| Summary: | A series of laboratory tests have been conducted with Port Hills loess from the Ahuriri quarry. Tests have been conducted to obtain a theoretical basis for suggestions of good design practice for retaining wall filtration/drainage systems. The study has concentrated on investigation of geotextiles as filtration options, with granular filters being tested for comparison. Two of the tests (stage 1 test and gradient ratio test) are designed to provide comparative information on filter performance with specific soil types. The third test is a series of three laboratory scale retaining wall simulations. Information from these tests provides evidence of potential problems with commonly used retaining wall filtration/drainage systems. In conjunction with laboratory testing, field observations have been carried out to assess the current state of the practice, and identify additional areas of concern. Results from both the gradient ratio and stage 1 tests indicate good performance of a range of needle punched nonwoven geotextiles for filtering the specific soil tested (Ahuriri quarry loess). Scanning Electron Microscope investigation of the soil and geotextile filter structures formed during testing indicate bridging to be the dominant filter network, with one example of a vault network for the needle punched range.
Small amounts of clogging observed within the geotextile structure have not resulted in significant reductions in permeability. Gradient ratio values for a selection of the needle punched range support evidence from the stage 1 test that satisfactory filter performance is provided.
Gradient ratio values of greater than 3.0 are recorded for two heat bonded nonwoven geotextiles, indicating a concerning level of clogging. Observations of tested heat bonded samples under the SEM show a degree of "blocking" (a specific form of clogging). These observations are supported by lower permeability values in both tests, comparative to the needle punched range, suggesting the needle punched range is better suited to filtering this specific soil type.
Indications from the retaining wall simulations suggest a need for impermeable drain channels under drainage pipes to prevent erosion of loess resulting from water flow under the drain pipes. Also indicated by these tests is the presence of salts on the backfill material obtained from a local quarry. Although further investigation is recommended, the potential exists for corrosion of steel reinforcing where adequate water proofing is not placed. The need for adequate surface drainage (particularly during construction) in combination with good retaining wall design is emphasised.
As a culmination of the testing and field work, a number of suggestions are put forward as aspects of good design practice for filtration/drainage systems for use in retaining walls on the Port Hills. Selection of an appropriate geotextile for a specific filtration project should ideally follow an in-depth design process, and should not be controlled by budget constraints, as is sometimes the case. Numerical design criteria, based on site specific soil properties and geotextile properties, as well as compatibility tests (such as the gradient ratio test, or the stage 1 test) are available to aid in the design process.
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