In Deep Level Mining, safety standards play a major role in everyday working conditions and it is for this reason that high early strengths in products are utilized to en-sure an early safe environ-ment...
In Deep Level Mining, safety standards play a major role in everyday working conditions and it is for this reason that high early strengths in products are utilized to ensure an early safe environment.
In order to maintain these stringent safety requirements it is important to grout with products that have early anchoring strengths, however you have to consider a number of factors in designing and developing these products namely:
1. Underground ambient temperatures (25 - 35°C)
2. Rock face temperatures (60 °C)
3. Pot life of the product underground
4. Heat of hydration of the grout product
5. Application of the products
6. Storage - minimum 6 months.
New technology has been developed that does away with all of the above problems. The technology is simply known as the cementitious “Spin” Capsule technology.
The technology describes a research programme to study the characteristics of a wet premixed cement and activator system, both housed in a single plastic sheath. The separate components (base and activator) can be stored under fairly extreme conditions for periods up to 6 months without hardening or deteriorating. The product performance relies on the mixing of the two pastes (base and activator) through mechanical spinning. The reaction rate of the finished product once mixed is very rapid, but controllable. It is a well known fact that temperature has a major impact on the chemical behaviour and performance of cementitious grouts. In designing the “spin” capsule which has as one of its components, an activator and therefore an allowance in formulating the capsule had to consider the effect of the high rock face temperatures (60 °C) which in itself acts as an activator. Hence the product has to react normally in ambient temperatures (25 °C) with a pot life of ± 3 - 5 minutes. As the mixing of the product takes place in these high rock face temperatures, the pot life is dramatically reduced to ± 2 minutes. The grout hardens rapidly. Very substantial strengths can be achieved in under 30 minutes.
The “spin” capsule will alleviate the following application problems in the Mining and Civil Engineering fields:
- Pot Life Constraints in high temperature areas (70 °C). The “Spin” capsule has no pot life problems and will only start reacting when the bar is span into the capsule mass.
- Storage Constraints. The spin capsule can be stored in temperatures exceeding 35 °C without any problems.
- Application Constraints. The spin capsule will alleviate the need to first dip the standard capsules in water before the application via applicators or pumps and only requires the insertion of the spin capsule into the drilled holes and mechanical spinning of the bar into place. No specialized application equipment is needed i.e. applicators (guns) or pumps.
- Size of Drilled Holes. There is no restriction on the size of holes to be drilled into the rock face. Annulus between anchor bolt and rock face is not a problem as in the case with resin applications. The number of capsules per hole is dependent upon the size of the hole diameter and the size of the reinforced support bar. The larger the bar diameter the greater the displacement of grout and the smaller the capsule diameter required, the reverse will hold true if the bar diameter is smaller
- High Early Strengths: Under normal conditions the standard high early strength grouts require a Full Hour to attain the specified strengths. In the case of the spin capsule the time taken to meet the required strength is 20 - 30 minutes.
- Over Spin and Under Spin:
(a) Over Spin: If the spin Capsule is subjected to excessive spinning the bond between bar and grout will be affected - bond broken. The fact that the set grout is still very “green” it will re-establish a new bond between the bar and grout. The bond will not be as strong as the original bond but will be sufficient in order to secure the bar in place - a mechanical bond.
(b) Under spin: If the spin capsule is subjected to under spinning the activator is powerfull enough to start the chemical reaction going. The strength gain will be slower but the final performance of the grout will be the same, although it be one to two hours later.