Method suitable for rapidly measuring bearing capacity of shallow silty clay natural foundation

Abstract

The invention provides a method suitable for rapidly measuring the bearing capacity of a shallow silty clay natural foundation, which comprises the following steps: S1, establishing a load-settlement displacement curve based on a plate load test (PLT); s2, considering the safety margin, and determining a permissible value of the bearing capacity of the foundation by adopting a double tangent method; s3, based on a light cone dynamic penetration test (DCPT), a regression analysis method is adopted to establish the relation between the cumulative penetration times and the cumulative penetration depth; s4, a dynamic penetration index (DCPI) is introduced, and a relational expression between the dynamic penetration index and the permissible value of the foundation bearing capacity is established; and S5, according to a light cone dynamic penetration test, rapidly measuring the bearing capacity of the foundation by utilizing the relational expression. Compared with a traditional in-situ test method, the problems that the site is limited, the test range and space are insufficient and the like can be effectively solved, meanwhile, the method is simple, convenient and efficient, the test working efficiency is greatly improved, the cost input is reduced, and a new technical means is provided for better and faster determination of the foundation bearing capacity.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of foundation bearing capacity determination, and particularly relates to a method for rapidly determining the bearing capacity of a natural foundation of shallow silty clay. BACKGROUND

[0002] The foundation bearing capacity is an important index for evaluating the load bearing capacity of a foundation and is also one of the important indexes for engineering investigation and design. For a soft foundation with low load bearing capacity, if the foundation bearing capacity is not evaluated before construction, problems such as excessive settlement or uneven settlement may occur in the later period, which brings great security risks to the long-term stability and normal use of the building (structure). At the same time, if the foundation design is too conservative, it will inevitably cause waste of materials, time and space, and increase the cost. Therefore, how to ensure the function of the foundation structure and avoid waste caused by conservative design, and how to scientifically and efficiently determine the foundation bearing capacity are particularly important. At present, common methods for evaluating the foundation bearing capacity include in-situ test method, theoretical analysis method, numerical simulation method, and local experience method.

[0003] 1. When the in-situ test method is used, the basic principle is to determine the bearing capacity through direct field test, mainly including plate loading test, static sounding test, dynamic sounding test, standard penetration test, etc. This method has a wide application range, but the test workload is large, the test time is long, and it is time-consuming and labor-intensive, and is greatly affected by the terrain, test range and space limitations.

[0004] 2. When the theoretical formula method is used, the basic principle is to determine the bearing capacity through theoretical formula calculation according to the soil performance parameter index. The adaptability is greatly affected by the differences in soil properties, and mainly depends on the samples obtained by test drilling and the test results indoors and outdoors, which is significantly limited by the engineering geological conditions.

[0005] 3. When the numerical simulation method is used, the basic principle is to simulate the solution of engineering practical problems by applying computer programs, combining finite element, discrete element, etc. to establish mathematical models. It is a macro-aided method. This method has a wide application range and can simulate the bearing properties of the foundation under different working conditions for optimization and selection, but the soil physical and mechanical parameters input by the program are similar to those of the theoretical formula method, and are greatly affected by the uncertainty of soil properties.

[0006] 4. When the local experience method is used, the basic principle is to determine the bearing capacity based on the statistical comparison of in-situ test data of a large number of existing engineering cases. Due to the influence of regional differences, stratum properties, geological conditions, construction environment, etc., the bearing capacity calculated by the engineering experience of different regions is not the same, and attention should be paid to the application conditions and scope of each method. SUMMARY

[0007] In view of the limitations and deficiencies of the above existing determination method, the present application aims to seek a method which can overcome the problems of high cost, space limitation, long time consumption and limited application range of traditional determination of foundation bearing capacity, so as to better and faster determine the foundation bearing capacity. The present application studies and determines a method for rapid determination of natural foundation bearing capacity of shallow silty clay, which comprises the following steps: firstly, a field plate load test (PLT) is carried out, the load value and the corresponding settlement value of each stage are recorded, and a load-settlement displacement curve is established. Then, based on the distribution characteristics of the load-settlement curve, considering the safety margin, the allowable value of foundation bearing capacity is determined by using the double tangent method. Secondly, a light dynamic cone penetration test (DCPT) is carried out, the measured data of the cumulative penetration times and the cumulative penetration depth of each group are obtained, and the relationship between the cumulative penetration times and the cumulative penetration depth is established by using the regression analysis method. The dynamic penetration index (DCPI) is introduced by using the parameters, and the relationship between the dynamic penetration index (DCPI) and the allowable value of foundation bearing capacity is obtained. Finally, the dynamic penetration index (DCPI) is converted from the measured data of the light dynamic cone penetration test, and the relationship is used to rapidly determine the foundation bearing capacity, thereby achieving the purpose of rapid and effective evaluation of the foundation bearing capacity.

