Chemically restoring pipeline position

Mar 17, 2011

Renovation

Current renewal methods for pipelines and culverts are very effective for pipe function decline and accident prevention, but offer little in the way of rehabilitating meandering and slacks of pipes. Continually expanding urbanisation means that restoring pipeline position using open cut methods has become less desirable, and as such has led to the development of new pipeline position restoration technologies, such as the chemical injection method.

Pipe renewal methods
One of the main causes of meandering and slacks is a lack of ground support. The pipelining renewal methods as shown in Figure 1 are all unable to repair the shrunk diameter or slacks of a pipeline as none of these lining methods were effective for ground improvement underneath the pipeline. Figure 2 shows a pipeline after renewal construction. 
  
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Figure 1. Classification of the renewal methods. (Source: Trenchless International)
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Figure 2. Illustration of pipeline after renewal construction. (Source: Trenchless International)
 
Restoring pipeline position
A technique commonly used for the improvement of subsided constructions is ground upheaval with injection grouting. However, it is difficult to define the amount of the injection grouting material that is necessary for position restoration of the pipeline, the shape of the pipe, and the impact to adjacent structures.

To overcome these problems, a new method has been developed that involves making a guide space formed primarily in the direction that the pipeline needs to be moved before construction. This process involves building casing pipes and a feeding pipe for injection grout. The feeding pipe is set at the guide space location. After pressurised slurry is sprayed from the feeding pipe to disturb the ground, the pipeline position may be improved.

This technique requires the consideration of drilling diameter, soil conditions, and the arrangement of the form and placement intervals of the feeding pipes. A suitable injection material requires a gelling time of around ten seconds with no change in strength, volume or weight after the injection. In cases of position restoration by grouting, a grouting material that uses two liquids, such as cement and sodium silicate, may be used. After each component is injected by a grouting pump into a guide space, both components are mixed in the same volume. However there are few materials that satisfy these requirements of cutting, mixing, and gelling. Moreover when using a pump, the injection performance and gelling time of the grouting material are not satisfied at the same time.

Thus, a new grouting material that suits the restoration method of the pipeline position via grouting was developed along with a method that enables the selection of the discharge quantity of different types of grouting material.

These days the extent of restoration and injection quantity of grouting materials is determined in advance, and optimal length and depth of injection are determined prior to construction. Figures 3 – 6 show the application procedures for the restoration method. After adopting the restoration method, snake-advancing and slack were rehabilitated with the underpinning effect and increased ground strength. Figure 7 shows the image of the soil condition after restoration.

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Figure 3. Guide casing pipe setting. (Source: Trenchless International)
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Figure 4. Chemical injection for preliminary soil compaction. (Source: Trenchless International)
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Figure 5. Sludge discharge. (Source: Trenchless International)
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Figure 6. Main injection for restoration. (Source: Trenchless International)
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Figure 7. Image of the soil condition after construction. (Source: Trenchless International)
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.

The top position measurement system involves preparing the casing pipe by drilling down to the pipe with pressurised slurry. The sprayed slurry is pumped from a tank on the surface and excavates and liquefies the soil, which is then continuously sucked from the casing pipe. After the hole has been extended to the top of the pipe a measuring rod is inserted into the drilled hole and the magnetised tip is set on top of the pipeline. This ability to establish existing conditions prior to construction works has resulted in improved site execution and management.
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Figure 8. Image of the water level measurement, gauge, and recording paper. (Source Trenchless International)
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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
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Figure 10. Upheaval pressure outbreak range. (Source: Trenchless International)

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

V= (tan30°)2 d23 π (5)

V is the amount of injected grouting material and d2 is the distribution depth of the pulse of injection of the grouting material.
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Figure 11. Injection range of grouting material. (Source: Trenchless International)
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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

 
The amount of injection for each pipe is calculated as the multiplication of V, porosity and the injection rate. The injection rate is defined as the percentage of the real amount of injection in relation to the porosity of soil. As such the total amount of injection is calculated as the multiplication of the amount of injection of each pipe and the number of injection pipes. Table 1 shows an example of the calculation for the amount of injection for restoring a sewage pipe. The condition of the site is as follows;
 
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
 
An example of successful pipeline position restoration
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.

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Masahi Komura, Koichi Araki, Hideki Shimada, Takashi Sasaoka, Kikuo Matsui and Takahisa Tomii

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