Chemically restoring pipeline position

Measurement of snake-advancing and slack
When restoring the position of a pipeline, an accurate survey of the location of the pipeline is typically required before developing a detailed layout of the restoration plan.

The measurement of the existing position is performed by optical survey and, where man-entry is possible, by manual logging with results contributing to background information for the restoration plan. However, in the case of gas or small conduit pipes when man-entry is not possible, two kinds of devices are used. The first is a camera with a survey instrument attached which is inserted into the pipe to measure its depth, as shown in Figure 8. The second is a device which directly measures the top positions of the pipeline as shown in Figure 9. This top position measurement system has been developed by the Department of Earth Resources Engineering at Kyushu University.

Figure 8. Image of the water level measurement, gauge, and recording paper. (Source Trenchless International)

Figure 9. Measurement system for position of pipeline. (Source: Trenchless International)

Amount calculation of restoration method construction
When planning restoration and construction works, it is necessary to calculate the amount of injection.

Step one: determining the injection depth

It is necessary to consider upheaval pressure to ensure appropriate planning when determining the injection depth for the restoration constructions. A pulse injection is adopted because ground upheaval can be easily monitored and controlled, making it easier to repair the damaged pipes.

It is assumed that the effect of the pulse injection is defined as a cone, the centre of which is defined as the outflow point of the injection hole. The height and width of the cone are the distance from the centre to the real arrival points of the grouting material, as shown in Figure 10. It is supposed that the injection pressure of the grouting is acting within the range of the cone as mentioned above. For example, if the upheaval is measured 0.15 MPa at an injection depth of 10 metres and the radius of the cone is 10 metres, the vertical angle is 60 degrees. If the upheaval pressure is defined as the pressure required project from the pipeline, the injection depth is determined as follows.

D= d1+ d2 (1)

Where D, d1, and d2 are the injection depth, sum of overburden of the pipeline and diameter of the pipeline, and distribution depth of the pulse of the chemical grouting, respectively, d2 is calculated as follows;

(a) If the effective range of the pressure is acted in 1/3 of the injection effective radius,

 

d2= d1/2 (2)

 

(b) If the effective range of the pressure is acted in 1/4 of the injection effective radius,

 

d2= d1/3 (3)

 

Step two: calculating the number of casing pipes

 

The number of casing pipes that are used for the injection to repair the pipelines is determined as follows.

 

N= 2n (4)

 

N, L, and L3 are the number of casing pipes, distance of the restoration, and the interval of casing pipes respectively. n is calculated as n= L / L3. The arrangement of casing pipe differs depending on whether the pipeline is a concrete pipe or a pvc pipe. When concrete pipes are used, casing pipes are set up at the joints of the each of the pipelines.

 

Step three: calculating the grouting material per pipe

 

The distribution of the grouting materials resulting from the injection site experiment is shown in Figure 11. From this result, it is supposed that the circular cone of the hatching part of the cylinder in Figure 12 is the range that grouting materials are uniformly distributed. It is also assumed that the other part of the cylinder is the consolidated range of infiltration of the grouting. Therefore, the volume of the circular cone is determined as follows

Figure 11. Injection range of grouting material. (Source: Trenchless International)

Figure 12. Distribution of the grouting materials. (Source: Trenchless International)

Step four: amount of injection for each pipe

Table 1: Comparison between prediction and the result of site investigation

Item	Prediction	Result
Setting depth of injection pipe	Gl-6.1 Metres	Gl-6.1 Metres
Number of pipe	14	14
Injection amount	8,147 Litres	8,304 Litres

 

Diameter of pipes: 200 mm

 

Type of pipes: pvc pipe

 

Soil condition: cohesive soil, N value = 3

 

Porosity: 70 per cent

 

Overburden: 4.1 metres

 

Length of restoration pipe length:

 

15.0 metre

 

Maximum height of restoration:

 

60 mm

 

This method of restoring pipeline slacks via chemical injection was applied to a 46.2 metre section of an 800 mm diameter pipeline. The pipeline was situated in sandy soil with an N value of 1–3 and had an overburden load of 9.5 metres due to inadequate support of the surrounding soil. Restoration using the chemical injection method saw the pipeline successfully lifted approximately 1,000 mm.

 

This article has been adapted from a paper by Masashi Komura, Koichi Araki, Hideki Shimada, Takashi Sasaoka, Kikuo Matsui and Takahisa Tomii from the Department of Earth Resrouces Engineering, Kyushu University, Fukuoka and Fuso Technology Co. Ltd., Tokyo Japan. Please refer to the original for more detailed information and references.