1. This expressway is located in Hebei Province. The whole line has been investigated and tested in detail. The distribution law of special soil (including saline soil, soft soil and soft soil) in the line crossing area is found out, and the distribution characteristics and liquefaction grade types of bad geology (sand liquefaction) in the line crossing area are found out.

Through investigation, geotechnical test results, standard penetration test, comprehensive analysis, well consolidation combined with static penetration test, statistics show that soft soil and soft soil are distributed in the route crossing area. One is continuous segment distribution, and the other is discontinuous segment distribution of lens shape. For soft soil and soft soil, the general guiding ideology is as follows: firstly, after analyzing the thickness and formation lithology of hard crust and the thickness and characteristics of each soft soil layer and soft soil, the foundation settlement and stability are checked according to the formation characteristics of hard crust, soft soil and soft soil; According to the checking results and the allowable settlement value in the Code for Design of Embankments on Soft Ground, the following treatment measures can be taken in turn for the settlement overrun section:

(1), sand cushion+geocell (geocell)+surcharge preloading (overload preloading) (mainly for general control section). Sand cushion+geogrid (geocell)+overload preloading is mainly aimed at low subgrade (fill less than 2.5m). If the surface layer is exposed to soft soil and soft soil, a sand cushion (geocell is used for the low subgrade section below 2.5m) will be set. Hard shell layer 1.5m above sand-free cushion. ..

(2) Treatment method of sand cushion+geogrid+vertical drainage body (bagged manhole)+surcharge preloading (mainly for general control section).

(3) Treatment method of geogrid+deep cement-soil mixing pile (mainly for the main control section and secondary control section of settlement on both sides of pile foundation, lower part of box foundation and both sides).

(4) Treatment method of dynamic compaction replacement method (mainly for the main control section and secondary control section of settlement on both sides of pile foundation, lower part of box foundation and unsaturated soft soil section).

2. The design standard is divided into three types of control sections to control post-construction settlement according to the different foundation types of bridge and culvert structures in soft soil and soft soil distribution areas along the whole line.

30m sections on both sides of abutment of pile foundation structure are used as box-shaped channels of main settlement section, 20m sections on both sides of culvert are used as secondary settlement control sections, and the rest are used as general control sections:

(1) The allowable value of post-construction settlement of the control section is not too large (10cm).

(2) The allowable post-construction settlement of the secondary control section is not more than 20cm.

(3) The general control section allows the post-construction settlement to be no more than 30cm.

3, soft foundation treatment scheme

(1) Design standard of sand cushion The design standard of sand cushion in the geological unit model above is: the material of sand cushion is medium sand and coarse sand, the silt content is not too large, 3%, and the width of sand cushion should be as wide as the bottom of embankment, so as to prevent the damage of construction machinery during construction from affecting the effective function of sand cushion (the width of both sides is about 0.5m, and the thickness of sand cushion is 0.5m, and at the same time, in order to increase the shear strength of foundation soil and improve the overall stability of embankment.

(2) Design standard of bagged manholes According to the stratum structure characteristics of soft soil and soft soil distribution areas in the working area, it is appropriate to use bagged manholes as vertical drainage bodies for surcharge preloading. Bagged manholes are arranged in an equilateral triangle. Bagged manholes have a diameter of 7 cm. The sandbag is made of geotextile (polypropylene fabric) with good water permeability. The manhole spacing is 1.2m, and the manhole depth should generally penetrate soft soil and soft soil. When conditions permit, the bottom of manhole should reach the permeable layer.

(3) Design standard of deep cement-soil mixing pile:

Deep cement-soil mixing piles are used on both sides of pile foundation, lower part of box foundation and both sides. The mixing piles are arranged in a regular triangle, with a pile diameter of 0.5m and a pile spacing of1.0-1.4m. The pile spacing gradually changes from dense to sparse, and the cement content is 15% of the quality of the reinforced soil. The water-cement ratio is 0.5. The 28-day unconfined compressive strength of the pile shall not be less than 1.5Mpa, and the bearing capacity of the single pile shall not be less than 150KN in 90 days. The bearing capacity of unit composite foundation is not less than 150Kpa, and the cement is 425 # slag cement. The buried depth of the foundation should be considered in the lower treatment section of the structure. Within the buried depth of foundation, shotcrete and mixing can be stopped.

