1. Engineering geological conditions.
The lithology and distribution law of foundation soil in the sub-central area of a city are divided into three categories: artificial accumulation layer, new sedimentary layer and Quaternary alluvial layer according to the genetic age; According to the lithology of the strata, it is further subdivided into 10 large layers and their sublayers. The lithology and characteristics of each layer are as follows from top to bottom.
The first type is the artificial accumulation layer. Layer: fill, yellow-brown, loose slightly dense, slightly wet, very wet, mainly silty clay clayey silt, containing plant roots, brick slag, ash slag; This large layer is widely distributed, and the maximum thickness exposed by the drill hole is 700m, the minimum thickness is 040m, the lowest floor floor elevation is 12390m。
The second type of strata is the recent sedimentary layer. This layer contains , two layers. Layer: clayey silt, sandy silt, fine sand layer, brown yellow, medium density, wet, containing mica, iron oxide, uneven soil quality. The maximum thickness of this large layer of drilling is 860m, the lowest floor bottom elevation is 11300m。Layer: silt, brownish, yellowish, yellowish, slightly dense, medium-dense, very wet, containing mica, quartz, feldspar, and a thin layer of sandy silt. The maximum thickness of this large layer borehole is 7100m, the lowest floor bottom elevation is 6520m。
The third type of strata is the Quaternary alluvial deposit. Layer: fine sand, medium sand, gray-yellow-gray, medium-dense and compact, very wet, containing mica, quartz, feldspar, locally round gravel, and the sand layer is hard. Silty clay is distributed only locally Heavy silty clay with a lenticular distribution. The maximum thickness of this large layer of borehole exposure is 12600m with a minimum floor elevation of 1840m。
The characteristics of each layer of soil engineering are shown in Table 1.
2 Overview of the composite foundation of long spiral pressure perfusion solidified soil.
Long spiral bored pressure perfusion solidified soil pile composite foundation refers to the long spiral bored pressure perfusion solidified soil pile arranged in the foundation soil, and the long spiral bored pressure perfusion solidified soil pile and the foundation ** bear the upper load. The basic structure is shown in Figure 1.
As can be seen from Figure 1, the composite foundation is an organism composed of building (structure) foundation, cushion, pile (composite foundation reinforcement) and pile, all of which are indispensable. Its mechanism of action is as follows: when the foundation is subjected to the load of the building, the load is transferred to the pile and the soil between the piles through the foundation, and the pile and the soil between the piles are deformed by force. The modulus of the pile is greater than the modulus of the soil between the piles, and the deformation of the pile is less than the deformation of the soil between the piles. Along with this process, the mattress layer is constantly adjusted and filled to the soil between the piles. Ensure that the pile and the soil between the piles are always involved in the force.
In the calculation of composite foundation, the deformation modulus of composite foundation is required, and it is found that the modulus is composed of the modulus of the pile and the soil between the piles. The soil modulus between piles can be obtained by experiments, and the determination of pile modulus is currently discussed.
The analysis shows that the vertical displacement of the pile top is generated under the load. s=sc+ss, sc is the compression of the pile itself; ss is the rigid displacement of the pile top. In other words, the ability of the pile to resist deformation is composed of two parts, one is the ability of the pile material itself to resist deformation, and the other part is the ability of the pile side and pile end soil to resist deformation.
The focus of the discussion is whether the modulus of the pile is based on the compressive modulus of the pile itself, or the deformation modulus generated by the settlement deformation of the pile under load. According to the analysis, it mainly depends on the material properties of the pile. For piles with general bond strength, the proportions of SC and SS in the total displacement S of the pile top are not negligible. The deformation modulus of the pile depends not only on the ability of the pile itself to resist deformation, but also on the action of the soil on the side of the pile and at the end of the pile.
