Since the advent of cone penetration test, the instrument has been updated several times, and the research on cone penetration mechanism is also very active. For example, the Second European Infiltration Conference (ESOFT) was held in 1974 and 1978; 1988 the first international sounding conference was held. At the same time, there are research articles on the mechanism of in-situ testing and sounding in the papers of previous international conferences on soil mechanics and basic engineering, engineering geology and geology in recent years. Since 1980s, many domestic units have done this work, such as Tongji University, Research Institute of Ministry of Railways, Fourth Survey and Design Institute, Changsha Railway Institute, former Changchun Geological Institute [2], China Geo University [3] and Wuhan University of Water Resources and Hydropower. , have done a lot of research work, published papers, published monographs or textbooks.
The experimental and theoretical research on the mechanism of cone penetration test is directly related to the application of its test methods and results. Therefore, it is of great significance to study the mechanism of vocalization. However, due to the uncertainty and complexity of soil properties and the large deformation of soil layer caused by detection, it is very difficult to study its mechanism. Therefore, so far, the theoretical research results of vocalization mechanism are far from satisfactory and are still in the exploration stage. At present, most known theories are obtained under the condition of undrained in saturated clay or drained in pure sand. These theories can be summarized into the following categories: ① bearing capacity theory; ⑦ reaming method; ③ Strain path method; ④ Other methods. These methods will be briefly analyzed and evaluated below.
1. Bearing capacity theory
Because the process of static cone penetration test is similar to that of pile, some people have tried to solve the end resistance qc of static cone penetration test by borrowing the ultimate bearing capacity theory of deep foundation for a long time, which is called bearing capacity theory or BCT for short. In this method, the soil is regarded as a rigid plastic material, and the slip line field is given according to the boundary stress condition, or the slip surface is assumed according to experiments or experiences, and the ultimate bearing capacity is obtained by the stress characteristic method or the limit equilibrium method. The qc obtained by BCT can generally be expressed as:
Soil in-situ testing and engineering investigation
Where: Cu is the undrained shear strength of soil; Is the overlying pressure; Related to soil depth: = γ h; Nc and Nq are one-dimensional bearing capacity coefficients, which depend on the choice of sliding surface.
The development of BCT bearing capacity theory is from plane strain and modified plane strain to axisymmetric bearing capacity theory.
The method can be evaluated as follows:
There is a difference between (1)BCT and stable penetration, the former is the theory used for the ultimate failure state; The latter is the process of destruction.
(2) Both slip line method and limit equilibrium method are statically determinate. The deformation of plastic zone is not directly considered when seeking qc, so compressibility, dilatancy and crushing effect cannot be considered. Both methods consider static load and do not involve high vertical and horizontal stress caused by penetration.
(3) Only in the whole shear failure soil can a complete failure surface appear, which can be solved by slip line method or limit equilibrium method. For most deep penetration, soil failure includes local shear and compression, and it is difficult to observe obvious sliding surface. Researchers often use parameters such as β to describe this kind of nonholonomic sliding surface in order to correct it.
(4) According to the rigid-plastic slip line method, before the plastic failure, the soil as a rigid body has no deformation, and when the force reaches the limit, the whole plasticity flows in the slip line field. Obviously, this is not in line with the reality, and the rigid-plastic simplification of soil constitutive relation will bring errors, but if we want to consider the relationship between elastic deformation and strain hardening and softening effects, it will cause great difficulties in mathematics, and the simplicity of slip line method will be lost.
(5) The velocity field of soil in plastic zone can be obtained according to the flow law, and the complexity of volume change can be considered. No one has done it, because the interest lies in qc, but the problem is static stress.
(6)BCT can't solve the pore pressure.
2. Reaming method
Cavity expansion method (CEM for short) originated from the viewpoint that cylindrical (or spherical) cavities in infinite homogeneous isotropic elastic bodies are subjected to uniform pressure in elastic theory. This theory is first applied to the analysis of metal pressure working, and then soil mechanics is introduced to explain the mechanism of tamping test and pile sinking with the expansion of cylindrical holes. Estimation of bearing capacity of pile foundation and influence of pile sinking on surrounding soil by spherical hole expansion. CEM is widely used in soil mechanics.
