First look at an article on how to find the undrained shear strength of cohesive soil based on CPT data.
"Research on Application of Static Cone Penetration Technology in Ocean Engineering Investigation" Chen Peixiong et al
With the increasing development and utilization of oceans, it is increasingly important to master the engineering characteristics of the most submarine soil. The submarine static cone penetration test (CPT) is a fast, efficient, low-cost, and high-precision in-situ test method to obtain this data. It is an important means to obtain the quantitative analysis of engineering geological problems and the soil mechanical parameters obtained from engineering design and construction. It is widely used in marine engineering survey and engineering geological disaster investigation and research. The application of offshore static cone penetration technology is very regional. In this paper, combined with the measured data in the western sea area of the East China Sea shelf, the static cone penetration parameters of the submarine shallow surface soil in this area are used for soil classification, and the undrained shear strength of the cohesive soil and the sensitivity of the cohesive soil are estimated. This is of theoretical and practical significance whether it is to further deepen the understanding of the characteristics of submarine soil engineering in this sea area or to promote the application of my country's submarine static cone penetration technology.
The research and application of my country's offshore static penetration technology started relatively late, and it is still in its infancy, with a large gap between technology and foreign countries. The application of static cone penetration technology in my country's sea began in the 1970s. It is mainly used in marine survey projects such as submarine pipelines, oil and gas field development, and submarine cable {optical cable> route surveys. The probes and instruments used are mainly developed abroad. Based on the regional engineering test data, some researchers have adopted statistical analysis methods to obtain the correlation between the static cone penetration parameters and various engineering properties of the soil, and then established some static cone penetration parameters and the soil The regional empirical formulas between the nature indexes of the preparation of the species.
4.2 Estimating the undrained shear strength of silty soil based on the study sea area static
cone penetration test. At present, the total cone tip resistance is commonly used in foreign countries to estimate the undrained shear strength Su, as follows:
In the formula, Su is the undrained shear strength, qc is the resistance of the cone tip of the static penetration probe, σv0 is the total overburden pressure (including water pressure), and Nkt is the probe coefficient.
SCHMERTMANN believes that there are many factors that affect the value of Nkt: such as the method used to determine the reference undrained shear strength, the stress conditions and stress history of the site soil, the structure of the soil, the sensitivity, the plasticity of the soil, the type of cone penetrometer and The operation method and penetration rate of static cone penetration. KJEKSTAD and LUNNE have shown through experiments that the range of Nkt value is mostly 15-20, and the average value is 17.
The author compared the Su value estimated by the above formula with the Su value obtained from the strength test of the sample by statistical analysis (where the value of Nkt is an integer value of 15 to 20), and found that in the research sea area, when the value of Nkt is 17 At the time, the estimated results are more consistent with the indoor experimental results. In statistics, because some soil samples contain fine layers of silt sand, the shear strength value measured by the test will be too small. At this time, Nkt can be added appropriately. Table 2 shows the comparison between the shear strength of cohesive soil estimated by static cone penetration and the Su value obtained from the strength test of the soil sample when the Nkt value is 17.
It can be seen from the article. The Nkt value is best to be obtained from indoor tests. Generally, we will conduct both manual sampling and static cone penetration on some boreholes for comparative tests. Because we borrowed CPT equipment from other companies and have no experience in using CPT, they gave the formula for Su as well as the formula for granular soil.
Then the density of granular soil has the following formulas:
These formulas are all from the literature. I originally wanted to search out the relevant literature, but later found that it was really difficult, but accidentally, I discovered that the "Pore Pressure Static Penetration Test Technical Specification" was published in 2017. This is easy to handle, according to the national standard.
Su formula for finding cohesive soil
Find the compactness and internal friction angle of granular soil
Classification formula of soil:
We used this to process our data. When Nkt=17, the value of Su, the classification of soil and the value of compactness are credible, and the internal friction angle is too large. It is recommended to use some empirical parameters in the Engineering Geology Manual to recommend internal friction Angle parameters. such as:
2. Abnormal problems of surface sound velocity instruments
In the past two days, I shot an XCTD to measure the speed of sound, which was 3-4m/s away from the surface sound velocity meter, and then I shot another one. The difference is still so much. I suspect that the surface sound velocity meter is broken, but I just did a sound speed comparison. There is no problem with the speed of sound. After pulling the surface sound velocity meter out of the well, it was found that the surface sound velocity meter was indeed broken. One of the three small pillars below it was broken. Since the surface sound velocity meter measures the speed of sound through the plane distance between the two ends of the column (about 10cm), the general sound speed is about 1500m/s, as long as the difference is 1mm, it will cause a difference of 1.5m/s, a difference of 3-4m/s, equivalent to two The distance between the end planes has increased by 2mm. This is because one of the columns is broken, which makes the time longer and the speed of sound smaller. Fortunately, I brought a surface sound velocity meter as a backup. After replacing it, it will be ready immediately.
For more information, please refer to xiaok marine surveying and mapping network and the public account of the same name