BioPolymerTrench

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BIO-POLYMER DRAIN TRENCHES
TECHNOLOGY OVERVIEW

TABLE OF CONTENTS

INTRODUCTION
APPLICATIONS
METHODS
ADVANTAGES
CONCLUSION

INTRODUCTION

Bio-polymer (BP) trenching is an innovative technique for installing permeable drainage trenches. BP trenches use synthetic biodegradable polymer water-based slurry to support the trench walls. This method provides several advantages over traditional construction techniques, particularly for deep applications that extend below the groundwater table. Additionally, the BP slurry stabilizes the excavation walls resulting in a safer project.

APPLICATIONS

BP trenches were first constructed in Europe for civil applications, such as long-term dewatering, to increase soil strength and to improve slope stability. Since being introduced in the United States during the late 1980’s, the technique has become widely used for environmental applications, such as the installation of collection systems; recharge systems; air sparge trenches; and reactive walls.

METHODS

BP trenching is a variation of the “slurry trench technique”, which involves excavating a narrow trench that is kept full of slurry. The slurry exerts hydraulic pressure against the trench walls and acts as shoring to prevent collapse. Traditional slurry trenches are excavated under bentonite or other clay slurries that form “filter cakes” on the trench walls and reduce the permeability of the soil. Conversely, BP trenches are excavated under biodegradable slurries that do not affect the final permeability of the in-situ soil or backfill materials.

Polymer is combined with water in a colloidal mixer and the resulting BP slurry is pumped through a pipe to the excavation. A variety of polymers can be used (guar gum, xanthan gum, and polyacrylamides), each having its own unique advantage.

BP trench excavations are typically performed with hydraulic excavators, with widths varying from 1.5 to 3.0 feet. Excavation depths in excess of 100 feet are possible with this technique. In some cases, the excavation will “key” into a low permeability stratum, such as clay or bedrock to allow the collection of dense liquids or materials dissolved in the groundwater. In other cases, the excavation penetrates the groundwater table and is used to collect floating contaminants. As an excavation segment is completed, the excavator backs up and begins a new overlapping segment. This creates a continuous trench. After sufficient excavation is complete, appurtenances are placed in the trench.

The BP technique facilitates the installation of manholes, sumps, and perforated horizontal or vertical piping, which allows extraction, collection, recharge or injection of contaminants in support of remediation activities. In addition, BP trenches can be used for in-situ remediation processes including air sparging, carbon adsorption, and chemical treatment such as dechlorination. In some cases the excavations are lined with filter fabric or impermeable down-gradient membranes that increase the effectiveness of the application.

After the appurtenances are installed, the BP trench is backfilled with permeable materials, such as sand or gravel to displace the slurry. For added economy, excavated soil can be placed above the trench's permeable zone.

Once the excavation/backfill is complete, enzymes or oxidizers are circulated throughout the permeable backfill to “break” any residual BP slurry; thereby, “developing” the trench and restoring the original permeability.

ADVANTAGES

BP trenches are particularly useful where site conditions would result in an unstable excavation without the use of shoring. The method eliminates the need for dewatering and associated treatment or disposal. In addition, soil disposal quantities are reduced, as compared to other methods. Other advantages of the BP technique include flexibility of design, quick installation, and superior depth potential. The method provides an enhanced safety environment; workers do not enter the trench and the slurry suppresses harmful vapors.

CONCLUSION

Numerous successfully completed projects have demonstrated that the BP technique is a safe and cost effective method to install permeable trenches for environmental and civil applications.