Soil substrate as a cascade of capillary barriers for conserving water in a desert environment:lessons learned from arid nature

Interaction between soil pedogenesis, subsurface water dynamics, climate, vegetation and human ingenuity in a desert environment has been found to result in a unique ecohydrological system with an essentially three dimensional sedimentation structure in the bed of a recharge dam in Oman. A 3-D array of silt blocks sand- wiched by dry sand-filled horizontal and vertical fractures was studied in pot experiments as a model of a natural prototype. Pots are filled with a homogenous sand-silt mixture （control） or artificially structured （smart design, SD） soil substrates. Rhodes grass and ivy （Ipomea, Convolvulaceae） were grown in the pots during the hottest season in Oman. Soil moisture content （SMC） was measured at different depths over a period of 20 days without irrigation. SD preserved the SMC of the root zone for both ivy and grass （SMC of around 25%-30% compared to 〈10% for control, 3 days after the last irrigation）. Even after 20 days, SMC was around 18% in the SD and 7% in the control. This, similar to the case of a natural prototype, is attributed to the higher upward capillary movement of water in control pots and intensive evaporation. The capillary barrier of sand sheaths causes discontinuity in moisture mi- gration from the micro-pores in the silt blocks to sand pores. The blocks serve as capillarity-locked water buffers, which are depleted at a slow rate by transpiration rather than evaporation from the soil surface. This creates a unique ecosystem with a dramatic difference in vegetation between SD-pots and control pots. Consequently, the Noy-Meir edaphic factor, conceptualizing the ecological impact of 1-D vertical heterogeneity of desert soils, should be generalized to incorporate 3-D soil heterogeneity patterns. This agro-engineering control of the soil substrate and soil moisture distribution and dynamics （SMDaD） can be widely used by desert farmers as a cheap technique, with significant savings of irrigation water.

Rationale for creation of capillary breaking layers in cold regions subgrade by pressure injection of waterproofing compounds

In the presents work,the authors have carried out a field study of moisture transfer processes in highway subgrades in a cold region during freezing and laboratory experiments on freezing samples of silicified soils used as a capillary barrier.The study showed that the creation of a capillary barrier from an injected solution,blocking the access of groundwater to the freezing zone,will reduce the amount of heaving to permissible values.Based on the results of laboratory studies,an exponential dependence of the relative deformation due to frost heaving on the relative height of soil silicatization in the freezing zone has been established.For creating a capillary barrier in the already existing subgrade,the authors have proposed designs of injection-silicified soils and substantiated the use of such soils.

Analysis of a landfill cover without geomembrane using varied particle sizes of recycled concrete

Previous studies have demonstrated the effectiveness of a novel three-layer landfill cover system constructed with recycled concrete aggregates(RCAs)without geomembrane in both laboratory and field.However,no systematic investigation has been carried out to optimize the combination of the particle sizes for fine-grained RCAs(FRC)and coarse-grained RCAs(CRC)that can be used for the three-layer landfill cover system.The aim of this paper is to assist engineers in designing the three-layer landfill cover system under a rainfall of 100-year return period in humid climate conditions using an easily controlled soil parameter D10 of RCAs.The numerical study reveals that when D10 of FRC increases from 0.05 mm to 0.16 mm,its saturated permeability increases by 10 times.As a result,a larger amount of rainwater infiltrates into the cover system,causing a higher lateral diversion in both the top FRC and middle CRC layers.No further changes in the lateral diversion are observed when the D10 value of FRC is larger than 0.16 mm.Both the particle sizes of FRC and CRC layers are shown to have a minor influence on the percolation under the extreme rainfall event.This implies that the selection of particle sizes for the FRC and CRC layers can be based on the availability of materials.Although it is well known that the bottom layer of the cover system should be constructed with very fine-grained soils if possible,this study provides an upper limit to the particle size that can be used in the bottom layer(D10 not larger than 0.02 mm).With this limit,the three-layer system can still minimize the water percolation to meet the design criterion(30 mm/yr)even under a 100-year return period of rainfall in humid climates.

Use of geosynthetics for performance enhancement of earth structures in cold regions

Earth structures, such as roadways, embankments and slopes, and earth retaining walls, have been commonly used in cold regions for transportation and other applications. In addition to typical design considerations for earth structures at normal temperature, a design must also consider the unique problems associated with low temperature, such as frost heave, lateral expansion, thaw settlement and weakening, and degradation of material properties. Geosynthetics have been used in cold regions to stabilize earth structures during construction and mitigate potential problems during their service at low tem- perature. This paper provides a state of practice review of the use of geosynthetics for performance enhancement of earth structures in cold regions. This paper starts with basic information on available geosynthetic products and their functions, evaluates properties and behavior of geosynthetics and soil-geosynthetic systems at low temperature, and discusses past studies and their key results on the use of geosynthetics to enhance the performance of roadways, embankments, and earth retaining walls in cold regions. This review reveals that geosynthetics at low temperature have higher tensile strength and stiffness, lower creep rate, and lower elongation at failure. The effect of temperature becomes significant when nonwoven geotextiles are subjected to moistening and soil intrusion at subfreezing temperature. Freeze-thaw cycles may degrade hydraulic and mechanical properties of geosynthetic-soil systems. The inclusion of geosynthetics in soil provides drainage and/or barrier to water flow, retains mechanical properties, and reduces frost heave during and after freeze-thaw cycles. Effectiveness of geosynthetics has been confirmed in the field in bridging over voids, stabilizing roadways over temper- ature-susceptible soils during thaw, and proving drainage and barrier to temperature-susceptible soils before freeze. To avoid frost heave and lateral expansion of backfill in earth retaining walls, granular fill without fines should be used. When backfill with fines is used for earth retaining walls, additional lateral earth pressure induced by soil freeze and thaw set- tlement should be considered in the design.