The above discussion shows that for the composite foundation of the long spiral bored pressure perfusion fluidized solidified soil pile, the pile modulus of the fluidized solidified soil pile is adjustable, and the adjustment range is large. The modulus of the pile body is high, and SC can be approximately equal to 0, that is, S ss, and the deformation modulus of the pile is controlled by the pile side and pile end soil. The modulus of the pile is low, and SC is not equal to 0, that is, S SC+SS. The deformation modulus of the pile is not only related to the pile material, but also related to the geometric size of the pile (pile diameter, pile length) and the properties of the pile end soil on the side of the pile.
In-depth study of the mechanical properties of materials and the selection of reasonable compressive modulus in engineering are one of the important technical topics in the design and construction of long-spiral borehole pressure perfusion solidified soil piles.
3. Design and calculation method of fluidized solidified soil composite foundation.
The design and calculation of the composite foundation of the long spiral borehole pressure perfusion flow state solidified soil pile can refer to JGJ79-2012 "Technical Code for Building Foundation Treatment", and the design basis of the composite foundation includes the superstructure load and the soil engineering characteristics of the foundation; The design content includes the bearing capacity of the composite foundation, the settlement deformation of the composite foundation and the stability of the foundation. The main parameters of the design of the composite foundation include pile length, pile diameter, pile spacing, pile plane layout, pile material and composite foundation cushion and other composite foundation structural parameters. The following is the calculation method for the design of composite foundations.
The calculation method of the bearing capacity of the composite foundation and the composite foundation of the long spiral pressure perfusion flow state solidified soil pile are calculated as follows:
where: FSPK is the characteristic value (kpa) of the bearing capacity of the composite foundation, which is determined by the on-site composite foundation bearing capacity test. FPK is the characteristic value (kpa) of the bearing capacity of the composite foundation pile, which is determined by the on-site single pile bearing capacity test. FSK is the characteristic value of the bearing capacity of the foundation soil after the foundation is strengthened (kn); n is the stress ratio of soil piles in composite foundation, which is generally determined by field tests; m is the replacement rate of the apparent area of the pile in the composite foundation, and the ratio of the cross-sectional area of the pile body of the solidified soil in the long spiral pressure perfusion state to the area of the composite foundation borne by the single pile; ESP is the deformation modulus (MPa) of composite foundation; EP is the settlement modulus (MPa) of the long spiral compression perfusion solidified soil pile, the settlement modulus of the loose material pile is the compression modulus of the material, and the settlement modulus of the flexible pile and the rigid pile.
According to the mandatory provisions of JGJ79-2012 "Technical Code for Building Foundation Treatment", the bearing capacity of composite foundation and composite foundation reinforcement should be determined by on-site static load test. In addition, because the material pressed into the long spiral pressure perfusion fluidized solidified soil pile is fluid, for the formation soil with a large permeability coefficient, there must be fluidized solidified soil infiltration, and the engineering characteristics index of the foundation soil will be changed, and the bearing capacity FSK of the foundation soil after reinforcement is a variable, and FSPK and FPK are determined by experiments. The method recommended in this paper can obtain the accurate soil pile stress ratio by multiple approximation calculations.
Long spiral pressure perfusion flow solidified soil pile composite foundation typeIt is believed that composite foundation is usually divided into three types: bulk material pile composite foundation, flexible pile composite foundation and rigid pile composite foundation. The composite foundation type determines the depth of the foundation soil for the reinforcement treatment of the composite foundation, and it has been proved in practice that when the foundation treatment depth h<12m, the bulk material pile composite foundation can be selected. When the foundation treatment depth is 12m15m, a rigid pile composite foundation should be selected.
The strength of fluidized solidified soil materials can be adjusted in a large range, usually in the range of 05 Adjust within the range of 15MPa. That is, the fluidized solidified soil pile composite foundation can be made into a cohesive flexible pile composite foundation, and can also be made into a rigid pile composite foundation. According to the data, the flexibility and rigidity of the pile can be judged by the following formula:
where: EP is the deformation modulus of the pile (MPa); gs is the shear modulus of soil (MPa); r is the radius of the pile (m); l is the length of the pile (m).
