Comprehensive recycling utilization technology of foundation engineering slurry

Foundation engineering slurry in the fields of underground continuous walls, horizontal directional drilling, shield, and pipe jacking has the characteristics of large amount of consumption and relatively single pollutant. With the implementation of various national environmental protection laws and regulations, the comprehensive treatment of slurries is particularly important. In this paper, a set of comprehensive recycling utilization technology of foundation engineering slurry was proposed. The technology of sand removal plus recycling of purified slurry and drilling cuttings can be adopted to the circulating slurry, and the technology of calcium removing, pH reducing, flocculation separation, pressure filtration and recycling of wastewater and mud cake was adopted to treat the waste slurry. Taking the slurry of an underground continuous wall project in Wuhan city as the example, the effect of comprehensive utilization of circulating slurry and waste slurry was investigated in detail. The results show that purified slurry and drilling cuttings obtained from the circulating slurry after sand removal, the former could be directly reused in foundation engineering, and the latter could be used to cultivate lyme grass and ryegrass seeds, with a germination rate of 100%. The addition of 5% sodium bicarbonate to the water slurry could reduce the calcium ion concentration from 703.5 mg/L to 173.6 mg/L. Adding 3.3% ammonium chloride could lower the pH from 13 to 9. To obtain obvious flocculating substance, it is suggested to add 300 mg/L flocculant A-2 into the waste slurry. Wastewater and mud cakes could be gained from the flocculating substance after pressure filtration. In addition of 2% sodium bicarbonate, the wastewater could meet the mud-making requirements of foundation engineering. Moreover, mud cakes mixed with 30% to 60% of nutrient soil could be used to cultivate lyme grass and ryegrass seeds, with a germination rate of 72%. Excess salt ions and high pH can be toxic to plant growth. Therefore, waste slurry should be first treated with calcium removal and pH reduction. Later, mud cakes obtained by pressure filtration can meet the requirements of plant growth. This technology can realize comprehensive recycling utilization of all drilling cuttings (or mud cake) and purified slurry (or wastewater) in foundation engineering slurry and have good enlightenment significance to the recycling utilization of similar engineering slurry, which has significant economic, environmental, and social benefits.