The Impact of a Planar Drainage Geocomposite in Unsaturated Soil
Date
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
ORCID
Type
Thesis
Degree Level
Masters
Abstract
The effects of infiltration in geoenvironmental systems characterized by unsaturated soils subject to occasional significant precipitation events has been a subject of concern as practitioners look for improved methods to reduce pressures and divert water from foundations, mechanically stabilized earth structures, cover systems, and other civil infrastructure. Drainage in these structures has typically been achieved with engineered fills, often combined with geosynthetics, and more recently with standalone geosynthetics designed for the express purpose of drainage. One such geosynthetic is DrainTube®, comprised of a PET (polyester) filter, PP (polypropylene) drainage blanket, and perforated PP mini-pipes spaced at regular intervals for diversion into a primary water collection system. The research presented focuses on laboratory characterization of DrainTube® and evaluation of its performance in an unsaturated system subject to rainfall.
The geosynthetic-water characteristic curve (GWCC) and permeability function (K-function) were characterized in the lab experimentally. A modified Tempe pressure plate cell with a high-flow, low Air-Entry Value (AEV) porous ceramic was used to obtain GWCCs for several variations of geotextiles used in the DrainTube® product line under a variety of vertical loads by use of hanging column and axis translation principles.
A custom permeameter was constructed to suit the unique task of measuring in-plane permeability of the geotextile under suction to experimentally measure the K-function. A series of tests were conducted to measure permeability of the DrainTube® geotextile under a range of suctions up to 20 cm of suction head (approximately 2 kPa).
The applicability of commonly used Water Characteristic Curve-fitting models and their related K-function predictions (van Genuchten 1980, Fredlund and Xing 1993, and Fredlund, Xing and Huang 1994) were evaluated against the measured GWCC and K-functions. Both the van Genuchten (1980) and Fredlund and Xing (1993) formulations for curve-fitting were found to be satisfactory models, while the K-function predicted by the Fredlund, Xing, and Huang (1994) formulation was found to better match the measured K-function over the range of laboratory data collected.
A soil column was constructed to experimentally evaluate the behavior of an unsaturated soil-DrainTube® system subject to infiltration. The apparatus was assembled with the geosynthetic situated in the middle of the column and tested with two soil materials: a commercially available abrasive blasting aluminum oxide grit media (referred to as alox), and a slightly coarser rock crusher dust which was further washed and processed (referred to as sand). A series of infiltration experiments were conducted with varying water table elevations and infiltration rates. The column was outfitted with six tensiometers to measure the suction profile during infiltration. Porewater pressure and suction measured by the six tensiometers showed that pressure profile remained hydrostatic below the geosynthetic but became subvertical to vertical above the geosynthetic, indicative of increased water content in the soil due to development of the capillary break effect.
The column featured unique outlets to allow independent collection of the outflows for the DrainTube® mini-pipe and geotextile components to determine the efficiency of the geosynthetic to capture infiltration and divert it out of the column. It was found that a certain infiltration rate was required to initiate capture of infiltrating water. The capture efficiency peaked as the infiltration rate approached the saturated permeability of the soil material in the column. Maximum capture efficiency was as high as 75% of infiltration captured with a shallow water table 2.5 cm below the geosynthetic, and as low as 40% with a deeper water table 22 cm below the geosynthetic. The mini-pipe generally accounted for the majority share of the total capture.
A 3-dimensional (3D) numerical calibration of the physical column experiment was created in GeoStudio 2021.3 Seep3D with the same dimensions as the physical experiment. Each combination of lab infiltration rates and water table elevations were modeled for both alox and sand materials in order to calibrate material parameters to the lab data. A reasonable fit to the lab data was achieved by making minor adjustments to the GWCC, SWCC, and permeability of the materials.
The calibrated materials were then utilized in a numerical simulation of a lab-scale embankment. The lab-scale embankment was constructed by PWRI et al. (1988) and has been the subject of other numerical studies (Iryo and Rowe 2005, Thuo et al. 2015). Where the prior studies were limited to 2D analyses and estimated the soil material parameters of the original physical study, The present research extended the geometry into three dimensions to incorporate the DrainTube® mini-pipes and utilized the calibrated alox and sand materials. Three geosynthetic layouts were assessed under infiltration rate conditions varying from 6 mm/hr to 36 mm/hr and mini-pipe spacings of 0.25 m, 0.5 m, 1 m, and 2 m compared to a geotextile-only scenario. Heatmaps were generated to compare the change in pressure head between cases incorporating mini-pipes against the base case (no geosynthetics) and also against the geotextile-only scenarios.
Capillary barriers were found to develop within the embankment at the interface of the geosynthetic and soil materials in the simulation. However, scenarios incorporating the mini-pipes generally reported an overall lower pressure head distribution within the embankment domain. Specifically, the severity of capillary break developed was reduced when mini-pipes were included in comparison to geotextile-only scenarios. This suggests that the inclusion of mini-pipes even in unsaturated conditions leads to a more favourable hydraulic condition.
Description
Keywords
geosynthetic, geotextile, nonwoven, unsaturated soil, draintube
Citation
Degree
Master of Science (M.Sc.)
Department
Civil and Geological Engineering
Program
Civil Engineering