1. Physical and mechanical properties of soft soil
1. The natural water content of soft soil is generally 5% ~ 7%, and the maximum is even more than 2%. The liquid limit is generally 4% ~ 6%, and the natural water content increases directly with the increase of liquid limit. The natural porosity ratio is between 1 and 2, and the maximum is 3 to 4. Its saturation is generally greater than 95%, so the natural water content has a linear relationship with its natural porosity ratio.
2. There are different quantities of thin layer or extremely thin layer powder, fine sand and silt in this soil layer, so the permeability in the horizontal direction is much larger than that in the vertical direction. Because of the small permeability coefficient, high water content and saturated state, this kind of soil not only delays the consolidation process of its soil, but also tends to have high pore water pressure at the initial stage of loading, which has a significant impact on the foundation strength.
3. Soft soil belongs to high compressibility soil, which increases with the increase of liquid limit and natural water content of soil. Due to the soil itself, the deformation of this kind of soil under the action of building load has the following characteristics: (1) The deformation is large and uneven; (2) The deformation stability lasts for a long time.
4. The shear strength of soft soil is small, which is closely related to the loading speed and drainage consolidation conditions. The shear strength obtained by undrained triaxial quick shear is very small and has nothing to do with its lateral pressure. The shear strength under drainage condition increases with the increase of consolidation degree.
5, obvious thixotropy and creep.
2. Common methods of road soft foundation treatment
1. Shallow soft foundation treatment technology
(1) Cushion method. Usually used in areas with low subgrade fill, it is required that the settlement value of soft foundation will not affect the expected design purpose during use. When setting the cushion, different materials can be used according to the specific situation. The commonly used materials are sand or gravel and lime soil, and geogrid, flaky squeezing and gravel cushion can also be used for comprehensive treatment.
(2) replacement method. In highway construction, when the water content is high, the weak layer is shallow, and it is easy to dig out unsuitable materials, the excavation and replacement method is generally adopted, including soft foundation and swamp areas with large compression settlement and even deformation. To deal with this kind of foundation, drainage protection should be done well before excavation, and the excavated unsuitable materials should be transported away or treated, and then backfilled in layers as required. The backfill materials can be sand, gravel, lime soil or other suitable materials according to the specific situation.
(3) crowding out method. When the expressway passes through puddles, fish ponds and deep muddy areas with strong liquidity, it often encounters high water content, high compressibility of silt, muddy clay soft foundation and underwater soft foundation, and this kind of soft foundation can be treated by crowding out method.
(4) surface drainage method. For the soft soil foundation with good soil quality caused by excessive water content, before filling, the ground surface should be excavated to remove surface water, and at the same time, the water content of the surface part of the foundation should be reduced to ensure the passage of construction machinery. In order to give full play to the effect of the excavated trench to achieve blind ditch in construction, gravel or gravel with good water permeability should be backfilled.
(5) additive method. When the surface layer is cohesive soil, human additives are added to the surface layer cohesive soil to improve the compressibility and strength characteristics of the foundation, so as to ensure the running of construction machinery. At the same time, it can also achieve the effect of improving the stability and consolidation of fill. The additive materials are usually quicklime, hydrated lime and cement.
2. Deep soft foundation treatment technology
(1) Bagged sand drain method. The drainage and consolidation measures of bagged sand drain have the advantages of simple construction, low cost and good reinforcement effect. During construction, put the bagged sand into the casing well, pack it tightly, pull out the casing section by section, and lay a horizontal sand cushion or drainage sand ditch on the top surface. Under the action of the upper subgrade filling load, the water in the soft foundation is communicated with the horizontal sand cushion or the vertical and horizontal drainage sand ditch through sand to form a drainage channel, so that the water in the soft foundation is drained away, thus achieving the purpose of drainage consolidation of the soft foundation.
(2) compaction sand pile method. It is formed by squeezing sand into the soil through impact and vibration by using machinery and methods similar to immersed cast-in-place piles. The main function of compaction sand pile is to compact and drain the foundation, so as to improve the overall shear strength and bearing capacity of the foundation and reduce the settlement and uneven settlement of the foundation. This method is generally suitable for sandy soil, but not for saturated soft clay foundation treatment. The standard requirements for sand used for compaction sand piles are basically the same as those for bagged sand wells, but the difference is that the compaction sand piles can also use a mixture of sand and breccia, and the silt content should not be greater than 5%.
(3) vibro-replacement gravel pile method. Gravel pile is a kind of pile which forms a composite foundation with the surrounding soil. The process of treating soft foundation with gravel pile is to use vibroflot to generate horizontal vibration, vibrate while rushing under the action of high-pressure water flow, make holes in soft foundation, and then fill the holes with gravel in batches. At this time, the vibroflot pulls out while vibrating, so that the gravel is compacted by vibration. In soft foundation treatment, especially in transitional sections such as bridge heads with high fill, in order to reduce the deformation of foundation soil, improve the bearing capacity of foundation soil and enhance the anti-sliding stability of foundation soil, it is one of the ideal methods to use gravel piles for reinforcement.
