How to calculate cohesive force and internal friction angle with

Publish: 2021-04-23 23:32:54

Cohesion c and internal friction angle of rock mass φ It can be determined by direct shear, uniaxial compression or triaxial compression experiments

2. The three reflect the bearing capacity and settlement capacity of the foundation soil. Cohesion is also called cohesion, which refers to the particles of the soil; Cohesion reflects the mutual attraction between soil particles. The internal friction angle refers to the shear strength of soil, which is composed of molecular particles, water and air

Cohesion, also called cohesion. The calculation method is: in the case of effective stress, the cohesion is obtained by decting the total shear strength from the friction strength. On the other hand, cohesion is the shear strength of the failure surface without any normal stress

The classical expression of internal friction angle is Coulomb's law τ=σ tan φ+ C

for cohesive soil, C is not 0, for sandy soil, C is 0, φ、 C can be obtained by triaxial test (or direct shear). Under different confining pressures, the maximum principal stress and minimum principal stress can be obtained, and the stress circle can be made. Under at least three different confining pressures, three stress circles can be made, and the common tangent of the three circles can be made. The inclination angle is the internal friction angle

The relationship between repose angle and internal friction angle:

1, repose angle and internal friction angle all reflect the internal friction characteristics of granular materials

The concepts of angle of repose and internal friction angle are different. The internal friction angle reflects the friction characteristics between layers of granular materials, and the angle of repose represents the rolling ability of a single granular material on the pile, which is the appearance of the internal friction characteristics

3. For similar materials with similar mass and moisture content, the angle of repose is always greater than the internal friction angle, and is greater than the sliding friction angle. The angle of repose is equal to the angle of internal friction for loose materials such as sand which lack cohesion

extended data

determination method of internal friction angle

in order to determine the internal friction angle of granular materials, the Mohr envelope of this material must be determined by experiment. Two methods can be used to determine the Mohr envelope of granular materials

Triaxial compression test the triaxial compression test device is developed by using the device to study the shear characteristics of soil. When the device is used for shearing test of bulk materials such as grain, the pre compacted grain is sealed in the rubber film and put into the compression chamber. The pressure in the compression chamber is graally increased to a predetermined pressure. The axial cutting load is applied to the grain column by universal testing machine or other loading device

In this way, the grain column is subjected to air pressure in the radial direction σ Under the combined action of compressed air pressure and axial load in the circumferential direction, the failure time is less than 3 σ The value of 1 can be measured by recorder. Repeat the above procere to get different results σ Principal stress of grain column failure at
3 σ The Mohr envelope of bulk material in a certain compaction state is obtained

Direct shear test

direct shear test can be carried out on the shear apparatus shown in the figure. The shearing instrument consists of three basic parts: shearing groove, loading device and recorder. The shearing groove comprises a base, a shearing ring and a top cover. The normal pressure is applied to the shear ring by the vertical compaction load, and the shear force is applied to the shear ring by the electric or mechanical transmission device. The transmission device is equipped with a force sensor or dynamometer to measure the shear stress acting on the contact plane between the base and the shear ring

reference source : network cohesion

reference source : network internal friction angle

4. The internal friction angle is measured by triaxial test. Step 1: fix the confining pressure, that is, the small principal stress (the small principal stress and the medium principal stress in the ordinary triaxial test are equal), and add the axial pressure until the specimen is destroyed, so as to determine the large principal stress. In this way, a group of small principal stress and large principal stress are obtained. Step 2, increase the confining pressure twice to determine the large principal stress of soil samples under the two confining pressures, and then get the values of two groups of small principal stress and large principal stress. Step 3, make Mohr circle according to the three groups of principal stress values. Step 4, draw the common tangent of three different Mohr circles above the coordinate axis. The angle between the common tangent and the horizontal direction is the internal friction angle, and the intercept between the common tangent and the Y axis is the cohesion. If you still don't understand, please continue to ask. If you solve your problem, please adopt it.

