Setting-out pipes

When laying pipes, it is critical to ensure the right position since the beginning because a small error in the gradient, level and direction could be unacceptable at the end of the line pipe.

Setting out is time-consuming, but it is common knowledge that site engineers are often rushed when setting out to give all the information to the squad to start laying pipes and run to the next job. Quality and safety should not be compromised for the program, but it is widely known that in construction, the productivity is in everyone's minds, time is money. Here I will try to compare several methods of setting out pipes analysing the pros and cons to meet the three goals mentioned previously: time, quality and safety. For this kind of task, the cost difference between methodologies is predominantly determined by the time needed to perform it. Therefore, the pure cost analysis has been overlooked.

Traditionally, the pipes have been set out placing pegs (position), and profiles (highs) along the pipeline with an offset from the centreline of the line (around 3m). The profiles and the pegs describe a reference line needed by the squad to start digging the track and laying pipes.

Layout, profile and section of the traditional setting out method (BM Sadgrove, 2007)

The availability of beam or line laser levels on sites gave more control, precision and productivity to these jobs. When laying pipes, this device creates a laser beam that gives a straight line at whatever slope needed that the squad uses as a reference. So once the laser is set, the construction process is monitored continuously increasing the accuracy and productivity of this task. The line laser levels, as expected, substituted the traditional straight line and also generated new methodologies. Then, the issue with this new technology is the method to put in place the laser at the start of the line with the right gradient, level and direction.

Pipe laser positioning (BM Sadgrove, 2007)

The laser could be set using the profiles and pegs of the traditional method. However, the information given by the profiles has to be transferred to the bottom of the excavation (usually 2-4 m depth and an offset of 3m), losing precision. It entails the acceptance of wide margins of tolerance. The increasing demand for high-quality standards implied that a site engineer (with a total station) join the squad to set the laser on the bottom of the track. Notice that the sitting out process, in this case, is a critical task in terms of time because the squad cannot start laying pipes until the laser is set. Also, it demands the presence of a site engineer when required.

From my professional experience, I have been applying two methods which I describe and compare next.

1. Setting out the first concrete pipe

The first method consists of setting out the first pipe. Being the first pipe on position and buried to ensure it doesn't move, the laser can be placed inside of the pipe, giving the required baseline. So, setting out the pipe is key, which is tedious, especially with large and heavy concrete pipes. Also with this method, the direction of the pipe could be compromised because the two control points are at the beginning and the end of the pipe which usually is only 2.5-3 meters long for a full pipe or 0,6-1m long when setting out the stub and rocket.

The first step for this method is to level the gravel bed where the pipe will lay. It is a good practice to level the bed a bit high (around 10mm) because when placing the pipe, this margin probably is lost (especially with heavy concrete pipes). Next, the pipe has to be positioned in the right direction, checking that the two extremes of the pipe are inside the reference line. The following step is to check the levels in both ends to make sure that the pipe has the right gradient on the correct elevation. If the pipe is low, the gravel bed has to be risen, going back to the first step. If the pipe is high, the pipe could be "shacked” or pushed down until meeting the desired position. It is an interactive process; it could be the possibility after levelling the pipe that the direction has been compromised, which means repeating some of the previous steps.

After the first pipe is on place, the laser is placed inside the pipe with the desired slope programmed. The end of the pipe could be taken as a reference point to point the laser in the right direction. Still, it is highly recommendable that the site engineer goes as far as possible along the reference line to give another point to ensure the direction of the laser beam. Despite being accurate laying the first pipe, it is recommended to double-check the line after 3 or 4 pipes.

In terms of security, sometimes when checking the position and highs of the control points the site engineer hast to stand on the pipe with the measurement tools (prism and controller), which can be risky when laying big diameter pipes.

2. Setting out the laser in a concrete block

In this method, the laser is placed on a concrete block at the beginning of the line, usually in the centre of the manhole. As well as the previous method, the site engineer with a total station will join the squad to set the laser.

First, the concert block must be positioned. The top of the concrete block (or log) has to be on the invert level and it roughly needs to be in the right direction. The direction accuracy is archived when positioning the laser on the block. The second step is to set the laser on the block with the gradient desired. Next, the site engineer will give a reference point on the block and another reference along the line to set the direction. In order to minimise errors, it is recommended to provide the second reference point as far as possible. It is also recommendable doing periodic checks along the line. Once the first pipe is on place, the laser could be paced inside the pipe, and the concrete block retired.

Site engineer setting out the laser on a concrete block

Conclusion

After putting into practice these two methods, setting out a concrete block has been proved to be less tedious and less time-consuming than setting out the first pipe. Additionally, as a consequence of this time optimisation, the site engineer has more time to do other tasks.

In terms of quality, the concrete block method is more simple and straightforward than the rest of the methods described. It offers more accuracy, mainly because the process needs fewer adjustments. It is essential to mention that the site engineer with this method has more control over the process. He knows exactly the level of the block where the laser is placed, so the gradient could be adjusted to meet the endpoint at the right level.

Despite setting the laser on a concrete block seems to be the proper method to lay pipes, setting out the first pipe could be the only solution in narrow places without space to place the concrete block.

The main drawback of the two methods exposed is that require a site engineer with a total station when the squad needs to start laying a pipeline. In contrast, the traditional approach the setting out could be independent of the task of laying pipes.

References:
- BM Sadgrove (2007). Setting-out procedures for the modern built environment.
 (C709) CIRIA, London.