1. After filling the embankment, it will settle rapidly or unevenly. The vertical and horizontal slopes of the road surface will become broken and the driving will be bumpy.

2. Longitudinal cracks appear on the road surface. When severe, the cracks become wider and extend toward the edge of the soil shoulder. The cracks appear staggered, forming a slip surface.

3. Transverse cracks appear on the road surface, and the cracks appear staggered.

2. Causes

1. Design aspects

1.1 Incomplete geological survey data, especially some original ditch pond areas that require foundation treatment are not surveyed It is clear that there are few boreholes or insufficient static cone penetration points for the lateral stratigraphic distribution, and the design basis is insufficient.

1.2 The proposed soft foundation treatment method is inappropriate, the design treatment depth is not enough, and the treatment effect is not obvious.

1.3 The soft soil foundation treatment design does not include a gradient section, resulting in sudden settlement changes at the junction of the treated road section and the non-treated road section.

1.4 The preloading period of equal-load or overloaded preloaded sections is expected to be insufficient.

1.5 The high-fill soil roadbed in the soft soil foundation road section is not equipped with a back-pressure berm in accordance with the regulations or the width of the back-pressure berm is insufficient, resulting in large lateral deformation of the ground during the filling process and operation. The strength is reduced and even causes sliding failure.

1.6 For high-fill soil sections, especially those with serious soil shortages, the investigation and selection of fillers was not thorough, and the changes in soil quality along the depth direction of the designed borrow pits were not listed, resulting in the use of fillers of different soil types during construction. Mixed filling or segmented filling will cause uneven settlement due to variations in compressibility and water resistance of different soil types.

1.7 The design length of the buried passage set up in the high-fill embankment is insufficient and cannot meet the ultra-wide rolling requirements.

1.8 The roadbed drainage design is imperfect.

2. Construction:

2.1 The soft foundation treatment did not reach the designed depth, and the raw materials entering the site were not strictly inspected according to product quality requirements, resulting in the treatment effect not meeting the design requirements.

2.2 The embankment filling speed in the road section with soft soil foundation is too fast.

2.3 Use inappropriate fillers and fail to take corresponding improvement measures or the measures are not in place.

2.4 Mixed filling or segmented filling of different soil types results in variations in water resistance and compressibility.

2.5 Settlement difference caused by failure to properly handle the interface between fill and excavation or non-full-width filling or segmented filling.

2.6 Failure to pay attention to subgrade drainage during construction caused the subgrade to be soaked by rain, and failed to repressurize in time during subsequent construction.

2.7 The moisture content of the embankment fill is not strictly controlled, and the compaction of the fill soil does not meet the requirements.

2.8 When the layered filling is rolled and compacted, the thickness of the compacted layer is thicker and the compaction quality is poor.

2.9 The layered fill has not been initially leveled and is compacted unevenly.

2.10 The construction inspection sampling was not carried out according to the regulations, and the actual measured compaction degree was false.

2.11 The size of the boulders contained in giant-grained soil or coarse-grained soil is too large and difficult to compact evenly.

2.12 The embankment filling process with highly plastic cohesive soil is discontinuous, resulting in a decrease in post-construction compaction.

2.13 The roadbed construction in special areas was not performed according to specifications.

3. Preventive and control measures

1. Design aspects:

1.1 The terrain, landforms and hydrogeological conditions along the route should be investigated in detail, especially the The design of special roadbeds provides more detailed information.

1.2 Soft soil foundation treatment design

Segment calculation and analysis must be performed based on different embankment heights (converted into additional loads), different soft soil burial depths, layer thicknesses and soil properties. Stability and settlement conditions under natural foundation conditions. Carry out calculation and analysis and compare multiple options to determine the option that meets the requirements of stability and post-construction settlement standards.

The design of soft soil foundation treatment must set up a gradient section to avoid sudden changes in subgrade settlement. The design of the gradient section should be carried out according to special design requirements, and lateral deformation needs to be taken into consideration.

1.3 Base design

In order to facilitate pre-fill compaction, if the ground surface is wet and soft, the design may consider measures such as soil replacement or adding lime, cement or laying geotextile; groundwater level For road sections with high or perennial water accumulation, in addition to improving drainage and drainage facilities, it is also advisable to set up a water-proof layer (such as gravel, gravel and other water-permeable materials).

