Abstract:
Abstract: Microbial mortar protective covering is a new technology for controlling silt slope surface dust and water erosion. The anti-erosion performance of the protective covering depends to a large extent on the strength and stability of covering. However, the factors that govern the protective property of covering have not been fully studied. Based on microbial induced calcite precipitation (MICP) technology, the surface protective covering experiments were carried out to treat silt slope surface against rainfall erosion. Indoor model tests on silt surface protective covering by microbial mortar were performed to investigate the properties such as strength, water stability and anti-erosion. Sand was covered on dredger fill silt surface to form a thin layer, and then some Sporosarcina pasteurii and a kind of cementing solution containing a mixture of CaCl2 and urea were sprayed on the sand layer surface to obtain a crustose protective covering. The micro-penetration test was adopted to investigate the penetration resistances change of uncovered silt and protected silt covered with microbial mortar crust. The property of protective strength ratio measured by micro-penetration test was used to evaluate the strength of microbial mortar protective covering relative to uncovered silt surface. The collapse ratio and strength loss ratio were tested on the microbial mortar protective covering by immersed in water for 12 h to assess its water stability performance. The change laws of protective covering strength and water stability were drawn from different covering thickness, mixed cementing solution concentration of CaCl2 and urea, and spraying times. On the basis of the tests of protective covering, the rainfall model experiments were conducted. A slope angle variable steel tank with the size of 1.4 m × 0.3 m× 0.06 m (length × width × height) was applied for holding test material and simulating the silt slope, which was subjected to strong rainfall scour under a rainfall simulating system. The soil detachment rates were investigated from the rainfall simulation experiment by changing slope angle from 5º to 25º and rainfall time from 4 to 20 min. The experimental results revealed that: 1) The minimum effective thickness of microbial mortar protective covering was 5 mm. The strength and water stability of protective covering increased with the cementing solution concentration, protective covering thickness and spraying times. 2) After treated 4 times with 0.50 mol/L cementing solution, a protective covering was formed, which had high strength, good anti-collapse property and excellent water stability. A 5 mm thick protective covering could reach penetration resistance of 310 kPa, protective strength ratio of 77.5, collapse ratio of 2.3%, and strength loss ratio of 5.4% when immersed in water. 3) Compared with untreated silt slope, the treated silt slope reduced soil detachment rate from over 29.6 g/(m2•s) to under 6.8 g/(m2•s). The soil detachment rate of treated silt slope was between 5.4 and 6.8 g/(m2•s) and insensitive to the slope angle. Under 20 min heavy rainfall scouring, the soil detachment rate of treated silt slope was only 3.6 g/(m2•s). The microbial mortar protective covering presented significant anti-erosion capacity. 4) The strength of protective covering and the permeability of substratum silt had coupling effect on the anti-erosion mechanism of microbial mortar protective covering. The coarse and porous covering played an important role in resisting raindrop splash and surface flow erosion by the high strength of crust. Meanwhile, the low permeability of silt under the covering resisted piping and suffusion erosion. The anti-erosion effectiveness of protective covering was dominated by the microbial mortar crust strength. For effective microbial mortar protection, at the microstructure level it was essential that the cement calcite from MICP formed surrounding cementation structure around soil particles surface. The results provide valuable information for applying MICP technology on silt slope anti-erosion.