in situ test

First, the role of in-situ testing technology in engineering investigation

In-situ testing is to measure the properties of rock and soil under the condition that the natural structure, natural water content and natural stress of rock and soil are basically kept in situ.

1. Advantages of in-situ testing

1) can be used to determine the relevant engineering parameters of soil layers (such as gravel soil, sand soil, plastic mud, etc.). It is difficult to obtain undisturbed samples.

2) Avoid the influence of stress release and structural disturbance during sampling.

3) The sample volume of in-situ testing is much larger than that of indoor samples, so it is also representative.

4) The stress state of the sample is closer to the engineering practice, and the test data is more reasonable.

5) The period of exploration test can be greatly shortened.

2. Disadvantages of in-situ testing

1) Various in-situ tests have their applicable conditions, and improper use will affect their effects.

2) The relationship between the parameters obtained from some in-situ tests and the engineering properties of soil is often based on statistical experience.

3) The factors affecting the in-situ test results are complex (such as surrounding stress field and drainage conditions), so it is difficult to accurately judge the measured values.

4) The direction of principal stress in in-situ testing is often different from that in actual geotechnical engineering problems.

5) Some in-situ test equipment is complex and huge.

Therefore, indoor test and in-situ test of soil have their own technical advantages. In the comprehensive study of various properties of rock and soil, we should not neglect one thing, but complement each other.

With the progress of science and technology, the theory, method and equipment of in-situ testing will be further developed and upgraded, and in-situ testing will play an increasingly important role in engineering investigation, and its coverage will be deeper and wider. It can be said that in-situ testing means is the development direction of engineering survey technology progress, and it is also a sign that survey technology is more mature.

Second, the in-situ test method

There are many in-situ testing methods for rock and soil. With the continuous development of geotechnical engineering technology and computer technology, new in-situ testing theories, instruments, equipment and testing methods are constantly emerging, which makes geotechnical engineering in-situ testing technology more and more important in engineering practice.

See table 2-2-69 to table 2-2-7 1 for the in-situ testing methods commonly used in engineering.

Table 2-2-69 In-situ Test Method of Rock Mass

Table 2-2-70 Soil in-situ Test Method

Table 2-2-7 1 Hydrogeological in-situ Test Method

sequential

In-situ testing is an indispensable and important technical means for survey work, so the rules and regulations related to engineering survey have technical requirements and working methods related to in-situ testing. Different in-situ testing methods are suitable for different engineering geological conditions, and the obtained geotechnical parameters also have their own emphasis. Table 2-2-72 shows the applicable scope of some test methods.

Table 2-2-72 Application scope of in-situ testing method

Thirdly, the application of in-situ testing in Shenzhen.

In-situ testing has been an important means of engineering investigation in Shenzhen since the establishment of the Special Zone. From 65438 to 0980, a special in-situ testing team entered Shenzhen with the survey team, and used in-situ testing methods in a large number of engineering survey projects. 1983, a comprehensive reconnaissance unit of an aviation unit systematically studied granite residual soil through in-situ testing, obtained the determination method of water content and deformation modulus of granite residual soil, and revealed the high bearing capacity of granite residual soil. During the period of 1985, the writing team of "Shenzhen Regional Stability Evaluation" of the Ministry of Geology and Minerals conducted in-situ stress measurements in many places in the region, which provided reliable data for regional stability analysis. In the following twenty years, the reconnaissance units in Shenzhen used vane shear test, static cone penetration test, pressuremeter test and spiral plate load test to determine the characteristics of soft soil, used standard penetration test to determine the liquefaction weathering degree of granite and sand, and used plate load test to determine the effect of foundation reinforcement. In recent years, some new methods, such as flat dilatometer test, have been introduced. It can be said that in-situ testing is a routine and indispensable means in engineering investigation.

(1) Load test of bottom plate of excavated pile in Jincheng Building

Six 26-story tower residential buildings in Jincheng Mansion, Luohu District, with a construction area of about 58000m2, adopt a frame-shear wall structure system. The total load of each tower is more than 20,000 tons, and the foundation pressure reaches 50t/m2. The geological conditions of the proposed site are complex, with many high dip fracture zones passing through, and the depth of local fracture zones is 40 ~ 50m. The survey was conducted in 198 1 year.