[0008] The present application provides a method for rapid determination of natural foundation bearing capacity of shallow silty clay, which comprises the following steps: S1, based on the plate load test (PLT), a load-settlement displacement curve is established; S2, considering the safety margin, the allowable value of foundation bearing capacity is determined by using the double tangent method; S3, based on the light dynamic cone penetration test (DCPT), the relationship between the cumulative penetration times and the cumulative penetration depth is established by using the regression analysis method;

[0009] S4, the dynamic penetration index (DCPI) is introduced, and the relationship between the dynamic penetration index (DCPI) and the allowable value of foundation bearing capacity is established; S5, according to the light dynamic cone penetration test, the relationship is used to rapidly determine the foundation bearing capacity.

[0010] Preferably, in S1, based on the plate load test (PLT), a load-settlement displacement curve is established, which comprises the following methods: selecting a typical test area, building a pressure platform reaction device, a bearing plate, a hydraulic jack, a pressure gauge, a manual oil pump, a steel pipe support as a complete loading tool, and cooperating with a dial indicator to measure the settlement value in the grading loading process.

[0011] Further preferably, the selection of the typical test area requires laying a sand cushion layer with a thickness of about 2 cm on the pressure test surface, and the sand cushion layer is compacted by beating to make the bearing plate and the pressure test surface flatly contact.

[0012] Further preferably, the load weight of the pressure platform reaction device is stacked by using standard sandbags with a weight of 35 kg per bag, the reaction beam is composed of I-shaped steel and steel plates, the grading load is applied by the hydraulic jack, and the load size is measured by the pressure gauge with a precision grade of 0.4.

[0013] Further preferably, the bearing plate adopts a circular steel plate with a diameter of 76 cm, a thickness of 25 mm, and an area of about 0.45 m 2 .

[0014] Further preferably, the hydraulic jack adopts a CES-50V1 type, with a range of 0-50T, a lifting piston diameter of 90 mm, an outer diameter of 160 mm, and a height of 281 mm.

[0015] Further preferably, the pressure gauge adopts a CES-10 precision pressure gauge, with a range of 0-60MPa, a dial diameter of 150 mm, and a measurement accuracy of 0.4 level.

[0016] Further preferably, the dial gauge has a range of 0-30 mm, a dial diameter of 58 mm, and a measurement accuracy of 0.01 mm.

[0017] Preferably, in S2, considering the safety margin, the allowable value of foundation bearing capacity is determined by using the double tangent method, including the following methods: according to the distribution characteristics of load-settlement curve, one tangent is taken as the linear stage passing through the origin, and the other tangent is taken as the tangent of the elastic-plastic stage corresponding to the upper load at the inflection point, the intersection point of the two tangents is the ultimate bearing capacity of the foundation and the corresponding settlement value, and the allowable value of foundation bearing capacity is calculated according to the ultimate bearing capacity of the foundation by considering the safety factor.

[0018] Further preferably, the load-settlement displacement curve of the plate loading test (PLT) has typical slow descent characteristics.

[0019] Further preferably, the safety factor is taken as 3. The safety factor is a technical parameter used in engineering design to ensure the stability of the foundation and the safety of the structure, in order to compensate for the deviation caused by soil parameter error, spatial variability, inaccurate load estimation or human factors, etc. The safety factor of 3 means that the foundation can withstand the maximum load without damage or excessive settlement, which reflects the design safety margin.

[0020] Preferably, in S3, based on the light dynamic cone penetration test (DCPT), the relationship between the cumulative penetration times and the cumulative penetration depth is established by using regression analysis method, including the following methods: using the light dynamic cone penetration instrument, the field implementation is carried out according to the relevant requirements of the test manual, the measured data of the cumulative penetration times and the cumulative penetration depth of each group are obtained, and the relationship between the cumulative penetration times and the cumulative penetration depth is obtained by using regression analysis method.

[0021] Further preferably, the light cone dynamic penetration instrument is adopted, and main technical parameters include that a hammer weight is 8.0 kg, a tolerance is 0.01 kg, a steel driving rod diameter is 16 mm, a sliding buckle fixing height is 575 mm, a tolerance is 1 mm, a cone probe tip angle is 60°, a tolerance is 1°, a bottom diameter is 20 mm, and a tolerance is 0.25 mm.

