Classification method of shallow silty clay strata based on CPTU field penetration test

Abstract

The present invention belongs to the field of geotechnical engineering field test technology, specifically a shallow silty clay stratum classification method based on CPTU field penetration test, using CPTU field penetration test, by modifying the cone tip resistance model, combined with the normalized friction ratio model; introducing the vertical stress of the overlying soil layer, the effective vertical stress of the overlying soil layer and the atmospheric pressure, to obtain the normalized cone tip resistance model; then based on the soil type index and boundary conditions such as the thickness of the silty clay stratum, the existing correction coefficient is optimized, and the normalized cone tip resistance model is further corrected; finally, the normalized friction ratio model and the modified normalized cone tip resistance model are brought into the soil type index model to obtain a modified soil type index model for dividing shallow soil. This method can meet the soil classification more accurately, and can meet the discrimination between silty clay, silt and silt sand, and has the advantages of being fast, simple, and having high reliability of test results.

Description

Technical Field

[0001] The invention belongs to the technical field of geotechnical engineering field test and particularly relates to a shallow silty clay stratum classification method based on a CPTU field penetration test. Background Art

[0002] In recent years, with the rapid development of urban underground space, underground pipelines, buildings and structures have increased day by day, and the safety of urban underground projects has become a focus of domestic attention. Soil mechanics indicators are key parameters for evaluating the stability of underground structures, facilities such as cables or oil pipelines, and slopes, and the determination of soil mechanics indicators depends on the accurate classification of soil layers. Reasonable classification of soil types can help designers better understand the geological structure of the site, grasp the main engineering geological problems, and formulate more reasonable and effective design plans to ensure the safety and stability of the project.

[0003] The pore pressure penetration test unit (CPTU) has the advantages of being simple, fast, highly automated, and having high reliability in test results. It has good application prospects in soil layer division, soil classification, and shallow geological disaster investigation, and has gradually become a commonly used in-situ test method in geotechnical engineering. However, there are still many problems in the application of the CPTU method in shallow geological surveys of silty clay, such as inconsistent soil classification methods, large fluctuations in test results, and insufficient data utilization; the test results of some silty clays with high permeability will be affected by drainage conditions, and the existing empirical coefficient method will cause inaccurate inversion results.

[0004] In addition, Robertson 2009 considered the influence of CPTU penetration depth on the test results and proposed a normalized soil behavior type (abbreviated as SBTn) classification diagram, which mainly solves the problem that the penetration resistance in sand is not proportional to the vertical effective stress. This difference is manifested in that the closer the soil is to sand, the stronger the nonlinear correlation between its penetration resistance and vertical stress. However, in shallow silty clay, due to the influence of factors such as partial drainage, overconsolidation and soil structure, the nonlinear characteristics of penetration resistance and stress are aggravated, resulting in a penetration resistance higher than the theoretical value range that can be generated under this stress condition, which makes the normalized cone tip resistance overestimated, and thus causes the soil type value I c As the depth increases, shallow silty clay is easily misidentified as sandy soil. This method has a large error in distinguishing shallow silty clay and is not applicable to shallow silty clay. Summary of the invention

[0005] In order to solve the problems of traditional soil classification methods, such as the difficulty in distinguishing shallow silty clay and the inaccurate acquisition of soil physical and mechanical parameters, the present invention proposes a shallow silty clay stratum classification method based on CPTU field penetration test. By correcting the soil type index, the soil classification can be more accurately met, and the test efficiency and reliability can be effectively improved.

[0006] The present invention is implemented by adopting the following technical scheme: a shallow silty clay stratum classification method based on CPTU field penetration test, comprising the following steps:

[0007] Step A, collecting mechanical characteristic parameters of shallow silty clay based on CPTU penetration test equipment;

[0008] The mechanical characteristic parameters include the cone tip resistance q c , side wall friction f s , pore water pressure u0, vertical stress of overlying soil layer σ v , effective vertical stress of overlying soil layer σ′ v and atmospheric pressure p a ;

[0009] Step B: constructing a normalized friction ratio model and a normalized cone tip resistance model according to the obtained mechanical characteristic parameters;

[0010] Step C: Introduce soil type index I c The normalized cone tip resistance model is modified by using the shallow silty clay boundary condition to obtain the modified normalized cone tip resistance model.