Figure 3-2 Expansion of Circular Hole
The expansion of the column (ball) point under the action of uniform internal pressure P is shown in Figure 3-2. When p increases, the area around the hole will change from elastic state to plastic state. The plastic zone expands with the increase of p value. It is assumed that the initial radius of the cavity is Rf, the expansion radius is Ru, the maximum radius of the plastic zone is Rp, the corresponding final value of the pressure in the cavity is Pu, and the soil outside the radius Rp remains elastic. CEM is similar to the general expression of elastic-plastic mechanical problems, that is, three groups of basic equations (equilibrium differential equation, geometric equation and soil constitutive relation) are listed and solved by failure criterion and boundary conditions. The difference between the solutions obtained by researchers mainly lies in the degree of deformation and the choice of constitutive relation involved in the problem. The choice of constitutive relation (including the flow law in plastic stage) is the key of CEM. With the development of soil mechanics theory and calculation method, there are mainly several models from simplicity to considering many complex properties of soil.
The main advantages of CEM are: using cylindrical cavity expansion or spherical cavity expansion, the three-dimensional problem of probe penetration is simplified and simulated as plane strain and spherical symmetry; Stress, strain and displacement are only functions of radial coordinate variable r, and the boundary conditions are very simple. Numerical method can be used to integrate various soil constitutive models and consider many complex characteristics of soil. It is obviously superior to BCT in obtaining pore pressure and considering permeability in high compressibility soil. It can be seen that the idea of CEM comes from treating probe penetration as a continuous expansion of conical surface, which is approximately replaced by cylindrical or spherical expansion, greatly simplifying the boundary conditions.
The main disadvantages of CEM are as follows: ① Obviously, cavity expansion in a fixed position cannot simulate the following two important characteristics of vertical penetration: a. The deformation of soil is related to vertical coordinates. In particular, column expansion can not simulate this point, and its displacement is in the horizontal plane, so spherical expansion can not explain the displacement inversion. B. continuity of stable infiltration. Because CEM always describes reaming in a fixed position. Therefore, even in the simplest homogeneous isotropic soil, CEM can not correctly simulate the deformation process (strain path) of each unit in the soil during penetration. ② The current CEM method does not consider the influence of permeability, although the influence of permeability on δ U (excess pore pressure) and qc exists.
3. Strain path method
The strain path method (SPM) was formally put forward by a group led by Baligh in 15 after years of research. SPM aims to provide a complete and systematic analysis method for reasonably explaining and predicting deep geotechnical engineering problems (relative to shallow foundation) such as pile penetration, static sounding and soil sampler.
The basic idea of (1)SPM
By observing the undrained penetration of the probe into saturated soft clay, Baligh( 1975) thinks that the strain of the soil around the probe is almost unaffected by the shear characteristics of the soil, because there are strict movement restrictions in the process of deep penetration (large overlying pressure, deep remodeling of the soil around the probe under high stress level, forced flow and incompressibility of the soil under undrained conditions, etc.). ), so Baligh said that such problems are caused by the shear characteristics of soil. Later theories and experiments also confirmed this hypothesis.
Therefore, it is reasonable to estimate the deformation and strain difference caused by penetration with relatively simple soil properties (such as isotropy). Approximate stress and pore pressure can be calculated by using the estimated strain and the constitutive model conditions that are in line with the actual situation.
For quasi-static penetration of axisymmetric probe into saturated cohesive soil, ignoring viscosity and inertia effects, plastic failure caused by undrained shear can be regarded as directional flow problem, that is, the probe is regarded as static, and soil particles flow in the opposite direction of probe penetration along streamline distributed around the probe. The deformation, strain, stress and pore pressure of each unit on different streamline can be obtained through some steps.
(2) Through the penetration simulation of 2)SPM.
The key to SPM simulation of stable penetration is to correctly predict the strain field. At present, soil is regarded as an inviscid incompressible fluid, and the strain field is estimated by solving the particle flow probe of soil. This can be divided into two cases, that is, the probe moves in a static fluid at the speed of U (generally 2cm/s); Or a static probe with a speed of u, for an infinite uniform beam with zero angle of attack.