1.4 When using subgrade fillers, the soil distribution, soil properties, and moisture content in the depth direction of the proposed borrow pit or borrow pit should be investigated and listed in order to avoid mixed filling of different soil properties. Or fill in sections.

Avoid using fillers that are not suitable for filling embankments. Modification measures and technical quality indicators must be proposed when difficulties arise, but they cannot be used in sensitive parts such as stability problems and subsidence.

1.5 The design of high-fill embankments should consider using soil shoulders and slope protection or using rapid troughs to divert water away from the embankment to ensure the stability of the high-fill embankment slopes. The width of the revetment should be based on the presence or absence of soft soil. Comprehensive design is carried out based on the nature of the foundation, fill material, and slope ratio.

1.6 In order to reduce the compression deformation of the cohesive soil embankment structure, a small amount of cement can be added in addition to the ash-mixing treatment (the amount of ash-mixing should be appropriately adjusted). In the design process, ash-mixing is recommended (advocated Use quicklime powder) to improve the soil properties and then add cement to further improve the soil properties to increase the structural rigidity of the embankment.

2. Construction:

2.1 The construction unit must prepare a construction organization design and propose a construction organization design in accordance with the relevant construction specifications, procedures, project quality inspection and assessment standards of the Ministry of Communications and the requirements of the construction unit’s bidding documents. Self-inspection requirements require effective quality control and management of the entire construction process. During handover and acceptance, the construction unit shall submit complete and authentic original construction records, test and inspection data, sub-project self-inspection data and other quality assurance information.

2.2 Regarding the buried depth of groundwater and the impact of surface water on the stability and construction of the fill subgrade, supplementary investigations should be carried out according to the design before construction, and corresponding water isolation and drainage measures should be taken.

2.3 Key points of foundation treatment

1) Construction sequence: No matter what treatment method is used, trenches should be dug for drainage according to the design requirements, then the surface should be cleared and leveled, and the ground should be filled before filling. Compact and create a certain cross slope.

For foundations designed to handle vertical drainage bodies, the drainage body can be laid only after laying the lower half layer of sand or gravel cushion. At the top of the drainage body, a certain length (about 30cm) should be reserved according to the design, and finally the upper half layer of sand or gravel layer should be laid.

For foundations designed to be treated with composite foundations, the original ground should be leveled and then properly rolled using light rolling machinery to ensure that it meets the specifications and design requirements before foundation treatment.

The ditch and pond must be dredged, replaced and rolled in layers to the adjacent ground elevation before foundation treatment can be carried out.

2) All materials used for foundation treatment must be purchased, stacked and used according to the quality indicators required by specifications and design.

3) Requirements for measures to ensure construction quality

When soft foundations are treated with plastic drainage boards, the performance of the mechanical equipment should be consistent with the grounding pressure and the bearing capacity of the treated foundation; Plastic drainage boards are strictly prohibited from kinking, breaking and tearing the filter membrane. When cutting the board body, leave enough exposed length. During construction, the elevation of the slab should be strictly controlled according to design requirements. When laying the geotextile, it is required to stretch without wrinkles.

When processing cement mixing piles, the construction machinery should use different construction machinery combinations according to the form of cement injection (i.e. powder spraying method or wet spraying method). When using the powder spraying method, the powder transmitter must be equipped with a powder metering device and accurately record the instantaneous injection amount and cumulative injection amount of cement. Before construction, an indoor formula test should be conducted with the actual cement used to ensure that it meets the design. After requesting, no less than 5 pile forming process tests should be carried out to obtain technical parameters such as drilling speed, lifting speed, stirring speed, jet pressure, and ash injection volume per unit time that meet the designed injection volume; determine the uniformity of stirring; Understand the resistance to lifting of drilled piles and choose reasonable technical measures; determine the re-spraying range according to the stratum and geological conditions. The depth of re-stirring must be guaranteed to be greater than 6m, and static cone penetration or core drilling test methods can be used to detect powder spray piles.