[0011] Step D: Based on the normalized friction ratio model obtained in step B and the modified normalized cone tip resistance model obtained in step C, a modified soil type index model for classifying strata is determined.

[0012] Furthermore, in step B, according to the cone tip resistance q c and pore water pressure u0 to obtain the modified cone tip resistance model q t ; Combined with the side wall friction f s , pore water pressure u0 to obtain the normalized friction ratio model F r ; and based on the modified cone tip resistance model q t , by introducing the vertical stress σ of the overlying soil layer v , effective vertical stress of overlying soil layer σ′ v and atmospheric pressure p a , construct the normalized cone tip resistance model Q tn ;

[0013] Among them, the modified cone tip resistance model q t It is expressed as:

[0014] qt =q c +u0(1-A a / A c )

[0015] Among them, q c represents the cone tip resistance; u0 represents the pore water pressure; A a and A c Respectively represent the cross-sectional areas of the top column and the cone bottom; A a / A c Generally, it is between 0.55 and 0.9. It should be noted here that the cone tip resistance q c The pore water pressure u0 can be directly obtained through the CPTU field penetration test, and the A a and A c This is to consider the influence of the cone head shape of the pore pressure static penetration rod on the pore water pressure, which in turn causes the cone tip resistance q t increase.

[0016] The normalized friction ratio model F r It is expressed as:

[0017] F r =f s / (q t -σ v )×100

[0018] Among them, F r represents the normalized friction ratio; f s Represents the side wall friction; q t represents the modified cone tip resistance; σ v represents the vertical stress of the overlying soil layer;

[0019] The normalized cone tip resistance model Q tn It is expressed as:

[0020]

[0021] Among them, q t represents the modified cone tip resistance; σ v represents the vertical stress of the overlying soil layer; σ v ' represents the vertical effective stress of the overlying soil layer; p a represents atmospheric pressure; n represents the proportionality coefficient;

[0022] The proportionality factor n is expressed as:

[0023]

[0024] Among them, σ v ' represents the vertical effective stress of the overlying soil layer; p a Indicates atmospheric pressure; Ic Represents soil type index.

[0025] In this scheme, the normalized cone tip resistance model Q tn Including the proportionality coefficient n, according to the normalized soil classification method proposed by Robertson 2009, in the case of clay, it can be considered that q t -σ v With σ v ' is proportional, the proportionality coefficient n = 1, but in the case of sandy soil, q t -σ v and The proportional coefficient n is 1 / 2, and the proportional coefficient n of the intermediate transition soil is between 1 and 0.5. c As the value of n increases, the value of n decreases. Therefore, it is necessary to accurately calculate the soil type index I c It is the premise for the reasonable classification of shallow silty clay, and the soil type index I c The determination of depends on the value of the proportional coefficient n.

[0026] According to the CPTU field penetration test, when the thickness of shallow silty clay is ≤10m, the penetration resistance and stress of the silty clay show a strong nonlinear relationship, which means that the proportional coefficient n of the shallow silty clay is ≠1, and the relationship of the proportional coefficient n is no longer applicable to the shallow silty clay.

