Dear Elected Officials of Charlestown, Walpole, and Claremont:
Work to construct the soil nail wall was halted due to movement of the rail lines and excessive groundwater at the construction site. The movement, recorded by the devices installed prior to construction beginning, caused the railroad to lower speeds to 10 mph for the Amtrak and freight trains.
The attached photos of the soil nail wall construction depict the issues created by the excess water. And while it may seem logical to de-water (removing the excess water by pumping), dewatering could lead to additional settlement of the rail tracks. De-watering may remove silty soil with the water, as well as creating more instability by leaving voids in the railroad embankment.
The Department has been actively working with the Railroad, the consultant, and the contractor to develop the best approach to repair Charlestown Rt. 12 with the poor soil strengths, high groundwater, and settlement experienced. As the contractor was conducting initial tests, it was found the soil did not have the required strength. In order to stabilize the embankment without dewatering, other more extensive retaining wall options were conceptualized; however all were determined to be cost prohibitive.
The Department is proposing a two-phased approach with a short-term and a long-term solution. The short-term solution would reopen Charlestown Rt. 12 in the interim. This approach was discussed last week with the Railroad, which was amenable to this short-term solution. Under the short-term plan, the current soil nail wall will be discontinued. The embankment will be stabilized. The road will be built back up with fill and gravel and paved so that the two-lane section of Charlestown Rt 12 can be re-opened. Although the Department considered opening a one-lane roadway, any future work to construct a second lane would require closures. Additionally, the roadway is not wide enough for construction equipment, workers, and vehicles to pass safely. (Attached photos show the constrained space.) It is a relatively short section of road to build back up and should proceed fairly quickly.
The re-opening of Charlestown Rt 12 is considered a temporary solution, as the conditions that caused the road closure still exist and poor soils will not be removed. The Department will install monitoring equipment to observe the conditions but cautions that the same situation could arise with another major weather event. The short-term solution would be the route until the long-term solution can be designed and implemented. This will likely involve full reconstruction and consideration of realignment alternatives to increase the distance between the road and the Railroad.
The long-term solution will be investigated within the comprehensive Ten-Year Plan project. The Department has already selected a consultant and initiated the scoping to determine the alternatives available. The efforts to date will be valuable in weighing the alternatives and investigations to determine the best location for NH Route 12.
A change order will be required, and the Department and consultants are working diligently to prepare revisions and minimize the contractor’s downtime. The Project Manager will meet with Town officials and other interested parties for further discussions on schedules, cost, and other pertinent information.
I will update you on further developments in the next 7-14 days.