Core tip: The deep mixing method is suitable for foundation reinforcement of buildings on saturated soft clay, muddy silty clay, newly dredged fill, carbonaceous soil in marsh areas, sedimentary silt, and similar strata. Deep mixing with cement grid-type retaining walls can serve as deep foundation pit support, waterproof curtains, and soft ground improvement for road and port foundations. The bearing capacity of the soil can be increased from 90 kPa to 248 kPa. (I) Construction Preparation 1. Materials (1) For soft clay reinforcement by the deep mixing method, ordinary Portland cement of grade 525# or above should preferably be used as the curing agent. The cement mixing ratio depends on the required reinforcement strength and is generally 7%-15% of the weight of the treated soil, with about 110-160 kg of cement added per cubic meter. The formula is: mixing ratio (%)=weight of cement/weight of soft soil to be treated×100%. (2) To improve the properties of cement-soil and the strength of the pile (wall) body, admixtures such as calcium lignosulfonate, gypsum, sodium chloride, calcium chloride, and sodium sulfate may be selected, and fly ash in different proportions may also be added. (3) When cement is used as the curing agent for deep mixing, the mix proportion is cement:sand=1:1-1:2. To improve the workability of the cement mortar and facilitate pumping, a water-reducing agent (calcium lignosulfonate) should be added at 0.2%-0.25% of the cement dosage, sodium sulfate at 1% of the cement dosage, and gypsum at 2% of the cement dosage. The water-cement ratio is 0.41-0.50, and the consistency of the cement slurry is 1-14 cm, which can provide rapid setting and early strength. 2. Working Conditions (1) Conduct indoor mix proportion tests based on geological survey data, and in combination with design requirements, select the optimal cement mixing ratio and determine the mixing process. (2) Prepare a construction plan based on the design drawings, make proper site layout arrangements, arrange the construction schedule, and set up slurry ponds for cement slurry preparation. Where conditions permit, install the cement slurry preparation system on a mobile truck for flexible supply. When pumping for placement, the pumping distance should preferably be less than 50 m. (3) Clear underground, surface, and overhead obstacles on site to ensure construction safety. (4) Conduct surveying and setting out, and mark each pile position. (5) Mechanical equipment configuration: deep mixer, crane, and guidance, measurement, and curing agent preparation systems. (6) Labor organization: each deep mixing machine crew consists of 12 people. (7) If the surface soil at the construction site is hard and water injection mixing is required, set up drainage ditches and sumps around the site. (II) Operating Process 1. Principle and Operating Process of Cement-Soil Solidification by Deep Mixing (1) Using cement-based material as the curing agent, a special deep mixer forcibly mixes soft clay with cement slurry in situ deep in the ground. First, the cement decomposes and hydration reactions generate hydrates; then the hydrates cement the particles and undergo ion exchange. Through granulation and hardening reactions, cement-reinforced soil with certain strength and stability is formed, thereby increasing the foundation bearing capacity and changing the physical and mechanical properties of the foundation soil, achieving the effect of soft ground reinforcement. (2) In deep mixing, two motors respectively drive the mixing shaft through reducers, causing the mixing head to cut the soft soil, while the curing agent is injected into the foundation soil through the central pipe and forcibly mixed into cement-soil. 2. Construction Process Characteristics of the Deep Mixing Method: According to the requirements of the superstructure, three reinforcement forms can be arranged: columnar, wall-shaped, and block-shaped. Column (pile) reinforcement form: install one mixing pile at a certain spacing. Wall-shaped reinforcement form: formed by partially overlapping and connecting adjacent mixing piles. Block-shaped reinforcement form: formed by connecting adjacent piles in both longitudinal and transverse directions. 3. Construction Process of Deep Mixing Piles (1) Positioning and centering (2) Pre-mixing and sinking (3) Preparing curing agent slurry (4) Spraying slurry, mixing, and lifting (5) Repeated mixing (6) Moving 4. Construction Process Flow for Wall-shaped Reinforcement: According to the columnar reinforcement process, construct adjacent piles in rows with longitudinal vertical overlap; the overlap distance between adjacent piles is determined according to design requirements. The shape is like the character "8". 5. Construction Process Flow for Block-shaped Reinforcement: According to the deep mixing construction process, construct adjacent piles with longitudinal and transverse overlaps to form a block-shaped reinforced body; the overlap distance between two rows of piles can be determined according to design requirements. (III) Quality Standards 1. Guaranteed Items The cement type, grade, water-cement ratio of the cement slurry, mixing ratio of the cement-reinforced soil, and types and dosages of admixtures used for deep mixing piles must comply with design requirements. Inspection method: check the factory certificate, conformity certificate test report, and construction records. 