Comparison of methods: dynamic versus hydrostatic testing of mine ventilation seals

From 2001 to 2007, the Pittsburgh Research Laboratory (PRL) of the National Institute for Occupational Safety and Health (NIOSH) conducted studies to develop alternative methodologies to full-scale explosion testing for deter­ mining the ultimate strength of mine seals. As a result, the PRL developed and proposes an alternative seal-strength evaluation method based on a hydrostatic pressure-loading concept. The researchers suggest pressure loading a seal using water to twice the expected dynamic design load.The hydrostatic chamber test offers a means of validating seal designs, establishing appropriate resistance functions and determining the ultimate strength of seals through testing to failure.This article contrasts the full-scale explosion and hydrostatic testing of mine seals using a simple dynamic system model and principles. Introduction Tragic mining disasters in West Virginia (Sago Mine) and Kentucky (Darby Mine) in 2006 greatly heightened interest in the development of quality seals to protect miners from blast effects and toxic gases produced by uncontained gob explosions. Following the enactment of the 2006 Miner Act and the U.S. Mine Safety and Health Administration (MSHA) issuance of the Emergency Tempo­ rary Standard (ETS) on the Sealing of Abandoned Areas in 2007, MSHA is conducting de­ tailed technical evaluations of all proposed seal designs for each underground sealing location. These technical evaluations are based on sound structural engineering design approaches followed by certification of as-built construction. MSHA will also accept the results from full-scale testing of mine seals. If full-scale performance testing is required to develop seal stress-strain response data, the authors propose a hydrostatic test method as an alternative to full-scale explosion testing.This method uses water to load the seal to pressures at least twice the expected dynamic design load to determine stress-strain data. This article contrasts the full-scale explosion and hydrostatic testing of mine seals using a simple dynamic system model and principles. There are many three-dimensional, finite-element codes for designing various load-bearing structures that can simulate not only the seal itself, but also the interac­ tion of the seal with the surrounding support strata. How­ ever, when studying a single mode of seal response, the basic analytical model used in most blast-design applica­ tions is the single-degree-of-freedom (SDOF) system. In the current study, the U.S. Army’s Wall Analysis Code (WAC) (Slawson, 1995) was used to identify test conditions where hydrostatic pressure loading can serve as an alternative yet equivalent test method to full-scale methane-air explosions for evaluating the strength performance of mine ventilation seals. Such hydrostatic tests would build confidence in design methodology through comparisons be­ tween pretest predictions and actual measurements, as well as provide a timely, cost-effective means of seal testing. Single-degree-of-freedom system (SDOF) All structures possess more than one degree of free­ dom regardless of how simple their construction. How­ ever, many structures can be adequately represented as a SDOF system for purposes of analysis.The accuracy of an SDOF approximation depends on how well the deformed shape of the structure and its resistance can be represent­ ed with respect to time. The procedure for obtaining the equivalent SDOF approximation for a structural compo­ nent is based on its deformed shape under the applied loading and the strain energy equivalence between the actual structure and the SDOF approximation. Equivalent mass, stiffness and loading are obtained through the use of transformation factors. Chapter 3 of the U.S. Department of Army, the Navy and the Air Force design manual TM 5-1300 (1990) contains tabulated transformation factors for typical structural elements, including slabs. The derivations of the equations for the transformation factors are also given in this reference and used in the WAC. The WAC is an SDOF model developed for the U.S. Army Engineer Waterways Experiment Station (WES), Structures Laboratory (Slawson, 1995). WAC was de­ veloped to provide a tool for the easy calculation of the response of typical walls subjected to blast loads. The WAC can calculate the resistance function (R(y), pres­ sure-deflection) of a wall given its construction details, including dimensions, material properties and support conditions or accepts user defined resistance functions based on experimental data. The WAC transforms the wall model to an equivalent SDOF model, calculates the actual and SDOF equivalent loads and solves the equa­ tion of motion to determine the response time history of a central point on the wall. The equation of motion for an SDOF system is M·yʺ(t) + Cd·yʹ(t) + R(y(t)) = F(t) (1) where M is the equivalent or “lumped” mass of the system; Cd is the damping coefficient taken as 5 percent of the critical value, i.e. very lightly damped; y(t) is the placement of the mass as a function of time t; yʹ(t) is the velocity of the mass or first derivative dis­ placement; yʺ(t) is the velocity of the mass or second derivative displacement; R is the structural resistance as a function of displace­ ment y; and F(t) is the structural load as a function of time, i.e. expected blast pressure history.