THESIS
2013
xvi, 154 p. : ill. ; 30 cm
Abstract
Vegetation is commonly found on slopes, embankments and landfill covers. In spite of many researches have been conducted to monitor the variation of matric suction in vegetated slopes, most of the studies ignored the influences of vegetation on matric suction. Any suction induced by vegetation during evapotranspiration or retained during rainfall may be crucial during assessment of slope stability....[
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Vegetation is commonly found on slopes, embankments and landfill covers. In spite of many researches have been conducted to monitor the variation of matric suction in vegetated slopes, most of the studies ignored the influences of vegetation on matric suction. Any suction induced by vegetation during evapotranspiration or retained during rainfall may be crucial during assessment of slope stability.
The principal objectives of this research were to investigate the retention of matric suction in vegetated soil after rainfall, and to identify influence zone caused by the induced suction in relation to plant characteristics. The effect of soil density on plant characteristics and its influence on matric suction in vegetated soil during rainfall were quantified and investigated. In this study, completely decomposed granite (silty sand), which is commonly found in Hong Kong was vegetated with Cynodon dactylon (Bermuda grass). Bermuda grass can be easily found in tropical and sub-tropical regions around the world and has high tolerance to drought.
A field monitoring programme (Part I) and laboratory investigation (Part II) were carried out in this study. The HKUST Eco-Park, located at Tseung Kwan O in Hong Kong was specifically constructed and instrumented. On the other hand, a comprehensive laboratory test programme was conducted in an atmospheric-controlled plant room. In both field and laboratory tests, silty sand were compacted to targeted soil density and vegetated with Bermuda grass. In each series of tests, one bare soil (non-vegetated) was also prepared identically as control. The observed distributions of matric suction between grassed and bare soil subjected to various test conditions were compared and discussed.
After a rainfall equivalent to 2-year return period of rainstorm in field, higher matric suction was retained in grassed soil than bare soil by 23 kPa. Consistently in laboratory, grassed soil retained 47 % higher matric suction than that in the bare soil after a simulated rainfall corresponding to a 10-year return period of rainstorm. Similarly, 40 % higher of matric suction was retained in grassed soil after ponding corresponding to a 100-year return period of rainstorm. The higher retention of matric suction in grassed soil as compared to bare soil may be attributed to higher matric suction was induced in grassed soil prior to wetting, resulted in lower permeability, and thus higher matric suction was retained.
With an increase in the degree of compaction by 20 % in laboratory, the shoot biomass, root biomass, shoot length and root depth decreased by 29, 14, 10 and 30 %, respectively. After a rainfall equivalent to 10-year return period of rainstorm, 4% of the applied rainfall was infiltrated in both grassed and bare soil compacted at 95 % of degree of compaction and matric suction of 17 kPa was retained. On the other hand, the applied rainfall fully infiltrated in soil with a degree of compaction of 70 % and no matric suction was retained. Higher matric suction was retained in soil compacted at degree of compaction of 95 % than 70 % is because of saturated permeability in the former was at least three orders in magnitude lower than that in the latter.
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