(4) The design standard of dynamic compaction replacement method is based on the distribution and development characteristics of soft soil in the project area. According to the characteristics of this part of soil, we adopt dynamic compaction replacement method to improve the bearing capacity of foundation and reduce settlement. Dynamic compaction replacement material is graded macadam, the particle content greater than 300mm should not exceed 30%, and the silt content should be less than 10%. Reduce investment. The filler can be replaced by construction waste, and it is required to be a coarse-grained material with hard texture and good grading, and the particle content greater than 300ram should not exceed 30%. The mud content is required to be less than 10%, which will not affect the protection of the environment and groundwater resources. The weight of the rammer is 5 ~ 10t, the falling distance is 10m, and the bottom of the rammer is round with a diameter of 2m. The rammed pits are arranged in an equilateral triangle with a spacing of 4m. After treatment, the bearing capacity of unit composite foundation shall not be less than 150Kpa. The depth of dynamic replacement pier is determined by the soil condition and the upper load, generally 3 meters, and the treatment range is to the toe of subgrade slope. Trial tamping shall be carried out before dynamic compaction construction in each treatment section. Determine tamping energy and tamping times, and adjust design parameters. The buried depth of the foundation should be considered in the lower treatment section of the structure. Before the construction of dynamic replacement pier, it should be excavated to the buried depth of foundation.

(5) Action mechanism of geosynthetics A layer of geosynthetics with certain strength is laid in the middle of sand cushion, which increases the shear strength of foundation soil and improves the stability of embankment. At the same time, the composite material has a certain stiffness. Effectively adjust the upper load, reduce differential settlement, and have good uniformity. Because the composite material can bear large tensile force, the stress on the foundation becomes smaller, and the settlement of the embankment center is obviously reduced. Due to the integral action of geosynthetics and sand cushion, not only the uneven settlement is reduced, but also the total settlement of foundation is reduced, which is suitable for the rapid filling of embankment and the rapid increase of load. Accelerated the consolidation of soft soil. So as to speed up the solution of the problem. Reduce late settlement and form a virtuous circle.

(6) Preloading embankment engineering; For natural foundation or vertical drainage foundation in soft soil area, soil and preloading are not needed for embankment filling preloading. Mainly reflected in the step-by-step loading, the stability of each loading depends on the improvement of strength after the previous preloading. In this case, the total settlement of soft soil foundation has not decreased. Only when most of the settlement is completed during the construction period can the post-construction settlement be effectively reduced.

Drainage system is a means, if there is no pressurization system, the water in the pores can't be discharged naturally without pressure difference, and the foundation can't be reinforced. On the contrary, if only the consolidation pressure is increased, the drainage distance of soil layer will not be shortened. It is impossible to complete the settlement required by the design as soon as possible during the preloading period.

Quality, investment and progress, so its importance has been more and more recognized by people.

Second, the purpose of foundation treatment

The purpose of foundation treatment is to reinforce the foundation soil by means of replacement, tamping, compaction, drainage, cementation, reinforcement and heating, so as to improve the engineering characteristics of the foundation soil.

(1) Improve the shear strength of foundation.

(2) reduce the compressibility of foundation

(3) Improve the water permeability of the foundation.

(4) Improve the dynamic characteristics of the foundation.

(5) Improving the bad geological characteristics of special soil The object of foundation treatment is soft soil foundation and special soil foundation.

Three, soft soil subgrade treatment to follow the construction principles

Construction season: give priority to the construction in non-rainy season, and choose the time with less continuous rainfall according to the weather forecast data. Soft soil foundation treatment includes the following methods: sand replacement cushion, dry gravel cushion, preloading and overload preloading, geotextile, unidirectional geogrid, bidirectional geogrid, geocell and mixing pile.