At the beginning of the construction of the special zone, several high-rise buildings in Luohu District were constructed with punched cast-in-place piles with a diameter of 1.0m, and the bearing stratum at the pile tip was slightly weathered rock. If Jincheng Building adopts this pile type, not only the pile spacing is too close, but also many piles are more than 50m long. After detailed technical scheme comparison, Jincheng Mansion decided to adopt large-diameter bored pile foundation, and the bearing layer at the pile end is strongly weathered rock. At that time, there was no successful experience in determining the bearing capacity of pile tip of strongly weathered rock in China, and there were not many data about in-situ load test of strongly weathered rock, so there were few available data. In order to accurately obtain the ultimate bearing capacity and deformation modulus of strongly weathered rocks as the design parameters of pile foundation, the fourth design room of Shenzhen Survey and Design Joint Company put forward a request, which was funded by Shenzhen Real Estate Company. Under the direct leadership of the Office of the Chief Engineer of the Municipal Construction Committee and the Luohu Project Construction Headquarters, the architectural research unit of the Ministry of Metallurgy was entrusted to carry out the load test of strongly weathered rock.

The test was carried out in engineering piles, with pile hole wall protection and reaction beam as reaction devices, and the field test was carried out by remote high-pressure loading, automatic strain recording system and closed-circuit television monitoring. The area of the circular bedplate is 1225mm2 and 1250mm2.

1982 trial excavation started in April, and test equipment was installed on May 5. /kloc-In May of 0/5, the load test of two piles with three deep and strongly weathered rocks was completed. The test results are shown in Table 2-2-73.

Table 2-2-73 Load Test Results of Strongly Weathered Rock in Jincheng Building

(II) Plate Load Test of Weathered Rock in Luohu Mountain

1983165438+1October, entrusted by Shenzhen Railway Station Construction Co., Ltd., according to the requirements of Ganghu Design Office for the load test of the strongly-moderately weathered phyllite at the bottom of the excavated pile of the proposed joint inspection building, in order to study its bearing capacity and deformation characteristics, the survey manager department of Shenzhen Survey and Design Joint Company conducted four flat load tests in Luohu Mountain (leveled).

The test was carried out in the fortified tunnel of Luohu Mountain, with the roof as the reaction device, using FQ 100 separated hydraulic jack to apply vertical pressure, using standard pressure gauge to observe the loading pressure value, and using dial indicator to measure the settlement value. The area of the circular platen is 800cm2, and the test results are shown in Table 2-2-74.

Table 2-2-74 Plate Load Test Results of Strongly-moderately Weathered Phyllite in Luohu Mountain

Through the above two 7-point load test results, we have a new understanding of whether strongly weathered rock and moderately weathered rock can be used as bearing layers of pile foundation, and their bearing capacity and deformation characteristics. Table 2-2-75 of allowable end bearing capacity of bedrock pile foundation with different weathering degrees in Shenzhen Special Zone is compiled and put forward. This table was adopted in the Trial Code for Structural Design of Reinforced Concrete Tall Buildings in Shenzhen (SJG 1-84), tried in 1984, and then continued to be compiled in the Trial Code for Design of Building Foundations in Shenzhen (SJG 1-88).

Table 2-2-75 Allowable End Bearing Table of Pile Foundation Rock in Shenzhen Special Economic Zone (kPa)

It should be noted that the circular pressure plate area of the two load tests is not large, and the diameter of the lower plane of the test point plate is much larger than three times the diameter of the pressure plate. Therefore, the two load test mechanisms still belong to shallow load plate test, and the test results are generally used to calculate the natural foundation and determine the bearing capacity of the pile tip, which is relatively safe in theory.

(3) Experimental study on granite residual soil

Granite residual soil has the characteristics of large void ratio, high compressibility and small compression modulus obtained by conventional geotechnical tests, which leads to the low bearing capacity obtained by looking up the table according to conventional engineering test indicators, which is quite different from the actual situation. At the beginning of the establishment of the special zone, the study of granite residual soil began with in-situ testing. First, the comprehensive survey unit of the Ministry of Aviation applied the in-situ testing method in the survey projects of Shangbu Industrial Zone, Shanghai Hotel and Baishaling Residential Area, and then the experimental study of granite residual soil in Huaxin Residential Area. In the investigation and study of these projects, in addition to drilling sampling, a certain number of tests such as plate load, pressuremeter, standard penetration and static penetration test are also used. The load test of Huaxin residential area was carried out in the well. The retaining wall and steel beam were used as reaction devices, the vertical load was applied by hydraulic jack, and the settlement was measured by displacement sensor. The circular bedplate covers an area of 2500 cm 2, and a dewatering well is set beside the test well. The comprehensive test results are shown in Table 2-2-76 and Table 77.