5. The internal friction angle of concrete is ≥ 45 °， The cohesion can be taken as ≥ 1000kPa. When the concrete strength meets the design strength, it is considered that it will not be damaged by shear. At this time, the internal cohesion can be taken as infinite. In the calculation, it can be taken as 1000. It is composed of cement and sand, so the internal friction angle should not be less than 45% of the block stone °， It is safe to calculate according to this value. Concrete, referred to as "concrete (T) ó NG) ": refers to the aggregate cemented by cementitious materials into a whole engineering composite. Generally speaking, the term concrete refers to the use of cement as cementing material and sand and stone as aggregate; Cement concrete mixed with water (including admixtures and admixtures) in a certain proportion, also known as ordinary concrete, is widely used in civil engineering.

6. 1. Shear strength of cement soil: the shear strength of cement soil increases with the increase of compressive strength. When FCU = 0.30 ~ 4.0Mpa, its cohesion c = 0.10 ~ 1.0MPa, which is generally about (20 ~ 30)%, and its internal friction angle changes between 20 ~ 30 degrees. &# 39; S/g mn
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in triaxial shear test, the specimen has a clear and flat shear plane, and the angle between the shear plane and the maximum principal stress plane is about 60 degrees. AFwdJte9e
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according to the regression analysis of the test results, the relationship between the cohesion c and the unconfined compressive strength FCU of cement soil is approximately a power function, and the formula is as follows: C = 0.2813 * (FCU ^ 0.7078). The condition of the equation is: FCU = 0.3 ~ 1.3mpa& gt; w< &# 39; 0
u W %#
compressive molus of cement soil: when the vertical stress reaches 50% unconfined compressive strength, the ratio of stress to strain of cement soil is called deformation molus E50 of cement soil. When FCU = 0.1 ~ 3.5Mpa, E50 = 10 ~ 550MPa. According to the linear regression analysis of the test results, it is obtained that E50 = 126fcu] 3sp w {= ^ (
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the same explanation is also given in the book of highway foundation treatment manual Foundation treatment and underpinning technology - the third edition has calculation examples for foundation pit slope treatment with mixing piles.

4.5.1 the design of deep foundation pit support is one of the hotspots and difficulties in geotechnical engineering. The success or failure of deep foundation pit support design is not only directly related to the smooth progress of engineering construction, but also linked with the huge economic benefits. The cost of foundation pit support and design is generally up to millions of yuan. At present, the design of deep foundation pit support in China is in the process of practice, summary and improvement. There are many theories and methods to be further improved

in the design of deep foundation pit support, the calculation of earth pressure is the fundamental basis and key to the design of deep foundation pit support, and in the calculation of earth pressure, the cohesion c internal friction angle of soil is the most important factor φ Value is the most basic parameter. The C φ Because of the different consolidation and drainage conditions, the value index can show different values. For example, the same kind of saturated clay has different internal friction angle in consolidated drained and consolidated undrained tests, but the internal friction angle in unconsolidated undrained shear tests is different φ＝ 0

in order to simulate various complicated drainage conditions in engineering as much as possible, the soil strength index C, C, C, C, C, C, C, C, C, C, C, C, C, C, C, C, C, C, C, C, C, C, C, C, C, C, C φ In the triaxial shear test, the unconsolidated undrained shear (Uu), consolidated undrained shear (Cu) and consolidated drained shear (CD) are considered. The corresponding direct shear tests are fast shear, consolidated fast shear and slow shear. Compared with direct shear test, triaxial shear test can simulate the actual stress state of soil and control the drainage condition more strictly, so its result is more reliable. The direct shear test is being eliminated because of many disadvantages

It is very important to fully understand the essence of shear under different consolidation and drainage conditions in deep foundation pit engineering. The correct choice of reasonable consolidation and drainage conditions is the guarantee of successful foundation pit support design. But at present, in the deep foundation pit engineering, many surveyors are sometimes uncertain about the reasonable triaxial shear test type for the specific deep foundation pit engineering, that is, whether to obtain the unconsolidated undrained shear index, or the consolidated undrained shear index or the consolidated drained shear index, which is not very clear. Some foundation pit support designers, when calculating the earth pressure, get different C, C, C, C, C, C, C, C, C, C, C, C, C, C, C, C, C, C, C, C, C, C, C, C, C, C, C, C, C, C, C, C, C, C φ I don't know how to choose. Sometimes for the sake of insurance safety and artificially take small value, resulting in waste; Or go to the other extreme, leading to foundation pit collapse accident. Therefore, it is necessary to clarify the essence of the three consolidation drainage conditions and their application in deep foundation pit engineering

