Methods of Setting Line and Grade in Construction of Sewer Sanitary System

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There are four techniques used to set grade and line in the construction of sanitary sewer system. One of these methods or combination thereof may be used in the construction. These methods are discussed in the following sections.

Fig.1: Sanitary Sewer Line and Grade

Methods of Setting Line and Grade in Construction of Sewer Sanitary System

1. Stakes or crosses installed on the surface on an offset from the centerline of sewer sanitary
2. Stakes are installed in the bottom of the trench on the sanitary sewer line as the rough grade for the sanitary sewer is completed
3. Elevations given for the finished trench grade and sanitary sewer invert while sanitary sewer construction progresses
4. A laser beam of light set in the manhole or a specified height above sanitary sewer flow line

1. Stakes or crosses installed on the surface on an offset from the centerline of sewer sanitary

By and large, this technique is suitable for small diameter sewer sanitary system. Practically, crosses, stakes, or spikes are placed at a uniform offset from sewer sanitary centerline. The excavated soil material should at one side of the sewer sanitary trench and the stakes should be placed at the other side. In order to perform an accurate and satisfactory job, proper sheet shall be prepare on which reference points of sanitary sewer stations, offset, and vertical distance from each reference point from suggested sanitary sewer invert must be recorded. Lastly, the line and grade of sewer sanitary is transferred to the bottom of the trench using tape and plumb bob unit or tape and level.

2. Stakes are installed in the bottom of the trench on the sanitary sewer line as the rough grade for the sanitary sewer is completed

This technique is suitable for large diameter sanitary sewer and for the sloped trench wall. In the latter case, the top width of the trench would be considerably wide, and the utilization of short offset would not be an option. In this method, surface references are transferred to the stakes that placed at the bottom of trench along sanitary sewer system. When stakes are installed at the trench bottom, string line need to be drawn between at least three points. The string line should be checked similar to the manner employed for batter boards.

3. Elevations given for the finished trench grade and sanitary sewer invert while sanitary sewer construction progresses

This method is suitable for sanitary sewer system with large diameter. The line and grade for each pipe length should be set using a transit and level at either the top or inside a completed conduit. When large diameter sewer sanitary is constructed in an open trench, it is possible to install both line and grade close to or at the bottom of the trench.

4. A laser beam of light set in the manhole or a specified height above sanitary sewer flow line

This method can be used to set the line and grade if all sizes of sanitary sewer system. In this technique, a laser device, which directs a thin laser beam along sewer sanitary pipe centerline, is used. The device is installed in the invert of the manhole and adjusted horizontally. A required slope will be established through laser device adjustment and then lunching the laser beam. It is required to check whether the required slope is achieved by the light beam or not by checking elevation at a distance of 30m from the manhole. When a laser device is used, one should be aware that it is influenced by temperature.