Ineffective excavation of and construction on cultural resource sites can cost a project time and money. Successful efforts by the Carpinteria Sanitary District (CSD) and South Orange County Wastewater Authority (SOCWA) highlight the benefits of replacing traditional excavation methods with geoprobes to achieve more accurate, cost-efficient evaluation of cultural resource sites.
Assessment of archaeological resources may be hindered by insufficient previous recording and mapping, ground disturbances, and the presence of vegetation and/or developed surfaces, such as paving.
Projects must also consider the potential heritage value of prehistoric resources to contemporary Native Californian populations, who wish to avoid all impacts. Mitigation for sites that are not avoided can cause substantial cost and time delays, particularly if unknown resources are encountered during construction.
Ineffective Traditional Excavation
To locate and mitigate damage to cultural resources, traditional methods such as hand excavation and backhoe trenching have been employed. But David Stone, Dudek’s cultural resources manager, said that these methods are arduous and invasive because:
- pavement may need to be cut and removed before digging
- the smallest size shovel test pit is no less than 18 inches wide
- pits can only be dug to a depth of 3–5 feet, and trenches to 6 feet
- workers excavating by hand can dig no more than 8 test pits per day
Traditional methods leave large pits to fill after excavation, and may fail to locate cultural resources that lie deeper than 5–6 feet.
Efficient Geoprobe Alternative
Alternatively, the use of hydraulically driven geoprobes to extract soil core samples provides a minimally invasive and far more accurate method for locating cultural resources and minimizing damage to them. Stone said geoprobes are more advantageous because they can:
- easily penetrate paved surfaces
- drill holes only 2 inches wide
- retrieve soil core samples beyond 50 feet deep
- excavate up to 12 locations per day
The core samples returned by the probes allow examination of all soil layers and determination of the depth and width of a site, as well as its condition. Then, if necessary, alternative construction methods can be used to avoid significant archaeological sites. Stone said this method can save up to hundreds of thousands of dollars to mitigate damage caused by construction informed by inaccurate or incomplete data gleaned from traditional excavation.
“Timing is everything, and the probes provide more accurate and complete data in a shorter amount of time,” Stone said. When it is time to fill the hole, it is only necessary to plug the two-inch opening at the surface, rather than filling an entire pit.
On the CSD and SOCWA projects, engineers were able to gather sufficient data to characterize the extent and quality of archaeological resources, reaching depths that would normally require greater costs and area exposures using conventional excavation methods. The feasibility of alternative construction methods to traditional pipeline trenching was then determined, ensuring project consistency with Local Coastal Plan and Coastal Act policies.
In addition to time and money efficiencies, Stone said the use of geoprobes is highly accepted by Native Californian representatives as legitimate and useful for preserving cultural resources. With money savings, time savings, and damage minimization, it’s a “win-win-win” solution for the applicant, engineers, and Native groups.
David Stone, RPA, has 32 years’ experience managing and directing archaeological and historic preservation technical studies, and is an expert on CEQA/NEPA process implementation. For more information, contact David at firstname.lastname@example.org.