By Joaquim Fanton*

A person who is not familiar with multi-pair cables cannot imagine how much cable weight can influence installation methods. Multi-pair cables have diameters between 20 mm and 70 mm and their weight range between 500 g and 7 kg per meter. 40 m of a 200 multi-pair cable weights 115 kg! All this weight is explained because the old multi-pair cables, besides copper core, had lead sheaths.

By the other hand, an optical cable is extremely light and slim. Its diameter ranges between 8 mm and 20 mm and weights between 50 g and 150 g per meter. A 40 m of an optical aerial cable span weights only 6 Kg.

This introduction was necessary since this story is about materials used in underground ducts for telecommunications cables.

Right after the end of World War II, the world was filled with new raw materials. Synthetic rubbers, polyethylene, polypropylene, polystyrene, acetates, acrylates, vinyl and so on.

This story encompasses only one from thousands of cases where new materials revolutionized the market. I refer to PVC ducts, which replaced the traditional clay products, which were being used in Brazil and in the world since Graham Bell invented the telephone.

The picture above was taken from Prática Telebrás 235-201-708, Network Series, issued on October 1976. At the time, sandstone ducts use was restricted only in recovery and maintenance of existing sections. They were in extinction process.

A DM-6 (6-hole one-meter unit) weighed 60 Kg. As a consequence, 1 Km of DM-6 consumed 1,000 units and required that 60 T of clay pieces were transported and stored near the construction site. The alignment between DM-6 was guaranteed by two steel pins inserted in the holes that appear in the drawn cut. Additionally, minimum trench wide was 50 cm, to provide proper work space for masonry activities. Yes, masons were used to lay the ducts, which required previous placement of a compacted and leveled clay layer. Because of clay’s fragility, it was almost standard practice to "envelop" the line duct with concrete. Settlement required a 3-man crew because of the weight. Before the "bagging" with cement mass, it was necessary to wind a strip of cotton fabric at the point of junction to avoid cement penetration in the duct. In short, one clay piece had a relatively low cost, but, its use required a high grade of logistics and construction method was expensive and rather complicated. All this together turned very high the final cost of Km duct line construction!

When I started working for TELEPAR, clay ducts were still used there. Cities under my responsibility, generally speaking, were very small and required exclusively aerial cables. But, there were some large cities requiring duct way construction of underground plumbing. Among them, was the city of Cascavel, one of my first projects. The design decided for the construction of 1.4 Km of DM-4 clay ducts.

Our best contractor, Lourival Frelik, was chosen to build the duct line. Six trucks containing DM-4 pieces followed to Cascavel. 225 Km of paved road from Curitiba to Guarapuava and 220 Km of non-paved road from Guarapuava to Cascavel. Three weeks later, Lourival asked for an additional load of DM-4 ducts and a truck containing 250 pieces (10 metric Tons) was sent to Cascavel. One week later, a new call requiring more pieces. At that point, I questioned him what was happening and he explained: “Most of the pieces are arriving broken after shaking during hours on the road between Guarapuava and Cascavel”.

In summary, Telepar purchased and payed for the transport of 2,000 pieces of clay ducts in a segment requiring 1,400 pieces. Six hundred pieces get broken during transportation. That caused delay and raised the forecasted cost.

Fortunately, someone told me that CTBC, a telephone company located in São Paulo ABC area was using PVC ducts and I decided travel to know this novelty. And I enjoyed very much what was seen.

One 6 m PVC piece corresponded to one DM-6, or to 1.5 DM-4. PVC price (at that time) was much more expensive than today’s PVC price. That was a true issue. But, benefits were countless.

For example, in a 120 m route, instead of 120 connections using cement plaster, made inside the trench, PVC ducts required only 24 connections (made with PVC adhesive and out of the trench). Only after the connection, PVC ducts were placed in the ditch. This detail permitted digging narrower tranches. The new trenches were 40 cm wide and represented 20% less in volume of excavation and in cost of repaving.

I remembered a classmate who had graduated in civil engineer, and was working at Tigre. We agreed to carry out an experimental building in cooperation. Tigre had a fleet of trucks to transport their products.

To reduce transportation cost, wall thickness was reduced and we also decided using two different diameters (100 and 75 mm). 100 mm ducts for 600 cables and above. 75 mm ducts for cable up to 600 pairs. The estimated duct required lengths divided roughly in half. Using PVC, the maximum loading volume is always reached before the weight limit. The idea of using two different diameters was fundamental to optimize load truck. In a first step, the 100 mm ducts were loaded. In a second step, the 75 mm ducts were placed inside the 100 mm ducts already boarded. One single truck was sufficient to carry all duct lengths in most of the cities we constructed underground facilities.

To prevent smashing due to small thickness of PVC ducts, we decided plugging ends and pressurizing them during construction work. First construction took place in Pato Branco. Experimental construction was a success and PVC ducts were standardized in Telepar underground duct ways. Over time, cost of PVC lowered, duct thickness was increased and pressurization needs disappeared. Clay ducts continued been used, but, restricted to existing underground repairs. Indeed, we had to face some problems created by clay duct manufacturers. But that's another story.

Over time, there have been other thermoplastics. It's been at least two decades that PVC ducts are no longer used in telecommunications plumbing. ´Polyethylene and polypropylene tube were introduced, which are flexible. Nobody remembers about duct "bars". Now we use the work "coils" that are provided with hundreds of meters. There are smooth and corrugated ducts, they are divided into simple and multiple ducts. In order to facilitate its identification, the ducts receive printed numbering or are colored. The only disadvantage presented by new thermoplastics with respect to PVC is that they cannot be glued. Polyethylene and polypropylene require mechanical connections.

Currently, Brazil is receiving a new line of ducts. They are called microducts, with diameter between 4 mm and 15 mm, aimed to receive optical micro-cables, which cannot be tensioned, being released through blowing. This technique shall be subject of another story!

*Joaquim Fanton is an electrical engineer from the Federal University of Paraná (UFPR), with 38 years of experience in optical networks. He worked as an outside plant engineer and manager for Telepar, Telesp, Telebrás and CPqD (Telebrás Research and Development Center). Today, he acts as RNP consultant and takes place in planning, designing, construction and acceptance of optical networks, including the Digital Cities Program. Travelling around the country, he accumulates stories that have been shared in this website.

[[{"fid":"6621","view_mode":"default","fields":{"format":"default","field_file_image_alt_text[und][0][value]":"Fanton","field_file_image_title_text[und][0][value]":"Fanton"},"type":"media","link_text":null,"attributes":{"alt":"Fanton","title":"Fanton","height":653,"width":641,"style":"width: 100px; height: 102px; float: right;","class":"media-element file-default"}}]]