Study on interpretation of Marine Geotechnical parameter based on static sounding

In recent years, the rapid rise of offshore wind power. Compared with other Marine exploration means and methods, pore pressure static penetration test is more economical, more efficient and more reliable. It has been widely used and is an indispensable exploration means in offshore wind power projects. Based on the survey results of the offshore windpower project in Guangdong in recent years, the applicability of using the pore pressure static sounding (CPTU) empirical formula to interpret various geotechnical parameters of offshore projects is analyzed in this paper, which can provide experience and reference for similar offshore projects.


Introduction
Static cone penetration test (CPT) is an in-situ test in which a cone probe is pressed into the soil through a series of probe rod at a constant penetration rate using static force, and the physical and mechanical properties of the soil are determined indirectly according to the penetration resistance measured.Suitable for soft soil, sticky soil, silt, (saturated) sandy soil and soil containing a small amount of gravel, with the following advantages: 1) continuous test, fast, high efficiency, multiple functions, both exploration and testing dual role; 2) After the use of electrical measurement technology, it is easy to realize the automation of the test process, and the test results can be automatically processed by the computer, which greatly reduces the work intensity of people.CPT is one of the most commonly used and indispensable test methods in both Marine engineering survey and land engineering geological survey, and plays an important role in geotechnical investigation.

Marine stratigraphy classification technology based on static sounding
An important application of static sounding is to classify and name the soil class along the test depth of the stratigraphic section.Especially for the coastal and offshore sites, it is difficult to drill and sample, and the sample is easy to be disturbed.Especially for the very complex strata such as interbedded, thinned, inclusions of shells, drifter and isolated strata, uneven deposition, and large soil property variation, the difficulty of conventional determination is aggravated.And the formation determination often depends on the engineer's experience, compared with the static penetration determination method has more advantages [1].

Rf-H curve characters u-H curve characters
Silt, silty clay 2. The qcvalue is large, which is obviously higher than that in the general soil layer Generally speaking, the typical CPT test curve has some different characteristics for different strata, such as cone tip resistance qc is high in sand, low in clay, friction resistance ratio Rf=fs/qc is low in sand, high in clay, etc.The specific soil properties and test curves are described in table 1, which is basically qualitative analysis.Engineers with different levels of experience make different judgments.The division of soil layers according to static sounding data should be carried out in the following steps.
The resistance and depth curves of the static probe are segmented.The segmentation is based on a comprehensive segmentation according to various resistance sizes and curve shapes.For example, the soil layer represented by the curve section with small resistance, large friction ratio, large excess pore water pressure and small curve change is mostly clay layer [2].The zigzag shape with large resistance, small friction ratio, small excess pore water pressure and sharp curve changes is sandy soil [3].
The interface depth of each soil layer is determined accurately according to the concepts of critical depth.In the process of static penetration from the surface, the cone resistance increases gradually (except the influence of the hard shell layer), and reaches a relatively constant value after a certain depth (critical depth), and the first relatively constant value section of the critical depth and curve is the first layer [4].When the probe continues to penetrate near the second layer, the probe resistance will be affected by the common influence of the upper and lower soil layers and change, becoming larger or smaller, the general rule is that the middle depth of the curve change section is the depth of the layer, the second layer also has a relatively constant value section, and so on [5][6].
After the above two steps, the probe resistance and other parameters of each layer of soil are arithmetically averaged respectively, and the average value can be used to determine the soil layer name, and the method of determining the soil layer (class) name can be carried out according to various empirical graphs.The soil profile of the site can also be obtained by using the porous static sounding curve.

Analysis of underlying parameters based on CPTU
Compared with the laboratory test method, the static sounding method is more reliable, economical, short in cycle and widely used.It can judge the moisture degree and density of soil, calculate the density of saturated soil, calculate the shear strength parameters of soil, obtain the basic bearing capacity of foundation soil, and explore the consolidation coefficient and permeability coefficient of saturated soil by using pore pressure contact.
After the test, the original data should first be checked and corrected, such as the zero drift value changes with the depth, the automatic recorded depth and the actual depth (calculated by the length of the probe rod) are different, the original data should be corrected according to the linear interpolation method.For the automatic recorder, the effect of zero drift value can be eliminated by raising the pen tip once every certain depth to zero.After the revised recorded data, the relevant sounding parameters of each test depth should be calculated in accordance with the following formula. ' ' / 100(%) Where, ps is the specific penetration resistance of the single-bridge probe; qc is the cone tip resistance of the two-bridge probe; fs is the side friction resistance of the two-bridge probe.ud is the penetration hole pressure measured on the cone of the probe; ut is the penetration hole pressure measured at the cylindrical surface above the base of the probe cone.kp, kq, kf and ku are the sensor calibration coefficients of the probe parameters.x'p, x'q, x'f, x'u are the corresponding corrected readings; qt is the total cone tip resistance; Rf is the friction-resistance ratio; Bq is pore pressure coefficient; Fa is the effective area of the cone tip surface; A is the full cross-sectional area of the cone tip.
Robertson defines the soil index Ic as follows [7] : Where qc is the tip resistance, fs is the cone side resistance, σ 'v0 is the effective overlying pressure, σv0 is the total overlying pressure, n is the stress index, Pa is the reference pressure with the same unit of effective overlying pressure σ'v0, Pa2 is the reference pressure with the same unit of qc and σv0.