There is no clear boundary between the rigidity and flexibility of the pile in the project, and it is found that according to the above formula, when k is less than 1, it is a flexible pile, and k is greater than 1 is a rigid pile. The flexibility and rigidity of the pile are related to the deformation modulus of the pile. Therefore, the type of long spiral bored pressure perfusion flow state solidified soil pile composite foundation is also related to the deformation modulus of solidified soil.
4. Study on physical and mechanical properties of fluidized solidified soil.
Fluidized solidified soil is a liquid mixture with water, soil and curing agent in a certain proportion to make it a liquid mixture with a certain fluidity and density. The long spiral borehole pressure irrigation construction technology is used to press the liquid mixture into the pile hole space, which plays a role in supporting the stability of the hole wall.
Liquid solidified soil relies on its own physical and chemical action to condense into a solid within a certain period of time to form a holistic, hard, water-stable solidified soil pile. Under the coordinated action of the cushion layer, the solidified soil pile and the pile act together to form the composite foundation of the building (structure).
From the above construction process, it can be seen that the solidified soil has undergone the transformation from liquid phase to solid phase in the process of forming pile body. When it is in the liquid phase, it needs to meet the requirements of pumping and pressure irrigation; After the solid phase is presented, it needs to have sufficient strength to meet the requirements of the upper load. Therefore, the study of solidified soil should be carried out in the whole process of liquid, solid and liquid to solid.
4.1. Study on the physical properties of liquid phase of fluidized solidified soil.
The physical properties of liquid solidified soil mainly include: density, fluidity, initial and final setting time, curing rate, etc. This performance is related to the mix ratio of the solidified soil, and specifically to the amount of water and curing agent.
The test finds that the solidification rate of the fluidized solidified soil reaches 100%, and the strength of the solidified soil material can meet the requirements of the bearing capacity of the composite foundation, and the amount of water should be calculated and determined according to the density of the solidified soil, the water content of the soil and the amount of curing agent. The formula is as follows:
where: mw is the mass of water (kg); The natural moisture content of the soil (%)0 is the moisture content of the air-dried soil (%), the slurry water-glue ratio (%), the curing agent incorporation ratio (%), m0 is the mass of the air-dried soil (kg).
The test results show that when the soil water content is saturated, not less than 50%, the solidification rate of the fluidized solidified soil will be less than 100%. In this construction process, the water consumption should be controlled to ensure the curing rate. Usually consider the fluidity of the fluidized solidified soil according to the slump of 180mm, determine the value.
4.2. Study on the mechanical properties of the solid phase of fluidized solidified soil.
After solidification, the fluidized solidified soil is a vertical reinforcement of the composite foundation, and its mechanical properties are directly related to the bearing capacity of the composite foundation. Therefore, in the process of research and development of the construction technology of long-spiral pressure perfusion flow-state solidified soil, the mechanical properties of solidified soil were specially studied.
The mechanical properties related to the engineering characteristics of solidified soil mainly include: compressive strength and deformation modulus of solidified soil, and the most relevant process technical problem with the mechanical indexes of solidified soil is the mix ratio of raw materials of fluidized solidified soil. During the research process, a series of mixing tests were carried out on the ratio of raw materials of fluidized solidified soil, and the test content mainly focused on the amount of curing agent and the optimal water content. The results of the optimal water content test are shown in Table 2. The results in the table should be used as the basis for the solidified soil mixture.
In order to grasp the mechanical properties of solidified soil, a variety of types of mechanical properties tests of solidified soil were carried out in this study, and only the representative test results are introduced below, and the test content is the stress-strain test of solidified soil.
Fig. 2-5 is the stress-strain curve of the solidified soil test sample. The dosage of curing agent in the sample was 5%, 10%, 15% and 20%, respectively. The optimal water content is used; The size of the test sample is the standard sample size; The curing time is 7d, and the curing conditions are: temperature 20 2, humidity not less than 95%. The test soil is excavated spoil in the site, and the soil is sandy silt. The results of the trial are summarized in Table 3.
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