[0027] Furthermore, in step C, the soil type index I is introduced c and shallow silty clay boundary conditions, the shallow silty clay boundary conditions are defined as 0m and 10m, and the function of the corrected proportional coefficient n is obtained (expressed as the correction coefficient n1); the functional relationship is substituted into the normalized cone tip resistance model Q tn , and then get the modified normalized cone tip resistance model Q tn1 , specifically:

[0028] The functional relationship of the correction coefficient n1 is:

[0029] When h≤10m,

[0030] 0≤n1=(a+bh)I c +0.05(σ v ' / p a )-(c+dh)≤0.5

[0031] When h>10m,

[0032] 0.5 <n1=0.381I c +0.05(σ v ' / p a )-0.15≤1

[0033] Where, h is the thickness of the silty clay layer; I c represents the soil type index; a, b, c, d represent empirical coefficients, which are obtained based on the thickness boundary conditions of shallow silty clay and field penetration tests;

[0034] Modified normalized cone tip drag model Q tn1 It is expressed as:

[0035] When h≤10m,

[0036]

[0037] When h>10m,

[0038]

[0039] Among them, q t represents the modified cone tip resistance; σ v represents the vertical stress of the overlying soil layer; σ v ' represents the vertical effective stress of the overlying soil layer; p a Indicates atmospheric pressure; I c Represents soil type index.

[0040] Furthermore, in step D, the modified soil type index I for classifying the stratum is obtained. c1 It is expressed as:

[0041] I c1 =[(3.47-logQ tn1 ) 2 +(1.22+logF r ) 2 ] 0.s

[0042] Among them, Q tn1 represents the modified normalized cone tip resistance; F r represents the normalized friction ratio.

[0043] Compared with the prior art, the advantages and positive effects of the present invention are:

[0044] The soil classification method of the modified soil type index proposed in this scheme can more accurately meet the soil classification requirements. The normalized cone tip resistance model is modified and combined with the normalized friction ratio model. The vertical stress of the overlying soil layer, the effective vertical stress of the overlying soil layer and the atmospheric pressure are introduced to obtain the normalized cone tip resistance model. Then, based on the soil type index and boundary conditions such as the thickness of the silty clay layer, the normalized cone tip resistance model is further modified. Finally, the normalized friction ratio model and the modified normalized cone tip resistance model are introduced into the soil type index model to obtain the modified soil type index for dividing shallow soil. It can meet the discrimination between silty clay, silt and silty sand, and has the advantages of being fast, simple and highly reliable in test results. It provides theoretical support for the safe construction of underground structures, cables or oil pipelines and other related facilities, and also shortens the time for on-site testing and calibration of soil types and the test process time, effectively reducing costs. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] Figure 1 This is a schematic diagram of the process of the shallow silty clay stratum classification method according to an embodiment of the present invention;

[0046] Figure 2 It is the standard diagram for soil classification;

[0047] Figure 3 This is the soil point arrangement diagram for the CPTU field penetration test;

[0048] Figure 4 This is a comparison chart of the soil layer classification calculation result of the measuring point 2-1 by the method described in the embodiment of the present invention and the calculation result of Robertson 2010; wherein (a) is the soil classification method of Robertson 2010, and (b) is the method of the present invention;

[0049] Figure 5 This is a comparison chart of soil layer classification calculation results of measuring points 1-5 by the method described in an embodiment of the present invention and the calculation results of Robertson 2010; wherein (a) is the soil classification method of Robertson 2010, and (b) is the method of the present invention. DETAILED DESCRIPTION

[0050] In order to more clearly understand the above-mentioned purpose, features and advantages of the present invention, the present invention is further described below in conjunction with the accompanying drawings and embodiments. In the following description, many specific details are set forth to facilitate a full understanding of the present invention, but the present invention can also be implemented in other ways different from those described herein, and therefore, the present invention is not limited to the specific embodiments disclosed below.

