to or has a cost for the project Owner. As the project develops and more of the actual scope and restrictions are known the schedule must be reevaluated and updated to reflect this new knowledge. The schedule development should be a living process that is used and revised constantly to be of maximum benefit to the project.
The realistic time needed to accomplish all aspects of the project must finally be reflected in the schedule. It makes no sense to handicap the tool (schedule) or the process by introducing artificial or incorrect restrictions or by putting unrealistic expectations into the schedule. In fact, these restrictions and incorrect assumptions always create problems later on in the project, usually in the form of delays, claims and higher costs. There can be a positive case made for an Owner to actually build some float time into the schedule, if possible, so that there is some way to cushion the effects of unknown occurrences that could impact the project schedule.
Unrealistic schedules sometime might result from external forces such as the desire to have a project completed in time for an upcoming event or election. These external forces always need to be acknowledged and addressed on a case-by-case basis. They can wreak havoc on a schedule, but they must be taken seriously. It should be noted that throughout a project's life, its schedule will be at the mercy of these external forces. Having said this, the best (and only) way to begin a project is with a realistic, well-thought out schedule and cost estimate. This will reduce the risk that the Owner Agency will be called on to defend a low-ball cost assumption and an inaccurate timeline necessary to complete the project. It is important to remember that the cost and schedule numbers that are initially released to the public are the ones that you will have to live with and defend throughout the project's life. It is much easier if these costs and schedules are reasonable and defendable, backed by professional experience and industry standards.
Numerous examples can be found where projects suffered from low cost and schedule pronouncements that were never achieved. In contrast, where realistic cost and schedules were developed, the Owner Agency managed the projects and was not constantly defending the numbers or the timeline. Having realistic schedules and budgets produces a "win-win" situation for both the Owner agency as well as the contractors by eliminating or at least minimizing the conflicts and finger pointing that can occur on a project that is squeezed for time and/or cash!
As the schedule of how the project is planned and built is developed, a timetable for the work also emerges. The schedule divides the work into discrete activities each with an amount of time needed for completion. Each activity is quantified with the important items of work such as linear feet of tunnel or cubic yards of concrete. Production rates are then applied to these activities and quantities. These discrete activities can then be combined in sequences that depict the way the designer anticipates the work to be constructed. These sequences can be linear or overlapping; but in the end, we have a roadmap of all the elements of the project, how they fit together and how long the project is expected to take.
Each of these discrete activities and the project as a whole are used to calculate the cost of doing the work. In the early stages of a project, these costs can be based on historical costs for similar size projects, in similar geologic conditions and in similar locations. These approximations of costs are useful for developing a potential cost for the work but, and these initial costs must not be used to develop an actual estimate.
The schedule is now the roadmap for developing the actual cost for the work. The Design Engineer should follow the procedures used by Contractors when they prepare their estimate for the bidding of the project. Typically, a contractor develops a crew of workers for each activity on the schedule. This crew is based on the work practices in the area, such as health and safety rules, where the project is located. The staffing is determined by the actual work to be accomplished, based on the local labor staffing requirements. After the crews are established the contractor will determine the productivity of the crew to accomplish the quantity of work associated with the activity This will determine the time required to do the work; or if the time is fixed, a determination is made as to how many workers are needed to perform the required quantity of work in the required time. To this labor, the contractor will add the equipment needed, the materials incorporated into the work and the materials consumed during the performance of the work.
This method is called a "bottom-up" estimate where all the components are established for each activity of work; then all these activities are combined into the total direct cost for the work. To this direct cost is added the indirect or costs not associated with any specific activity but needed for the overall construction of the project such as insurance, bonds, non union labor and costs of running the project and home offices.
By using a bottom-up estimate prepared by an estimator with some construction or contractor background, the Engineer's Estimate will be more accurate and will better reflect the true costs for the work. This is the goal.
So why is a realistic schedule important? There are several reasons. The schedule gives the Owner an expectation of when the project is to be completed and ready for use. The schedule is used to coordinate the interfaces with other construction contracts within the project or external to the project, equipment procurement contracts and other interfaces. The schedule is also used to determine the cash flow and financing requirements, such as bond sales.
A schedule is used as the basis to determine the cost of the work. Labor makes up close to 30% of the cost of a tunnel estimate, so an accurate picture of the length of time that labor will be used on the project is important to the total cost the Owner, Contractor and Public will eventually have to pay.
There is an additional benefit that comes from using a realistic schedule as the basis of the engineer's cost estimate. Once this is done then the schedule and estimate can be used to determine the magnitude of any claim proposed by the contractor (based on the contractor's schedule and compared to the costs and schedule impacts claimed by the contractor) for delays or the impacts to the budget of Owner initiated extra work.
