Snowdonia Visual Impact Provision (Snowdonia National Park, Wales)

Ground Modeling Optimizes TBM Launch Design in a Sensitive National Park

The Snowdonia Visual Impact Provision (VIP) project involves relocating a section of the existing overhead transmission line between Pentir and Trawsfynydd substations into a 3.4 km-long TBM-driven tunnel. This work significantly improves visual amenity and landscape character within Snowdonia National Park. During early design, an opportunity was identified to optimize the proposed Garth Sealing End Compound shaft. This alternative approach improved constructability, TBM launch efficiency, and safety during tunneling.

Project Scope:	Geostatistical modeling of rockhead conditions to support shaft optimization and TBM launch design
Unique Features:	Major visual impact mitigation project within a protected national park, layered 3D model
Our Services:	Geostatistical modeling, 3D ground model development, uncertainty assessment, and TBM launch risk evaluation.
Emprise Personnel:	Geotechnical Engineer: Jacob Grasmick

Impacts & Benefits

• Improved TBM Launch Safety: Reduced uncertainty in shallow ground conditions supporting safer launch and early tunneling operations.

• Optimized Shaft Design: Enabled adoption of a shallower, rectangular cofferdam solution that improved constructability over a conventional deep shaft.

• Risk-Informed Ground Treatment: Supported development of slurry pressure balance and grouting strategies to manage split-face and shallow soil conditions.

• Sensitive-Environment Delivery: Helped enable undergrounding of transmission infrastructure within a protected national park while minimizing construction risk.

Project Gallery

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Project Summary

The Snowdonia Visual Impact Provision (VIP) project involves relocating a section of the existing overhead transmission line between Pentir and Trawsfynydd substations into a 3.4 km-long TBM-driven tunnel. This work significantly improves visual amenity and landscape character within Snowdonia National Park. During early design, an opportunity was identified to optimize the proposed Garth Sealing End Compound shaft. The original concept—a 15 m diameter, approximately 30 m deep circular shaft—was revised to a longer, shallower rectangular piled launch box cofferdam (partial drift). This alternative approach improved constructability, TBM launch efficiency, and safety during tunneling. Advanced ground modeling was critical to managing geotechnical risk while enabling a safer, more efficient construction approach in a highly sensitive environment.

Project Writeup

National Grid engaged Emprise Concepts in partnership with Maxwell Geosystems to develop a layered, three-dimensional geostatistical model of rockhead conditions beneath and ahead of the proposed cofferdam structure to support TBM launch planning and risk management. The modeling approach systematically incorporated sequential geotechnical investigation data, beginning with early GI phases and progressively integrating CPT results, additional boreholes, and topographic and construction-derived data from piling (including pile refusal and bedrock encounters). The model was extended beyond the cofferdam to assess likely rockhead conditions ahead of the TBM launch, informing expectations of transition to full-face rock and supporting slurry pressure balance and grouting strategy development. By explicitly modeling both rockhead geometry and uncertainty, the work provided National Grid and the project team with a defensible basis for TBM launch design, ground treatment planning, and construction risk mitigation in shallow, variable ground conditions.