There are two methods to solve the flow around axisymmetric bodies, namely Bankine method and conformal mapping method. The evaluation of this method is as follows:
Its advantages are as follows: The advantages of SPM method mainly lie in considering and simulating the characteristics of vertical penetration for the first time, and overcoming two main shortcomings of CEM. According to the basic assumption, the strain field is obtained by cone flow method, which avoids the calculation difficulty of constitutive relation under complex boundary conditions and complex stress paths. The main disadvantage of SPM method lies in the applicability of its basic assumptions. Clark and Meverhof( 1972) and Steenfelt (198 1) observed that the influence range of piling on the radial displacement field of surrounding soil is 4 times and 8 times the pile diameter respectively. Some researchers have concluded that the influence range of δ U is 4 ~ 25 times the pile diameter. Therefore, the strain produced by penetration depends on the soil properties. At present, SPM method actually advances its basic assumption one step, and equates the flow field in soil with that of inviscid incompressible fluid flowing around a cone. As we all know, inviscid flow cannot resist any shear force (no matter how small), and the viscosity of soil is generally 8 ~ 16 orders of magnitude greater than that of water. Therefore, only the cone of inviscid incompressible irrotational fluid can be used to simulate the flow field caused by deep penetration, which is effective for completely saturated soft clay (refer to first-order approximation). For hard clay with OCR > 4, it is easy to produce discontinuous sliding surface during penetration, so it is not suitable to use continuous fluid motion to simulate it. If the viscosity, compressibility and friction of the pile-soil interface are considered, the flow equation is difficult to solve.
Despite the above difficulties, SPM method is ingenious in conception. It calculates the strain field and stress field separately, which opens up a new way to solve the problem of deep penetration and has great development prospects. Many useful results have been obtained by using it, such as estimating the bearing capacity coefficients of qc and δ U, which can be referred to Baligh's article.
Second, the working principle of static sounding probe
1. probe forms a resistance sensor.
Static penetration probe, also known as stratum resistance sensor, is a key component to measure the penetration resistance of foundation soil. It is an element that directly feels the resistance of the soil during the penetration process, converts it into an electrical signal, and then displays it by the instrument. In order to realize this process, different types of sensors can be used, among which the resistance strain sensor is the most commonly used. The resistance strain sensor is made by applying Hooke's law, resistance law and bridge principle.
2. The electromechanical principle of the static cone penetration test site.
(1) When the P → E conversion probe (Figure 3-3) is pressed into the soil, the hollow column (deformation column 4) installed inside the probe will be deformed due to formation resistance; If the hollow pile is regarded as a member, the relationship between its resistance and deformation can be expressed as Hooke's law:
Soil in-situ testing and engineering investigation
or
σ=Eε (3-3)
Where: e is the elastic modulus of the material; F is the cross-sectional area of the hollow column; P is the pressure resistance of the probe; ε is the strain of hollow column under pressure p; L is the effective deformation length of the hollow column. For a given probe, both are given. So as long as the strain ε is measured, the magnitude of stress σ can be obtained, and then the magnitude of stress P can be known.
(2) ε→δ r conversion In order to measure ε, a resistance strain gauge with a resistance value of R is attached to the periphery of the hollow pile (Figure 3-4). The hollow pile is deformed by tension, and the resistance wire also becomes longer. According to the formula of resistance law:
Soil in-situ testing and engineering investigation
Where: L is the length of the resistance wire; ρ is the resistivity of the resistance wire. Because the stress of hollow pile causes the change of δ L, the corresponding resistance R will also cause the change of δ R, and the relationship can be expressed as:
Soil in-situ testing and engineering investigation
Where: k is the sensitivity coefficient of the resistance strain gauge.
Figure 3-3 Schematic Diagram of Single Bridge Probe Structure
Figure 3-4 Transformation between Strain and Resistance Change
(3) Δ r →Δu conversion formula (3-5) shows that the conversion from non-electric quantity ε to electric quantity Δ r has been realized. However, the deformation of steel is very small in the elastic range, so the δ R value of the resistance change caused by it is also very small. It is difficult to accurately measure the change of force by using tiny resistance change, so it is necessary to stick a set of strain gauges on the hollow pile by using the bridge principle and amplify it by the amplifier to realize the measurement of tiny voltage.
Let's analyze the principle of the bridge: the bridge circuit is shown in Figure 3-5. The bridge voltage is U, and the voltage on R2 drops to UBC. In the ABC or ADC loop, the resistor R 1 and R2 are connected in series, and the current is I 1. According to ohm's law:
Soil in-situ testing and engineering investigation
Therefore, the BC potential difference is:
Soil in-situ testing and engineering investigation
Similarly, on the ADC loop, the potential difference UDC of DC is:
Soil in-situ testing and engineering investigation
The output voltage δ u of the bridge is the difference between UBC and UDC, that is:
Soil in-situ testing and engineering investigation
Figure 3-5 Bridge Principle
Obviously, in order to balance the bridge, that is, the output voltage is zero (the galvanometer has no current), there should be:
R2 R4-r 1 R3 = 0； Or r 1 R3 = R2 R4 (3-7)
Equation (3-7) is the bridge balance condition.
