The following is the relevant content about the special roadbed treatment construction plan brought to you by Zhongda Consulting for your reference.

1. Soft soil roadbed

(1) Shallow soft soil:

Shallow soft soil is commonly developed in the mountain valleys along the line, with a thickness of 3m. (up to 5m in some mountainous depressions), replacement treatment is adopted. The replacement material is preferably strong and weathered flake stones. In order to restore the original groundwater recharge path, a drainage cushion is provided within 0.3m of the bottom; if The horizontal groundwater in this section is extremely abundant, so all the filling is made with unscreened gravel to ensure smooth drainage.

The surface silt of other water (fish) ponds and paddy field sections is directly dredged and replaced. Some water (fish) ponds do not completely encroach on the road section and are passed through submerged embankments. When the water depth in the pond is large, it is difficult to use full drainage. At that time, cofferdam construction was adopted.

(2) Deep soft soil:

Deep soft soil foundations are developed in some intermountain alluvial plains along the line. The conditions for excavation and replacement of such soft foundations are severely limited, and it is necessary to Adopting a composite foundation solution, this project mainly uses cement slurry mixing piles and gravel piles, combined with overload preloading according to the rock and soil conditions.

1) Overload preloading:

The total time of soft foundation construction (filling + preloading) in this section is controlled according to 180-270 days (6 to 9 months). The construction period generally does not exceed 6 months in order to achieve a longer pre-pressure period. The loading rate is controlled at 25cm/week, and the construction should be limited to the compacted thickness of each layer. On the premise of meeting the allowable post-construction settlement and stability requirements, a shorter preloading period should be adopted as much as possible. The preloading time is generally not less than 6 months.

On the premise of meeting the allowable post-construction settlement and stability, this section mainly adopts overload preloading, and uses 3m of fill soil on top of the road bed for preloading.

2) Cement slurry mixing pile (wet spray pile)

1. Design of wet spray piles

The diameter of wet spray piles is 0.5m. The spacing is 2m. Arranged in an equilateral triangle on the plane. The designed unconfined compressive strength of the pile body is: R28≥1. OMPa.

2． Material requirements

The cement uses 42.5MPa ordinary Portland cement. The purchased cement is a national inspection-free product and should be used within the validity period. It is strictly prohibited to use inferior cement that is damp, agglomerated or deteriorated. For cement produced by non-inspection-exempt plants, test reports on relevant grades, stability, etc. should be provided in batches.

The water-cement ratio is generally 0.4 to 0.5. In order to increase the workability and stability of the cement slurry for slurry-jet piles and improve the early strength, and taking into account the high natural moisture content of the soft soil in this section, high-efficiency water-reducing admixtures can be added when preparing the cement slurry if conditions permit. Or early strength agent, the dosage is determined by indoor mix proportion test.

3. Determination of cement content

The cement content of cement slurry mixing pile is determined by indoor mix ratio test. Meet design and specification requirements.

4. Construction technology

① Remove the topsoil, level the site, pave 30cm of natural gravel, and dig temporary drainage ditches.

② After the mixing machinery is transported to the construction site, it should be installed and debugged first. After the speed, air pressure and metering facilities are normal, it can then be put into place.

③Align the mixing head at the designed pile position, start the motor, and after the mixing head rotates normally, rotate and cut the soil while sinking until the reinforcement depth is reached.

④Grout upward from the bottom of the pile while stirring and lifting until it is 50cm above the ground. Then re-spray while stirring to the bottom of the pile. Stop drilling after reaching the bottom of the pile and continue to spray for 1 minute. Finally stir and lift to the ground.

⑤Turn off the power, move the device, and repeat the above steps.

5. Construction quality control

Quality control during construction should comply with the following regulations:

① The curing agent slurry should be mixed strictly according to the predetermined ratio. The prepared slurry must not segregate or stay for too long (no more than 2 hours); a sieve should be added to filter the slurry when it is poured to avoid caking in the slurry and damaging the pump body.

② Before pumping slurry, the pipeline should be kept moist to facilitate slurry transportation. When mixing slurry on site, a dedicated person should record the amount of curing agent and external admixtures, and record the start and end times of pumping slurry.