There are different levels of cost estimates. In early stages of a tunnel project, often a decision is made that for budget level or order of magnitude estimates, a bottoms up estimate is not necessary or appropriate since the project definition is not far enough along. Instead, a quick estimate can rely on unit price methods such as $-inch foot of tunnels in similar ground conditions. However, once an unrealistic number is estimated, it often stays with the project and establishes unrealistic expectation through out the life of the project as discussed previously. The sooner an experienced construction based scheduler and estimator gets involved the better the schedule and cost numbers will be, even if the estimator needs to make assumptions on typical design details.
14.8 Claims Avoidance and Disputes Resolution
Uncertainly and change in site condition on underground projects often leads to disputes, change orders and claims. Owners usually have years to plan a project, perform geotechnical investigations needed to understand the ground through which the tunnel will be built, and deal with all the regulatory agencies and third party abutters. Contractors are in business to make money. They usually have no input to the project plans, specifications, schedule or contracts but must accept these as given and in the space of a few months come up with a cost to perform the work and beat out all other contractors bidding the work. Underground projects are expensive, linear, and sequential, so any delay to the project leads to extra expense that the contractor will look to recover from the owner.
Recognizing the uncertain nature of underground construction and the need to make the contracts fairer, the federal government has mandated the use of a differing site condition clause in underground projects. This clause says in effect, that if the ground conditions differ from what was predicted or from what reasonably could have been anticipated in similar work then the owner would recognize this as additional costs and the contractor would be issued a change order to cover a portion of this extra cost and schedule. The alternative would be for the contractor to include into its bid a contingency to cover the potential costs if an unknown or unusual event occurred. If the contractor does this and the event does not occur then the owner is stuck paying for this uncertainty. The other option the contractor has is to not include any costs for these potential occurrences but to sue the owner to recover any additional cost should a risk event occur.
How can claims be avoided? One way is to incorporate a change condition clause into the contract. This is one indication that the owner is willing to share the risk on the project. Risk should be given to the party to the contract that is in the best position to control the risk. More and more owners are recognizing that they own the risk of the underground.
Another indication of the owner's stance on risk sharing to a contractor bidding the work is how the contract is worded in areas like, time related impacts of delays caused by the owner of outside agencies or third parties. Contracts that indicate that there will be "No Damage for Delay" make too plain to the contractor that the owner is not willing to share risk but is actively looking to transfer to the contractor all risks that they are not legally required to retain.
14.8.1 Disputes Resolution
Since disputes are inevitable in underground construction: how should they be dealt with? Suffering with these same issues the practitioners of underground construction got together and in 1974 produced a manual dealing with, "Better Contracting Practices for Underground Construction". This publication contained 14 recommendations to improve the way underground projects were managed. One of these recommendations was the use of a Disputes Review Board (DRB) and the use of Escrow Bid Documents.
A Dispute Review Board is usually a trio of underground experts experienced in the design and construction of underground projects that are brought together by both the owner and contractor to, on a regular basis, become familiar with the project, its progress and problems and to offer their opinion about who is right and wrong in any disputes that arise on the project that cannot be settled by the contracted parties. These "three wise men" as they are sometime referred to, must be impartial and have such standing in the underground in industry that their decisions are accepted.
In any dispute that the DRB is asked to weigh in on, both sides are allowed to lay out their positions and refute the positions of the other side. The DRB is allowed to ask questions and evaluate the "evidence" supplied by both sides. Usually the DRB issues a written decision that then is used as the basis of settling the dispute. One of the side benefits of using a DRB is that often contractors will work hard to reach a settlement with the owner instead of going to the DRB and in fact the presence of a DRB will prevent a contractor presenting frivolous or questionable issues to the DRB so as not to look bad to their peers.
Escrow Bid Documents was another recommendation in the Better Contracting Practices publication. An owner will require that all bidders submit with their bids or the low 3 bidders submit within several days of submitting their bids, all the documents, quotes and other information that the bidders used to produce their bids. These documents usually must conform to minimum formats and are sealed. The owner and the low bidder then open the sealed documents to ensure that all the required information is present and if not the additional info is then added. The complete documents are then sealed and stored with an independent agent. The documents are then available if there is a dispute and can be opened in the presence of both owner and contractor, to determine what was and was not included by the bidder in the cost at the time of the bid. After the project is complete the Escrow Bid Documents are returned to the contractor.
There are other methods of dispute resolution used to help settle issues that arise on underground projects, arbitration and mediation to mention a few.
14.9 Risk Management
By its nature, risk sometimes defies definition, and the most onerous risks are those that were not anticipated by designer, contractor, owner or by anyone else. A well structured risk management process will anticipate, to the extent possible, the potential risks, weigh their probability and effects, and plan for handling the risk to the degree necessary to de-risk the project through every phase from conception to completion. The project owner who does not use risk management often fails to control the cost, schedule, quality and safety of the work.
