Introduction to Measurement in Roadbed Construction

Do a good job in measurement and ensure the quality of the project

Abstract: Surveying is an important basic work for grass-roots construction units. This paper introduces the commonly used methods in surveying and setting out work in roadbed, pavement and bridge structures.
Keywords: surveying; setting out; engineering quality

In recent years, high-grade highways have gradually occupied an increasing proportion of newly built roads across the country. Expressways have become the top priority due to their large investment scale, complex construction technology, and high technical standards. During the construction of expressways, how to do a good job of measurement has become a concern for every construction technician.

1. Surveying work before construction
For construction units, doing a good job of technical disclosure before construction and clarifying the design intention are the guarantees for smooth construction. Expressways are linear projects. With the widespread use of electromagnetic wave rangefinders, the road construction measurement control network has become less and even no longer uses the triangulated network form commonly used in the past, and more single wire network forms with precise distance measurement are used. Therefore, before construction, the work of restoring the alignment should be carried out first. From the end of the survey to the period before construction, the guide points and leveling points set by the design unit are often lost or moved, so the origin should be reviewed and corrected before construction. After the guide points and leveling points are restored, the center line of the route should be calibrated. Since it is the first excavation and site cleaning of the original ground, the accuracy of the center pile of the route can be slightly lower.

2. Measurement work in roadbed construction

(1) Polar coordinates layout method for roadbed plane position In road construction, the layout methods commonly used in the past are: deflection method, tangent offset method, auxiliary tangent method, etc. However, with the improvement of highway grade, the weaknesses and inconveniences of these methods have become obvious. Expressways pass through fields, ditches, rivers and hills. Therefore, the amount of earthwork for roadbed filling and excavation is very large. The embankment filling is more than 6m high, and even the highest point is more than ten meters. In addition, the roadbed filling is carried out by layered compaction. The center piles on each layer will inevitably be buried and damaged during construction. If the deflection method is used, the main point will often be destroyed, which brings great inconvenience to the measurement work. At present, total stations have been widely used as measurement means in highway construction, and the labor intensity of measurement work has been greatly improved. Therefore, the corresponding layout method has been further simplified. The angle measurement accuracy of the total station is generally within "1.5"~3"", and the distance measurement accuracy is within 1mm+2ppm~3mm+5ppm. Therefore, the convenient and simple polar coordinate method can be used for layout, which can fully achieve the accuracy. Its advantages are that first, there is no need to worry about the destruction of the center piles of the route during roadbed filling, and the center piles of any pile number on the roadbed can be set at any time according to needs. Secondly, the macro control function is good, and a road section of about 1km can be laid out in one mirror placement. In terms of the overall accuracy, it is fully guaranteed.

(2) Control of roadbed elevation The roadbed is filled and compacted in layers, so the elevation of each layer needs to be strictly controlled. On the one hand, it is a requirement for construction quality, and on the other hand, it is the basis for earthwork measurement during the construction process. The construction leveling points should be densely packed on the first-level leveling points designed, with one set every 150m or so, and the leveling point closure error f≤±20L (L is the length of the leveling route). Since the elevation control accuracy of the roadbed is slightly lower than the accuracy of the pavement structure layer, considering the advanced nature of the total station, the trigonometric height method can sometimes be used for joint measurement of leveling points. The formula is as follows: h=stga+i- L+K, i is the instrument height, L is the mirror height, K is the spherical aberration, and the atmospheric light stripping correction value. When using this method to arrange leveling points, opposite observations are used to take the average of the height differences between the two. The current total station memory module has the function of trigonometric height measurement. You only need to enter the instrument height, sighting height, temperature and air pressure values ​​to automatically calculate. When the visibility is good and the temperature and air pressure are appropriate, trigonometric height measurement can completely replace ordinary leveling, and the accuracy can reach the third-class leveling. However, the spherical aberration correction of trigonometric height measurement is based on empirical formulas. When measuring distance and angle, there is too much external interference. For example, when electromagnetic waves pass through the water surface and rice fields, the spherical aberration of the local ground has a very large impact. In addition, the instrument is far away from the target, the point accuracy is low, and the angle measurement accuracy is poor. Therefore, the actual height difference measured is often not ideal.

(3) Observation of roadbed deformation (settlement observation) In the densely water-networked areas of the south, soft soil foundations are widely distributed. During road construction, the settlement of the roadbed is generally large. Therefore, strengthening the settlement observation of soft soil foundation is also a relatively important task. The settlement of soft soil foundation has a great impact on the quality of the roadbed. The maximum settlement measured can reach 26cm.

The settlement observation of soft foundation should be carried out at the beginning of roadbed construction. When the soft soil foundation is treated, the settlement plate should be buried immediately, the initial elevation of the settlement plate should be measured, and regular observations should be carried out to calculate the settlement amount. When the roadbed fill height reaches the top of the settlement rod, the rod should be connected immediately to measure the elevation. It must be noted that the effective protection of the settlement rod is the most basic and most important condition for good settlement observation. The damage to the settlement plate caused by the carelessness of the on-site construction personnel cannot be compensated for the losses caused by the subsequent observation work. Some construction technicians do not attach importance to the observation work of soft soil foundation. In fact, the losses to the construction unit due to the settlement of the roadbed are quite large. Therefore, the settlement of soft soil foundation must be given enough attention.

(4) Surveying and staking out structures and bridges Generally speaking, as long as the accuracy of the center pile number and the geometric dimensions of the channel are ensured, the accuracy of the entire structure will be easier to achieve. However, the surveying and staking out of bridges, especially large bridges, must be carried out with caution. Small and medium-sized bridges are short in length, and one guide wire can often be used to complete the measurement task. Therefore, their overall connection accuracy is easier to ensure. It should be noted that: after completing the bridge layout, the connection between the starting and ending points of the bridge and the piles in the roadbed should be checked as a check for the bridge layout. For extra-large bridges, due to the large number of piers, in order to ensure the accuracy of the geometric dimensions between them, the accuracy of the bridge control points must be improved. In addition, the coordinates of the guide wire points should be checked frequently, and if there is a large change in the guide wire points, they should be corrected immediately. In addition, during construction layout, the measurement accuracy can be improved by using methods such as increasing the number of guide wire points, using the full circle method for angle measurement, and using the average of three times for distance measurement.

3. Measurement of pavement structure layer
The thickness and flatness requirements of the pavement structure layer are relatively strict, and the construction quality of the pavement subbase and base layer has a greater impact on the surface layer. Therefore, when controlling the elevation of the subbase and base layer, it should be done carefully and strictly. The spacing between leveling points should be small and checked frequently. In addition, in actual construction, the author found that due to the large load of the pavement structure layer, some soft soil sections, sections with insufficient compaction, and individual structures (channels, bridges) will have obvious local settlement. At this time, cracks will appear on the road surface, affecting the appearance and quality of the road surface. In addition, the settlement of the road surface will increase the thickness of the next layer, causing great economic losses. Based on this, after the construction of each layer is completed, when the elevation is accepted, the section elevation should be measured in detail. Before the next layer is laid, the elevation of each section should be re-measured and compared with the previous elevation. If the difference is large, compensation should be considered. In this way, the local "sag" phenomenon of the road surface caused by settlement can be greatly reduced.

4. The role of surveyors in construction
Since surveying is the first step in engineering construction, each surveyor must maintain a serious and responsible attitude. Each surveyor must undergo necessary technical training before taking up the post, strengthen the ability to use instruments, and be familiar with the engineering situation to ensure that the construction task is completed with high quality and efficiency.