When processing compacted gravel piles, the DE40-60 series tubular vibrating pile sinking machine needs to be used, with a built-in flat-bottom loose-leaf pile tip. The diameter of the pile tube is generally 377mm or 426mm, and it is equipped with secondary feeding. mouth, the maximum pile sinking depth can reach 20m. The construction sequence starts from the periphery and proceeds to the center. Two adjacent piles must be driven by jumping. During the construction process, the soil brought out by the pile pipe should be excavated in time, and the soil from the hole must not fall into the hole. During construction, the depth of pile sinking, pile making time, the amount of gravel poured each time, the number of reverse insertions, etc. should be recorded, and quality self-inspection records should be made according to regulations. If major changes in the soil layer are found during construction, the amount of material input or pile sinking should be recorded. If the speed is abnormal, work should be stopped immediately and reported to the supervisor.

2.4 Key points of embankment filling

1) Embankment filling and rate control: After the foundation treatment is completed, embankment filling should be carried out in a timely manner. After the foundation for vertical drainage body treatment is completed, it can be filled; for the foundation treated with cement mixing piles, it should be filled after one month. The filling rate must be dynamically controlled. When the daily deformation and settlement is not greater than 10mm/d and the horizontal displacement is not greater than 3mm/d, normal filling can generally be performed; if the daily deformation (settlement and displacement) increases sharply, the number of measurements must be increased. Analyze the cause and take necessary measures in a timely manner (such as stopping loading or slowing down the filling rate or unloading, etc.).

2) The filling width should be controlled according to the design construction slope and ultra-wide rolling requirements. The paving thickness should be controlled by wires and checked frequently.

3) Stacking preloading and settlement supplement: the longer the stacking preloading time, the smaller the post-construction settlement will be. Therefore, for road sections with preloading requirements, stacking should be arranged as early as possible during construction. The top surface of the stack should be flat, dense and have a cross slope. After settlement, the filling should be made in time. The thickness of one filling should not exceed the thickness of one layer of filling and should be properly compacted. The construction unit shall measure the settlement amount every month and report to the supervisor once. It is strictly prohibited not to fill during the preloading period, but to fill in the later stages of preloading or during road construction to avoid excessive settlement.

4) Displacement observation: For the safety and stability of embankment construction, displacement observation is more important than settlement observation.

During construction, displacement observation piles must be buried according to regulations, and normal observation records must be maintained.

2.5 Fine-grained soil (including silt, clay soil, etc.) is susceptible to the comprehensive impact of rainfall and temperature. In the construction organization design, the construction period should be reasonably arranged and continuous construction should be organized. Re-pressurization must be carried out after rain. Pay attention to covering during the winter, and re-inspection must be carried out before subsequent construction.

2.6 The stability of the silty soil during the filling process (such as rainwater erosion, etc.) should be analyzed and temporary drainage measures should be taken.

2.7 During the roadbed construction process, different compaction thicknesses should be selected according to different properties of fillers and rolling tool performance. For example, light steel wheel rollers are suitable for pre-pressure and leveling of various fillers, and heavy steel wheel rollers are suitable for For fine-grained soil, sandy soil and gravel soil, heavy-duty tire rollers are suitable for all types of soil, especially fine-grained soil. Goat-foot rollers need to be used in conjunction with steel wheel rollers, which have better compaction effects on fine-grained soil. Vibratory rollers are suitable for sandy soil, gravel (crushed) stone soil and large-grained soil. If used for rolling fine-grained soil, the effect is relatively poor.

2.8 The moisture content of the filler has a great influence on the compaction effect. Before construction, appropriate adjustments should be made based on the data obtained from the standard compaction test, the construction climate conditions and the pressure test results.

2.9 For the treatment of over-wet clay soil, it is recommended to use quicklime powder instead of lump ash to improve the soil properties, and add 2% to 3% cement (it needs to be determined by testing, and the amount of lime can be adjusted appropriately) to increase the pressure. Early strength and stiffness of solid layers, reducing compression settlement.

2.10 When using coarse-grained or large-grained soil to fill the roadbed, the layered compaction thickness should be reasonably determined based on the performance of the compaction machine, and the maximum particle size should be controlled. Generally, the largest particle size in the lower layer of the road bed is The diameter should be no larger than 2/3 of the compacted layer thickness. Giant particles exceeding the limited particle size should be removed before discharging the material. The particle size of the uppermost layer of the road bed should be controlled to be less than 10cm to facilitate the top surface of the road bed. Flatness control.