Next, the relationship between the output voltage δU and the resistance change δR is analyzed, and then the deformation ε is further analyzed.
The object of analysis is the full-bridge measuring circuit with equal bridge arms, one for each arm, that is, R 1=R2=R3=R4. Obviously, when there is no force, the bridge equilibrium condition is satisfied. The pasting method of the four blocks is shown in Figure 3-6, that is, pasting R2 and R4 along the axis direction of the hollow column to make it change positively; R 1 and R3 are attached across the hollow column, which makes them change negatively and the four pieces compensate each other. The expression of the output δU of the bridge thus formed is deduced as follows:
Soil in-situ testing and engineering investigation
Obviously, Kε( 1-μ) in the formula is a nonlinear term, that is, δ u in the above formula is not proportional to ε. For conventional strain gauges with small resistance, the term Kε( 1-μ) is very small, because the value of k is very small (about 2), even if the strain is large, it can be omitted, so the formula (3-8) becomes:
Soil in-situ testing and engineering investigation
For a full-bridge measuring circuit with two pieces of tension and two pieces of stress, it is not difficult to prove the relationship between its output voltage δ u and strain ε as follows:
Soil in-situ testing and engineering investigation
From the analysis of the above two formulas, it can be seen that the output voltage of the former is (1-μ) times that of the latter under the same conditions of k, ε and u, just because the methods of attaching strain gauges are different. In order to obtain greater output, the strain gauge in the static probe adopts the former pasting method at present.
According to formula (3-9) or formula (3- 10), the output voltage Δ u of the bridge is directly proportional to the sensitivity coefficient k of the strain gauge, the strain ε and the power supply voltage u of the bridge. For a certain sensor, the assembly mode of the bridge has been determined, k and ε are constants, and Δ u only changes with the strain ε of the space-time stem when the working voltage u is selected. In connection with Equation (3-2), it is easy to see that since E and F have also been determined, the output voltage Δ u only changes with the magnitude of the stress p on the hollow column.
To sum up, the cone penetration test is to measure the strength of soil through a series of transformations such as formation resistance → hollow column deformation → resistance change → voltage change → application of electronic recording instruments.
3. Structure type of probe
Probe is the key component of cone penetration tester to measure penetration resistance, which has strict specifications and quality requirements. Generally, it is divided into two parts: the conical end and the cylindrical friction cylinder at the back. At present, probes used at home and abroad can be divided into three forms:
(1) Special (bridge) probe: It is a unique probe type in China, and can only measure one parameter, namely specific penetration resistance ps, with low resolution (accuracy), as shown in Figure 3-3 and Figure 3-8.
(2) Dual-purpose (bridge) probe: It is a probe that separates the cone head from the friction cylinder, and can simultaneously measure two parameters, namely the cone head resistance qc and the side wall friction resistance fs, with high resolution, as shown in Figure 3-7 and Figure 3-8.
Figure 3-6 Full-bridge Circuit with Four Walls Working
Figure 3-7 Schematic Diagram of Double Bridge Probe
Figure 3-8 Static Penetration Probe Types
(3) Multi-purpose (pore pressure) probe: Generally, a dual-purpose probe: a water permeable filter and a sensor for measuring excess pore water pressure generated during penetration is reinstalled. The resolution is the highest, and it should be preferred in areas with shallow groundwater level.
The cone angle of the probe is generally 60, and the bottom area is 10cm2, and there are also 15cm2 or 20cm2. The larger the bottom area of the cone head, the higher the compressive strength the cone head can bear; The probe is not easy to be damaged; And there is more room to install other sensors, such as sensors for measuring well deviation, temperature and density. In the same test project, it is advisable to use probes with uniform specifications for comparison. See table 3- 1 and table 3-2 for the relevant provisions of the technical standard for static penetration test (CECS 04: 88).
Figure 3-9 shows a set of physical probes, including 10cm2 single double bridge probe, 15cm2 single double bridge probe and 50× 100mm2 probe and blade sensor.
Table 3- 1 specifications of single-bridge and double-bridge probes
Table 3-2 Specifications of Common Probes
4. Problems related to probe design
Briefly explain some points of this problem:
(1) The hollow column of the probe should be in good contact with its top column, and the top column should be in the best contact, which can make the sensor stress uniform and easy to process.
(2) Steel for processing hollow columns (elastic elements) should have the characteristics of high strength, good elasticity, stable performance, small thermal expansion coefficient and corrosion resistance. Hollow columns are generally made of 60 Si2Mn (spring steel) and 40 CrMn steel in China. Other parts can be made of 40 Cr or 45 steel, which needs heat treatment.
