Investigation of Tree Root Penetration into a Levee Soil-Cement-Bentonite Slurry Cutoff Wall

摘要

Regulatory policies have been recently promulgated calling for the removal of all woody vegetation and trees that have grown and matured on or near levees. Due to the significant financial and environmental impacts of removing this mature vegetation, many levee owners and maintaining agencies are looking to retain at least some woody vegetation and are interested in exploring whether structural components such as steel sheetpiles or slurry cutoff walls installed through the top of the levee could act as effective root barriers. To investigate the feasibility of using a soil-cement-bentonite (SCB) wall as a potential root barrier, an 18-year-old SCB wall installed in a Sacramento River levee in Sacramento's Pocket Area was partially excavated in October 2009. Trench excavations were completed opposite a large black walnut tree growing at the landside edge of the levee crown. The purpose of the trenching was to determine if roots from this tree had penetrated into or through the 18-inch-wide SCB wall during the 18 years following wall construction. Trenching excavations revealed that roots up to 13/s inches in diameter had been encountered during the original construction of the wall and had apparently been cut or broken at the edge of the wall by construction equipment. However, over time, many of the broken ends of the roots had regenerated new branch roots. These new root extensions were found to have penetrated into the SCB wall a few inches, and then to have divided into smaller branches that travelled laterally within the outer few inches of the wall, perhaps absorbing moisture from the wall which still retained a high water content. Some root branches were found to have travelled more than 10 feet laterally within the outer few inches of the wall. The SCB wall was also found to have numerous, small vertical cracks that were regularly spaced about 10 to 20 inches apart, presumably caused by shrinkage of the wall over time. Where the roots encountered the cracks, small branch roots sometimes exploited the cracks and travelled through the wall. An important finding was that the tree roots were not observed to penetrate the wall except through the small cracks. This paper presents some of the observations made during the trenching investigations, laboratory test results for samples from the levee fill and the SCB wall, and information on root characteristics encountered.of the intended purpose of the SCB wall. Future investigations examining whether such cracks occur in more modern SCB walls, and if so, to what depths, would be extremely valuable.