Vertical Engineered Barrier at Dover Air Force Base, Groundwater Remediation Field Laboratory National Test Site, Dover, Delaware

Site Name:

Dover Air Force Base

Location:

Dover, DE

Period of
Operation:

10/96 - 9/98

Cleanup
Type:

Field Demonstration

Technology:
Vertical Engineered Barrier (VEB)
- VEB consisted of cement bentonite thin diaphragm walls, installed using high-pressure jet grouting
- Project conducted in two phases - Phase 1 consisted of creating four small barrier boxes to refine grouting parameters; Phase 2 consisted of creating two cofferdams, each 34 feet (ft) in diameter, and consisting of 12 interconnecting thin diaphragm walls (each wall 13 to 16 ft long)
- The cofferdams were constructed by jetting a cement/bentonite slurry 7 ft into the underlying clay confining unit (a total depth of 43 ft bgs); one cofferdam was covered with a geomembrane to prevent infiltration
- The integrity of the cofferdams was tested using pump tests, pulse tests, flood tests, and geophysical techniques

Cleanup Authority:
Not provided

Technical Contacts:
Richard Landis
DuPont Engineering
302-892-7452

Allen Furth
Hayward Baker, Inc.
410-551-1980
Management Contacts:
Skip Chamberlain
DOE SCFA
301-903-7248

Scott McMullin
DOE Savannah River
Operations Office, SCFA
803-725-9596

Andrea Hart
MSE Technology Applications, Inc.
406-494-7410

Contaminants:
None

Waste Source:
Not applicable - clean test site

Type/Quantity of Media Treated:
Groundwater
- Depth to groundwater 26 ft
- The upper aquifer consists of sand with lenses of gravel and clay, and has hydraulic conductivity ranging from 2.8 x 10-3 to 1.2 x 10-2 cm/sec
- Clay confining unit present at 36 ft bgs

Purpose/Significance of Application:
Demonstration of vertical engineered barrier using thin diaphragm walls

Regulatory Requirements/Cleanup Goals:
Project objectives were to demonstrate:
- Construction of a high-pressure, jet grouted, thin-diaphragm containment barrier
- Performance of the barrier using several verification and monitoring methods
- Hydraulic conductivity performance goal of 1 x 10-7 cm/sec

Results:
- Hydraulic testing showed no defects in the upper 25 ft (unsaturated zone) of the cofferdam; one defect was identified in the lower 10 ft of the second cofferdam - the defect was associated with a thin zone of higher hydraulic conductivity
- Hydraulic testing also showed that the cofferdams met the hydraulic conductivity performance goal, at all except one location

Cost Factors:
- A unit cost of $6.71/ft2 was projected for constructing two single-wall cofferdam barriers using thin-diaphragm panels with a depth of 43 ft and a length of 40 ft; this cost did not include cement or bentonite materials, mobilization/demobilization, spoils disposal, or costs associated with a guidance tool
- A unit cost of $8.21/ft2 was projected for constructing a thin-diaphragm wall cofferdam at the Dover AFB test site, including mobilization and demobilization; costs for construction at a low level radioactive waste site were projected to be significantly higher

Description:
During fall 1997, a demonstration of high-pressure jet grouting to emplace thin-diaphragm walls was conducted at the Groundwater Remediation Field Laboratory (GRFL) National Test Site at the Dover Air Force Base. Two cofferdams were constructed, consisting of 12 interconnecting cement bentonite, thin-diaphragm walls. Hydraulic testing also showed that the cofferdams met the hydraulic conductivity performance goal of 1 x 10-7 cm/sec, at all except one location. A projected unit cost of $8.21/ft2 was identified for constructing a thin-diaphragm wall cofferdam at the Dover AFB test site, including mobilization and demobilization; costs for construction at a low level radioactive waste site were projected to be significantly higher.