Test Result Analysis
Interpretation and application of CPT test results, including the application of test results, identifying the changes of foundation soil in the horizontal and vertical directions, dividing the soil layer, determining the physical and mechanical parameters of the foundation soil, testing the compactness and uniformity of the filled soil and other artificially strengthened foundations, determining the possibility of pile sinking, estimating the bearing capacity of a single pile, determining the compactness of sand and the possibility of liquefaction under earthquake action, etc.
Robertson summarized the applicability of testing the physical and mechanical parameters of static penetrating rock mass.He proposed five applicability levels. 1 is highly applicable, including the overconsolidation ratio and undrained strength of viscous soil. 2 is suitable for medium height, including the lateral pressure coefficient, sensitivity, permeability, consolidation coefficient of viscous soil and the relative density of sandy soil, state parameters, friction Angle, and small strain shear die; 3 for moderate application, including the Young's modulus (elastic modulus), shear modulus, compression coefficient of clay and sandy soil; 4 is suitable for medium and low degrees, including the friction Angle of viscous soil; 5 is suitable for low degrees, including the overconsolidation ratio of silty soils.The soil property index Ic along depth curve is shown in figure 1.

Figure 1. Soil property index Ic along depth curve
In this paper, the empirical relationship and correlation between CPT test index and soil physical and mechanical parameters are discussed.After summary and induction, the empirical relationship between the compression modulus Es (MPa) of the previous soil deformation index and the ratio of the CPT parameter of a single bridge to the penetration resistance ps (MPa) is as follows, as shown in Table 2 and Table 3  The interpretation results of the relative density Dr And internal friction Angle φ are only applicable to soil layers at 5, 6, 7 and 8 in the SBTn region, Young's modulus E is only applicable to soil layers with Ic less than 2.60, undrained shear strength Su and overconsolidation ratio OCR are only applicable to soil layers at 1, 2, 3, 4 and 9 in the SBTn region.
Therefore, according to the in-situ test results of CPT/CPTU, the results of geological exploration can be compared with the data, including the basic engineering geological information of the seabed, the physical property index, the mechanical property index, the water permeability index, the compression and deformation property index, etc., and the in-situ test and test results including the standard penetration number and shear wave velocity value can be obtained.Therefore, based on the representative values such as the range, average value and standard value of each index feedback from each layer, and combining the laboratory test results, the engineering geological characteristics of each layer can be more accurately determined and evaluated.

Conclusion
The actual strata are extremely complex in the sedimentary history and formation environment, which are generally non-uniform and gradual, and often one type of soil is mixed with other types of soil, which is also the general geological law of quaternary sedimentary strata, and a reasonable and economical stratigraphic classification method should be adopted on the basis of understanding its basic geological engineering concepts.Through the discussion and application of this study, the following brief conclusions can be drawn: (1)It is necessary to optimize the original test data, which can remove the noise error, smooth the test curve, improve the human identification degree, and grasp the overall law of the test curve.
(2)The chart classification method of stratum soil property index can obtain the continuity index of stratum characteristics along the depth, and then understand the continuous and progressive change of stratum soil property distribution characteristics with depth.At the same time, combining the test data of multiple drilling holes in the site, it can grasp the distribution law of stratum soil property and stratum fluctuation, which is of great significance for foundation engineering, especially pile foundation engineering.
(3)The stratigraphic classification method based on CPT soil property index and its application examples can be found that the determination concept of this method is more clear and clear, especially for complex strata with prominent phenomena such as interlayer and interlayer, which reduces human operation error and misjudgment.It is a fast, reproducable, high-precision and quantitative analysis and determination method, which is worthy of promotion and application.

Table 2 .
. Relation Between Compression Modulus ES And Specific Penetration Resistance PS

Table 3 .
Empirical formula of deformation modulus E0 Through an example of CPTU data of a sea-bed pore pressure static sounding, the results of interpretation of Marine geotechnical test indexes and physical and mechanical indexes are as follows:Cone tip resistance test value qc, side friction resistance fs, test indicators: