• RECON utilized an excavator mounted on swamp pontoon tracks to install several drainage ditches in excessively soft sediment/ sludge. This coupled with simple sump dewatering techniques proved to lower the water elevation in the sludge, forming a two-foot thick crust. This allowed low-ground pressure dozers to traverse most of the area. Other very soft areas within the impoundment were stabilized using fly ash, a by-product generated from a local power plant. Long-stick excavators were used to mix the fly ash with the sludge. By excavating drainage ditches, RECON used 50% less fly ash during the project. In addition, we were able to use normal strength bi-axle geogrid in lieu of high-strength.
• Clean sand was to be placed above geogrid for purposes of serving as a pressure-relief layer and GCL bedding layer. Since the drainage ditches were installed, sand was no longer required to serve as a pressure-relief layer. RECON substituted a more readily available sandy clay [60-40] material in place of the sand. RECON crews installed both geogrid and GCL.
• In order to speed-up the construction schedule to receive a minimum of 200 loads of dirt per day, RECON developed and operated a borrow pit and purchased 17 dump trucks. With the help of local hauler, RECON was able to haul in excess of 400 loads per day to UNOCAL.
• Last but not least, our field crews make the difference. Much of our success on this UNOCAL project was directly attributed to their ingenuity and willingness to get the project done in a safe and quality manner, productively.

• Stabilize approximately 300,000 cubic yards of sludge, and
• Design and place an engineered composite drainage cap.

Phase 1: Sludge Stabilization
The stabilized reagent was a mix of Class C flyash, cement and bed ash. Prior to the work, RECON performed several test studies to determine the ratio of reagent components that would best meet the performance requirement of an unconfined comnpressive strength of 25 psi at 28 days in accordance with ASTM D2166.

The sludge was solidified in place to an approximate depth of 12 feet using five track-mounted excavators equipped with reagent injectors and special rake attachment sludge mixers which, when used correctly, also control the reagent dust problems inherent to this type of solidification work. Depending on the density and physical characteristics of the encountered sludge, a customized bulk material deployment tent was also used for controlled off-loading of the reagent, which was then carefully placed and mixed with the insitu sludge using a standard excavator bucket. During the solidification process, RECON maintained a zero air or sediment release. Following solidification, the material was excavated and re-graded to form a sub-base in preparation for the Phase 2 construction of the engineered cap.

Phase 2: Drainage Cap Construction
Construction of the engineered cap consisted of:

• Placement of a one foot thick cohesive soil barrier, with a hydraulic conductivity less than or equal to 1x10-5 cm/sec, over the re-graded material.
• Compaction of the soil barrier to meet a requirement of at least 95% by dry unit weight, as measured by ASTM D698.
• Installation of a 40-mil polyethylene geomembrane liner.
• Placement of a non-woven geotextile fabric, a one-foot thick granular drainage layer and a second geotextile fabric.
• Placement of a six-inch protective soil cover, followed by six inches of topsoil in preparation for seeding.