The origins of risk in tunneling and underground construction often stem from unanticipated obstructions, natural or manmade, soil and groundwater conditions differing from those anticipated; ground behavior differing from that ordinarily expected; and misinterpretation of ground conditions leading to the choice of inappropriate construction methods or equipment. Analysis of historical records, photos and maps, as well as a comprehensive geotechnical investigation plan and other exploratory work, help determine the ground conditions along the tunnel horizon and location of existing or abandoned structures along a tunnel alignment, thereby reducing risk. Administrative risks (e.g., site unavailability for external reasons) are as important to eliminate. Interface risks between adjacent contracts, including items such as potential for late delivery of site or facility by one contractor for use by another, are another type of risk that can derail a construction schedule. Underlying mitigation for risks on tunneling projects include design of features that reduce or eliminate the identified risk; selection of tunnel alignments that, where possible, avoid adverse ground conditions or avoid above ground sensitive structures; specification of minimum requirements for methods of tunneling and shaft construction coupled with monitoring and controls to be implemented during construction that identify adverse trends and warn against impending risks.
Risk assessment, risk analysis and risk management are required to assure the project is kept on schedule and within budget, and to provide greater accuracy in the application of project contingency. A comprehensive risk management process includes the use of risk workshops, development of an "actionable" risk register, risk analysis and the development of risk management and action plans. What's important is early identification and communication of potential risk factors that might create delays and bottlenecks, followed by proactive management of threats to cost and schedule adherence and to identify opportunities for improvement (as shown in Figure 14-7).
Typically risk management starts by an owner and design engineer conduct a risk workshop in which all participants are encouraged to write down any and all events that could happen on and to the project and that could have impacts on the cost, schedule, quality, viability and/or safety of the project. In addition the participants need to try to determine the owner's risk tolerance. What is insignificant, tolerable and intolerable to the owner for each of the major drivers of the Project? The Owner's risk tolerances must be categorized on some scale so that they can be compared and weighed against cost drivers. On the schedule is 1 day delay acceptable? Is a week or a month tolerable? Is several months intolerable? The same for costs, depending on the size of the project, is $5M tolerable? Is $50 M intolerable? A scale or matrix (Figure 14-8) must be developed that rates risks consequences from inconsequential all the way to unacceptable so that choices can be made as to which to ignore, which to watch, and which to deal with or eliminate. These matrixes can be a 3x3, 5x5 or even 10x10. The more categories contained in the matrix the more effort is needed to manage this technical phase of the risk management process.
Figure 14-7 Risk Management Process
Figure 14-8 Typical Project Risk Matrix
A risk register is used as a way to catalogue the events that might happen on the project, and the probability and consequences if they occur. In addition it is also a tool to compare the risks, catalogue and mitigation measures chosen by the project team to either lessen the probability that the events occur or to lessen the consequences should they occur. The register also allows the project to keep track of all the mitigation efforts and the residual risks that remain. Knowing these residual risks allows an owner to then decide what to do with these residual risks. Residual risks can be accepted by the owner, passed on to the contractor, given to the insurance or bonding companies or can be candidates for additional mitigation. Once these events are catalogued then the workshop participants are asked to identify the probability that these events actually happening and if they happen, what would be the consequences or impacts on the project's cost, schedule, quality, viability and safety. Risk is actually the possibility of an event happening times the consequences that occur should the event happen.
The risk management process forms the basis of design development, accurate cost estimates and development of confident construction schedules. Risk Management and Action plans are developed based on the residual exposure after the anticipated reduction of the risks have been achieved. Costs can then be attributed to the mitigation of these risks. However, the process does not stop there. Through each phase of the project identified risks should be further evaluated in terms of ultimate risk exposure in schedule uncertainty, monetary value, probability and mitigation costs. Figure 14-9 illustrates the risk management process throughout phases of a project cycle.
Figure 14-9 Risk Management throughout the Project Cycle
On complex projects design support technologies such as virtual design and construction (VDC) combined with risk management and risk analysis software provides added value in managing risk in the design phase and during construction. Using virtual design and construction and risk analysis models project managers are be able to visualize the impacts of unmitigated risk on the project, perform interference checking and clash detection to mitigate risk and control schedule overruns. Project managers can see or experience the project in a highly visual, consistent and interactive manner, and individual teams can drill deeper into the modeling database to evaluate specific project elements, options, layers, disciplines and construction phases of any contract package or combination of packages, that will support critical decision making and mitigate risk. By combining the attributes of VDC and risk analysis projects can avoid costly design and construction errors before they happen and improve communication and coordination during construction. A collaborative risk analysis and VDC approach to risk management takes the guess work out of the project.
Once the underground facilities are in place, some might suggest that most of the risks have been overcome, and the facility will operate for its scheduled life as planned. This is only true only if certain operational risks are mitigated. In fact, the long-term consideration of the operational risk for a tunnel sets a number of design criteria for the works.