In order for people of all ethnic groups in our great motherland to steadily march towards a new socialist society, we diligent road builders must lay a solid foundation. Otherwise, soft and squishy, so scary!

So what if we accidentally encounter soft soil foundation during construction? How to deal with it? The methods for soft soil foundation treatment have undergone significant changes in the 2017 edition of the highway textbook. Based on the worry that every change will be tested and a responsible attitude towards all students of our Construction Engineering Education Network, we must summarize the applicable conditions, construction machinery, and construction regulations of each construction method. We hope all students roll up their sleeves and read carefully; no one can fall behind on the way to the second-level constructor exam!

1. Cushion and shallow treatment

(1) Cushion and shallow treatment are suitable for shallow soft foundation treatment where the surface soft soil thickness is less than 3m. Shallow treatment can adopt methods such as replacement cushion, throwing stones to squeeze mud, stabilizer treatment, etc., and the treatment depth should not exceed 3m.

(2) Sand-gravel cushion should use well-graded, hard medium-coarse sand or sand-gravel; fly ash cushion can use silicon-aluminum low-calcium fly ash discharged from power plants; lime-soil cushion should use grade III calcareous hydrated lime or grade II magnesian hydrated lime; throwing stones to squeeze mud should use large-particle unweathered stone materials.

(3) Crushed stone, sand-gravel, stone chips, and slag cushion construction can adopt rolling method, plate vibrator, or immersion vibrator; the cushion should be laid horizontally. When the ground has undulating slopes, steps should be excavated, and the step width should be 0.5-1.0m.

(4) The maximum dry density and optimum moisture content of fly ash cushion construction should be determined by indoor compaction tests.

(5) Lime-soil cushion construction should first implement drainage facilities; during segmented construction, the construction joints of the upper and lower layers should be staggered by no less than 0.5m.

(6) During stone throwing to squeeze mud construction, when the underlying stratum has a significant transverse slope, it should expand from the higher side of the underlying layer to the lower side; after the stones are above the water surface, heavy machinery should be used for compaction, then a reverse filter layer should be set on top, and then fill and compact.

2. Vertical drainage body

(1) Vertical drainage bodies are suitable for soft soil foundation treatment with depth greater than 3m. When used for treating muddy soil and mucky soil foundations, they should preferably be used in combination with loading preloading or vacuum preloading schemes. When using vertical drainage bodies to treat soft soil foundations, sufficient preloading period should be ensured.

(2) Bagged sand wells and plastic drainage boards can be constructed with casing-driven pile drivers, and plastic drainage boards can also be constructed with board inserters. Bagged sand wells should use circular casings, and the inner diameter of the casing should be slightly larger than the sand well diameter; plastic drainage boards should use rectangular casings, and circular casings can also be used. Equipment capable of detecting the construction depth of drainage bodies should be provided.

(3) The length of the sand bag top embedded in the sand cushion should not be less than 0.3m, and it should be embedded vertically, not horizontally.

(4) Construction process of bagged sand wells: level the original ground → spread the lower sand cushion → position the machinery → drive the casing → sink the sand bag → extract the casing → move the machinery → bury the sand bag head → spread the upper sand cushion.

(5) Construction process of plastic drainage boards: level the original ground → spread the lower sand cushion → position the machinery → put the plastic drainage board shoe → insert the casing → extract the casing → cut the plastic drainage board → move the machinery → spread the upper sand cushion.

3. Vacuum preloading

(1) The vacuum preloading method is suitable for soft soil foundation treatment where the soft soil properties are very poor, soil sources are scarce, and the construction period is tight.

(2) Conditions for stopping air extraction in vacuum preloading construction: the measured settlement rate for 5 consecutive days and nights is less than or equal to 0.5mm/d; the requirements for settlement and bearing capacity of the project are met; the foundation consolidation degree reaches more than 80% of the design requirement.

4. Granular pile

(1) Granular piles can be formed by vibro-replacement method or vibration-driven casing method. The vibro-replacement method is suitable for treating soft soil foundations with a vane shear strength of not less than 15kPa; the vibration-driven casing method is suitable for treating soft soil foundations with a vane shear strength of not less than 20kPa.