4.5.2 at present, C φ In the design of foundation pit engineering support, it is popular to use consolidated undrained shear C φ The experience shows that this method is successful in many foundation pit support design, but there are also some foundation pit collapse accidents or great waste

generally, the soil of foundation pit has a certain degree of original consolidation (except for soft soil and newly accumulated soil), and the excavation of foundation pit is usually carried out by machinery, which is faster, and there is no time to drain the pore water in the soil φ The indicators are understandable. But its index is usually not directly used, usually multiplied by an empirical rection factor, based on the following:

(1) the direct shear apparatus used today can not simulate the actual stress state of soil, can not strictly control the drainage conditions, artificial shear failure surface, and many other factors, which make the test results larger

Even if the triaxial shear apparatus overcomes many disadvantages of the direct shear apparatus, it can not completely simulate the stress state of soil as the true triaxial apparatus does σ2＝ σ 3 , but the actual situation σ2≠ σ 3 , so that the test results still have errors compared with the actual

(2) in the test process, the stress path of the excavation side wall soil is opposite to that of the conventional triaxial test

(3) the stress history of soil samples (P cm ) is sometimes ignored ring the test

(4) sampling disturbance and vibration in the process of sample transportation will destroy the natural state of soil samples, which is bound to affect the test results

(5) in the design of foundation pit supporting structure, C φ The most important way to obtain the value is the indoor soil test, the other way is the in-situ vane shear test, and the vane test only provides the undrained shear C U value of soil φ＝ At this time, the Rankine active earth pressure coefficient of soil is equal to the Rankine passive earth pressure coefficient, which is not suitable for the design of foundation pit support. If C φ There is no other way to measure C φ It is difficult to find and judge the value of the test, and the result is a serious foundation pit accident, which makes people hold a cautious attitude towards the test results and avoid it by multiplying the rection factor by the engineering experience

The test results can be directly used without rection. Because of its C φ The value is mainly used to calculate the bearing capacity of foundation soil. In addition, it is determined by many methods, such as field standard penetration test, load test, static exploration test, water content, void ratio e, liquid index I L and so on. Even if C φ There are some differences in the value of the index, which has little effect on the bearing capacity of the foundation soil

In fact, compared with the actual situation, the indoor test results may be too large or too small, which is generally difficult to judge, and only multiplied by a rection factor to ensure the safety of foundation pit support design. When the test value is smaller than the actual value, and then multiplied by a rection factor, the result is more conservative, which is one of the important reasons why the actual measured earth pressure is much smaller than the calculated earth pressure; When the test result is larger than the actual value, multiplying by a rection factor can make the foundation pit support successful. However, if the test deviation can not be made up after multiplying the rection factor, the foundation pit collapse accident will be caused. This example is also common

in the calculation of deep foundation pit pressure, it should be considered comprehensively according to many factors, such as the structure and distribution of soil layer, the permeability of soil, the groundwater condition of the site, the excavation method of foundation pit and so on φ Value results

Unconsolidated undrained shear wall

thick marine silt, muddy soil and other soft soils are widely distributed in coastal areas of China, which have large void ratio (E = 1-3) and weak permeability (permeability coefficient k = 10 - 6 - 10 - 8 cm / s). This kind of soil is often the main soil layer that forms the earth pressure of deep foundation pit. For this kind of soil deep foundation pit support design (C φ In many cases, the earth pressure is smaller than the actual value, resulting in the collapse of foundation pit. In these areas, because the excavation of foundation pit is generally carried out by mechanical construction, the speed is fast, and the water in the foundation pit can not be discharged in time. Therefore, in soft soil area, the soil C φ At the same time, attention should be paid to:

(1) before the unconsolidated undrained test, the soft soil should be pre consolidated under the self weight stress, so as to avoid the influence of soil sample disturbance on the test results and make the soil sample return to the original stress state as much as possible. For example, in the design of foundation pit supporting structure in Shenzhen, the soft soil is pre consolidated first, and then the shear test is carried out to obtain C φ It's worth it, and it's successful. However, this point is usually ignored in the unconsolidated undrained shear test, and the specimen is not pre consolidated φ The value index is too small, which brings difficulties to the design

