How we laid the cable under the bottom of the river
Hi, Habr. Probably many of you have already seen our previous material on the construction of a base station with a tower on the roof of the building. And today we want to tell you about a completely different kind of work that we recently had to carry out. It will be about the conquest of two elements - land and, in part, water. More precisely, about how we laid fiber-optic cable under a river bottom on a 980-meter-long section.
The task was set - to connect an additional fiber optic channel to the Vladimir and Nizhny Novgorod regions, along the Nizhny Novgorod – Murom highway. The problem was that in this place, by an absurd accident, the Oka River flows. The width of the channel is decent, the depth reaches 6 meters.
Well, we don’t get used to the difficulties. It was decided to lay fiber-optic cable under the surface of the river bottom. Since this is a normal cable for underground / underwater laying, and not a special reinforced cable, it was necessary to protect it from external influences. The fact is that the bottom soil is quite mobile, under the influence of the flow, displacements of the bottom surface masses constantly occur. And if the cable is not protected, then sooner or later it will break. For the same reason, you can not lay directly on the bottom, this is also fraught with mechanical damage.
Therefore, this solution is applied: at a depth of several meters, a steel tight pipe is laid under the bottom surface, through which a cable is drawn from one bank to another. However, the soil throughout the site may be heterogeneous in composition, primarily in density. So dense rocks can be found that laying a pipe through them is impractical or simply impossible. And in order not to encounter such an obstacle, a bottom reconnaissance is being carried out on the proposed laying section. Within a week, the bottom topography was examined with the help of divers, and geological engineering surveys were carried out. As a result of these surveys, a specific site for laying was selected. Although it must be said that even this measure does not guarantee successful laying the first time. But we are lucky
A trench for the cable, about 1.5 meters deep, was laid on the shore at the start of work.
At a distance of about 70 m from the water edge, a horizontal directional drilling rig FORWARD RX44x160 was installed.
With the help of this installation, the inlet channel was first drilled, and then it was gradually pressed into it, pushing a steel pipe with a diameter of 110 mm and a wall thickness of 8 mm directly through the ground. It is in it that the fiber-optic cable will be laid later.
Pipe sections are pre-connected to each other on the thread, and the joint is welded to ensure tightness.
That is why it is so important that the soil of the river bottom is relatively low density. After all, a drilling rig that develops a pulling force of 44 tons has to push a rather thin pipe almost 1 km long through the ground! And if a sufficiently dense rock gets in the way of the pipe, then the pipe may break, crumple or go to the side. And in such cases, you have to look for a new site and start all over again.
While the drilling rig pushed a pipe under the river, a bulk pit was dug on the other side, at the site of the alleged exit to the surface. A week after the start of laying, the pipe successfully exited the pit wall.
The most difficult stage is behind. Now you need to stretch the cable through a narrow, sealed pipe of kilometer length. Of course, just pushing it is impossible. Instead, the end of the cable is attached to a small “wad” that is inserted into the pipe. Then, using the installation for blowing fiber optic cable - injector PSI.1 - air is pumped into the pipe.
The compressor serves as a source:
Under air pressure - up to 10 MPa - the “wad” began to move along the pipe and pull the cable along with it. That is why the pipe must be airtight, otherwise it simply will not be possible to blow through the cable through it. A few hours later, the "wad" came out on the other side. The cable is stretched under the river!
The total duration of the work, including bottom surveys, was less than two weeks. This section of the communication line has become the longest of us laid under the bottom of the river. Thanks to him, he managed to close the ring of the fiber optic highway with a total length of over 750 km, which increased the throughput of the MegaFon network in two regions.