HDPE Vertical Barriers
Junction, UT: Bio-Polymer Slurry Drainage Trench, Piute Dam
This project was awarded the Association of State Dam Safety Officals 2005 "National Rehabilitation Project of the Year".
Trench excavation was accomplished with a 3-foot wide bucket specifically designed for difficult excavations, at the end of an approximately 50 foot long "stick" attached to the boom of a Komatsu PC750 hydraulic excavator. Excavation rates for the approximately 30 foot deep trench were about 60 feet per day.
Fresh bio-polymer slurry ranging in viscosity between about 60 to 122 Marsh seconds, and averaging about 80 Marsh seconds, and consisting of approximately 50 percent powdered guar bean and 50 percent synthetic polyacrylamide solution mixed with water and small quantities of other chemicals used to buffer the pH and extend slurry life was used to temporarily support trench walls. Slurry levels were maintained approximately 3 feet below the top of the trench during trench excavation and backfilling. Overnight slurry losses generally amounted to a few inches, and never exceeded 11 inches. In-trench slurry viscosity ranged from 64 to 132 Marsh seconds, with an average of about 98 Marsh seconds.
Protective non-woven geotextile panels were placed in overlapping fashion, followed by 60 mil HDPE sheets as soon as sufficient trench had been excavated and cleaned to preclude operational conflict. Bottom layer drain materials, approximately 14 feet thick were placed by tremie following geotextile/geomembrane placement and consolidated using a 2-inch diameter concrete vibrator. The vibrator was inserted the full depth of the filter/drain material at 1.5 foot intervals, in an off-set pattern approximately 8 inches from each face of the trench excavation.
Drain pipeline placement was accomplished with each pipeline sequence fully assembled, including attaching weights, before being suspended in the trench. Setting/positioning these pipeline sequences (approximately 100 feet in length) in the trench was accomplished using 3 trackhoes. Pipelines were held in vertical alignment by cables attached to pipeline weights and then to 4X4's spanning the trench.
Drain materials were placed to depths of approximately two feet over the pipeline using a tremie to bed the pipe and secure both vertical and horizontal alignments. Following that, the tremie was used to build up a drain material slope at one end of the pipeline, after which drain materials were introduced to the trench using a front-end loader. Upper layer drain material consolidation was completed the same as described above for the lower layer.
Pumping wells were installed during drain material placement and were used during slurry degradation phases of the work to air lift fluid from the bottom of the trench to the surface. Chemicals to assist degradation were primarily introduced at the top of the trench and then recirculated by pumping/air-lifting until a Marsh funnel viscosity of less than 30 seconds, and a neutral pH were obtained.