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Preparation and Rehabilitation of Permanent Ventilation Seals

30-01-2012

Preparation and Rehabilitation of Permanent Ventilation Seals

Background of Sub-Technical Inc

A fifteen year old contracting company, specializing in Ground and Water control in the Mining and Civil Industries. Utilising a variety of proprietary Polyurethane Formulations. Over nine hundred projects completed to date, approximately half of them within Underground Coal.

Background in development, Installation and Rehabilitation of Permanent Ventilation Seals in Coal Mines

The Principals of Sub-Technical were deeply involved in the development and testing of a variety of explosion proof seals from the late nineteen eighties onward. Designs included steel sets with interlaced, imbedded rebar and an internal bladder filled with cementitous grout, (similar to the "Precision Seal", currently installed by Strata Services, Inc.).Additionally a variety of Composite Barrier Seals using Block walls to contain the core.

Being established in 1996 Sub-Technical undertook as an approved installer for the "Minova" "Plug Seal" for a number of years. In addition we were appointed by the "Mowlem Group, Plc." (Australia) to install and advise per Testing and Approval at the Lake LLyn Facility. These were also an imbedded steel/Rebar interlaced structure which was then Gunnited.

Our portfolio of projects related to Sealing also includes Bulkheads, deep under Lake Superior to contain Sulphuric Acid leaching solution. Also many Bulkheading projects for Coal Mining, in which we incorporated a proprietary “Burst Hose” installation, prior to filling of the forms. This system allows complete “ring grouting” once the cement is cured, (and shrunk), without ever drilling a hole.

Current and past materials used for Consolidation/Sealing in and around Ventilation Seals

  • High Density (70#PCF) Two Component Polyurethane Systems. An industry standard for many years, its immense strength and ability to be rapidly deployed make it a prime choice for Roof Control, however its use in - seam and in floor measures can produce issues around seal areas which complicate the procedures necessary for efficient prosecution of the work. The internal pressures generated by Polymerization will produce up to 50 P.S.I, this in areas where a less aggressive medium is definitely an advantage with respect to strata displacement.
  • Lower Density (20#PCF) Two Component Polyurethane Systems. These are generally a product of simply including a predetermined amount of water on the “B” side in a 70# system, they work well for void filling and consolidation but have the disadvantage as a sealing medium as they produce an end product which is in fact a “Rigid Foam”, this in area’s around seals which are far better served by materials which have more elastomeric properties.
  • Polyurethane Urea Silicate systems. Recently produced as a solution to perceived problems with conventional 70# Two Component Polyurethane Systems. Much of the chemistry is derived from "water glass" systems common over thirty years ago. These exhibit compromises in physical strength and set time which truly hinder its purpose, when compared to the 70# systems it is much weaker in every aspect especially flexural and shear which are two of the most critical elements of ground control. From a practical standpoint they produce problems with the ability to cure leaks (gel time) and as they are non-expansive do not produce the economies of True Polyurethanes, which of course foam in contact with water.
  • Coatings and Linings. An expansive array of materials, including cements, clay based, spray applied P.V.A mixtures and Polyurethanes, also composites such as Carbon fiber/Poly-Urea. Their primary disadvantage is the fact that they are surface applied; they may provide a brief temporary result when applied in areas which are on “Draw” but generally Temporary is the key word of importance.

"Hyperflex" as a grouting medium specifically as refers to Preparation of, or Rehabilitation of Permanent Ventilation Seals.
HYPERFLEX is a single component, water activated, Hydrophobic, Pre-Polymeric, Pre-Catalyzed, Polyurethane. It’s product is a low density, closed cell, flexible foam.