(2) The vibro-replacement method can use a vibrator, crane, or special construction flatbed truck and water pump. Lifting machinery can use crawler or tire cranes, self-propelled derrick special flatbed trucks, or torsion-resistant rubber hose special vehicles, etc. The vibration-driven casing method should preferably use a vibratory pile driver and steel casing.

(3) Process flow: level the ground → position and center the vibrator → drill the hole → clean the hole → add material and vibrate to compact → stop the machine and water → move the vibrator.

5. Reinforced soil pile

(1) Reinforced soil piles are suitable for treating soft soil foundations with a vane shear strength of not less than 10kPa and an organic matter content of not more than 10%.

(2) During dry jet mixing pile construction, changes in relevant parameters such as powder injection pressure, instantaneous powder injection amount, cumulative powder injection amount, drilling speed, and lifting speed should be recorded at any time. During wet jet mixing pile construction, slurry exceeding 2 hours should be discarded, and changes in relevant parameters such as slurry injection pressure, slurry injection amount, drilling speed, and lifting speed should be recorded at any time.

6. Cement-fly ash-gravel pile

(1) Cement-fly ash-gravel pile (CFG pile) is suitable for treating soft soil foundations with a vane shear strength of not less than 20kPa.

(2) CFG piles should preferably be formed by the vibration-driven casing cast-in-place method, and the construction equipment should preferably be a vibration-driven casing pile driver.

(3) The mixing time of the mixture shall not be less than 1min; the over-pouring height at the pile top should not be less than 0.5m; when the design pile spacing is small, construction should be carried out in a sequence of skipping every other pile. The interval between driving a new pile and an already driven pile should not be less than 7d.

7. Rigid pile

(1) Rigid piles are suitable for high embankment sections, bridge heads, or channel-embankment connection sections with large loads and strict deformation requirements on deep soft soil foundations.

(2) Prestressed concrete thin-walled pipe piles should preferably be constructed with a static pile driver, and can also be constructed with a hammer pile driver. The construction site should be equipped with lifting equipment, and its lifting capacity should preferably be greater than 5t. Cast-in-place concrete large-diameter pipe piles should preferably be constructed with vibration-driven casing equipment.

8. Explosive squeezing of mud

(1) The explosive squeezing method is suitable for treating mucky and muddy soil foundations such as bay tidal flats. The treatment thickness should not exceed 15m.

(2) Construction units engaged in blasting work should obtain a blasting operation permit issued by the local public security department, and personnel engaged in blasting work should hold certificates. When electric detonators are used as initiation devices, two parallel-connected electric detonators from the same factory and same batch number should be used.

9. Embankment foundation isolation wall

(1) Embankment foundation isolation walls are suitable for isolation between two adjacent embankments, or between an existing embankment and a widened embankment when mutual interference occurs and adversely affects foundation seepage, deformation, stability, etc.

(2) Isolation walls can be divided into anti-seepage isolation walls and retaining isolation walls according to their role and function. For adjacent embankments, when the embankment to be built adopts foundation treatment methods such as dewatering preloading, vacuum preloading, enhanced consolidation, or engineering measures such as deep well dewatering, anti-seepage isolation walls should be set; in other cases, retaining isolation walls should be set to isolate the mutual interference between new and old roadbeds.

(3) Cement mixing pile anti-seepage or retaining isolation walls should preferably use wet jet mixing piles; high-pressure jet grouting cement-soil anti-seepage isolation walls should preferably be constructed with the single-pipe swing jet grouting method.

10. Dynamic compaction and dynamic replacement

(1) The dynamic compaction method is suitable for treating foundations such as gravel soil, low-saturation silt and Mongolian soil, miscellaneous fill, and soft soil.

(2) The dynamic replacement method is suitable for treating high-saturation silt and soft-plastic, flow-plastic soft clay foundations, and the treatment depth should not exceed 7m.

(3) The mechanical equipment for lifting the tamper should preferably be a crawler crane, and the tamper can be a reinforced concrete tamper or a cast steel tamper. When dynamic compaction is used to reinforce Mongolian soil foundations, a tamper with a larger bottom area should be used. For dynamic replacement, a slender cast steel tamper should be used.

(4) Before dynamic compaction and dynamic replacement construction, trial compaction should be carried out to determine parameters such as single-click compaction energy, number of compactions, number of passes, and interval time.