(4) DJM piles. DJM pile is to spray solid materials such as cement powder into the soft soil to be reinforced in the form of mist with the help of compressed air after drilling holes by using powder jet mixing machinery. After in-situ mixing, compression and absorption of water, a series of physical and chemical reactions are generated, which harden the soft soil, form a pile with strong integrity, good water stability and high strength, and form a composite foundation with soil between piles, thus improving the strength of subgrade. It is characterized by rapid strength formation, short preloading time and small foundation settlement.
(5) plastic drain board. Plastic drainage board is a vertical drainage material that can accelerate the drainage consolidation of soft soil foundation. When it is inserted into soft soil foundation under the action of mechanical force, it can collect the pore water discharged from the surrounding soil with low water inlet resistance and discharge it along the vertical drainage channel, so as to consolidate the soil and improve the bearing capacity of the foundation. The plastic drainage board has good mechanical properties, sufficient longitudinal water flow capacity, strong membrane permeability and soil isolation.
(6) reinforced geotextile. Reinforced geotextile is generally laid at the bottom of embankment to adjust the stress distribution of upper load on foundation. Through the longitudinal and transverse tensile strength of reinforced geotextile, the local shear strength and overall anti-sliding stability of foundation are improved, and the lateral extrusion of foundation is reduced. The material strength of reinforced geotextile should not only meet the design requirements, but also the strain at fracture. When the filler is gravel and earth-rock mixture, it must also meet a certain bursting strength. During construction, the reinforced geotextile should be leveled and close to the lower bearing layer, and its overlapping, stitching and anchoring should meet the design requirements.
(7) Steel slag pile method. The steel slag pile method is to use converter steel slag, an industrial waste, as a reinforcement material, to pour it into pre-formed pile holes, and then form a pile body through vibration compaction and water absorption consolidation. The reinforcement mechanism is that the converter steel slag absorbs the moisture in the soft foundation, and the pile body expands to form a main body that is compacted with the surrounding soil, forming an integral stress structure with the foundation.
(8) concrete piles. Low-strength concrete pile is a new type of pile developed in recent years. The composite foundation formed by low-strength concrete pile as vertical reinforcement is generally called low-strength concrete pile composite foundation. Because the low-strength concrete pile composite foundation method can effectively improve the foundation bearing capacity and reduce the foundation settlement, it can deal with various soil foundations such as cohesive soil, silty soil and muddy soil, and the applicable foundation forms are also diverse.
(9) deep mixing. Using other materials such as cement or lime as the main agent of curing agent, soft soil and curing agent are forced to stir in the depth of foundation by special deep mixing machinery, and a series of physical-chemical reactions between curing agent and soft soil are used to form a hard mixing column, which plays the role of composite foundation together with the original soil layer. In the design, the concrete conditions should be analyzed, and the mixture ratio, replacement ratio and pile length should be adjusted according to different geological conditions and load conditions to meet the requirements of bearing capacity and settlement.
iii. detection of soft foundation
the quality detection of replacement and filling includes: layered construction quality inspection and engineering quality acceptance, mainly to check whether the cushion meets the design requirements. The quality inspection of layered construction of cushion can be carried out by ring knife pressing and sand filling. After the replacement, the quality of the project can be accepted according to the requirements of the project, and the acceptance method can be through load test or static penetration test.
pile quality detection methods mainly include load test, static (dynamic) cone penetration test and wave velocity test, etc. The load test mainly tests the bearing capacity of composite foundation, while the static (dynamic) cone penetration test mainly tests the reinforcement effect of soil between piles, and the wave velocity method mainly judges the treatment effect by measuring the change of wave velocity in the base before and after foundation treatment. The main function of cone penetration test is to evaluate the allowable bearing capacity of soil before and after foundation treatment, but it is not suitable for large gravel strata and bedrock.
the detection methods of surface treatment such as dynamic compaction are divided into in-situ test and indoor geotechnical test. Static (dynamic) cone penetration test, pressuremeter test, vane shear test test and plate load test can be used for in-situ test, and the quantity of quality inspection should be determined according to site conditions and highway grade.
The quality inspection methods of deep mixing method include sampling inspection, field load test and excavation inspection. During the construction period, the construction records should be checked in time to ensure the quality of the project.
in practical engineering, various detection methods are often used synthetically, and there are many detection methods for a foundation treatment method. When selecting the detection method, it should be determined according to the site conditions.
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