The cushion method is to dig out the weak soil layer within a certain range under the base of the foundation, then fill it in layers with strong sand, gravel, plain soil, lime soil, and other materials with stable performance and non-erosive properties, and compact it ( or tap) to the required density.
Cushion Method Classification
1. Classified by cushion material: sand cushion, gravel cushion, plain soil cushion, lime soil cushion, fly ash cushion, reinforced soil cushion, etc.
2. Classify according to the function of the cushion: soil replacement cushion, drainage cushion, and reinforced soil cushion.
Applicable conditions for the cushion method: It is suitable for shallow treatment of silt, silty soil, collapsible loess, plain fill soil, miscellaneous fill soil foundations and underground trenches, underground ponds, etc., and the building load is not too large. The treatment depth of the replacement and filling method should usually be controlled within 3m, which is more economical and reasonable, but should not be less than 0.5m.
The Role of Cushion
In various projects, the main functions of the cushion are sometimes different. For example, the cushion under the foundation of a building mainly plays the role of replacing soil; while in projects such as embankments and earth dams, the cushion is mainly used for drainage and solidification. knot effect.
1. Replace soil cushion
(1) Improve foundation bearing capacity
Shear failure in the foundation starts from the bottom surface of the foundation and gradually develops in depth as the foundation pressure increases. Therefore, if the weak soil layer is replaced with stronger sand or other filling materials, the bearing capacity of the bearing layer can be increased and foundation damage can be avoided.
(2) Reduce foundation settlement
The stress in the shallow part of the foundation under the foundation is relatively large, and its settlement generally accounts for a large proportion of the total settlement of the foundation. If the shallow soft soil is replaced with dense sand or dense filling materials, the size of the foundation can be reduced. Partial settlement. In addition, due to the stress diffusion effect of the dense cushion layer, the pressure acting on the underlying soil layer is smaller, so the settlement of the underlying soil layer is also reduced accordingly.
(3) Accelerate the drainage and consolidation of weak soil layers
Due to the high water permeability of cushion materials such as sand or gravel, when the soft soil layer is under pressure, the cushion layer can be used as a good drainage surface, allowing the pore water pressure under the foundation to quickly dissipate, accelerating the degeneration of the soft soil layer under the cushion layer. Consolidation, thereby improving the strength of the foundation soil.
(4) Prevent frost heaving of foundation soil
Since the pores of coarse-grained cushion materials are larger and capillary phenomena are less likely to occur, it can prevent frost heaving caused by freezing in the soil in cold areas.
2. Drainage cushion
Drainage cushion refers to the horizontal drainage layer laid on a soft soil foundation such as a dam or a large-area loading base. It is generally filled with medium-coarse sand or gravel with good water permeability. If necessary, add additional layers on the bottom and upper surface of the cushion. Geosynthetic materials with reverse filtration properties prevent the sand and gravel cushion from being blocked and cracked.
Its main functions are:
(1) As a drainage channel for the horizontal drainage layer and the underlying soft soil layer, it accelerates the drainage and consolidation of the foundation, improves the shear strength of the shallow foundation, and cooperates with sand wells to reinforce the deep soft soil layer;
(2) Constrain the lateral deformation of the weak underlying layer, improve the stability of the foundation, and improve its deformation properties.
3. Reinforced soil cushion
Reinforced soil cushion refers to a composite cushion composed of various types of reinforced materials added to sand, stone, and plain soil cushions. Such as reinforced soil cushions, geocell cushions, etc.
Its main functions are as follows:
(1) Diffuse stress to make compressive stress evenly distributed;
(2) Enhance the integrity and stiffness of the sand and gravel cushion and reduce uneven settlement;
(3) Constrain the lateral deformation of weak soil.
Soil replacement design
Main contents: selection of cushion material, determination of thickness, cushion laying range foundation settlement calculation, etc.
1. Cushion material selection
Cushion materials: sand, gravel, plain soil, lime soil, fly ash, slag, reinforced soil, etc.
Selection principle: Reasonable selection based on local conditions and specific conditions of the project.
2. Determination of sand cushion thickness and width
For cushions made of coarse-grained replacement materials such as gravel, pebbles, sand with gravel, sand, and slag, the compression deformation of the cushion itself has been completed during construction, and the magnitude is very small. Therefore, in the calculation of foundation deformation, it can Ignore this part; but for fine-grained materials, especially thicker replacement cushions, the deformation of the cushion itself should be taken into account.
In the calculation of s1, the stress can be taken as the average value of the pressure on the top and bottom surfaces of the cushion. Generally, Es=20~30MPa for sand cushion; Es=8~20MPa for fly ash, and 30~50MPa for gravel and pebbles. Whether it is a foundation or a cushion, its settlement calculation can be calculated using the traditional layered summation method.
Foundation lime soil cushion construction
1. Determination of thickness
The determination of soil cushion thickness on soft soil foundation is the same as that of sand cushion.
(1) The thickness of the cushion on the non-self-gravity collapsible loess foundation should ensure that the pressure on the natural loess layer is less than its collapse initial pressure value.