[0051] like Figure 1The above is a flow chart of the shallow silty clay stratum classification method based on the CPTU penetration test according to an embodiment of the present invention. The correction coefficient is obtained by combining the soil type index and the shallow silty clay boundary conditions through the CPTU field penetration test, and then the normalized cone tip resistance is corrected to obtain the corrected soil type index for dividing the stratum category, which specifically includes the following steps:

[0052] Step A, collecting mechanical characteristic parameters of shallow silty clay based on CPTU penetration test equipment;

[0053] The mechanical characteristic parameters include the cone tip resistance q c , side wall friction f s , pore water pressure u0, vertical stress of overlying soil layer σ v , effective vertical stress of overlying soil layer σ′ v and atmospheric pressure p a ;

[0054] Step B: Determine the normalized friction ratio model F according to the obtained mechanical characteristic parameters r and normalized cone tip resistance model Q tn ;

[0055] Step C: Introduce soil type index I c Effect of shallow silty clay boundary conditions on the normalized cone tip resistance model Q tn After correction, the modified normalized cone tip resistance model Q is obtained. tn1 ;

[0056] Step D: Normalized friction ratio model F obtained based on step B r and the modified normalized cone tip resistance model Q obtained in step C tn1 , calculate the modified soil type index I for dividing the stratum categories c1 .

[0057] In order to more clearly understand the present invention, this embodiment selects the shallow silty clay in the lower reaches of the Yellow River as the research object of the field penetration test. The groundwater resources in the lower reaches of the Yellow River are relatively abundant, the stratum structure is complex and the soil moisture content is high. The precipitation operation is extremely difficult and there are a large number of soil instability phenomena. The easily liquefied silt layer and the underlying soft soil layer with high moisture content are the main disaster-causing strata in the area, which seriously threaten the safety of underground pipelines and other structures in the area.

[0058] In this embodiment, the test equipment used in the on-site CPTU penetration test is a multifunctional digital static penetration vehicle, which is a relatively mature system and is not the focus of this solution. The CPTU system includes a vehicle-mounted static penetration system and a crawler vehicle power part; the cone angle of the pore pressure static penetration rod is 60°, and the surface area of ​​the side wall friction cylinder is 150cm 2 , the probe cone bottom area is 10cm2 The pore pressure test element is 0.5 cm thick and is located at the cone shoulder position (u0 position). The probe penetration rate is 20 mm / s. During the test, the data acquisition system can set the sampling frequency to achieve uniform distribution of data acquisition.

[0059] Firstly, based on the CPTU field penetration test, the cone tip resistance q in the shallow silty clay in the lower reaches of the Yellow River was obtained. c , side wall friction f s , u0 represents the pore water pressure, and the vertical stress of the overlying soil layer σ v , effective vertical stress of overlying soil layer σ′ v and atmospheric pressure p a Parameter information such as Figure 3 The figure shows the soil point arrangement diagram for the field penetration test. Then, according to the cone tip resistance q c and pore water pressure u0 to obtain the modified cone tip resistance model q t ; Combined with the side wall friction f s , pore water pressure u0 to obtain the normalized friction ratio model F r ; and according to the modified cone tip resistance q t By introducing the vertical stress σ of the overlying soil layer v , effective vertical stress of overlying soil layer σ′ v and atmospheric pressure p a , the normalized cone tip resistance model Q is derived by calculation tn .

[0060] Among them, the modified cone tip resistance model q t It is expressed as:

[0061] q t =q c +u0(1-A a / A c )

[0062] Among them, q c represents the cone tip resistance; u0 represents the pore water pressure; A a and A c Respectively represent the cross-sectional areas of the top column and the cone bottom; A a / A c Generally, it is between 0.55 and 0.9. It should be noted here that the cone tip resistance q c The pore water pressure u0 can be directly obtained through the CPTU field penetration test, and the A a and A c This is to consider the influence of the cone head shape of the pore pressure static penetration rod on the pore water pressure, which in turn causes the cone tip resistance q t increase.

[0063] The normalized friction ratio model Fr It is expressed as:

[0064] F r (%) = f s / (q t -σ v )×100

[0065] Among them, F r represents the normalized friction ratio; f s Represents the side wall friction; q t represents the modified cone tip resistance; σ v represents the vertical stress of the overlying soil layer;

[0066] The normalized cone tip resistance model Q tn It is expressed as:

[0067]

[0068] Among them, q t represents the modified cone tip resistance; σ v represents the vertical stress of the overlying soil layer; σ v ' represents the vertical effective stress of the overlying soil layer; p a represents atmospheric pressure; n represents the proportionality coefficient.

