Microtunneling: Key Planning Elements to Know
A microtunneling project typically includes data collection, core testing of soil and rock samples, and a geotechnical survey, but the most important factor to consider is the line and grade of the trenchless project.
The most important factor to consider is the line and grade of the trenchless project.
Here, we will take a look at components of the project which are essential to determining the line and grade for a microtunneling project. (Also read: The Basics of Trenchless Microtunneling Projects.)
Essential Elements of Planning
- Gather information about past projects in that area.
- Identify existing subsurface utilities and manholes.
- Establish a sightline between shafts.
- Data collection.
- Lab testing.
- Geotechnical investigation.
Knowledge about past projects in the area provides a head start in the planning process. This eliminates speculation and assumptions, and provides the ideal situation for analysis and planning.
For example, railroad embankments have been known to contain old rail pieces, fish plates and ties that can damage a microtunnel boring machine (MTBM).
If the area was under the influence of contaminating material, consideration must be given to the proper disposal of the muck.
Along the tunnel route, there may be manholes or existing utilities that may cause interference at the point of project execution.
Noting these potential issues and including them in the data while planning, will help prepare a more accurate picture of the tunnel route with minimum to no deviation.
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Having information about subsurface conditions is very important. Hiring a geotechnical engineer with the proper knowledge, and reliable equipment to carry out such an investigation, may seem like an expensive affair, but it may actually prove to be a time and cost saving decision. (Learn more about this in "Why a Detailed Geotechnical Report Means Success for Your Trenchless Project").
Strong partnering between the owner, the designers and the contractor is necessary to eliminate risks and save costs. It is necessary for the owner to provide adequate information about subsurface conditions to the contractor.
Identifiable obstructions, like piers and sheet pile walls, can be easily identified by physical observation along the proposed route. The length and location of the individual shafts should be determined depending on the scope of the project.
Long drives can prove to be time consuming and costly as they will require the use of multiple interjacking stations. Survey work to confirm the elevation should be performed at different intersecting points of the proposed microtunnel bore.
Sample Collection and Lab Testing
Collecting data from test pits and bore holes is a crucial element of planning for a microtunneling project. Ideally, initial borings should be taken at wider intervals, about 300 feet apart, and also at the launch and reception points.
In the case of notable variations in subsurface conditions, additional borings may be required.
Sometimes bucket borings, or large test pits, and auger drilling may be required if there is a reason to suspect the presence of rocks, stones and boulders nestled together that could hinder the progress of an MTBM.
Soil samples are tested in a lab as a standard procedure to help determine the type of accessories to be used with the MTBM for a specific project including screening and separation equipment.
For example, this data can be used to determine whether or not a centrifuge system is required. If the site offers rocky conditions, core rock samples should then be tested to determine inherent properties.
A geotechnical investigation is a mandatory prospectus of the size and budget of the project and includes:
Geotechnical data report
The geotechnical data report (GDR) addresses sampling, laboratory analysis, and the interpretation of site specific engineering properties.
Geotechnical baseline report
The geotechnical baseline report (GBR) provides a contractually binding, meaningful description of current geotechnical conditions expected to be encountered during the microtunneling process.
These reports, along with the bidding documents, constitute the geotechnical investigation and provide a clear picture of the scope of the project and potential expenditures, helps to reduce claims and creates a basis for shared risk.
The Work Space, Methods and Equipment
While the above planning factors are essential to determine subsurface conditions to prevent surprises, it is also necessary to choose the correct microtunneling system to carry out the work. (Find out more on trenchless tunneling in "An Overview of Trenchless Tunneling").
Before setting up the MTBM, it is necessary to look at the following:
- Ensure a safe working space is available including needed surface space for launching and retrieval shafts for the MTBM.
- Determine if the MTBM system is capable of attaining the drive lengths and depths specified in the tender.
- Determine the cutter head configuration based on the tunnel face conditions.
- Understand the method for removal of excavated material to the slurry chamber and the need for the separation of particles based on size.
A failed MTBM project can incur high costs in terms of machinery, material, labor and reputation. Planning should be the first concern in any project, especially when dealing with subsurface construction that relies heavily on unseen subsurface data, past construction outcomes and current utility mapping.
Gathering this data should be foremost in planning to prevent accidents, such as drilling through gas pipelines, existing sewer lines and cables.