History of Soft Soil Foundation Development 10, What is Soft Soil Foundation?

Weak soil refers to silt, silty soil and partially alluvial soil, miscellaneous fill soil and other highly compressible soils. Most of the physical characteristics of this type of soil are saturated, containing organic orange build-up, the natural water content is greater than the liquid limit, and the porosity ratio is greater than 1. When the natural porosity ratio is greater than that, it is called silt; When the natural porosity ratio is greater than 1 and less than, it is called silty soil.

The shear strength of this kind of soil is very low, the compressibility is high, the permeability is very small, and it has structure, which is widely distributed in the southeast coastal areas of China and the surrounding of inland rivers and lakes, and is the main soil type of soft soil, commonly known as soft soil. Generally, it has the following engineering characteristics: (1) high water content and large porosity ratio.

Generally, the water content is 35% 80%, and the porosity ratio is 1 2. (2) The shear strength is very low. According to the results of geotechnical tests, the natural undrained shear strength of soft soil in China is generally less than 20kpa, and its variation range is 5 25kpa. The effective internal friction angle is about 20° 35°; The internal friction angle of the consolidated undrained shear is 12° 17°.

The undrained shear strength of the normally consolidated soft soil layer tends to increase with the increase of depth from the surface, and the growth rate is about 1 2 kPa per meter. Accelerating the consolidation rate of soft soil layer is an effective way to improve the strength characteristics of soft soil. (3) High compressibility.

Generally, the compressibility coefficient of normally consolidated soft soil is about 1-2=, and the maximum can reach 1-2=; The compression index is about cc= (4) and the permeability is very small. The permeability coefficient of soft soil is generally about 1 10-6 1 10-8cm s (5), which has obvious structural properties. Soft soils are generally flocculent, especially marine clays.

Once this kind of soil is disturbed, the strength of the soil is significantly reduced, and even it is in a flowing state. The sensitivity of coastal soft soil in China is generally 4 10, which belongs to high sensitivity soil. Therefore, if the foundation treatment and foundation pit excavation are carried out in the soft soil layer, if the structure of the soil is not disturbed, the soil deformation will be aggravated, the strength of the foundation soil will be reduced, and the foundation treatment effect will be affected.

6) It has obvious rheology. Under the action of load, the soft soil bears the action of shear stress to produce slow shear deformation, which may lead to the attenuation of shear strength, and may continue to produce considerable sub-consolidation settlement after the completion of the main consolidation settlement. The treatment methods of soft soil foundation mainly include replacement cushion method, preloading method, strong compaction method and strong compaction replacement method, vibroflotation method, sand and gravel pile method, cement fly ash gravel pile method, cement soil pile method, cement soil mixing method, high-pressure jet grouting method, lime pile method, ash soil compacting method and soil compacting method, column hammer punching and expanding pile method, single liquid silicification method and alkali method.

Please refer to the "Code for Bending and Lifting Techniques of Building Foundation Treatment Techniques" JGJ79-2002 for the scope of application and principles of various foundation treatment methods. Reference: Foundations and Foundations (3rd Edition) China Architecture & Building Press.

1. High compressibility: soft soil has high compressibility due to its porosity ratio greater than 1, large water content, small bulk density, and a large number of microorganisms, humus and combustible gases in the soil, so it is not easy to achieve stability for a long time. Under the same conditions, the higher the plastic limit of the soft soil, the higher the compressibility.

2. Low shear strength: Therefore, the shear strength of soft soil is best tested in situ.

3. Low permeability: the permeability of soft soil is very low, and the vertical layer is almost impermeable, which is not conducive to drainage consolidation, which is reflected in the long duration of building settlement. At the same time, in the early stage of loading, high pore water pressure often occurs, which affects the strength of the foundation.

4. Thixotropy: soft soil is a pure sediment with a flocculated tung matrix structure, and it often has a certain structural strength when the undisturbed soil is not damaged, but once disturbed, the structure is damaged, and the strength is rapidly reduced or quickly becomes a diluted state. This property of soft soils is called thixotropy.