(2) reasonable degree of consolidation should be adopted. The test results are different with different degrees of consolidation. For the same soil sample, the higher the degree of consolidation, the greater the strength. In some coastal areas, the deposition time of saturated soft clay is not too long, and the original degree of consolidation is not high. If 100% degree of consolidation is still used, its strength will be too large, and the design will be dangerous. We know that the consolidation degree of soil layer can be expressed as u = 1-u d / U 0 (U d is the current pore water pressure, U 0 is the initial pore water pressure). If the pore water pressure U d can be measured, the original consolidation degree of soil can be calculated

Therefore, it is suggested to add a clause to the existing code for geotechnical tests (SL 237-1999) concerning unconsolidated undrained shear test: "unconsolidated undrained shear test shall be carried out for soft soil, and the pre consolidation under self weight stress shall be carried out before the test, and the degree of consolidation adopted shall be determined according to the original consolidation degree of soil layer."

4.5.4 consolidated undrained shear

at present, most of the domestic foundation pit support structure designers use the C, C, C, C, C, C, C, C, C, C, C, C, C, C, C, C, C, C, C, C, C, C, C, C, C, C, C, C, C, C, C, C, C, C, C, C φ The value index is used as the basis of design. Theoretically, the consolidated undrained shear index is applicable to most soil layers except soft soil. The reason is very simple. Mechanical construction is used for foundation pit excavation, and the speed is fast. The water in the soil can not be discharged in time, so the undrained shear is used; In addition, the soil itself has a certain degree of consolidation (except for soft soil or newly accumulated soil), so the consolidated undrained shear test is in line with the reality. Consolidation undrained shear index C φ The rection standard of value should be different, and the method of multiplying by 0.7 rection factor cannot be used

(1) permeability: when the permeability coefficient of foundation pit soil is relatively small, the degree of consolidation of soil may not reach 100%. If the consolidated undrained shear index is adopted, its value can be reced

(2) ground water: if there is no ground water in the site, or in the later stage of foundation pit dewatering, or for silt and silt, the consolidation degree is relatively high, less rection should be considered. On the contrary, we should rece more

(3) construction speed: if the foundation pit construction is slow and the foundation pit soil is allowed to be drained and consolidated for a long time, it should be reced; If it is mechanized and rapid construction, the rection should be more

4.5.5 consolidation drainage shear results C d φ d and the effective stress C ' φ＇ Because of the long test time and high cost, it is not widely used in engineering

for deep foundation pit engineering, the foundation pit soil layer is silt, silt, fine sand and other soil layers with good water permeability, such as Wuhan on both sides of the Yangtze River, and the silt, silt and fine sand layers on both sides of the Lijiang River in Guilin, the consolidation drainage shear Index C d , C d , C d d , C d d d d d value, but the excavation speed of foundation pit is required to be slow or manual excavation φ< The results of sub > d are in good agreement with the actual situation

Conclusion (1) different consolidation and drainage conditions, the shear test indexes of deep foundation pit soil C φ The values are different

(2) the results of unconsolidated undrained shear test are generally used for the design of foundation pit in soft clay area with low degree of consolidation and weak permeability. Before shear test, proper pre consolidation should be carried out under self weight pressure

(3) the results of consolidated undrained shear test can be used to remove soft soil

8. In general, in geotechnical engineering investigation, direct shear and quick shear are used if there are no special requirements. For example, when the foundation pit is excavated for 1-3m, the surrounding Level 3 foundation pit is relatively open
when the foundation pit is excavated for more than 4-5m, and there are other buildings around, the foundation pit support is needed. In this case, some geotechnical parameters of the soil layer above the foundation need to be used, and the three pumping test is needed. When the building load is relatively large, the soil layer below the foundation is relatively thick, and the high-pressure consolidation test is needed, In this case, at least 6 groups of tests should be carried out for each layer of soil
like other soils with expansibility, expansion tests should also be carried out

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How to calculate cohesive force and internal friction angle with

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