2. Basic Items (1) The depth, cross-sectional dimensions, overlap conditions, overall stability, and wall and pile body strength of deep mixing piles must comply with design requirements. Inspection methods: 1) Generally, within two weeks after pile formation, use a drilling rig to take samples for inspection, excavate to check cross-sectional dimensions, observe the overlap conditions and mixing uniformity of the pile body; there should be no water seepage in the pile body. 2) For quality inspection of mixing piles, use a lightweight penetrometer and judge the strength of the cement slurry in each section based on the penetration blow count. (2) On-site load test: use this method to test the effect of engineering reinforcement. Because the quality of mixing piles is closely related to the pile formation process and construction technology, the bearing capacity obtained from on-site load tests fully reflects actual conditions. (3) Regular settlement observation: for projects formally using deep mixing to reinforce foundations, regular settlement observation and lateral displacement observation are ideal methods for intuitively checking the reinforcement effect. 3. Allowable Deviations The allowable quality deviations and inspection methods for deep mixing piles shall meet the requirements in the table below. Inspection quantity: sample 5% according to the number of walls (columns). Item Allowable deviation (mm) Inspection method Pile top displacement 10(20) Check with ruler Pile (wall) verticality 0.5H/100 Check with measuring instrument, plumb line, and ruler (IV) Construction Precautions 1. Avoid Common Quality Problems (1) The deep mixer should basically remain vertical; pay attention to flatness and the verticality of the guide frame. (2) After the deep mixing blade sinks to a certain depth, start preparing cement slurry according to the design mix proportion. (3) The cement slurry must not segregate. The cement slurry must be prepared strictly according to the designed mix proportion. The cement must be sieved. To prevent segregation of the cement slurry, keep stirring it in the mixer and pour the cement slurry into the hopper only before grouting. (4) Pre-mix according to reinforcement strength and uniformity; the soft soil should be fully pre-cut and broken to facilitate uniform mixing of the cement slurry. 1) During the grouting stage, slurry interruption is not allowed, and the slurry delivery pipe must not be blocked. 2) Strictly control the spraying, mixing, and lifting speeds according to the determined design data. 3) Control the sinking and lifting speeds during repeated mixing to ensure that every depth within the reinforcement range is fully mixed. (5) During pile formation, if shutdown occurs due to excessively low voltage or other reasons and the pile formation process is interrupted, to prevent pile breakage, after restarting the mixer, lower the deep mixing blade by half a meter before continuing pile formation. (6) The interval between construction of two adjacent piles shall not exceed 12 hours (columnar). (7) To ensure the continuity of the wall-shaped reinforced body, when the pile body needs to overlap by a certain length according to design requirements, in principle each construction section should be constructed continuously, and the interval between construction of adjacent pile bodies shall not exceed 24 hours (wall-shaped). (8) Considering that the contact part between the mixing pile and the foundation or pile cap of the superstructure bears relatively large forces, it is usually also possible to add one more grouting pass within 1.5 m of the pile top to increase its strength. (9) In the construction of mixing piles, according to the stress characteristics of friction-type mixing, a variable mixing ratio construction process can be adopted, that is, using different lifting speeds and grouting speeds to meet the required cement slurry mixing ratio. Under the condition of a fixed-displacement pump, to mix different amounts of cement slurry into soft soil, only the lifting speed can be changed, and this is detected by a lifting speed detector. 2. Main Safety Technical Measures (1) The cooling circulating water of the deep mixer must not be interrupted during the entire construction process. Frequently check the inlet and return water temperatures; the return water temperature should not be too high. (2) During soil cutting and lifting mixing by the deep mixer, if the load is too large and the motor working current exceeds the rated value, reduce the lifting speed or supply clean water. Once sticking or stoppage occurs, cut off the power supply and force the mixer up before restarting the motor. (3) If the power grid voltage for the deep mixer is lower than 380 V, construction should be suspended to protect the motor. (4) Slurry pump and slurry delivery pipeline 1) Before pumping cement slurry, the pipeline should be kept moist to facilitate slurry delivery. 2) There must be no hardened lumps in the cement slurry to avoid suction into the pump and damage to the cylinder body. After daily work is completed, thoroughly clean once. During spraying and mixing construction, if a fault causes shutdown for more than half an hour, it is advisable to dismantle the pipeline, remove the slurry, and clean it properly. 3) The slurry pump should be disassembled and cleaned regularly, and the lubricating oil in the gear reducer should be kept clean. (5) When deep mixing machinery and lifting equipment are constructed in soft surface soil conditions, the site should be paved with stone and crushed stone, leveled and compacted, and according to soil conditions, sleepers, steel plates, or specially made track boxes should be laid. 3. Product Protection After the construction of deep mixing piles is completed, heavy objects are not allowed to be randomly stacked nearby to prevent deformation of the pile body.