The Ground is Speaking: What the Howard Hanson Dam Rocks Mean for Construction

Listening to the Rock    

The bedrock at the HHD site is composed of Tertiary volcanic units – Andesite, Pyroclastite and Basaltic Andesite. While broadly categorized, these formations exhibit overlapping features and varying degrees of weathering, which adds natural complexity to interpretation during field logging and core review.     

What makes this site particularly challenging is its variability. In some areas, the rock is soft and weak, breaking down easily when exposed. In others, it transitions sharply – from dense and intact to fractured material. These differences can occur over very short distances, reinforcing the need for flexible planning and adaptable construction approaches.    

Beyond material type, the core samples reveal important structural features: joints, shear zones and contacts between units. Together, these form a network of discontinuities that influence excavation and support decisions.    

The Value of Geotechnical Characterization    

For the construction team, geotechnical characterization is more than technical — it’s central to constructability, safety and schedule planning.     

• Excavation Methods: Depending on rock hardness and excavation geometry, a range of approaches – from mechanical excavation to controlled blasting – may be necessary. Steep-cut geometries and proximity to existing infrastructure require careful consideration of equipment limits and sequencing.    
• Ground Support: Varying rock conditions call for tailored stabilization measures, including spiles, rock bolts, steel ribs and shotcrete. One focus area is the rock pillar between the planned excavation and the existing outlet tunnel – an element identified for close monitoring and potential reinforcement to support excavation safety and tunnel protection.    

• Groundwater: Groundwater management is a significant factor. Perched or pressurized zones can impact stability and excavation performance. Planning for effective dewatering is a priority.     

   

Tools, Models and Interpretation    

The technical team, led by the U.S. Army Corps of Engineers and supported by Shannon & Wilson, utilized a suite of established tools and methods to characterize the subsurface. These include drilling and core recovery, empirical classification systems (RMR, Q), structural mapping and laboratory testing.    

Technology played an integral role throughout the broader design process. State of the art tools assisted the design team to simulate failure mechanisms, assess rock mass behavior, and understand stress redistribution in critical areas like the tunnel-adjacent rock pillar. These analyses help translate geologic observations into engineering data for design and construction planning.    

Managing Uncertainty     

Even with extensive investigations, subsurface conditions always carry some degree of uncertainty. No program can identify every fracture, joint or groundwater pathway in advance. Managing this variability is part of every complex project and requires close coordination among the owner, designer and builder.    

We don’t just observe the ground – we interpret it, challenge it and adapt to it. Progress depends on understanding the complexities beneath our feet and using that knowledge to build safely and wisely.  

The ground is speaking at the Howard Hanson Dam, and the team is listening.