CN107059956B - Simple test method for measuring long-term shear strength of pile-soil contact surface - Google Patents
Simple test method for measuring long-term shear strength of pile-soil contact surface Download PDFInfo
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Abstract
(1) The invention belongs to the field of geotechnical engineering, and particularly relates to a simple test method for measuring long-term shear strength of a pile-soil contact surface. The simple test method for the long-term shear strength of the pile-soil contact surface comprises the following specific steps: (1) preparing a concrete sample; (2) making a soil sample and solidifying the soil sample; (3) measuring the physical and mechanical properties of the soil sample; (4) combining the soil sample and the concrete sample; (5) carrying out conventional triaxial tests on the combined sample under different confining pressures to obtain short-term shear strength; (6) carrying out long-term creep triaxial test on the combined sample under different confining pressures, and monitoring long-term shear strain; (7) and analyzing the long-term shear strength of the pile-soil contact surface under different confining pressures by an isochronous stress-strain curve method. The invention can simulate the long-term shear strength of the pile-soil contact surface in the actual engineering, has wide application range, can be completed by directly utilizing the static triaxial apparatus, and has simple operation.
Description
Technical Field
The invention belongs to the field of geotechnical engineering, and particularly relates to a simple test method for measuring long-term shear strength of a pile-soil contact surface.
Background
The pile foundation is an ancient foundation type, and after thousands of years of development, both pile foundation materials and pile types and construction methods have been developed greatly, so that the pile foundation becomes a main foundation type for buildings, bridges and the like. And the long-term stability of the pile foundation is of great importance to the engineering safety. In fact, when the pile end soil is non-compact sand soil or silt soil and residual deficient soil is not cleaned, under the action of load, piercing damage is easy to occur. At this time, the pile tip resistance is small, and the pile side resistance is large. Therefore, under certain engineering conditions, the research on the long-term shear strength of the pile-soil contact surface is a very important problem.
At present, indoor test research methods for shear strength of pile-soil contact surfaces at home and abroad are single, direct shear tests are mostly carried out based on a shear apparatus, and the influence of time effect is rarely considered. Because the side friction resistance of the pile gradually attenuates along with time in the actual engineering, a test which has high fitting degree with the actual working condition and considers the creep influence needs to be found for researching the long-term shear strength of the pile-soil contact surface.
Therefore, the invention provides a simple test method for measuring the long-term shear strength of the pile-soil contact surface. According to the method, a conventional static triaxial testing machine is adopted, a pile-soil combined sample is adopted to perform a long-term shear test on the pile-soil contact surface, a long-term shear strength parameter value of the pile-soil contact surface is obtained, and the method is expected to provide reference for damage caused by insufficient pile side shear friction bearing capacity in actual engineering.
The current research situation of the current domestic related pile-soil contact surface long-term shear strength research method is as follows:
1. the text of the shear test research on the contact surface of the side of the soil and different pile side roughness introduces an indoor test method for researching the side friction force between the pile and the soil by using a direct shear apparatus (see structural engineer 2011, 3 rd, author: Zhangshun, Huabin), in which a large-scale multifunctional interface shear apparatus developed by Tongji university is used for carrying out test research on the interaction and mechanical characteristics between the cohesive soil in Shanghai region and the contact surface of the concrete with different surface roughness, but the influence of the interaction between the soil sample and the contact surface of the concrete under the long-term action of load is ignored;
2. a method for researching the side friction force between the pile and the soil through a large-scale field test is introduced in a model test research on the influence of the pile-soil interaction (see the 6 th stage of 2008 of geotechnical mechanics, an author: Tao), the results of the model test and the field actual measurement are compared with an elastic theoretical solution, the elastic theoretical solution is pointed out to exaggerate the influence of the pile-pile, pile-soil and soil-soil interaction, cause a settlement calculation value to be larger and excessively estimate the nonuniformity of the pile top counter force, and a method for correcting the elastic theoretical solution is provided. The method has large sample size and huge cost;
3. a method for performing horizontal unidirectional and bidirectional vibration tests on a group pile foundation model by using a centrifugal machine bidirectional vibration table is introduced in the text of the preliminary exploration of pile-soil interaction under horizontal multidirectional load (see the third 3 th stage of the soil and soil mechanics in 2008, authors: threatened land and leaf stem of Japanese cabbage), and the test method has high requirements on test instruments and is difficult to popularize;
4. the experimental study on pile-soil sliding friction introduces an indoor direct shear test on a modified instrument (see the rock and soil mechanics, 2002, 4 th edition: Zhang Ming Yi, Deng Anfu), the test method of the test is simple, but the experimental model is greatly different from the actual model.
Disclosure of Invention
In order to overcome the defect that the existing indoor test method is few, the invention provides a simple test method for measuring the long-term shear strength of the pile-soil contact surface.