Process arrangement: Mechanized rapid construction is adopted, and excavation, replacement, protection and reinforcement, waterproof and drainage facilities and other processes are completed in one go, so as to shorten the exposure time of the working face as much as possible. Construction shall be carried out in strict accordance with the technological requirements of various treatment methods. Culvert engineering in soft foundation section should be slotted after subgrade preloading expires and settlement is basically completed.

Third, the foundation treatment method

The methods of foundation treatment can be classified according to the principle, purpose, nature, opportunity and motivation of foundation treatment.

1. General embankment shallow treatment construction

Drainage sand cushion is adopted, with geogrid at the top of the drainage cushion, dry masonry for slope protection, and geotextile filter behind the slope protection.

1. 1. Replace the gravel soil cushion.

1.2 sand cushion construction.

Sand should be medium and coarse sand. Before construction, the sand field should be investigated and sampled and analyzed in time. The fineness modulus, silt content and harmful substance content are mainly determined. Only sand that meets the design standards can be used. During construction, firstly, the turf and sundries on the ground within the scope of reinforcement shall be removed, and the soil with the same soil quality shall be filled into a cross slope with a gradient of 3-4% and compacted.

Layered filling: the sand cushion is filled in two layers, and each layer is compacted 25cm thick. According to the qualified fillers and process parameters determined by the test, the sand cushion is filled and compacted in layers.

Paving and leveling: In order to ensure that the embankment is compacted evenly and the thickness of the filler layer meets the requirements, the filler is leveled initially by bulldozer and leveled twice by scraper, so that the smoothness of the filler paved surface meets the requirements.

Sprinkling water or airing: the water content of sand directly affects the compaction density. Under the same rolling condition, the compactness is the largest when the optimum water content is reached, and the fluctuation range of filler water content is controlled within the range of +2% ~-3% of the optimum water content. When the optimum water content exceeds 2%, it is dried, and when the water content is lower than the optimum water content, it is sprayed. Sprinkle water with sprinkler, dry it naturally, and turn it over with rotary cultivator if necessary.

Mechanical rolling: Rolling is the key process to ensure that the sand cushion meets the compactness requirements. Rolling should be carried out according to "static pressure first, then vibration rolling"; "light before heavy"; "Slow first and then fast"; The principle of "two sides first, then the middle".

Inspection visa: K30 load meter detects foundation coefficient, and nuclear density meter detects compaction coefficient.

Construction waterproof and drainage: After the construction of sand cushion is completed, temporary drainage ditches shall be dug on both sides so that the water discharged into sand cushion can be discharged in time. Strict management of construction water and domestic water, so as not to scour the various parts of subgrade and borrow soil.

1.3. Construction of treating soft soil foundation with unidirectional single-layer geogrid

During construction, firstly, the turf and sundries on the ground within the scope of reinforcement shall be removed, and the soil with the same soil quality shall be filled into a cross slope with a gradient of 3-4% and compacted. Fill it with 30cm thick medium-coarse sand, compact it to the design requirements, level the surface, remove the stones on the surface, fill up the pits formed after the stones are removed, and then lay a one-way geogrid on it. The laying of geogrids requires that the vertical joints between the two webs should be closely attached, and the horizontal joints should be connected by connecting rods or lap joints. The connection strength should not be lower than the design strength, and the horizontal joints should be staggered not less than1m. When laying, make the grid close to the soil layer, and fix the grid on the soil layer with U-shaped nails at regular intervals. After the grid is laid, cover it with sand or other permeable materials in time, with a thickness of 20cm, and lay back the folded sand according to the design requirements, and fold it back to 2m one by one, and hold it down with sand. And then leveling and compacting to meet the design requirements before subgrade filling. In the process of subgrade filling and rolling, the construction machinery shall not be directly used on the unfilled grille to avoid rolling the grille.

1.4. construction of dry rubble and geotextile filter layer at slope angle

Dry masonry is generally used to protect the drainage cushion for shallow treatment of embankment. After the construction of drainage cushion and geogrid is completed, dry masonry slope protection angle construction will be carried out.

(1), measure and set out before the construction of dry-laid rubble, set horizontal and elevation control piles for the slope angle of dry-laid rubble every 10m, and do a good job of protection after setting control piles.

