Boulders in Tunnel Boring - Alleged DSC
---on behalf of owner---
claimed having encountered unanticipated boulders, one of which is illustrated
An examination of the
anticipated and encountered conditions revealed that the contractor actually
encountered a shorter length of tunnel containing boulders than indicated in the
geotechnical information and experienced on a previous section of the project as
Furthermore, the contractor actually made better progress when excavating in
tunnel sections with boulders as illustrated below.
In fact, most of the delays suffered by the contractor were associated with
replacing broken lagging as a result of rib spacing beyond the capacity of the
lagging used as show below. A membrane behind the lagging reduced the skin
friction between lagging and soil, thereby placing the TBM's forward thrust
force into the lagging, forcing it to act as a beam in compression.
CONTRACTOR WITHDREW CLAIM FOR TUNNEL BOULDERS AND SETTLED
FOR ORIGINAL OFFER FOR SHAFT BOULDERS