In order to achieve the purpose, the invention adopts the following technical scheme:
a simple test method for measuring long-term shear strength of pile-soil contact surface comprises the following steps:
(1) preparing a concrete sample;
(2) making a soil sample and solidifying the soil sample;
(3) measuring the physical and mechanical properties of the soil sample;
(4) combining the soil sample and the concrete sample;
(5) carrying out conventional triaxial tests on the combined sample under different confining pressures to obtain short-term shear strength;
(6) carrying out long-term creep triaxial test on the combined sample under different confining pressures, and monitoring long-term shear strain;
(7) and analyzing the long-term shear strength of the pile-soil contact surface under different confining pressures by an isochronous stress-strain curve method.
Compared with the prior art, the invention has the following beneficial effects:
1. the invention can simulate the long-term shear strength of the pile-soil contact surface in the actual engineering and has wide application range;
2. the invention can be completed by directly using the static triaxial apparatus, and the operation is simple.
Drawings
FIG. 1 is a schematic flow chart of a simple test method for long-term shear strength of a pile-soil contact surface;
FIG. 2 is a schematic diagram of an experimental model;
FIG. 3 is a schematic diagram of a long term intensity determination method;
FIG. 4 is a schematic diagram of the meaning of each letter in the test model;
Detailed Description
As shown in fig. 1, a simple test method for measuring the long-term shear strength of a pile-soil contact surface comprises the following steps:
1. concrete samples were prepared as follows:
two semi-cylindrical concrete samples with the diameter of 60mm and the height of 50mm are prepared by utilizing a mould, and the parameters of the samples such as strength and the like refer to the parameters of a pile body in actual engineering.
2. The method for manufacturing the soil sample and consolidating the soil sample comprises the following specific steps:
taking soil at an engineering site, and preparing an initial soil sample of a cylinder with the diameter of 60mm and the height of 120mm by using a mold;
and (3) solidifying the initial soil sample by using a static triaxial apparatus, adopting drainage solidification during solidification, and applying certain confining pressure for 7 days. And (3) carrying out consolidation on a plurality of soil samples with certain initial water content in sequence, and taking the plurality of soil samples with the same initial water content as a group. One of the two methods is used for measuring the physical and mechanical properties of the soil sample, such as density, water content after consolidation and the like; and the rest of the test pieces are combined with the concrete sample to simulate the pile-soil interaction process to carry out a triaxial test.
And the physical and mechanical parameters for preparing the soil sample are consistent with the actual engineering parameters by taking the soil layer parameters in the actual engineering geological survey report as reference.
3. The method for measuring the physical and mechanical properties of the soil sample comprises the following steps:
the moisture content of the soil sample is measured by using a dryer, the density of the soil sample is measured by using a measuring cylinder and an electronic scale, and the cohesive force and the internal friction angle of the soil sample are measured by using a static triaxial apparatus.
4. The concrete method for combining the soil sample and the concrete sample comprises the following steps:
referring to a test model (see figure 2), the soil sample is symmetrically cut into 4 blocks, and 2 blocks and 2 concrete samples are combined into a complete standard sample in a staggered mode.
5. Carrying out conventional triaxial tests on the combined sample under different confining pressures to obtain short-term shear strength;
the combined sample of the concrete and the concrete is subjected to a non-drainage test on a static triaxial apparatus. Applying confining pressure sigma first3And held constant, and then subjected to an axial stress σ1The increase was gradual until shear failure was tested and the shear stress τ -shear strain γ curve was recorded.
Suppose the height of the specimen is h and the diameter is d, sigma1For axial stress, σ3For confining pressure, γ is the soil sample shear strain, as shown in FIG. 4.The shear stress on the vertical contact surface is
Positive stress of sigma ═ sigma3。
And measuring the axial maximum stress under a plurality of confining pressures, drawing a shear stress tau and a positive stress sigma curve, and determining the cohesion force c and the friction angle phi of the pile-soil contact surface.
6. The combined sample is subjected to a long-term creep triaxial test under different confining pressures, and long-term shear strain is monitored, and the method specifically comprises the following steps:
the combined sample of the concrete and the concrete is arranged on a static triaxial apparatus, and the confining pressure sigma is firstly applied3And held constant, and then subjected to an axial stress σ1And held constant at a shear stress of
The combined sample is subjected to shear creep deformation under the action of a constant load, and a shear stress tau-shear strain gamma-time t curve is recorded; at the same confining pressure sigma3Lower, varying axial stress σ1Size, similar tests were performed a number of times. And obtaining a plurality of shear strain gamma-time t curves under different shear stresses tau.
According to the program, the confining pressure is further changed, and then a plurality of shear creep tests under different axial stresses are carried out, so that a shear stress tau-shear strain gamma-time t curve under different confining pressures is obtained.