(2) For self-weight collapsible loess foundations, the thickness of the cushion should be greater than that of non-self-weight collapsible loess foundations, or the remaining collapsibility should be controlled to not exceed 20cm to achieve good results.
2. Determine the width
According to the treatment scope, plain soil cushion or lime soil cushion can be divided into partial cushion and whole cushion.
(1) Partial cushion: It is generally set under the bottom surface of a rectangular (or square) foundation or strip foundation. The width of its plane treatment range beyond the bottom surface of the foundation should not be less than half of the thickness of the cushion layer.
(2) Entire pad: generally set within the plane of the entire building (except for industrial plants with large spans). The width of each side beyond the outer edge of the building wall foundation shall not be less than the thickness of the cushion and shall not be less than 2m.
Fly ash cushion
The chemical composition of fly ash is very similar to that of natural soil. It has the characteristics of volcanic ash and has pozzolana under humid conditions. It reacts with silica, aluminum oxide, and other substances to form hydration products. , cementing and solidifying the compacted fly ash particles to form a block structure, which can improve the strength of fly ash, reduce compression deformation, and enhance impermeability and water stability.
2. Physical properties
The compaction curve of fly ash is similar to that of cohesive soil, with a relatively wide optimal moisture content range. Its dry density is less sensitive to moisture content than cohesive soil, and the optimal moisture content is easy to control.
Cushion construction
1. Soil compaction mechanism
Soil is generally composed of solid particles, water, and air. The proportion of solid, liquid, and gas phases in the soil reflects the degree of dryness, wetness, and density, which is of great significance to the evaluation of the engineering properties of the soil and is the basis for selecting cushion compaction parameters.
Practice has proved that to achieve the best compaction effect of soil, its moisture content must be appropriate. Rolling over wet soil will produce “rubbery soil”, which will not increase the compactness of the soil. Rolling very dry soil will not fully compact the soil.
1. Optimal moisture content
The peak value of dry density on the dry density-moisture content curve is the maximum dry density, and the corresponding moisture content is the optimal moisture content.
2. The physical meaning of dry density-moisture content curve
(1) When the water content of the clay soil is small, the hydration film is very thin, mainly composed of bound water, and the gravity between particles is large. Under the action of a certain external compaction work, this gravity cannot be overcome and the soil particles are relatively opposite to each other. Movement, poor compaction effect, and low dry density of soil;
(2) When the water content of the soil is increased, the combined water film gradually thickens, and the gravity between particles weakens. The soil particles are easy to move and compact under the same compaction function. The compaction effect is improved, and the dry density of the soil is also increased. improve;
(3) When the water content in the soil increases to a certain level, free water begins to appear in the pores, and the expansion of the combined water film is not obvious. The gravity between particles is very weak, but the free water fills the pores and prevents the soil particles As the water content continues to increase, the movement resistance gradually increases, the compaction effect decreases, and the dry density of the soil decreases.
2. Cushion construction method
There are three methods: mechanical rolling method, heavy hammer tamping method, and vibration compaction method.
(1) Mechanical rolling method
The mechanical rolling method uses road rollers, bulldozers, sheep’s foot rollers, vibrating rollers, or other compaction machinery to compact weak foundation soil or layered soil cushions. It is often used in projects with a wide foundation pit area and a large amount of earthwork to be excavated.
Construction steps:
1. Excavate the soft soil within a certain depth within the proposed construction area to the design requirements;
2. Roll and consolidate the bottom soil;
3. Fill in layers and densify layer by layer.
Construction parameters: such as construction machinery, paving thickness, number of rolling passes and filling moisture content, etc. are determined by construction site tests.
To ensure effective compaction quality, the rolling speed must be controlled. The flat roller is controlled at 2.0km/h, the sheep foot roller is controlled at 3.0km/h, and the vibrating roller is controlled at 2.0km/h.
The depth of surface foundation reinforcement by the rolling method can generally reach 2~3m. Generally, the foundation-bearing capacity of clay soil can reach 80~100kPa after the surface layer is compacted.
Quality inspection:
During construction, the average dry density and moisture content of each layer will be tested. Use the ring knife method or penetration measurement method. Set up an inspection point for each foundation pit or 100~500m2.
(2) Heavy hammer tamping method
The heavy hammer tamping method is to use a lifting machine to lift the tamping hammer (15~30kN) to a certain height (2.5~4.5m), then drop the hammer freely, and repeatedly pound the hammer to strengthen the foundation.
1. Construction sequence
Heavy hammer tamping should be carried out in sequence, one ramming after another. In independent column foundation pits, tamping should be carried out in the order of first outside and then inside. When the bottom elevations of the same foundation pit are different, they should be compacted layer by layer in the order of first deeper and then shallower.
2. Construction points
(1) On-site test compaction;
(2) The moisture content of the soil in the pit should be checked before ramming;
(3) Construction ramming technology;
(4) When there are slopes or adjacent buildings on site, necessary protective measures should be taken.