[0022] Further preferably, the test operation manual mainly includes the following contents:

[0023] Equipment inspection: before starting the test, check whether the equipment has fatigue damaged parts, especially the coupler assembly and handle, and whether the driving rod and replaceable probe are excessively worn;

[0024] Equipment operation: the tester holds the equipment in a vertical position through the handle, makes the top of the widest part of the probe tip flush with the surface of the foundation to be tested, then lifts and releases the hammer from the standard drop height, each group of 5 hammer blows is recorded, the cumulative penetration depth of each group of hammer blows is recorded, the measurement accuracy is 1 mm, and generally, the total penetration depth is required to be less than 900 mm;

[0025] Equipment pulling out: after the test is completed, the equipment is pulled out by slowly lifting the hammer upward against the handle to avoid shaking as much as possible.

[0026] Preferably, in S4, the dynamic penetration index (DCPI) is introduced, and a relationship between the dynamic penetration index and the allowable value of the foundation bearing capacity is established, including the following method: the dynamic penetration index (DCPI) is defined as the ratio of the cumulative penetration depth after each group of hammer blows to the cumulative number of penetrations, and a regression analysis method is used to establish the relationship between the dynamic penetration index (DCPI) and the allowable value of the foundation bearing capacity.

[0027] Preferably, in S5, the foundation bearing capacity is quickly determined according to the light cone dynamic penetration test, including the following method: through the light cone dynamic penetration test (DCPT), the measured data of the cumulative number of penetrations and the cumulative penetration depth of each group are obtained, the dynamic penetration index (DCPI) is converted, and the foundation bearing capacity can be quickly determined according to the relationship in S4.

[0028] Compared with the prior art, the method described in the application solves the problems of high cost investment, space limitation, long time consumption and limited adaptability range of the traditional in-situ test evaluation method of the foundation bearing capacity, can greatly reduce the test workload and solve the influence of site limitation, test range and space shortage on the construction progress, is simple and efficient, greatly improves the test work efficiency and reduces the cost investment, and provides a new technical means for better and faster determination of the foundation bearing capacity. BRIEF DESCRIPTION OF DRAWINGS

[0029] Figure 1 is the basic flowchart of the present application;

[0030] Figure 2 is the schematic diagram of the test device of the present application;

[0031] Figure 3 is the result diagram of the plate load test (PLT) of the present application;

[0032] Figure 4 is the result diagram of the light dynamic cone penetration test (DCPT) of the present application;

[0033] Figure 5 is the diagram of the relationship between the dynamic cone penetration index (DCPI) and the allowable bearing capacity of the foundation of the present application.

[0034] In the figure: a typical test area is selected (1), a counterforce device of the pressure platform is built (2), a bearing plate is built (3), a hydraulic jack is built (4), a pressure gauge is built (5), a manual oil pump is built (6), a steel pipe support is built (7), a dial gauge is built (8), a standard sand bag (2-1) is selected, an I-shaped steel (2-2) is selected, and a steel plate (2-3) is selected. DETAILED DESCRIPTION

[0035] In order to make the purpose, technical scheme and advantages of the present application more clear, the present application is further described in detail below in combination with the drawings and implementation cases, so as to prove the actual significance and value thereof.

[0036] The present application provides a method for rapidly determining the bearing capacity of a natural foundation of a shallow silty clay, comprising the following steps:

[0037] S1, based on the plate load test (PLT), a load-settlement displacement curve is established, including the following methods: three test areas are selected, a counterforce device of the pressure platform is built, the load counterweight is stacked by using standard sand bags with a weight of 35 kg / bag, the bearing plate, the hydraulic jack, the pressure gauge and the manual oil pump are used as the matching loading tools, the typical test area is required to be paved with a sand cushion layer with a thickness of about 2 cm on the pressure testing surface, and the sand cushion layer is beaten to be compacted, so that the bearing plate is in flat contact with the pressure testing surface. The load counterweight of the counterforce device of the pressure platform is stacked by using standard sand bags with a weight of 35 kg / bag, the counterforce beam is composed of the I-shaped steel and the steel plate, the graded load is applied by the hydraulic jack, and the load size is measured by the pressure gauge with a precision grade of 0.4. The bearing plate is a circular steel plate with a diameter of 76 cm and a thickness of 25 mm, and the area is about 0.45 m 2 . The hydraulic jack is a CES-50V1 type, the range is 0-50T, the lifting piston diameter is 90 mm, the outer diameter is 160 mm, and the height is 281 mm. The pressure gauge is a CES-10 precision pressure gauge, the range is 0-60 MPa, the dial diameter is 150 mm, and the measurement accuracy is 0.4 grade. The dial gauge range is 0-30 mm, the dial diameter is 58 mm, and the measurement accuracy is 0.01 mm.

