There are two main tasks of geotechnical engineering detection and monitoring: one is to control the construction quality of geotechnical engineering; The second is to understand the effect of geotechnical construction and provide basis for further utilization of geotechnical body. There are similarities and obvious differences between geotechnical engineering test and experimental test in geotechnical engineering investigation stage. The similarity lies in that both of them need to quantitatively understand the engineering characteristics of engineering rock and soil through experimental tests; The difference is that the test object of the former is the rock and soil before reinforcement, which aims to provide a basis for geotechnical engineering design, while the test object of the latter is the rock and soil after reinforcement, which aims to control the construction quality of geotechnical engineering and test the construction treatment effect. From the similarity between them, we can know that many test methods in geotechnical engineering detection are the same as those in geotechnical engineering investigation. For example, the in-situ static load test of rock and soil is not only one of the important test methods to test the bearing capacity of foundation soil layer in geotechnical engineering investigation, but also an important means to test and treat reinforced composite foundation and pile foundation in geotechnical engineering inspection. However, the engineering rock and soil after treatment and reinforcement will change greatly compared with the rock and soil before treatment and reinforcement. After treatment and reinforcement, some artificial materials and functional components are often added to the original natural geotechnical matrix. For example, in the surrounding rock support of cavern, anchor cable can be used to strengthen surrounding rock, and concrete lining can also be used for support. At this time, geotechnical engineering testing should detect whether the anchoring force of anchor cable to surrounding rock can meet the design requirements, and also whether the quality of concrete lining can meet the design requirements. For another example, when the foundation soil cannot meet the requirements of the superstructure on the deformation or bearing capacity of the foundation, the bearing capacity and integrity of the pile foundation must be tested in geotechnical engineering inspection to ensure that the pile foundation can achieve the design function. It can be seen that the test in geotechnical engineering detection often needs to extend beyond the engineering geotechnical body.

In addition to the routine tests in geotechnical engineering investigation, the monitoring tests often carried out in geotechnical engineering investigation mainly include pile foundation detection, foundation deformation observation, slope deformation monitoring and surrounding rock observation of underground cavern.

A, pile foundation detection

With the increasing scale of urban construction, it has become a common engineering scheme to improve the bearing capacity of foundation soil by using pile foundation. The main contents of pile foundation detection include two aspects: bearing capacity of pile and integrity of pile. It is necessary to test the horizontal resistance of slope protection piles and anti-slide piles, and sometimes it is necessary to test the uplift force of piles. However, a large number of tests are mainly carried out as the vertical bearing capacity of pile foundation under the load of superstructure.

The main methods of pile foundation detection are drilling coring, static load test, high strain dynamic test, low strain dynamic test and acoustic wave transmission test. Static load test is one of the conventional means in engineering geological detection, and it is also the most direct and reliable method to detect the bearing capacity of pile foundation. However, the load test method is expensive and time-consuming, and only a small number of samples can be taken. Although a new Osterberg pile test method has appeared in recent years, it is still limited by the cost, and it is difficult to achieve the ideal detection rate. Therefore, the convenient dynamic testing technology of pile foundation has been popularized and applied in engineering inspection. Modern dynamic testing technology of pile foundation is developed on the basis of wave theory. In 1930s, D.V.Isaacs first proposed a one-dimensional wave equation to describe the wave propagation along the pile after the pile top was impacted by the pile hammer. 1960, E.N.Smith published the famous paper "Wave Equation Method for Pile Driving Analysis", which made the wave equation analysis method enter the practical stage. 1972, Professor Zhou Guanglong of Hunan University put forward the dynamic parameter method of dynamic testing of pile foundation, which promoted the research of dynamic testing method of pile foundation in China. Dynamic test method can not only detect the bearing capacity of pile foundation, but also detect the integrity of pile body, which overcomes the limitation that load test can only detect the bearing capacity of pile foundation. In the high strain dynamic test, a free drop hammer (the weight of the hammer should be greater than 1% of the ultimate bearing capacity of a single pile) is used to impact the pile top, and a stress wave propagating along the length direction of the pile is generated in the pile. The acceleration and stress of particle vibration on the pile section near the pile top are measured by acceleration sensor and force sensor, and then the axial force and pile side friction of each section are obtained by wave equation analysis and fitting. In the low strain dynamic test, the portable hammer is used to excite the fluctuation at the pile head, and the bearing capacity of the pile foundation is calculated according to the vibration and fluctuation signals of the pile foundation measured by the sensors arranged at the pile top, so as to judge the integrity of the pile body. Acoustic transmission method is a common method to detect the quality of large diameter pile foundation. Before acoustic wave test, 2 ~ 6 steel pipes shall be symmetrically buried according to the cross-section size of the pile, and the steel pipes shall be ensured to be parallel to each other. During the test, the transmitting probe emits ultrasonic waves at a certain depth in the pipe, and the receiving probe receives the penetrating wave signals at the same depth in the symmetrical steel pipe, thus measuring the sound speed of the pile material. According to the magnitude and change of sound velocity, the quality of pile can be judged.

Second, the foundation deformation and slope deformation observation

The main task of deformation observation is to observe the observation points set on the measurement target repeatedly on a regular basis, so as to obtain the changes of the points or elevations of the observation points with time, and provide technical basis for evaluating the construction quality of geotechnical engineering, understanding the stability of foundation and slope and the rationality of design parameters.

The observation of foundation deformation mainly includes the observation of foundation pit rebound, the observation of the influence of foundation pit lateral deformation and excavation on adjacent buildings, the observation of building settlement and displacement, and the observation of site settlement. The observation of foundation deformation mainly has the following significance: first, estimate the possible recompression deformation of foundation to improve foundation design; The second is to estimate the influence of foundation pit excavation unloading on adjacent buildings so as to take protective measures in time; Third, check the stability of supporting structure to ensure the safety of engineering construction; Fourthly, the engineering experience is accumulated through the observation of foundation deformation, which provides the basis for the back analysis of long-term deformation of foundation.

Slope deformation observation includes surface deformation monitoring, borehole deformation monitoring and slope rock and soil acoustic emission monitoring. Among them, the monitoring items of borehole deformation mainly include settlement, inclination, deflection, groundwater level or seepage pressure. The main purpose of slope deformation observation is to grasp the stable state of slope in time, and provide basis for predicting the development trend of slope stability and formulating the treatment scheme of slope rock and soil.

Three. Observation of surrounding rock of underground cavern

Before the excavation of underground cavern, the rock mass is in a state of stress balance. The excavation of the cavern destroyed the original stress balance state, and with the development of surrounding rock deformation, the stress redistributed. Practice shows that it is an effective method and way to adjust the construction scheme and reinforcement measures in time and predict the dangerous situation by using the information provided by the observation of surrounding rock of underground cavern, which is also the characteristic of the so-called new Austrian tunneling method. In recent years, based on the development of information and computer technology, some people put forward a new idea of intelligent geotechnical engineering, which should be said to be the further development of NATM. The observation of surrounding rock of underground cavern mainly includes the observation of convergence of surrounding rock of underground cavern, the observation of axial displacement and lateral displacement of drilled rock mass, the observation of anchor stress, the observation of anchor load of rock mass and the observation of groundwater seepage pressure.