3. Quality inspection
It is necessary not only to check the construction records and the final settlement amount of the test tamping but also to check the reinforcement quality. The foundation trench has one point every 30m, and the entire foundation has more than two points every 100 m2. The bearing capacity of the foundation compacted by a heavy hammer can reach 100~150kPa.
(3) Plate vibration method
The vibration compaction method uses various vibration compactors to vibrate and compact loose soil.
It is suitable for treating non-cohesive soil or loose miscellaneous fill soil foundations with low clay content and good water permeability.
The working principle of the vibrating compactor is that the motor drives two eccentric blocks to rotate in opposite directions at the same speed.
Large vertical vibration force.
Compaction effect: After compaction of generally mixed fill soil, the characteristic value of the foundation bearing capacity can reach 100~120kPa.
The effect of vibration compaction is related to the fill composition, vibration time, and other factors. Generally, the longer the vibration time, the better the effect. However, after the vibration time exceeds a certain value, the subsidence caused by the vibration is stable, and the vibration will not be effective if the vibration time is continued. further compaction. Therefore, a vibration test needs to be conducted before construction to determine the relationship between stable subsidence and time.
Reference: For construction waste mainly composed of slag, broken bricks, and tiles, the vibration time is about 1 minute or more; for fine-grained fill soil containing furnace ash, the vibration time is about 3 to 5 minutes, and the effective vibration depth is 1.2 to 1.5 m.
The vibration range should be about 0.6m from the edge of the foundation. The two sides of the foundation trench should be vibrated first, and then the middle. The standard for vibrating is that the vibrator vibrates in place and no longer sinks, which is qualified, and is supplemented by a light penetration test. Its uniformity and depth of influence.
3. Sand (gravel, gravel) cushion construction
1. Cushion material
Sand and gravel: Sand and gravel with good particle gradation and hard texture (the unevenness coefficient of the sand material cannot be less than 5, medium-coarse sand is better. A certain amount of gravel can be mixed into the sand, but it must be evenly distributed). Containing grassroots and garbage, the mud content should be less than 5%. For sand cushions with drainage requirements, the mud content should be controlled to no more than 3%.
Lime soil: Silty clay should be used as the soil material, lumpy clay, and sandy silt should not be used, and fresh-slaked lime should be used as lime.
2. Construction points
(1) Construction machinery should be selected according to different cushion materials, and compaction parameters should be determined by field tests.
(2) Different construction machines and tools should use different optimal moisture contents, especially plain soil, and lime soil cushions should be strictly controlled.
The construction moisture content of silty clay and lime soil should be controlled within the optimal moisture content of wop±2%; fly ash should be controlled within the optimal moisture content of wop±4%; for sand and gravel materials, a flat vibrator should be used It can be 15% to 20% when using flat rolling or frog tamping, it can be 8% to 12%, and it should be saturated when using a plug-in vibrator; gravel and pebbles should be fully watered and soaked before tamping.
(3) Undesirable geological phenomena (such as ancient wells, ancient tombs, caves, old foundations, hidden ponds, etc.) under the cushion should be cleaned first before constructing the cushion.
(4) During the construction process, it is strictly prohibited to disturb the soft soil layers such as silt and silty soil under the cushion. To avoid disturbing the soil layer at the bottom of the pit, the soil layer about 200mm thick can be left unexcavated for the time being and then excavated to the design elevation before paving the cushion. For gravel or pebble cushions, a 150~300mm thick sand cushion or a layer of geotextile should be laid at the bottom to prevent local damage to the soft soil surface.
(5) The bottom surface of the cushion should be laid at the same elevation. If the depths are different, the soil surface at the bottom of the foundation pit should be dug into a ladder shape or a slope overlap, and the cushion construction should be carried out in the order of first deep and then shallow, and the overlap should be tamped Press tightly.
(6) When the groundwater level is higher than the bottom of the foundation pit, drainage measures should be taken.
(7) Each layer of cushion should be paved with a thickness of 200~300mm, layered and compacted. Only after each layer is inspected and meets the design requirements can the next layer be constructed.
3. Quality inspection
Construction quality inspection – ring knife method or penetration measurement method.
(1) Ring knife method: Use a ring knife with a volume of no less than 200cm3 to press into the depth of 2/3 of each layer to take samples and measure the dry density. The dry density should not be less than the dry density value of the sand and gravel in the medium-density state.
(2) Penetration measurement method: First scrape off about 3cm thick sand on the surface of the sand cushion, and then use a penetrometer, steel fork, or steel bar to qualitatively check the quality of the sand cushion based on the degree of penetration.
Project quality acceptance – load test.
Things to note when replacing the soil cushion method:
a. Shallow layer (deep construction is difficult);
b. The replacement thickness is 0.5 to 3.0m (too thin is useless, too thick is difficult to construct and expensive);
c. Layered backfilling and compaction (to ensure quality);
d. Check deformation;
e. For large-area and deep cushions, the impact on deformation (the impact on itself and adjacent buildings) should be considered when the replacement material is heavier than the natural soil.
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