Performances and mechanism of sand fixation and growth promotion based on modified hydrophilic polyurethane

Abstract: In order to study the mechanism of sand fixation and growth promotion by modified hydrophilic polyurethane (W-OHC), experiments both in laboratory and in field were conducted. In the laboratory experiment, the sand was taken from flowing sand dunes around Qinghai Lake. The permeability of W-OHC on sand was tested through the free infiltration method and the penetration thickness was gauged by a plastic ruler (precision accuracy of 2 mm). The performance of compressive strength of the sand-fixing layer was assessed in accordance with GB/T50123-1999 and the specimen was tested on a AG-X plus universal testing machine. As an important indication of vegetation germination and growth, the surface hardness was measured using a Yamanaka soil hardness tester. The wind tunnel test for wind erosion resistance evaluation was conducted in the Key Laboratory of Desert and Desertification in Cold and Arid Regions Environmental and Engineering Research Institute，Chinese Academy of Sciences. The spraying concentration of W-OHC was 3% and the wind with velocity of 10, 15, 20, 25 m/s lasted for 20, 15, 10, and 7 min, respectively. The property of freezing-thawing of the sand-fixing layer was evaluated. The freezing-thawing cycles was 50 times under -25 ℃ for 22 h and 25 ℃ for 2 h. Both the wind erosion resistance and freezing-thawing performances were assessed by compressive strength and surface hardness. Then the experiment of water retention was carried out in different spraying amounts and concentrations of W-OHC solution, and the rate of water content in sand-fixing layer was obtained. Finally, a field test in the flowing sand dunes around Qinghai Lake was carried out. The 2% and 3% W-OHC with spraying amount of 3 L/m2 were compared. The results showed that: 1) W-OHC solution had good permeability on sand and a flexible sand fixation layer with porosity was formed when different concentrations of W-OHC solution were used. The penetration thickness decreased with the increase of W-OHC concentrations. When the concentration of W-OHC was about 3%, the penetration thickness was about 14 mm; 2) the compressive strength of sand-fixing layer increased with the increase of concentrations of W-HOC and it could reach as high as 1.2 MPa. The surface hardness presented an increasing trend with increasing concentrations of W-OHC and it could reach 23.2 mm. When the W-OHC concentration was 2% and the spraying amount was 3 L/m2, the surface of sand-fixing body showed some small cracks in the strong wind. While in the condition of 3% W-OHC, the sand-fixing layer could resist against the wind velocity of 25 m/s, and no obvious damage occurred on the surface in 7 min. In the freeze-thaw cycles of 50 times, the loss rates of compression strength and surface hardness were 10.2% and 6.5%, respectively; 3) A spatial network structure would be formed when W-OHC solution was solidified, and it would play an important role on water retention and growth promotion. The evaporation rate of water could decrease by more than 50% in the same conditions, the growth circle of vegetation would be extended in drought region, and thus the efficiency of vegetation restoration would be improved. The study will provide a strong support in both theory and application when W-OHC is widely applied to sand erosion control and growth promotion in desert.