Wuhan-Guangzhou Railway Passenger Dedicated Line is the passenger dedicated line with the longest mileage, the highest technical standard and the largest investment in China, and the requirements for post-construction settlement of subgrade are also very strict. In order to meet the requirements of settlement rate and post-construction settlement, the vertical deformation of soft foundation must be observed during construction. In addition to strictly detecting and controlling the compaction quality of embankment filling, the settlement observation of embankment as a whole is also an indispensable quality control means.

Second, the settlement observation

Subgrade settlement observation mainly focuses on subgrade surface settlement and foundation settlement observation. Settlement deformation observation sections are set according to different foundation conditions and different structural parts. Bury the inclinometer in the transition section. The inclinometer adopts special plastic hard pipe, and its bending stiffness should meet the requirements of vertical displacement of the soil to be measured, and the joint of the pipe end is tight.

(1) Installation and protection of inclinometer paper

1, use total station (or theodolite combined with level) to determine the center elevation and embedding direction of observation section.

2. In the process of subgrade filling, after the embankment filling height is 30cm higher than the buried pipe position, a trench with a width of 20cm and a depth of 30cm is dug along the vertical line, the trench bottom is leveled, and a layer of fine sand with a thickness of 5 ~ 10cm is paved at the trench bottom.

3. When laying inclinometers, they should be connected in sections, with each side extending a certain distance from the toe of subgrade slope (the specific length shall be subject to the design, generally extending 50cm). In order to prevent the inclinometer tube from being damaged, C20 concrete protection wells are poured at both ends of the tube head, with a wall thickness of 30cm, a section size of 90cm×90cm and a well depth of 100cm.

One is reserved in the inclinometer? 2 ~ 3 mm steel wire rope, the length is about 2m longer than the length of the inclinometer tube, which is convenient to connect with the hole covers at both ends, and the end shall not be retracted into the inclinometer tube.

(ii) Working principle

The mobile inclinometer is mainly composed of NCX-2 inclinometer probe, outgoing cable, manual winding wheel, NDA151transmitter signal indicator, etc. And an inclinometer tube embedded in the position to be monitored in advance.

The main part of the inclinometer probe is the servo acceleration sensor, and there are four guide wheels on the shell, which are installed on two wheel carriers respectively, and the wheel carriers can rotate around the axis. The distance l between the rotating shafts of the two wheel carriers is called the gauge distance of the inclinometer.

The cable led from the inclinometer plays the role of pulling the probe and determining its position. The special cable with reinforced core has good dimensional stability, and the cable is marked every 0.5 meters to determine the position of the probe in the inclinometer.

NDA 15 1 1 Transmitter signal indicator is the secondary instrument for data output of inclinometer, which is used to complete the functions of measuring, displaying and storing probe data, and can output the measured data to PC through communication port.

(3) Measurement documents

The inclinometer is measured by forward and backward pulling method. During forward measurement, firstly, connect the wire rope laid in the inclinometer with the pull ring at the front end of the inclinometer probe firmly, then push and pull the inclinometer to make its guide wheel completely enter the vertical guide groove of the inclinometer, and pay attention to the direction of the guide wheel. Then, according to the mark on the cable, measure and read the spacing (0.5m) of each standard measuring point, and make records until the measurement of the whole pipeline is completed.

After the positive measurement is completed, pull the cable back to the inclinometer, then remove the reserved wire rope and fix the rope head at the orifice. Pay attention to protect the reserved wire rope from being retracted into the inclinometer.

Then the inclinometer enters the inclinometer from the other end of the inclinometer tube (assumed to be the left end), and the steel wire drum moves to the right end. As shown in figure 3. On the left, firmly connect the wire rope placed in the inclinometer with the pull ring at the front end of the inclinometer head, and then push and pull the inclinometer to make its guide wheel completely enter the vertical guide groove of the inclinometer. Roll the cable reel to make the inclinometer probe slide to the right, and pay attention to the direction of the guide wheel.

Look at the marks on the wire rope, which correspond to the surveying and mapping punctuation marks set on the pier, and ensure that the starting point of reverse measurement is the same as that of positive measurement. Then pull the cable according to the mark marked on the cable, and measure the displacement of the inclinometer axis relative to the horizontal line every standard measuring point spacing (0.5m) until the measurement of the whole pipeline is completed.

After the reverse measurement is completed, pull the cable back to the inclinometer, then remove the reserved wire rope and fix the rope head at the orifice. Pay attention to protect the reserved wire rope from being retracted into the inclinometer.

Third, data processing.

(A) the principle of systematic reading of information papers

The mobile inclinometer moves along the guide groove of the inclinometer at a set interval of 0.5m, and outputs a voltage value u corresponding to the sine value of the inclination angle of each measuring point. That is: (1) where: is the output voltage of the inclinometer in V (Volts); Is the included angle between the inclinometer axis and the horizontal plane, in degrees.

(2) Arrangement of observation data

During observation, the first measured value of the borehole is the initial measured value, and the difference between the daily measured value and the initial measured value is called the change value (change value = measured value? —initial measured value), when observing the vertical displacement, the algebraic sum of the changing values is accumulated to another point of the inclinometer, and the calculation result is the vertical displacement of this point relative to one end of the inclinometer. The difference between each observation displacement and the initial observation displacement is the cumulative settlement of this observation, and the difference between each observation displacement and the previous observation displacement is the settlement. On this basis, displacement curves and distance-settlement curves can be drawn.

Four. conclusion

As a monitoring tool for vertical deformation of subgrade, soil inclinometer is very fast and efficient in application. It can not only monitor the whole deformation of subgrade, but also detect the vertical displacement of any point on the cross section of subgrade and the horizontal displacement of different depths. The two-way pulling method of inclinometer effectively eliminates the influence of vertical inclinometer slot and overcomes the error caused by field calibration. The field operation shows that the horizontal inclinometer has the advantages of being unaffected by construction and weather, convenient to operate, flexible and high precision, which provides a guarantee for timely and accurate measurement of subgrade settlement and has broad application prospects in future high-speed railway projects.

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