(2) The bottom elevation of dry rubble is 20cm higher than the bottom standard of drainage cushion, and the drainage gravel cushion should be touched as little as possible when the foundation of slope protection angle is excavated. After the foundation of slope protection angle is excavated to the design elevation, measure the hanging line and dry build by laying bricks or stones for slope protection angle.

(3) Rubble used in masonry must meet the specification requirements.

(4), dry build by laying bricks or stones flaky slope protection Angle adopts layered masonry, masonry stones face down, pay attention to the staggered joints between the upper and lower stones, build by laying bricks or stones joint width is 2 ~ 4cm；; When building, fill the joints with sand and pebbles between them. Avoid straight seams, empty seams and blind seams when laying.

(5) Set drainage holes according to design requirements. The drainage holes are made of hard plastic pipes, with a diameter of 15cm and a spacing of 1.5m, and a slope of 2% with the ground. The thickness of the filter layer is the angle height of the slope protection, and the filter layer is made of geotextile wrapped with gravel.

2. General subgrade mixing pile preloading treatment construction

Drainage gravel cushion is adopted, and double-layer bidirectional geogrid is adopted. Geogrids are arranged at the bottom and top of the drainage cushion, and preloading is carried out by preloading earthwork. Back-pressure berm is used to pre-press the part crossing ditches and ponds.

2. 1. Construction of gravel cushion

Gravel with good gradation should be selected, the maximum particle size should not exceed 8cm, and the silt content and stone powder content of gravel should meet the specification requirements. During construction, firstly, the turf and sundries on the ground within the scope of reinforcement shall be removed, and the soil with the same soil quality shall be filled into a cross slope with a gradient of 3-4% and compacted.

Layered filling: Gravel cushion is filled in two layers, each layer has a compaction thickness of 25cm, and it is filled and compacted in layers according to the qualified fillers and process parameters determined by the test.

Paving and leveling: In order to ensure that the embankment is compacted evenly and the thickness of the filling layer meets the requirements, bulldozers are used for initial leveling, and manual stringing is used for secondary leveling, so that the surface smoothness of the filling meets the requirements.

Vibration compaction: vibration is the key process to ensure that the gravel cushion meets the compactness requirements. Adopt frog rammer and flat rammer to tamp. Compaction should follow the principle of "two sides first, then the middle".

Inspection and visa: K30 load meter is used to test the foundation coefficient of gravel cushion.

2.2. Two-way double-layer geogrid construction

During construction, firstly, the turf and sundries on the ground within the scope of reinforcement shall be removed, and the soil with the same soil quality shall be filled into a cross slope with a gradient of 2% and compacted. The laying of geogrids requires that the vertical joints between the two webs should be closely attached, and the horizontal joints should be connected by connecting rods or lap joints. The connection strength should not be lower than the design strength, and the horizontal joints should be staggered not less than1m. When laying, make the grid close to the soil layer, and fix the grid on the soil layer with U-shaped nails at regular intervals. Fill it with gravel cushion. After the construction and compaction of the gravel cushion meet the design requirements, level the surface, remove the stones on the surface, fill up the pits formed after the stones are removed, and then lay the upper geogrid on it. After the grating is laid, it should be covered with sand or other permeable materials in time. The folded sand should be laid back according to the design requirements, and the outer side should be folded back by 2m one by one, and pressed with sand. Then leveling and compaction are carried out to meet the design requirements. During subgrade filling, the thickness of each layer is 25cm, and the spacing between upper and lower geogrids is 50cm. The longitudinal lap or binding of adjacent grid volumes shall be no less than 30cm, and the binding lap length shall be no less than 10cm. The binding method is to spirally bind the upper and lower grid strips together with polyethylene rope. When laying the grid, make the grid stick to the soil layer smoothly along the foundation direction, and the grid should not wrinkle. The overlapping place should be fixed on the soil layer with U-shaped nails, and fixed at regular intervals with U-shaped nails, so that the grid can be closely attached to the soil and the laying quality of the grid can be guaranteed. After laying the grid, fill the grid in layers according to the design requirements until a layer of grid is laid. During the rolling process, the construction machinery shall not be directly used on the unfilled grid to avoid rolling the grid. The overlapping position of the upper and lower grids shall be staggered by no less than1m.

