Logistics Is Key During Pre-Design For Linear Pipeline

By Dale M. Smith, P.E. Collinsville Branch Manager, Geotechnology Inc. | November 2010, Vol. 65 No. 11

Observations in the overland borings focused on concerns related to trench excavation and backfill such as depth to bedrock within the excavation zone, groundwater level, soil stratigraphy and corrosion potential. Laboratory testing included index tests and soil corrosivity tests. Index test results are used for soil classification and in situ characteristics. Corrosivity test data are used in assessing buried pipe material type and the need for or extent of corrosion resistant systems to protect the pipeline. Environmental-related construction permits included strict controls on replacement of top soil in trench backfill. Therefore, boring logs included topsoil thickness at each boring. During construction this material will be set aside and replaced as the final course of trench backfill.

For the HDD alignments, two borings were typically located on each side of the feature, such as a railroad crossing, highway or river, and were cored into bedrock to an elevation approximately 10 feet below the proposed HDD placement. One boring was near the entrance point, one each at the one-third and two-thirds points and one at the exit point. The borings at the third points were near where the vertical HDD alignment was deepest. Physical access features led to field adjustment of boring locations from the ideal positions.

In the deliverable reports for the HDD sites, Geotechnology provided foundation recommendations and addressed footing loads from the directional drill and associated equipment. Design and construction recommendations for the mainline overland sites included trench excavation considerations, suitable trench backfill recommendations and temporary shoring recommendations.

Getting the work done
It was important to understand the client’s schedule for the various aspects of their project and the work was planned accordingly to complete the 500-plus bores. Twenty-four reports were delivered as produced over a 1½-year period. Organization of reports fell into two categories: site-specific HDD reports and per-county reports for the overland borings. The field work and report preparation and delivery was executed in the sequence preference identified by the owner.

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