[0038] Test apparatus such as Figure 2 As shown, in accordance with the relevant requirements of the "Technical Specification for Testing Building Foundations" (JGJ340-2015), the slow-speed sustained load method was used for graded loading and unloading. According to the design requirements, each load applied to the bearing plate was 26 kPa, with a maximum applied load of 596 kPa. If the settlement rate of the bearing plate was less than 0.1 mm / h for two consecutive hours after each loading stage, it was considered to have reached a relatively stable state, and the next loading stage could be carried out until the maximum load value required by the design was reached and the loading termination condition was met. The settlement value of the bearing plate under graded loading was measured using a dial gauge during the test (see Table 1).

[0039] Table 1. Measured data of graded loads and settlement in plate load tests.

[0040]

[0041]

[0042] S2. Considering a safety margin, the allowable bearing capacity of the foundation is determined using the double-tangent method, including the following steps: Based on the load-settlement curve distribution characteristics, one tangent line is taken as the linear stage tangent line passing through the origin, and the other is taken as the tangent line corresponding to the elastic-plastic stage of the previous load with a significant inflection point. The intersection of the two tangent lines yields the ultimate bearing capacity P of the foundation. cr And the corresponding settlement values, the test results are as follows Figure 3 As shown, the plate load test (PLT) graded loading-settlement displacement curve exhibits typical slow-descent characteristics. A safety factor of 3 is considered. The safety factor is a technical parameter used in engineering design to ensure foundation stability and structural safety, compensating for deviations caused by uncertainties such as soil parameter errors, spatial variability, inaccurate load estimation, or human factors. A safety factor of 3 represents a value that ensures the foundation can withstand the maximum load without failure or excessive settlement, reflecting a design safety margin. By considering a safety factor κ = 3, the allowable bearing capacity P of the foundation is calculated based on the ultimate bearing capacity. a =P cr / κ.

[0043] S3, based on the light dynamic cone penetration test (DCPT), the relationship between the cumulative penetration number and the cumulative penetration depth is established by regression analysis. The following methods are included: a light dynamic cone penetration instrument is used, and the main technical parameters include a hammer weight of 8.0 kg, a tolerance of 0.01 kg, a steel driving rod diameter of 16 mm, a sliding buckle fixed height of 575 mm, a tolerance of 1 mm, a cone probe tip angle of 60°, a tolerance of 1°, and a bottom diameter of 20 mm, a tolerance of 0.25 mm. According to the relevant requirements of the test manual, the field implementation is carried out, and three test sections are arranged in the selected three test areas, and two measuring points are symmetrically arranged in each section. Before starting the test, check whether the light dynamic cone penetration instrument has parts that are fatigued and damaged, especially the coupler assembly and the handle, and the driving rod and the replaceable probe are excessively worn. During the test, one tester holds the equipment in a vertical position through the handle, so that the top of the widest part of the probe tip is flush with the surface of the foundation to be tested, and the other tester lifts and releases the hammer from the standard drop height. Each group of hammering times is recorded, and the cumulative penetration depth of each group of hammering times is recorded (see Table 2), and the measurement accuracy is 1 mm. Usually, the total penetration depth is required to be less than 900 mm. After the test is completed, the hammer is slowly lifted up to the handle to pull out the equipment, and shaking is avoided as much as possible. The measured data of the cumulative penetration number and the cumulative penetration depth of each group are shown in Table 2. Figure 4 The relationship between the cumulative penetration number and the cumulative penetration depth is established by regression analysis.

[0044] Table 2 Measured data of the cumulative penetration depth of each group of hammering times

[0045]

[0046] S4, the dynamic penetration index (DCPI) is introduced, and the relationship between the dynamic penetration index and the allowable value of the foundation bearing capacity is established. The following methods are included: the dynamic penetration index (DCPI) is defined as the ratio of the cumulative penetration depth after each group of hammering to the cumulative penetration number, and the relationship between the dynamic penetration index (DCPI) and the allowable value of the foundation bearing capacity is obtained by regression analysis [P a ] = 276.186-6.37[DCPI], and the test results are shown in Table 2. Figure 5

[0047] S5, according to the light dynamic cone penetration test, the relationship formula is used to quickly determine the foundation bearing capacity. The following methods are included: through the light dynamic cone penetration test (DCPT), the measured data of the cumulative penetration number and the cumulative penetration depth of each group are obtained, the dynamic penetration index (DCPI) is converted, and the relationship formula in S4 is used to quickly determine the foundation bearing capacity.

