From A Mixed Face to A Full Face of Rock: Northeast Remsco Delivers in High-Strength Rock Northern Virginia Project for a Private Client.

A contractor was hired to construct approximately 70’ of 36” steel casing to install a water main beneath existing wetlands for a new development. The ground was expected to be competent rock with an Unconfined Compressive Strength (UCS) of 16 ksi or less. The contractor expected to self-perform the casing installation using a Robbins Small Bore Unit (SBU) fitted with a rock head.

As the contractor prepared for the crossing, it was discovered that the ground was actually a mixed face condition. The top 20% of the face was not competent rock. Together, the limited cover (~24”) and the mixed face condition would make it very difficult to maintain grade using the SBU. The contractor investigated further and found that the competent rock in the balance of the rock face was tested at 24 ksi, significantly higher than predicted. Furthermore, the strength was expected to increase if the profile were lowered to facilitate a full-face rock condition for the SBU.

Ultimately, the client agreed that lowering the crossing to engage a full face of competent rock was the best way to address the change. However, this raised the question as to whether the SBU would be able to excavate the stronger rock. With this unknown, the contractor offered to complete the casing installation on a time-and-material basis, with no guarantee of how long it would take to complete.

This was not an ideal situation for the owner, so the contractor reached out to Northeast Remsco (NRC) to discuss alternate ways to install the casing. After several conversations, during which the owner was educated on the available options, NRC was hired to use its Geonex HZR-1200 horizontal hammer boring machine to install the casing. Unlike pipe jacking, the horizontal hammer boring machine excavates the hole in the rock along the design line and grade while pulling the casing in behind it.

The jacking pit entry wall was already an established vertical rock face, which facilitated the launch. But as the casing progressed, it became apparent that the 24 ksi rock compressive strength was becoming much higher than the 24 ksi that was identified.

The actions of the hammer boring machine were more consistent with encountering 40 – 45 ksi material. There were no rock chips. The hammer was no longer breaking the rock; it was pulverizing the rock. This created a dust problem, so water was introduced into the air circuit for dust control. Unfortunately, this created another problem: the addition of water turned the rock dust into a weak concrete slurry. Our crews had to clean the air system continuously or else the wetted dust would have contaminated the air lines.

Additionally, we initiated the casing installation using two 1,350 CFM air compressors. The strength of the rock and resulting slow progress compelled us to add a third compressor, for a total available airflow of approximately 4,000 CFM. This improved the production considerably.

Ultimately, Northeast Remsco, using the horizontal hammer, successfully delivered the job to the project owner, proving it was a good choice.