Kaneohe/Kailua, HI

Kaneohe/​Kailua Wastewater Conveyance Tunnel


The Kaneohe/Kailua tunnel is a 16,000-foot long (4,877 m), 10-foot (3 m) internal diameter storage and conveyance tunnel that was designed to replace a 42-inch (1,070 mm) internal diameter force main between the Kaneohe Wastewater Pretreatment Facility (WWPTF) and the Kailua Regional Wastewater Treatment Plant (WWTP). The new tunnel provides storage for peak wet-weather flows, thereby preventing spills that would occur when wastewater flows exceeded the capacity of the previous Kailua plant.

Scope of Work

As the lead tunnel geotechnical engineering firm for the project, Delve Underground provided design and construction support services for this $194 million conveyance/storage tunnel. Work performed included analysis of alternative alignments, design of deep shafts for construction, tunnel construction and lining evaluations, construction cost estimates, and risk assessment.

The scope included the development of the geotechnical baseline report (GBR) for the tunnel and deep shaft. The GBR included detailed mapping of the rock mass, assessment of the kinematic stability relationship of the major blocks with the alignment of the tunnel and shaft, and ground characterization. The GBR outlined support requirements including rock bolts and shotcrete.

Challenges & Innovations

During design, it was learned early that ground conditions along the tunnel alignment would be challenging for construction. More than 95% of the tunnel was expected to be in strong volcanic rocks, which are great conditions for hard rock tunnel boring machine (TBM) mining. However, the last 5% of the tunnel, or about 1,000 feet (305 m), was highly variable, with conditions ranging from soft mud to dense sands and gravels to highly weathered rock that could not be excavated by a hard rock TBM. To be able to construct the remaining 1,000 feet of tunnel, jet grouting was used to stabilize the ground prior to tunneling, and conventional” tunneling methods (excavators and roadheaders) were used to mine the last stretch of tunnel. This simplified the construction approach, reduced the significant costs associated with deep open-cut excavations, and eliminated the need to mobilize additional specialized tunneling equipment.

Fast Facts
  • 16,000-foot-long (4,877 m) tunnel
  • 10-foot (3 m) internal diameter tunnel
  • Tunnel slopes from 39 to 77 feet (12 to 23.5 m) below ground level