It should be noted that the following table only shows that various in-situ testing methods can be used to understand the characteristics of soil more comprehensively. The values in the table should not be referenced as specific items.

Through the experimental study of several projects, it is proved that granite residual soil has high bearing capacity and small overall deformation. The comprehensive survey team of the Ministry of Aviation put forward the relationship between the deformation modulus (Eo) of granite residual soil and the number of standard penetration blows:

Eo=2.2N (MPa)

The above formula was adopted by the Trial Code for Design of Building Foundation in Shenzhen (SJG 1-88) and included in the provisions of the code. According to the comparison between a large number of standard penetration blows and the state of soil and the degree of mineral weathering, the survey and measurement unit in Shenzhen suggested that 50 standard penetration blows (revised) should be used as the demarcation standard between granite residual soil and strongly weathered rock. This standard is also included in SJG 1-88. After that, through a lot of engineering practice, the current code for geotechnical engineering investigation (G B 5002 1-200 1) stipulates that the weathering degree of granite rocks is divided according to the standard penetration blow count (uncorrected), and n is more than or equal to 50 for strongly weathered rocks; N is less than 50, and more than or equal to 30 is completely weathered rock; N less than 30 is residual soil.

Table 2-2-76 Comprehensive Summary of In-situ Testing of Residual Soil Gravel Clayey Soil in Shangbu Industrial Zone (1983)

Table 2-2-77 Comprehensive Summary of In-situ Testing of Gravel Cohesive Soil of Residual Soil in Huaxin Village (1985)

(4) In-situ stress measurement

1985, in order to obtain the data of stress state in this area, the writing group of "Shenzhen regional stability evaluation" of the Ministry of Geology and Minerals used piezomagnetic induction method and hydraulic fracturing method to measure the stress in this area. SYL-2 digital piezomagnetic stress generator and horizontal confining pressure tester are used in piezomagnetic induction method, and directional indentation instrument is used to determine fracturing direction in hydraulic fracturing method. See table 2-2-78 and table 2-2-79 for the measurement results.

Table 2-2-78 1985 Summary of in-situ stress measurement results by piezomagnetic induction method in Shenzhen

Table 2-2-79 Summary of in-situ stress measurement results of Shenzhen Special Economic Zone 1985 to 1986 hydraulic fracturing method

(5) In-situ test of soft soil foundation investigation

Because it is difficult for soft soil (silt and muddy soil) to obtain samples in natural state, and it is even more difficult to ensure that the samples will not be disturbed and damaged during transportation, storage and testing, it is particularly important to determine the engineering characteristics of soft soil by in-situ testing methods (mainly static cone penetration test and vane shear), and to supplement and confirm with the indoor test results. Especially because of the inhomogeneity of soft soil, there is often a thin layer of silt. By using the static cone penetration test method, the continuous cone penetration parameters can be obtained from top to bottom. Shenzhen has a large area of soft soil. In large-scale development areas such as bonded area, Shenzhen airport, Houhai and Qianhai reclamation project, engineering investigation units have done a considerable number of in-situ tests on soft soil foundation, and obtained a large number of test data, which provided more accurate design parameters for soft soil reinforcement. Taking the engineering investigation of the Shenzhen-Hong Kong Western Passage in Houhai as an example, a large number of soil samples were taken for indoor tests to study their physical indexes and mechanical properties (shear strength, consolidation coefficient, etc.). ) sludge. At the same time, a considerable number of cone penetration tests and vane shear test were carried out. See tables 2-2-80 and 2-2-8 1 for the test results.

Table 2-2-80 Static Penetration Test Results of Silt

Table 2-2-8 1 Sludge vane shear test Results

In the large-scale soft foundation reinforcement project, in-situ testing before and after treatment is even more indispensable to judge the timeliness and quality of reinforcement.