Abstract

Nanjing generates approximately 60 million cubic meters of construction engineering spoil and about 4 million tons of waste slurry each year. The traditional disposal of engineering spoil is landfill at spoil grounds, which not only wastes resources but also places a certain degree of pressure on ecological and environmental protection. Fluidized solidified soil is a new type of building material prepared using engineering spoil as raw material. It can be used for backfilling the trench on the side walls of building foundation pits, backfilling the side walls of deep subway foundation pits, backfilling the side walls and upper portions of utility tunnels, as well as backfill pouring for various other foundation pits, pipe trenches, etc. It can not only consume large quantities of engineering spoil but also further improve the quality of fill engineering and promote the resource-based recycling of construction waste. The strength of fluidized soil can be designed according to backfill requirements, with cement content of 5%-30% and strengths ranging from 0.3, 0.5, 0.8, 1.0, 2.0, 3.0, to 5.0 MPa.
Keywords

Fluidized soil backfill, construction spoil, waste slurry treatment, foundation pit side wall backfill, utility tunnel backfill, in-situ mixed cement soil, low-strength cement soil, variable-strength cement soil

1. Definition of fluidized solidified soil

Fluidized solidified soil is a new type of geotechnical engineering material prepared according to engineering needs and geotechnical characteristics by using local solid waste to prepare a dedicated high-efficiency geotechnical solidifying agent, taking local soil and adding the solidifying agent and water to form a flowable mixture, which is finally poured by concrete pumping and cured to solidify into a material with a certain strength, low permeability, and water stability. It is equivalent to another form of "concrete" and can be used for building backfill, foundation pit side wall backfill, replacement filling, and ground treatment, etc.

Fluidized solidified soil is a new type of environmentally friendly material with characteristics such as flowability, controllable strength, self-density, convenient construction, and suitability for backfilling narrow gaps, giving it broad applicability for engineering backfill.

2. Performance advantages of fluidized solidified soil

Fluidized solidified soil not only has a short solidification time but also requires a relatively small working area. In addition, because the material is in liquid form, it does not generate dust pollution, making it both economical and environmentally friendly. As a self-compacting material, fluidized solidified soil can replace manual and equipment rolling and compaction by being poured directly, enabling simultaneous construction of multiple sections, reducing the construction cycle, and effectively avoiding workers having to work continuously in narrow spaces for long periods, greatly improving construction safety.

The Guidelines address the problem in engineering that mechanical equipment finds it difficult to compact narrow areas to the required density. Using dewatered construction slurry filter cake and engineering spoil as raw materials, fluidized solidified soil with flowability and hardening strength that meets engineering backfill requirements is prepared. As a green building material capable of co-processing multiple types of waste, it not only ensures engineering quality but also enables large-scale on-site consumption of spoil, consumes and reuses slurry generated during construction, and promotes the resource-based recycling of construction waste.

3. Preparation of fluidized solidified soil

The preparation of fluidized solidified soil is divided into two modes: on-site mixing and plant mixing. Before mixing, foreign matter mixed into the raw soil, such as wood chips, metal objects, and plastics, needs to be removed. For sandy soil materials, crushing and screening equipment can be used; fine-grained soil tends to clump and form lumps, so a soil-dissociation step should preferably be added, with water added to prepare slurry and then screening.

▲ Process flow of fluidized solidified soil

4. Placement of fluidized solidified soil

Placement can be carried out using chutes, tremie pipes, pumping pipelines, etc., and direct impact on trench side walls or embedded pipes should be avoided as much as possible. Pump trucks and mixing equipment need to be close to the placement surface to ensure sufficient working area for construction. After pouring is completed for the day, residual fluidized solidified soil in the tremie pipes and pumping pipelines should be cleaned promptly to avoid dilute slurry generated during cleaning being discharged into the already poured working surface.

The time from completion of mixing to placement of fluidized solidified soil should preferably not exceed 3 hours. When the interval exceeds the specified duration, adjustment and addition of retarder are required. During transportation, the Guidelines stipulate that the fluidized solidified soil should be kept uniform and free from segregation, and no water or other admixtures may be added before placement. The flow value needs to be tested to meet placement requirements.

5. Application scenarios of fluidized solidified soil

The Guidelines specify that fluidized solidified soil can be used for backfilling foundation pit trenches, pipeline trenches, top slabs of underground structures, foundation cavities, the back side of retaining walls, and roadbeds in construction projects. For example, the foundation pit trench on the west side of Buildings A1 and A2 in Phase I of the China Mobile Yangtze River Delta (Nanjing) Science and Innovation Center project used this new material. It is understood that the backfill volume for the project's trench was nearly 20,000 cubic meters, and the first layer of backfill has now been completed, effectively saving a large amount of natural sand and gravel resources.

Because the working area in the side wall backfill zone of the basement level B1 in the Phase I project was narrow, traditional backfill methods were difficult to construct, machinery could not operate, and compaction was difficult. Using fluidized solidified soil for backfill can effectively solve these technical pain points. During backfill, direct discharge and pumping methods can be used for continuous operation and rapid construction. At the same time as pouring, initial fit-out within the B1 main structure can be organized and implemented in parallel, effectively shortening the construction cycle and greatly saving manpower and machinery use.

At present, this technology has been widely tested and applied in many projects in Nanjing, including water pipeline backfill, internal road backfill within the sludge treatment center plant area, urban pipeline backfill, top slab backfill of the open-cut section of the Jianning West Road river-crossing tunnel, Phase I of the Chip City Science and Innovation Base in the Jiangbei New Area Research and Innovation Park, overall backfill of the three-story basement foundation pit near Suning Yuantong Station, and top slab backfill projects for underground stations of Nanjing Metro Lines 5 and 9. Because fluidized solidified soil has working performance similar to concrete, it can also be used as construction bedding material or temporary solidified ground, for example, for temporary infrastructure, material storage yards, parking lots, and other locations on projects.

6. Jiangsu Province's Technical Guidelines for the Application of Fluidized Solidified Soil Placement

Jiangsu Province's Technical Guidelines for the Application of Fluidized Solidified Soil Placement (Trial) have been released. Organized by the Nanjing Municipal Urban and Rural Construction Commission and the Nanjing Municipal Urban Management Bureau, and jointly compiled by the Resource Regeneration Company of Nanjing Water Group, the release of the Guidelines provides a technical basis for design, construction, acceptance, and evaluation for the promotion of fluidized solidified soil in Nanjing, and is a strong practice focused on the "dual carbon" goals and supporting the construction of "zero-waste cities." The Municipal Construction Commission will further promote the high-quality development of green buildings across the city through measures such as strengthening scientific research and technical breakthroughs, improving policy mechanisms, and highlighting demonstration leadership. The release of the Technical Guidelines provides a new option for the treatment and reuse of construction waste.

These technical guidelines mainly address the problem in engineering that mechanical equipment finds it difficult to compact narrow areas to the required density. Using dewatered construction slurry filter cake and engineering spoil as raw materials, they prepare fluidized solidified soil with flowability and hardening strength that meets engineering backfill requirements. On the basis of consuming large quantities of engineering spoil, they improve the quality of fill engineering and promote the resource-based recycling of construction waste.

The Guidelines consist of 10 chapters, including General Provisions, Basic Requirements, Materials and Performance, Construction, and Quality Inspection and Acceptance, along with 6 appendices. They set the scope of use of fluidized solidified soil, clarify requirements for raw materials and backfill locations, standardize inspection methods, and play a good guiding role in improving the application of new materials and enhancing quality control.

Attachment download:

Notice on Issuing the Technical Guidelines for the Application of Fluidized Solidified Soil Placement (Trial).pdf