Mix Ratio Study Of Roadbed Replacement Lightweight Soil During Sponge City In Collapsible Loess Regions

The sponge city is a new urban development model,which aims to solve urban environmental disasters such as urban waterlogging,the heat island effect and urban water shortages.However,there is a contradiction between the subgrade water immersion and the water permeability requirement of during the sponge city road construction in the collapsible loess areas,which needs to be solved.For solving the technical problem,replacing the roadbed with lightweight soil is an important measure,but the research on lightweight soil currently could not meet the requirements of the water permeability.Furthermor,the requirements for the construction of sponge city in collapsible loess areas could not be met.In this paper,orthogonal design was used as the experimental method.Based on the rich cotton stalk resources and the unique mechanical properties and natural hydrophilic properties of cotton stalk fiber in the Northwest area,lightweight soil with cotton stalk fibers suitable for the construction of sponge city in the collapsible loess areas was developed.The density,strength and permeability characteristics of lightweight soil with cotton stalk fibers under different ratios were studied by orthogonal test.And the water absorption,water retention rate and freeze-thaw resistance of lightweight soil with cotton stalk fibers under the optimal mix ratio were investigated by water absorption characteristics,water retention characteristics and freeze-thaw cycles test.In addition,the soil water characteristic curves of lightweight soil with cotton stalk fibers were measured by centrifuge method,and the influences of cement content and cotton stalk fiber contents on the water-holding characteristics of lightweight soil with cotton stalk fibers were investigated,and the differencesof soil water characteristic curves between undisturbed loess and lightweight soil with cotton stalk fibers was compared,which evaluated the water-holding capacity of lightweight soil with cotton stalk fibers.Finally,through the replacement calculation,the feasibility of reducing the weight of lightweight soil with cotton stalk fiber to eliminate or weaken the collapsibility is measured.The main conclusions of this study were as follows:(1)Compared with conventional lightweight soil,lightweight soil with cotton stalk fibers not only has the same light weight and high strength characteristics as conventional lightweight soil,but also unique good water permeability and certain water absorption and water retention properties,which could solve the contradiction between submerged water collapsibility and road water permeability during the construction of sponge city in the loess areas.The optimum mix ratio is that the content of cotton fiber,cement,sand and EPS is 4,25,120 and 12 g/kg.(2)The water absorption rate increases with the immersion time and mainly occurs in the early stage of water immersion,but the effect of age is reversed.The water retention rate is rapidly reduced at the initial stage of maintenance,and is basically stable after the cement reaches initial setting.(3)Compared with undisturbed loess,lightweight soil with cotton stalk fibers has better water-holding capacity,and the water-holding capacity gradually increases with the increase of cement content.And the cotton stalk fiber content has no significant effect on water-holding capacity.The air-entry value of light soil with cotton stalk fibers is 10 times higher than that of undisturbed loess,and there is hysteresis on the soil water characteristic curve,which is not affected by the change of cotton stalk fiber and cement content.(4)Based on the Van Genuchten model,the soil water characteristic curve fitting model of lightweight soil with cotton stalk fiber is proposed,which could uniformly describe the change of the suction of the lightweight soil under the coupling of cement content and water content.And the model is in good agreement with the experimental data.(5)The freezing and thawing cycle causes the damage of lightweight soil with cotton stalk fiber surface structure,and the strength and density of lightweight soil are reduced.When lightweight soil is applied to the construction of sponge city in seasonal frozen-soil areas,it should cause pay special attention to avoid the harm caused by freeze-thaw cycles.(6)It is feasible to reduce the pressure of the underlying loess layer by reducing the weight of the lightweight soil,thereby eliminating or weakening the collapsibility of the site.