[0069] Among them, the normalized cone tip resistance Q tn The relationship includes the proportional coefficient n. According to the CPTU field penetration test, the reasonable value of the proportional coefficient n is the basis for distinguishing the shallow silty clay in the lower reaches of the Yellow River. When the thickness of the shallow silty clay is ≤10m, the penetration resistance and stress of the silty clay show a strong nonlinear relationship, which means that the proportional coefficient n of silty clay and other clays is ≠1, and the relationship of the proportional coefficient n is no longer applicable to the shallow silty clay.

[0070] This embodiment introduces soil type index I c and shallow silty clay boundary conditions, combined with Figure 2 As shown in Table 1, Figure 2 Table 1 is the soil classification standard diagram obtained in related studies. Figure 2 The corresponding soil type index value range and the soil category it represents.

[0071] Table 1 Soil classification name map

[0072] Soil type <![CDATA[I c ]]> Soil type 1 N / A Sensitive fine-grained soil 2 <![CDATA[I c ≥3.6]]> Organic soil-clay 3 <![CDATA[2.95≤I c <3.6]]> Clay type: silty clay-clay 4 <![CDATA[2.6≤I c <2.95]]> Silt mixture: clay silt-silty clay 5 <![CDATA[2.05≤I c <2.6]]> Sand-soil mixture: silty sand-sandy silt 6 <![CDATA[1.31≤I c <2.05]]> Sandy soil: pure sand-silty sandy soil 7 <![CDATA[I c <1.31]]> gravelly sand-dense sand 8 N / A Very hard sand-clay sand 9 N / A Very hard fine sand

[0073] Because the existing soil classification methods cannot distinguish the soil types within 10m, this embodiment defines the boundary conditions of the shallow silty clay as 0m and 10m, combines the relationship of the proportional coefficient n, and then establishes a functional relationship with the soil layer thickness h to obtain the functional relationship of the correction coefficient n1, and brings the functional relationship into the normalized cone tip resistance model Q tn , and then obtain the modified normalized cone tip resistance model Q tn1 ;

[0074] The relationship between the proportionality coefficient n is:

[0075]

[0076] Among them, σ v ' represents the vertical effective stress of the overlying soil layer; p a Indicates atmospheric pressure; I c Represents soil type index.

[0077] The functional relationship of the correction coefficient n1 is:

[0078] When h≤10m,

[0079] 0≤n1=(a+bh)I c +0.05(σ v ' / p a )-(c+dh)≤0.5

[0080] When h>10m,

[0081] 0.5 <n1=0.381I c +0.05(σ v ' / p a )-0.15≤1

[0082] Where, h is the thickness of the silty clay layer; I c represents the soil type index; a, b, c, d represent empirical coefficients; they are obtained based on the boundary conditions of the silty clay layer thickness (0 m and 10 m) and the CPTU field penetration test of silty clay. In this embodiment, the values ​​of the empirical coefficients a, b, c, d are a=0.25, b=0.013, c=0.75, d=-0.06, respectively.

[0083] Then the modified normalized cone tip resistance Q is obtained tn1 Relational formula:

[0084] When h≤10m,

[0085]

[0086] When h>10m,

[0087]

[0088] Among them, q t represents the modified cone tip resistance; σ v represents the vertical stress of the overlying soil layer; σ v ' represents the vertical effective stress of the overlying soil layer; p a Indicates atmospheric pressure; I c Represents soil type index.

[0089] Finally, based on the obtained normalized friction ratio model F r and the modified normalized cone tip drag model Q tn , the modified soil type index model I for dividing the stratum categories is calculated c1 :

[0090] I c1 =[(3.47-logQ tn1 ) 2 +(1.22+logF r ) 2 ] 0.5

[0091] Among them, Q tn1 represents the modified normalized cone tip resistance; F r represents the normalized friction ratio.