[0048] ​Further preferably, a corresponding distribution interval of the allowable value of the foundation bearing capacity and the dynamic cone penetration index (DCPI) is proposed: (1) when 20 < DCPI ≤ 28, the foundation bearing capacity is considered to be not less than 100 kPa; (2) when 12 < DCPI ≤ 20, the foundation bearing capacity is considered to be not less than 150 kPa; (3) when DCPI ≤ 12, the foundation bearing capacity is considered to be not less than 200 kPa.

[0049] The application effectively overcomes the problems of high cost, space limitation, long time consumption and limited adaptation range of the traditional in-situ test evaluation method of the foundation bearing capacity, and the foundation bearing capacity can be determined better and faster, and the application provides a reference basis for quickly determining the foundation bearing capacity in engineering investigation and design.

Claims

1. A rapid method for determining the bearing capacity of shallow silty clay natural foundations, characterized in that, include; S1. Based on the plate load test, establish the load-settlement displacement curve; S2. Considering a safety margin, the double-tangent method is used to determine the allowable value of the foundation bearing capacity. S3. Based on the lightweight cone dynamic penetration test, the relationship between the cumulative number of penetrations and the cumulative penetration depth is established using regression analysis. S4. Introduce the dynamic penetration index and establish the relationship between the dynamic penetration index and the allowable value of the foundation bearing capacity. S5. Based on the light cone dynamic penetration test, the bearing capacity of the foundation is quickly determined using the formula.

2. The method for rapid determination of bearing capacity of shallow silty clay natural foundations according to claim 1, characterized in that, In step S1, the specific method for establishing the load-settlement displacement curve is as follows: select a typical test area for plate load test, build a counterweight platform reaction device, use a bearing plate, hydraulic jack, pressure gauge, and manual oil pump as supporting loading tools, and use a dial gauge to measure the settlement value of the bearing plate during the graded loading process to establish the test graded loading-settlement displacement curve.

3. The method for rapid determination of bearing capacity of shallow silty clay natural foundations according to claim 1, characterized in that, In step S2, the specific method for determining the allowable value of the foundation bearing capacity is as follows: based on the gradual descent characteristic of the load-settlement curve, one is taken as the tangent line passing through the origin in the linear stage, and the other is taken as the tangent line corresponding to the elastic-plastic stage of the previous load with a clear inflection point. The intersection of the two tangent lines yields the ultimate bearing capacity of the foundation and the corresponding settlement value. By considering the safety factor, the allowable value of the foundation bearing capacity is calculated based on the ultimate bearing capacity of the foundation.

4. The method for rapid determination of bearing capacity of shallow silty clay natural foundations according to claim 1, characterized in that, In step S3, the specific method for establishing the relationship between the cumulative number of penetrations and the cumulative penetration depth is as follows: a lightweight cone dynamic penetrometer is used, and the device is kept in a vertical position by the handle so that the top of the widest part of the probe tip is flush with the surface of the foundation to be tested. Then, the drop hammer is lifted and released from the standard drop height. Every 5 hammer blows constitute a group, and the cumulative penetration depth of each group of hammer blows is recorded. The relationship between the cumulative number of penetrations and the cumulative penetration depth is established by regression analysis.

5. The method for rapid determination of bearing capacity of shallow silty clay natural foundations according to claim 1, characterized in that, In step S4, the specific method for establishing the relationship between the dynamic penetration index and the allowable bearing capacity of the foundation is as follows: the dynamic penetration index is defined as the ratio of the cumulative penetration depth after each set of hammer blows to the cumulative number of penetrations, and regression analysis is used to establish the relationship between the dynamic penetration index and the allowable bearing capacity of the foundation.

6. The method for rapid determination of bearing capacity of shallow silty clay natural foundations according to claim 1, characterized in that, In step S5, the specific method for determining the bearing capacity of the foundation is as follows: by conducting a light-duty cone dynamic penetration test, the measured data of the cumulative number of penetrations and the cumulative penetration depth of each group are obtained. The dynamic penetration index is calculated based on the measured data. Then, the bearing capacity of the foundation can be quickly determined by using the relationship between the dynamic penetration index and the allowable value of the foundation bearing capacity.

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