Therefore, after the soft soil foundation is subjected to vibration load, it is easy to produce phenomena such as lateral sliding, settlement and extrusion on both sides of the bottom surface.

5. Rheology: refers to the characteristic that the deformation of soil increases with time under the continuous action of a certain load. so that its long-term strength is much less than the instantaneous intensity.

This is not good for the stability of slopes, embankments, docks, etc. Therefore, the shear strength value is obtained by general shear test, and an appropriate safety factor should be added.

6. Inhomogeneity: Due to the inclusion of fine sand lenses in the soft soil layer, there are obvious differences in the plane and vertical directions, which is easy to produce uneven settlement of the building foundation.

When it comes to soft foundations, relevant construction people are still relatively unfamiliar, the basic definition of soft foundation treatment? What are the soft ground treatment technologies? What are the common problems of soft foundation in the construction of engineering projects?

Weak foundation is a foundation formed by silt, silty soil, miscellaneous fill, flush fill or other highly compressible soil layers, which are basically rarely affected by geological changes or topography, and have never been affected by physical effects such as ** and load, and have not been affected by chemical interactions between soil particles.

In general, in the treatment of soft foundations, what are the common problems of soft foundations?

1) The bearing capacity and stability of the foundation: under the action of the building (structure) load (including static load and dynamic load), when the bearing capacity of the foundation cannot meet the requirements, the foundation will produce local or overall shear failure, which will affect the normal use of the building (structure) and even cause the failure of the building (structure). ——Nansha Coastal Garden.

2) Settlement, horizontal displacement and uneven settlementUnder the action of load (including static load and dynamic load), the foundation is deformed in a trembling manner. When the settlement of the building (structure), or the horizontal displacement, or the uneven settlement exceeds the corresponding allowable value, it will affect the normal use of the building (structure), and even the posture of the bench may cause damage. When the settlement of buildings (structures) is large, the uneven settlement is often larger.

The harm of uneven settlement to buildings (structures) is greater. -- The entire collapse of 13 floors in Shanghai.

3) When the seepage flow or hydraulic ratio drop of the foundation exceeds its allowable value, a large water loss will occur, or the building (structure) damage will be caused by the instability of the foundation due to latent erosion and pipe surge, resulting in an engineering accident.

The drainage consolidation method is one of the effective methods for the treatment of weak foundations.

Vertical plastic drainage board construction.

a Method of construction.

a.Restore the center line, release the side piles of the working section, and clean and level the original foundation. Do a good job in the drainage system to ensure that the drainage channels are smooth to facilitate the drainage and consolidation of soft soil.

b.The sand of qualified quality is transported to the construction site, discharged according to the calculated amount, and the sand cushion is laid and compacted according to the design thickness with a manual bulldozer.

c.Draw a square grid diagram and stake out on site, control it with a square grid, mark the position of the insert board, and control the board spacing error within the allowable range.

d.Choose a plug-in machine (with tire type, chain type, track type), and the machine should be engraved with obvious footage mark. If the track type inserting machine is selected, the track perpendicular to the center line of the roadbed should be laid in advance.

e.In place of the board inserting machine, adjust the flatness of the frame and the verticality of the casing, so that the verticality deviation of the drainage board is controlled within the allowable range.

f.Insert the drainage plate into the casing, start the vibratory hammer, and press the casing and drainage plate into the soil.

g.The length of the drainage board should be sufficient, and the drainage board with lap continuation is not allowed. The depth of the drainage board shall not be less than the design depth.

h.The conveying roller is reversed, the drainage plate is loosened, the casing is lifted, the queuing water plate is left in the soil to meet the design depth, and the length of the casing is not more than 50cm when the casing is lifted.

i.Cut off the drainage plate 20cm above the ground and move to the next hole for construction.

j.After the insertion plate of an operation section is completed, the inserting machine is removed, the sand pad is leveled, the board head is buried, and the construction of the next process is transferred as soon as possible.

b Process flow block diagram.

Main machinery and equipment: plugging machines, bulldozers, generators, dump trucks.

It refers to the land with fewer rocks in the soil, which is generally more humid. Not conducive to construction.