Settlement Risk Assessment and Monitoring for Direct Pipe® Construction Beneath Critical Infrastructure

Direct Pipe® (DP) is an innovative trenchless construction method that combines aspects of micro-tunneling and horizontal directional drilling (HDD). In the DP method, ground excavation and pipeline installation are simultaneous. In this method, the jacking frame in micro-tunneling is replaced with a pipe thruster and coupling is eliminated. Additionally, reaming the bore hole, which is common with the HDD method, is eliminated. The DP method of construction is typically applied when subsurface conditions beneath a surficial obstacle are not conducive for the HDD method of construction due to final hole diameter, granular soils, and/or high risks of hydraulic fracture leading to inadvertent drilling fluid returns. During tunneling operations, the microtunneling boring machine (MTBM) excavates a space or tunnel that is typically a few to several centimeters larger in radius than the jacking pipe; which is thrust into place continuously behind the TBM. This overcut annular space and the potential, occasional temporary extraction of the MTBM and jacking pipe represent settlement risks during construction. For this reason, the pipeline construction team should consider assessing settlement risks associated with DP construction, and subsequently prepare and deploy a settlement monitoring plan during the pipeline construction phase, particularly where the proposed DP alignment will cross railroad, highway, and/or other sensitive features that may be adversely affected by settlement. This paper presents an adopted settlement risk assessment and monitoring plan for a recent trenchless crossing where the DP method of construction was utilized to install a pipeline section beneath two major interstates and a railroad outside of Washington D.C., United States. The paper also includes a Finite Element Analysis (FEA) using Plaxis software with the purpose of exploring the time variable in settlement analysis. Furthermore, the paper expands on the planning characteristics of settlement monitoring utilized during construction, which consisted of conventional surveying and vehicle-mounted LiDAR scanning.