Technically Speaking,

  • "Single Component", its ingredients begin life as a two component system, which is reacted as one, under a controlled environment and allowed to proceed until only one component is lacking to allow full Polymerization, that component is water. Hence its water activated nature.
  • "Hydrophobic" refers to its action during Polymerization, which is to repel water during the final phase and be permanently cured.
  • "Pre-Polymeric" describes its condition prior to use.
  • "Pre-Catalyzed", it is unique in that all necessary catalysts are pre-blended. This solves a primary problem with all other of these type of materials which must use a separate catalyst to be of use. This must be “Field mixed” prior to use, this presents a problem as the two liquids are almost non miscible by conventional hand mixing techniques, therefore when performed in this fashion it creates “Hot Spots” in the mixture, which in turn causes differential expansion and SHRINKAGE, an undesirable by product in a sealant/adhesive.

Practically Speaking. We have as a company championed the cause of Single Component materials in the Mining Industry for most of our History, they have sat well with our other formulations to allow us a full range of offerings. Five years ago we were fortunate to engage the services of one of the Nation’s foremost Polyurethane Chemists, (ex-Mobay/Bayer) and have benefitted since. As a result all of our formulations have been improved to a finite degree, including of course HYPERFLEX.

Although we began use of this versatile material many years ago, its urgency of use in Coal Mining is increasing along with operators needs to remain in compliance in light of MSHA’s increased regulatory activity.

Applications for Seal areas prior to seal installation are to be honest understated, there have been many instances where the expense involved up front would have saved time and money when compared to retrofitting at a later date. This is understandable to some degree as the cost of a given seal has escalated significantly, since we left the use of the 20PSI and progressed to 50 and 120 PSI ratings for the units. Nevertheless a more thorough examination of the sites and some predictive engineering as relates to future mining plans and the effect they will or may have on Ground conditions in these critical areas could well save the time and expense of a reactive approach brought about by regulation at a later date.

This being said current reality is that the majority of our work in Seal areas takes place at a point where significant gaseous exchange is occurring, often the leakage has progressed to the point where Roof, Interface and floor are all incompetent to a greater or lesser degree. In addition there is often need for supplementary support in the adjacent entry way, sometimes beyond.

Generally if apparent the worst zone receives primary injection but in instances where it is not obvious we begin with the roof unit and progress downward, there are instances where both injection and some supplementary spot bolting is required although the properties of Hyperflex most often make the latter unnecessary.

Mechanical Strengths

Although as mentioned previously the stated strengths of Hyperflex are measured in a “free rise” situation, the reality of pumping these materials means that when pumped under pressure, they can not expand fully and the density of the cured material increases. For example the free rise density of Hyperflex would be 2.25 #PCF, this gives a compressive strength of 25 PSI, Shear 28 PSI, Tensile 30 PSI. Whereas at 08 #PCF, compressive 70 PSI, Shear 80 PSI, Tensile 85 PSI. So physical strength characteristics as may be expected increase along with Density.

Pumping tests performed in roof strata, where crack systems are well defined, with an input pressure of 1,000 PSI (having subtracted pumping losses); give an average cured in place Density of 40 # PCF, which provides a compressive strength in the region of 2,000 PSI, with very high adhesion values. This provides very competent support in unstable roof conditions while still allowing a large margin for movement without "Compromising the materials sealing ability".

Hyperflex's advantages as a Sealing/Support Medium.

  • As a Single Component Material, equipment and application are simple
  • Requires only 2 % Moisture to activate
  • Will Migrate in excess of 20 LFt
  • Extremely low reactive pressure (8 - 10 PSI), (competitive materials exhibit in excess of 50 PSI)
  • FLEXIBLE, absorbs movement, (300% Elongation)
  • "Self-Injection", following cessation of pumping
  • Viscosity drop during reaction allows penetration of hairline fractures and the pores of coal
  • Extremely tenacious adhesion
  • Foaming action promotes economic filling of void space
  • Increased Density as desired with pumping pressure variance provides extremely competent support
  • Due to Pre-Polymeric nature significantly reduced NCO exposure hazard (Isocyanate)
  • Closed cell structure
  • N.S.F. 61, Approved for use in Drinking Water Applications

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