(3) According to Formula (3-2), the strain of hollow column is related to formation resistance and annular cross-sectional area of hollow column. Under the same formation resistance, the greater the strain (that is, the more sensitive it is), the smaller the maximum load it can bear. To give consideration to both, as mentioned above, steel can be selected. But even this would not be enough In order to meet the penetration needs of different areas and different soft and hard soil layers, manufacturers generally produce several probes with different rated loads (when the hollow column material is fixed, it is equivalent to different cross-sectional areas). Generally, a more sensitive probe with a smaller rated load can be selected in soft soil areas; On the contrary, choose the probe with larger rated load.
Figure 3-9 Photo of Physical Probe
(4) The Technical Rules for Static Cone Penetration Test (TBJ37-93) of the Ministry of Railways stipulates that the probe specifications, machining tolerances of various parts and updating standards should meet the requirements of the rules.
(5) The insulation performance of the probe shall meet the following requirements: the insulation resistance of the probe shall be greater than 500 mΩ when it leaves the factory, and it shall not be less than 500 mΩ under the constant pressure of 500kPa for 2 hours. The insulation resistance of the probe used for field test shall be not less than 20MΩ.
(6) For all kinds of probes, the diameter of any bar connected with the cone within the length range of 1000mm from the bottom of the cone shall not be greater than the diameter of the probe; In order to reduce the frictional resistance between the probe and the soil, when it is necessary to add an antifriction resistor, it can only be installed above this specified range.
(7) Probe storage should be equipped with a special probe box (box) that is moisture-proof and shockproof, and stored in a dry and cool place.
5. Resistance strain gauge and adhesive
Figure 3- 10 foil resistance strain gauge
At present, foil resistance strain gauge (Figure 3- 10) is widely used to make sensors, which has the advantages of good heat dissipation and large allowable current (so large input voltage can be used). So as to obtain a larger output voltage), long fatigue life, good flexibility, small creep and the like. Linear rubber-based resistance strain gauges can also be used, but semiconductor strain gauges are rarely used because of their serious shortcomings such as large nonlinearity and poor temperature stability, which can not meet the quality requirements of sensing circuits.
When measuring with resistance strain gage,120Ω chip can be selected. When using automatic recorder, you can choose 240Ω or 360Ω film. The resistance values of the four pieces should be as equal as possible, and the maximum difference should not exceed 0. 1ω, otherwise it will be unfavorable to the initial balance of the bridge. Instruments such as DC single bridge can be used to measure the resistance of strain gauges.
There are many kinds of adhesives suitable for sticking strain gauges. At present, phenolic glue 1720 glue is commonly used; Polyimide adhesives are also used. When selecting adhesive, attention should be paid to make it consistent with the rubber base of strain gauge.
I won't introduce the specific patch technology here, because there are many kinds of commercial sensors produced by domestic factories, which can be selected by engineers and technicians. The quality is generally good and the price is not expensive. There is no need for users to make them except in special circumstances.
6. The influence of temperature on the sensor and its compensation method.
When the sensor is not stressed and the temperature changes, the limit value of the resistance wire (also called wire grid) in the strain gauge will also change. At the same time, because the linear expansion coefficient of wire grid material is different from that of hollow column material, the additional tension or compression of wire grid will also change the resistance of strain gauge. Generally speaking, the relationship between the resistance changes of the strain gauge attached to the hollow column due to the change of temperature (t) can be expressed as follows:
Soil in-situ testing and engineering investigation
Where: αt is the temperature coefficient of resistance of the strain gauge attached to the hollow column. Combined with equation (3-5), the thermal output εt of the strain gauge caused by temperature change is:
Soil in-situ testing and engineering investigation
This heat output has nothing to do with formation resistance, so it must be eliminated to make the test results meaningful. In the static sounding technology, the influence of temperature on the sensor can be basically controlled within the test accuracy by adopting the following two methods. In addition, temperature self-compensation strain gauges can also be actively used if conditions permit.
(1) The bridge compensation method is to select four strain gauges with the same batch, specification, resistance and sensitivity coefficient when making the sensor, and stick them on the hollow column with the same glue and patch technology to form a full-bridge four-arm measuring circuit (four working plates compensate each other, or two working plates compensate each other), so that when the temperature changes, the compensation plate and the working plate (Δ r/r).
(2) The temperature correction method is to measure the change of initial reading in field work and eliminate it when sorting out indoor data.