[0092] It can be shown that the modified soil type index I c1 The relationship function includes the normalized friction ratio F r and the modified normalized cone tip resistance, and the relevant parameters obtained through the CPTU field penetration test, and then the normalized friction ratio F is modified r The normalized cone tip resistance can more accurately meet the soil classification and can distinguish between silty clay, silt and silt sand. It has the advantages of being fast, simple and having high reliability of test results.

[0093] Combination Figure 4-5 In order to ensure the accuracy of the formula obtained by analyzing the results of the CPTU field penetration test, this embodiment relies on the Jinan area near the lower reaches of the Yellow River, quotes the soil classification method of Robertson 2010, and selects typical shallow silty clay on site for calculation. The results show that the shallow silty clay stratum classification method of the lower reaches of the Yellow River based on the CPTU penetration test proposed in the present invention can more accurately meet the soil classification and can meet the discrimination between silty clay, silt and silt sand, thereby improving the overall accuracy of the detection work.

[0094] The above description is only a preferred embodiment of the present invention and does not limit the present invention in other forms. Any technician familiar with the profession may use the technical content disclosed above to change or modify it into an equivalent embodiment with equivalent changes and apply it to other fields. However, any simple modification, equivalent change and modification made to the above embodiment based on the technical essence of the present invention without departing from the content of the technical solution of the present invention still falls within the protection scope of the technical solution of the present invention.

Claims

1. A classification method for shallow silty clay strata based on CPTU field penetration test, characterized in that: The following steps are involved: Step A: Collect the mechanical characteristic parameters of shallow silty clay based on the CPTU penetration test equipment; the mechanical characteristic parameters include the cone tip resistance q c , side wall friction f s , pore water pressure u0, vertical stress of overlying soil layer σ v , effective vertical stress of overlying soil layer σ′ v and atmospheric pressure p a ; Step B: constructing a normalized friction ratio model and a normalized cone tip resistance model according to the obtained mechanical characteristic parameters; Among them, the normalized cone tip resistance model is expressed as: Among them, Q tn represents the normalized cone tip resistance, q t represents the modified cone tip resistance, n represents the proportional coefficient, I c Represents the soil type index; the proportional coefficient n is expressed as: Step C, correcting the proportionality coefficient n in combination with the boundary condition of shallow silty clay to obtain a correction coefficient n1, the correction coefficient n1 is expressed as; When h≤10m, 0≤n1=(a+bh)I c +0.05(σ' v / p a )-(c+dh)≤0.5 When h>10m, 0.5 <n1=0.381I c +0.05(s) v / p a )-0.15≤1 Wherein, h is the thickness of the silty clay layer; a, b, c, and d represent empirical coefficients, which are obtained based on the boundary conditions of shallow silty clay and field penetration tests; Then the modified normalized cone tip resistance model Q is obtained tn1 , expressed as: When h≤10m, When h>10m, Among them, Q tn1 represents the modified normalized cone tip resistance, q t Indicates the modified cone tip resistance; Step D: Determine a modified soil type index model I for classifying strata based on the normalized friction ratio model obtained in step B and the modified normalized cone tip resistance model obtained in step C. c1 , expressed as: I c1 =[(3.47-logQ tn1 ) 2 +(1.22+logF r ) 2 ] 0.5 Among them, Q tn1 represents the modified normalized cone tip resistance, F r represents the normalized friction ratio.

2. The shallow silty clay stratum classification method based on the CPTU field penetration test according to claim 1 is characterized in that: In step B, the normalized friction ratio model is expressed as: F r =f s / (q t -σ v )×100 Among them, F r represents the normalized friction ratio; q t represents the modified cone tip resistance, q t =q c +u0(1-A a / A c ), A a and A c They respectively represent the cross-sectional areas of the top column and the cone bottom of the probe rod of the CPTU penetration test equipment.

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