③Construction shall be carried out according to the technical parameters determined by the pile test. The operator should record the sinking time and lifting time per meter, and record the changes in relevant parameters such as pulp delivery time and pulp stop time.

④The pulp supply must be continuous and the mixing must be uniform. Once the grouting is stopped for any reason, in order to prevent pile breakage and lack of grout, the shotcrete mixer should be sunk to 1.0m below the grouting stop surface, and the shotcrete mixer should be lifted after the grout supply is restored. If the machine is shut down for more than 3 hours for any reason, in order to prevent the slurry from hardening and blocking the pipe, the slurry pipeline should be disassembled and cleaned before use.

⑤ When the mixer is lifted to 2m below the ground, it is advisable to use a slow speed; when the gunite port is about to lift out of the ground, the lifting should be stopped and stirred for a few seconds to ensure that the pile head is uniform and dense.

⑥If it is found that the amount of shotcrete is insufficient during the construction of grouting piles, one pile should be added next to it, and the amount of shotcrete added should still not be less than the designed amount.

6． Quality Inspection

①Construction Allowable Deviation

Construction allowable deviation inspection includes pile diameter, pile spacing, pile length, verticality, single pile shotcrete amount (determined by cement content and cement slurry (calculated from the water-cement ratio) and strength, which meet the design requirements.

②Within seven days after the pile is completed, the construction unit shall first perform a self-inspection on excavation, observe the shape of the pile body and the uniformity of mixing, and keep records truthfully. The inspection frequency is 1%, and the excavation depth is not less than 1.5m. If poor condensation is found, the piles should be scrapped and replaced.

3) Gravel piles

Gravel piles have a pile diameter of 50cm and are arranged in an equilateral triangle. The pile spacing is reasonably set according to the calculation results of the design bearing capacity. A 50m gravel cushion is laid on the top of the piles. To spread the base stress and give full play to the bearing capacity of the soil between piles.

Construction technology:

According to the soil lithology, porosity ratio and groundwater conditions, the gravel piles are constructed using the vibrating immersed tube method, using the vibration piling mechanism to pile, immersed tubes, feed, Vibrate and extubate. The construction sequence adopts the form of jumping and is carried out from the outer edge to the inside.

The specific construction process is as follows:

Level the construction site and stake out pile positions.

The vibrator is positioned and centered, the vibrating hammer is started, and the pile pipe is penetrated to the designed depth.

The continuous feeding method is used to make piles section by section from bottom to top. The amount of each feeding is determined according to the soil quality and construction conditions, and the pipe is vibrated for 1 minute before pipe removal. The pipe should be lifted every 1m. Insert 30cm backward, leave vibration for 10-20 seconds, and extubate the tube at a speed of 1m/min. If the filler is less than the designed value, the tube can be inserted again.

The above are empirical values ??and need to be adjusted based on the pile test results.

Move the hole position and repeat the above steps.

Pile test:

The key to the construction of gravel piles lies in the control of electricity, material addition, time and other aspects, which must be confirmed by on-site pile making tests. The number of test piles in each test section is 9 to 16. The amount of gravel, vibration frequency, vibration retention time, reverse insertion depth and other parameters are determined based on the quality inspection results to guide large-scale machine construction.

Quality inspection:

The soil between the piles should be subjected to standard penetration tests before and after the pile test, and the gravel piles should be subjected to dynamic cone penetration (heavy type II) tests after the piles are completed. , single pile bearing capacity load test, three pile composite foundation bearing capacity load test. There should be no less than 4 test points for various tests in each test section.

The load test uses a rigid bearing plate, the single pile bearing capacity load test uses a circular bearing plate, the size is the same as the pile cross section, the three pile composite foundation bearing capacity load test uses a rectangular bearing plate, the size is the same as the three pile The pile composite foundation reinforcement area is the same.

The soil between piles, the bearing capacity of a single pile, and the bearing capacity of a three-pile composite foundation must meet the design requirements. The number of hammer blows per 10cm of penetration in the pile body dynamic penetration test (heavy type II) must reach 7 to 10 hit.

