A Geotechnical Baseline Report —GBR— It is a technical-contractual document that defines the assumed geotechnical reference conditions for designing, putting out to bid, and executing an underground construction project. In projects involving microtunneling, is used to determine which soil conditions, water table levels, risks, and excavation parameters are considered foreseeable, and which situations could be treated as different or unforeseen conditions during construction.
In projects of pipe ramming, microtunneling in terrestrial and subway applications, Direct Pipe o infrastructure crossings, the GBR helps reduce ambiguities between the client, the engineering team, the contractor, and the construction management team.
What is the purpose of GBR in microtunneling?
The GBR does not replace the geotechnical report; rather, it translates it into Reference Conditions for the Project. Its value lies in the fact that it allows technical and contractual decisions to be made on a common basis.
In microtunneling, GBR helps define:
- What types of terrain are expected along the route?.
- What groundwater level or water pressure is considered the reference value?.
- What assumptions are made regarding the presence of rock, boulders, mixed soils, abrasiveness, or permeability?.
- What type of ground conditions are considered compatible with the planned tunnel boring machine?.
- What risks should be included in the proposal?.
- What conditions might require a technical review, a change in methodology, or a contractual adjustment?.
What information should I include?
A GBR applied to microtunneling should include, at a minimum:
Reference geological and hydrogeological conditions: stratigraphy, lithology, changes in rock type, water table, permeability, water pressures, and expected variability.
Relevant geotechnical parameters: strength, grain size, cohesion, angle of friction, abrasiveness, presence of boulders, rock, fill material, cavities, or mixed soils.
Expected excavation conditions: face stability, excavability, behavior of the excavated material, expected thrusts, soil-pipe friction, and lubrication requirements.
Identified geotechnical risks: boulders, blocks, karst, cavities, abrasive terrain, saturated soils, abrupt changes in strata, subsidence, water ingress, or fine-grained material transport.
Implications for the selected technology: compatibility with open shield, EPB tunnel boring machine, hydro shield or other excavation methods.
Metrics and Change Management Criteria: a definition of which conditions are included within the intended scope and which situations could be considered deviations from the geotechnical baseline.
Difference between a geotechnical report and a GBR
| Document | Main function | Use in Microtunneling |
|---|---|---|
| Geotechnical Report | Describe and interpret the study area | It provides surveys, tests, profiles, and technical specifications |
| GBR | Defines the contractual geotechnical reference conditions | Helps with bidding, risk allocation, and managing unforeseen circumstances |
| Construction Project | Define the technical solution to be implemented | Includes route, manholes, piping, method, and controls |
In practical terms, the geotechnical report states “What has been studied and how the terrain is interpreted”. The GBR defines “What conditions are taken as the basis for designing, proposing, and implementing”.
How It Is Used During the Bidding Process
During the bidding process, the GBR allows bidders to evaluate the project using consistent criteria. This is especially important in microtunneling, where small changes in geotechnical conditions, water, abrasiveness, or the presence of boulders can affect the type of tunnel boring machine, performance, tool wear, shaft design, and cost.
A well-defined GBR helps:
- Compare bids on the same technical basis.
- Reduce price uncertainty.
- Avoid conflicting interpretations of the terrain.
- Identify included and excluded risks.
- Justify contingencies or design alternatives.
- Improve the preparation of RFQs and contract documentation.
How it is used during execution
During construction, the GBR serves as a reference for comparing the actual conditions encountered with the anticipated conditions. If significantly different conditions arise during pile driving—for example, more boulders, higher water pressure, more abrasive rock, cavities, or unexpected changes in strata—the GBR helps assess whether these are to be expected or represent a significant deviation.
When running, it can be used for:
- Check feed parameters.
- Interpret changes in thrust, cutting torque, or excavated volume.
- Assess geotechnical issues.
- Justify procedural changes.
- Activate contingency plans.
- Document claims, deviations, or modifications.
- Adjust auscultation and safety controls.
Why It's Important in Trenchless Construction
In a trenchless project, much of the route is not directly visible during excavation. For this reason, determining the geotechnical conditions in advance is particularly critical.
A GBR adds value because it connects three levels:
- Design: helps you choose the route, depth, wells, and technology.
- Offer: It allows risks and costs to be assessed on a common basis.
- Execution: It allows you to compare actual conditions with predicted conditions.
This is especially useful in projects with high water table, mixed terrain, abrasive rock, crossings under roads or railways, outfalls, water intake structures, or critical infrastructure.
Minimum Checklist for a GBR in Microtunneling
A GBR for microtunneling should include: alignment and longitudinal profile, geotechnical units, available borehole data, water table, permeability, strength, grain size distribution, abrasiveness, expected presence of boulders or rock, risks of cavities, face conditions, predicted thrusts, friction, TBM selection criteria, shaft conditions, known uncertainties, assigned risks, and change management criteria.
Request a Technical Review of Geotechnical Engineering and Reference Conditions for Microtunneling Before finalizing the design, preparing the request for proposals, or determining the most appropriate trenchless technology.
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