A new look at compressive loading forces

A new look at compressive loading forces

Experiments reveal that industry standards for fiber-optic outside plant should be re-evaluated.

Rick Watson and Charles Marion Siecor Corp.

Because of their environmental applications, fiber-optic outside plant (osp) cables must be designed to withstand static compressive loading forces. To evaluate cable compliance, several industry specifications have been established, specifically, those from the Insulated Cable Engineers Association (icea 640), the U.S. Department of Agriculture Rural Utilities Service (rus 7 cfr 1755.900), and customer detail specifications. Siecor Corp. decided to experiment with compressive loading specifications to determine the actual compressive loading forces placed on osp cables in burial deployment.

Fiber-optic test procedure fotp-41, Compressive Loading Resistance of Fiber Optic Cables, defines the test apparatus and procedure for evaluating the compressive resistance of an osp cable. Many cable specifications require a compressive load resistance of 220 N/cm for nonarmored cables and 440 N/cm for armored cables. The test apparatus uses a 10-cm circular plate to place the compressive load on a cable; therefore, the cable`s ability to withstand a crush force of 2200 N results in a normalized value of 220 N/cm.

Before Siecor began its experiment, it investigated how the fotp-41 was developed. The fotp-41 originally presented the test apparatus and the procedure for performing the test. In addition, this research indicated that cable compressive resistance values contained in industry standards resulted from a Department of Defense specification (dod-std-1678) derived from a specialized optical-fiber cable application. The compression requirements were developed from a cable deployed on the ground surface connecting a satellite downlink antenna to an equipment hut. This provided a benchmark to establish requirements for other optical-fiber cables.

However, the dod application was not representative of typical buried telecommunications cable applications. Optical-fiber cable manufacturers are required to meet industry standards but have questioned the functionality of the 220- and 440-N/cm crush force. The study documented in this article was designed to derive a functional compressive resistance value for osp cables supported by empirical experimentation.

The experiment

The compressive loading due to the weight of the soil (approximately 2000 kg/cu m) was obtained from Introduction to Soil Mechanics and Foundations, by George B. Sowers and George F. Sowers. At a depth of 122 cm, a compressive force of 20 N/cm was estimated based on the use of a 10-cm diameter compression plate. Based on this estimate, an experiment was conducted to measure the compressive force in buried simulations using different backfill scenarios.

The experiment measured the compressive forces applied to load cells at the bottom of a trench 122 cm (48 inches) deep, 60 cm (24 inches) wide, and 330 cm (11 ft) long. To simulate different burial scenarios, four different trials containing different media were used to backfill the trench to the original ground level. The backfill media consisted of excavated soil, sand, small gravel, and large rocks (commonly known as "rip rap"). In addition, for trenches in which rocks were used as the backfill media, concrete slabs were placed at the bottom of the trench to simulate a solid bottom.

Four different burial trials were conducted in the experiment. For each trial, 15 load cells were buried (see figure) as follows:

Trial 1--Load cells were placed at the bottom of the tamped trench and covered with the excavated dirt. The backfill soil was tamped. This trial measured the loading conditions using the local soil--a combination of sandstone and clay.

Trial 2--Load cells were placed at the bottom of the trench with a 0.3-m-thick bedding of sand above and below the load cells. The remaining portion of the trench was filled with the excavated soil. This scenario measured the compressive forces of a typical industry-recommended procedure for buried optical-fiber cable.

Trial 2a--After the readings were recorded for Trial 2, the front and rear tires of one side of a vehicle (estimated weight 33,000 kg) were parked on the trench and the measurements were repeated. This allowed an observation of how much the compressive load increased under the weight of a vehicle.

Trial 3--Load cells were placed on a bedding of 25.4-mm-thick concrete slabs, and the trench was filled with #57 gravel, creating a condition in

which the trench had a solid bottom and rocky backfill material.

Trial 4--Load cells were placed on a bedding of concrete slabs with a 0.3-m-thick layer of #57 gravel on the load cells, and the remaining part of the trench was filled with Class B rip rap. This created a scenario in which the trench had a solid bottom and the backfill material contained large rocks. Placing the #57 gravel directly on the load cells avoided any damage to the load cells by dropping large heavy rocks directly on them.

Analysis

The data in the table outlines agreement between the calculated compressive force and the actual measured values. The observed measurements resulted in an overall average compressive force of 24 N/cm (26 N/cm when scenario 2a is included), compared to the compressive force of 20 N/cm estimated at the beginning of the experiment. A comparison of the empirically derived compressive forces to industry specification values revealed a difference of an order of magnitude in the case of nonarmored cable designs.

Considering that some magnitude of variability existed for every measurement, analysis of the standard deviation values displayed an average standard deviation value of 14 (16 when scenario 2a is considered). Assuming that for most cases the actual compressive force was within six standard deviations of the mean, a functional requirement for a cable`s compressive resistance was on the order of approximately 100 N/cm for a buried optical cable. Essentially, this represented less than half of the value currently specified for nonarmored cables. Therefore, the compressive resistance specification value applied to buried cables should apply to all osp cables, regardless of the cable construction (i.gif., armored or nonarmored).

Of particular interest was the data collected in the burial scenarios that involved large and small rocks. Before the experiment, it was believed that rocks would transmit higher forces in direct burials. However, as seen in the data collected, forces scatter (i.gif., bridge) in different directions as much as, or more than, they do with backfill containing mostly soil.

To understand the compressive loading forces applied to optical-fiber cables in burial scenarios, data was obtained in the different approaches. The calculated and experimental results indicated that the current compressive resistance requirements contained in industry standards and customer specifications differ by an order of magnitude from the calculations and measurements in this experiment. Considering the results of this study, the static compressive resistance requirements for telecommunications outside plant optical-fiber cables should be modified to 100 N/cm to reflect actual, functional deployment conditions.u

Rick Watson is an applications engineer and Charles Marion is a product development technician at Siecor Corp., Hickory, NC.