During the normal construction stage, standard penetration tests are conducted on the soil between piles, with a detection frequency of every 5 points/1000m2. The gravel piles are subject to dynamic cone penetration (heavy type II) tests based on 1% of the number of piles. . In areas where the number of soil mark penetrations between piles is insufficient, additional detection points should be added to adjacent gravel piles. The inspection work should be carried out 15 days after the pile is completed.

2. Karst subgrade

Different treatment measures are adopted according to different parts and characteristics of karst development. The specific treatment principles are as follows:

① Backfilling rubble: used for To deal with the karst funnels exposed on the surface in the roadbed passing area that cannot be supported and avoided, small mechanical compaction (compaction) is used for layered filling, and strong compaction and pressurization are used on the top surface.

② Covering board: used to treat sinkholes, karst ditches, and gaps that are exposed on the surface and have regional underground karst drainage pipes or connected to underground rivers.

③Strong compaction breakthrough (inspection): It is used to treat the fill roadbed passing through the dissolution trench. The terrain is relatively flat and there are conditions for strong compaction construction.

④ Mortar masonry pillars (walls): For the large caves that have been exposed, they are used when there are conditions for entering the cave.

⑤Grouting filling: Shallow dry caves that affect the stability of the roadbed: When the caves are small filled caves, the drilling grouting method can be used. The grouting process should use low-pressure intermittent grouting ( The grouting pressure is 0.3~0.4MPa, the interval time is 7~8 hours), and the mortar label is M7.5; otherwise, the method of drilling and blasting is used to collapse the cave from the roof into an open ditch, and then fill the cave with excavated stones.

⑥ Retaining and avoidance: When the toe of the fill slope is close to the karst sinkhole, use shoulder or embankment retaining wall support to avoid dealing with karst.

⑦ No treatment: If the thickness of the karst roof is greater than 4 meters and the span is not greater than 6 meters, if the integrity of the roof of the karst cave is good, no treatment will be carried out. In addition, if the burial depth of the karst cave is greater than 20 meters, due to additional stress It is already very small at 20 meters and can be ignored, so deep treatment will not be carried out.

3. Collapses, rock piles and unstable slopes

The geological conditions along the project are complex and there are many exposed lithologies. Affected by tectonic movements and physical and chemical erosion, some It is difficult to avoid collapses, rock piles and unstable slope sections on the route. Measures such as supporting seepage ditches, anti-slip retaining walls, and anti-slide piles will be adopted according to different scales and characteristics. Some sections of the road will be protected by passive flexible protection nets. The following matters should be paid attention to during the construction of anti-slide piles (pile board walls):

1) Check the sliding surface conditions at any time during pile foundation excavation, and catalog lithological data in a timely manner. When the actual conditions do not match the design, time, it should be processed.

2) Anti-slide piles should be constructed in the dry season. During the rainy season, a canopy should be set up at the opening, the lock should be made, and a cofferdam should be built on the ground of the opening.

3) During construction, the deformation and movement of the slope should be monitored.

4) Anti-sliding piles should be excavated in sections by jumping piles. The height of each section should be 0.6 to 0.2m. Sections should not be divided at places where the soil and rock layers change or on sliding surfaces. One section should be excavated and one section should be supported immediately. . Excavation of adjacent piles can only be carried out when the strength of the pile body is not less than 75%.

5) Excavation should be carried out after the final setting of the retaining concrete in the previous section. The support of the retaining concrete formwork should be removed only after the concrete strength reaches the point where the retaining structure can be maintained without deformation.

6) It is strictly prohibited to pile waste residue on the slope. And the slope top ramp construction can only be carried out after the anti-slide pile construction is completed.

7) The concrete on the pile body should reach the design strength before the retaining board can be installed. When the retaining board is installed, it should be backfilled while being installed, and drainage facilities behind the construction board should be prepared. The construction quality standards of anti-slide piles meet the design requirements.

To sum up, combined with the actual roadbed conditions of this project, I have made a brief discussion on several corresponding treatment methods for special roadbeds. Due to my limited experience and level, I only hope to provide some guidance for such projects. The technicians were helpful and referenced.

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