Influence of geologic setting on microtunneling

Abstract

A thorough and accurate characterization of geologic setting and definition of geotechnical conditions is critical to the planning, design and construction of microtunneling projects. Geologic setting controls soil, bedrock and groundwater conditions. An understanding of where subsurface materials originated (provenance) and how and in what setting they were transported and deposited (paleoenvironment) and their subsequent burial, compaction, consolidation, cementation, erosion, and structural deformation are key elements in characterizing geologic setting. Specific unique aspects of microtunneling impacted by geologic setting and geotechnical conditions include: cutter-head configuration and tunnel face stability; cutter-head performance, steering corrections and line and grade control in mixed and changed-face conditions; cutter-head abrasion; crushing chamber configuration for oversize materials; spoil handling; slurry and slurry additives; mechanical slurry separation systems; hydrofracture and slurry loss; overcut stability; lubrication and pipe friction loads; jacking forces and drive lengths; intermediate jacking stations; thrust block and reaction wall capacity; pipe selection; long-term jacking and receiving pit stability; and ground settlement. Understanding the unique aspects of pipeline installation by microtunneling and how subsurface conditions are controlled by geologic setting is a critical step to successful planning, design and construction of all microtunnel projects.