2.3. Mixing pile construction

(1), material ①, ordinary portland cement of 42.5 or above is used as curing agent, and the cement content is generally 15% of the weight of reinforced soil according to the reinforcement strength, so as to ensure that the unconfined compressive strength of cement-soil after 90 days is not less than 1.5 MPa(2) In order to improve the performance of cement-soil and the strength of pile, it is approved by the supervision engineer according to the test. ③ The mixing ratio is cement: sand = 1: 1 ~ 1: 2. In order to improve the workability of cement mortar and facilitate pumping, water reducing agent can be added, the dosage of which is 0.2 ~ 0.25% of the cement dosage, and sodium sulfate can be added, the dosage of which is 1% of the cement dosage.

(2) Before site cleaning, find out the buried depth and location of underground obstacles, dig out deep pits and make clear signs; Clean up the sundries in the construction area, and then bulldoze and compact them.

(3) Check the datum points, axes and leveling points of the construction site by measuring and positioning; Clearly mark and number the calibrated datum points and properly protect them; Use total station, theodolite, etc. Determine the boundary line of pile location area by coordinate method; Measure and locate all piles in the construction area, and make obvious and reliable pile location marks. At the same time, make measurement records for review.

(4), pile machine in place ①, the specialist is responsible for the unified command of pile machine in place, before moving the pile machine, first find out whether the pile machine is up and down, left and right, before and after the parties are barrier-free, if there is any resistance, eliminate it in time. After moving, check the positioning of the pile driver, and correct it in time if there is a big deviation. (2) After the pile machine is in place, the geometry is stable, its power head and mixing head are on the same vertical line with the center of the pile position, and the positioning deviation of mixing head is not more than 5cm.

(5) Ready-mixed sinking ①. Hang the mixer on the crane with steel wire rope, and connect the mortar pump with the deep mixer with a slurry hose. After the cooling water of the mixer circulates normally, start the mixer motor and loosen the steel wire rope of the crane, so that the mixer can stir and cut soil along the guide frame by the self-weight of the equipment. (2) During construction, the sinking speed is strictly controlled by the current monitoring table of the motor, which is 0.38 ~ 0.75 m/min. (3) Observe the working load of the power head closely, and control the working current index to be no greater than the rated value, so as to avoid burning the motor. (4) When the sinking speed is too slow, a small amount of mud is pressed through the central pipe to make the soil moist, thus accelerating the sinking speed.

(6) Prepare cement slurry. While the mixer is sinking, use mortar mixer to mix cement slurry according to the design. When the deep mixer sinks to a certain depth, pour the cement slurry into the hopper before grouting.

(7) After the mixer sinks to the design depth, raise the mixing head by about 0.2m, and then start the mortar pump to continuously press the cement slurry into the foundation from the central pipe of the mixer. When the slurry reaches the nozzle, spray the slurry and lift the deep mixer at a constant speed of 0.3 ~ 0.5m/min. When the concrete sprayed by the deep mixer is lifted to 0.5m above the designed pile top elevation, the mortar pump is turned off to complete the first grouting lifting and mixing. (8) Repeatedly stir up and down (secondary grouting lifting mixing) When the mixer is lifted to the top elevation of the designed reinforcement depth, all the cement slurry in the hopper will be discharged. In order to mix the soft soil and cement slurry evenly, the mixer is sunk into the soil while rotating again, and then lifted off the ground after the reinforcement depth is designed. When the mixer is lifted for the second time, the cement slurry continues to be pressed into the stratum. Finish the second grouting, lifting and mixing, so as to complete a columnar solid, which is in the shape of "8" and superimposed one by one, that is, a wall-shaped solid.

(9) Cleaning: inject a proper amount of water into the hopper, start the mortar pump, clean the residual cement slurry in all pipes until it is basically clean, and clean the soft soil adhered to the mixing head.

(10). Move the pile driver. After the mixer is lifted off the ground, turn off the motor, then move the pile driver to a new pile position, and repeat the above steps for the construction of the next pile.

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