In China's coastal areas and along rivers and lakes, there are often highly compressible silt or mucky soil with different thicknesses and low bearing capacity, which is plastic and extremely poor in strength. If small bridges and culverts are built on this basis, if they are not handled properly, there may be instability phenomena such as local damage or even overall sliding of the foundation. In recent years, the state-owned Huaihai Farm in Jiangsu Province has adopted the wet spraying method of deep cement mixing pile to strengthen the soft soil foundation of small bridges and culverts, which has solved this engineering problem well.
design
Design pile length 15m, pile diameter 55cm and pile spacing 1 10cm.
Construction preparation work
① Mechanical equipment. According to the design quantity, length and construction period of cement mixing piles, L ~ 2 STB-3 single-head deep cement mixers are selected, each mixer is equipped with L mortar mixers, L UBJ- 1.8C 1 extruded mortar pumps, and corresponding lifting guide devices and electrical control panels. The distance between the cement slurry pump station and the deep mixer should be less than 50m, and the closer it is, the better.
② Labor organization. Each machine shift of the mixer consists of 8 ~ 10 people.
③ Three links and one leveling. Introduce water and electricity to the construction site before construction. General 1 deep cement mixer needs 60kW,100kw; Suitable for 2 mixers. The water supply pressure must reach 6 ~10 kg/cm2; Temporary access roads should be repaired; The site should be flat; Cofferdams should be built in river ditches to drain stagnant water and cover them with gravel.
④ Determine the pile position. Before mechanical equipment enters the site, the axis, anchor point and leveling point of pile foundation shall be determined, and the pile positions shall be numbered for subsequent construction.
Mud preparation and transportation
(1) mixture ratio. Slurry for deep mixing mainly adopts 425# ordinary portland cement, the cement dosage is12% ~15% of the cement wet soil weight (υ =1.8t/m3), the water cement ratio is 0.45 ~ 0.50, and calcium lignosulfonate water reducer (0.2% of the cement weight) is added.
(2) Preparation and transportation. When mixing mortar, water should be added first, and then cement, water reducer and gypsum should be added in turn. Each mortar mixing time is not less than 2 minutes, and the cement slurry should be fully mixed. When the cement slurry is poured into the hopper from the mortar mixer, the screen must be filtered to remove the cement blocks. The capacity of the hopper is generally 0.2m3, so that a certain margin can be ensured, and the pile will not be broken due to insufficient slurry supply, and the slurry concentration will not be insufficient due to precipitation due to too much slurry.
The cement slurry is pumped into the hose with the inner diameter of φ32 by the extrusion mortar pump, sent to the drill pipe of the deep mixer, and finally injected into the slurry outlet of the mixing blade.
Deep mixing construction
① The equipment is in place. The drill pipe of the mixer should be vertical and aligned with the pile position.
(2) the first drilling. After confirming that the slurry is sprayed from the slurry outlet of the mixing blade, the mixer can be started. At the drilling speed of 60r/min and 1m/min, drill and grout clockwise until the designed pile length, stop the pump after shotcreting 15min, and stir counterclockwise to the designed pile top.
③ The second drilling. Drill and grout clockwise again in the same way. Stop pouring position shall be based on the cement dosage reaching the design dosage of each pile, but it shall not affect the mixer to continue drilling until the designed pile length is reached after re-mixing, and stir counterclockwise until the mixing head is exposed to the ground.
④ Pile-forming double circulation drilling. After two cycles of drilling and lifting, the cement slurry is stirred in the pile hole for four times. Finally, a circular steel plate with a diameter of 500mm, a thickness of 10mm and a hole of 100mm in the middle is placed on the top of the pile, and the pile is pressed down by a mixing head for 20-30cm, and the operation is completed. Then, move the machine to the next pile location for construction.
quality management
(1) Strictly control the water cement ratio, and the cement slurry must be prepared by the measurer.
(2) The vertical deviation of the mixing rod shall not exceed 65438 0%, the centering error between the pile driver and the pile position shall not be greater than 2cm, and the deviation of the pile position after pile formation shall not be greater than 8cm.
(3) The foundation pit shall not be excavated within 7 days after the cast-in-place pile, and mechanical excavation is prohibited. Pile heads should be carefully arranged, and heavy hammers are not allowed. The pile head should be leveled, 2 ~ 3 cm above the basement elevation.
quality test
Sampling 2% ~ 5% of the piles for quality inspection. Within 7 days after the completion of the pile, the continuous light detection test (N 10) was carried out on the cement pile with the top depth of about1m. Important stress parts are cut and sampled according to design requirements, and 70.7mm×70.7mm×70.7mm test blocks are made for compressive test. Generally, the strength can reach 7 MPa ~ 8 MPa in 28 days, and it is stable at 10 MPa ~ 12 MPa after 90 days. During the excavation of foundation pit, we require the construction unit to supervise and inspect the pile diameter on site and observe the appearance mixing uniformity. At the same time, the design unit also carried out the vertical bearing capacity test of single pile, and the bearing capacity can reach 160 kN ~ 180 kN, and the bearing capacity of composite foundation can reach 250 kPa ~ 300 kPa, which meets the design requirements.
Technical and economic analysis
① Cement dosage: the cement dosage of cement mixing pile with a diameter of 55cm per meter is 29 ~ 36 kg.
② Work efficiency: According to the field records, each STB-3 deep mixer can reinforce 95 linear meters on average.
(3) Project cost: According to the current unit price, including materials, machinery and labor costs, the wet-sprayed cement mixing pile is 78.65 yuan /m3, which is 765,438 yuan +0.20 yuan /m3 less than the dry-sprayed pile under the condition of meeting the design bearing capacity. Compared with diving piles and sinking pipe piles, it saves more investment.
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