7. The long-term shear strength of the pile-soil contact surface under different confining pressures is analyzed by an isochronous stress-strain curve method, which comprises the following steps:
under a certain confining pressure, drawing an isochronous stress-strain curve (see figure 3) according to a plurality of obtained shear stress tau-shear strain gamma-time t curves, wherein the plurality of curves have a horizontal asymptote, and the corresponding shear stress is the long-term shear strength tau of the pile-soil contact surface of the combined sample under the confining pressure∞。
Plotting Long-term shear Strength τ∞And determining the long-term cohesion c of the pile-soil contact surface according to the positive stress sigma curve∞And long term internal friction angle phi∞。
Claims (5)
1. A simple test method for measuring long-term shear strength of a pile-soil contact surface is characterized by comprising the following specific steps:
(1) preparation of concrete samples
Preparing two semi-cylindrical concrete samples with the diameter of 60mm and the height of 50mm by using a mould, wherein the strength of the samples refers to the parameters of a pile body in actual engineering;
(2) making soil sample, solidifying the soil sample
(3) And measuring the physical and mechanical properties of the soil sample
(4) Combining the soil sample with the concrete sample
Symmetrically cutting a soil sample into 4 blocks, and combining 2 blocks and 2 concrete samples in a staggered manner to form a complete standard sample;
(5) carrying out conventional triaxial test on the combined sample under different confining pressures to obtain short-term shear strength
Carrying out a non-drainage test on a combined sample of the concrete and the concrete on a static triaxial apparatus; applying confining pressure sigma first3And held constant, and then subjected to an axial stress σ1Gradually increasing until the test shear failure, and recording a shear stress tau-shear strain gamma curve;
suppose the height of the specimen is h and the diameter is d, sigma1For axial stress, σ3For confining pressure, gamma is the shear strain of the soil sample, and the shear stress on the vertical contact surface is
Positive stress of sigma ═ sigma3；
Measuring the axial maximum stress under a plurality of confining pressures, drawing a shear stress tau and a positive stress sigma curve, and determining the cohesion c and the friction angle phi of the pile-soil contact surface;
(6) the combined sample is subjected to long-term creep triaxial test under different confining pressures, and long-term shear strain is monitored
(7) And analyzing the long-term shear strength of the pile-soil contact surface under different confining pressures by an isochronous stress-strain curve method.
2. The simple test method for measuring the long-term shear strength of the pile-soil contact surface according to claim 1, characterized in that a soil sample is prepared and consolidated, and the method comprises the following steps:
solidifying the initial soil sample by using a static triaxial apparatus, adopting drainage consolidation during solidification, and applying confining pressure for 7 days; a plurality of soil samples with certain initial water content are solidified in sequence, and the plurality of soil samples with the same initial water content are used as a group; one of the two methods is used for measuring the physical and mechanical properties of the soil sample; the rest of the concrete sample and the rest of the concrete sample are combined together to simulate the pile-soil interaction process, and a triaxial test is carried out;
and the physical and mechanical parameters for preparing the soil sample are consistent with the actual engineering parameters by taking the soil layer parameters in the actual engineering geological survey report as reference.
3. The simple test method for measuring the long-term shear strength of the pile-soil contact surface according to claim 1, characterized in that the physical and mechanical properties of the soil sample are measured by the following method:
the moisture content of the soil sample is measured by using a dryer, the density of the soil sample is measured by using a measuring cylinder and an electronic scale, and the cohesive force and the internal friction angle of the soil sample are measured by using a static triaxial apparatus.
4. The simple test method for measuring the long-term shear strength of the pile-soil contact surface according to claim 1, wherein the combined sample is subjected to a long-term creep triaxial test under different confining pressures, and the long-term shear strain is monitored by the following method:
the combined sample of the concrete and the concrete is arranged on a static triaxial apparatus, and the confining pressure sigma is firstly applied3And held constant, and then subjected to an axial stress σ1And held constant at a shear stress of
The combined sample is subjected to shear creep deformation under the action of a constant load, and a shear stress tau-shear strain gamma-time t curve is recorded; at the same confining pressure sigma3Lower, varying axial stress σ1Size, onMultiple similar tests; obtaining a plurality of shear strain gamma-time t curves under different shear stresses tau;
according to the program, the confining pressure is further changed, and then a plurality of shear creep tests under different axial stresses are carried out, so that a shear stress tau-shear strain gamma-time t curve under different confining pressures is obtained.
5. The simple test method for measuring the long-term shear strength of the pile-soil contact surface according to claim 1, wherein the long-term shear strength of the pile-soil contact surface under different confining pressures is analyzed by an isochronous stress-strain curve method, which comprises the following steps:
drawing an isochronous stress-strain curve according to a plurality of obtained shear stress tau-shear strain gamma-time t curves under a certain confining pressure, wherein the plurality of curves have a horizontal asymptote, and the corresponding shear stress is the long-term shear strength tau of the pile-soil contact surface of the combined sample under the confining pressure∞；
Plotting Long-term shear Strength τ∞And determining the long-term cohesion c of the pile-soil contact surface according to the positive stress sigma curve∞And long term internal friction angle phi∞。
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