DEVELOPMENT OF FAST SETTING PUMPED FOAMED CEMENT PACKS. R.M. Smart Minova RSA, Technical Director A.T. Harrison Minova RSA, Consultant

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1 DEVELOPMENT OF FAST SETTING PUMPED FOAMED CEMENT PACKS ABSTRACT R.M. Smart Minova RSA, Technical Director A.T. Harrison Minova RSA, Consultant Foamed cement packs offer a viable alternative to conventional timber or other forms of packs for permanent underground support. These packs comprise steel mesh and hoop containment with an outer bag for the grout; woven, laminated polypropylene fabric being ideal for the purpose. The grout is supplied in 25kg bags and fed into a purpose designed foam placer machine which delivers the foamed grout directly to the packs from 60 metre long delivery hose. Foamed cement grout as a fill material offers attractive properties for support packs. In summary these are: Variable Grout Strength Grout strength is controlled by the grout density being delivered from the placer machine. This can be changed by the machine settings. Strength Development Fast rate of strength development is possible so that support is available quickly. Pack yield The foamed grouts are compressible so that yielding of the grout pack can be achieved without loss of support. Rate of Filling Three sizes of placer machine are available. These use one, two, or three 25kg bags per minute of grout. Pack filling time is therefore a matter of minutes, a typical 0.75m diameter pack x 1.2m high taking some 17 minutes to fill. Tests in a 1000 tonne press have confirmed that the above characteristics can be achieved with foamed cement packs at acceptable cost. INTRODUCTION Grout packs filled with foamed cement may offer a viable alternative to conventional timber, composite, or grout packs for permanent underground support. Foamed cement is currently used in the form of low density blocks for the construction of packs, usually a combination of foamed cement and internal reinforcing steel cage and fibre. A number of proprietary products of this type are available. Cement-based packs offer the immediate advantage of increased safety, removing the risk of underground fires, an ever present hazard with conventional timber. Page 159

2 Progress towards a foamed cement pack system offering advantages over cement packs constructed from discrete foamed cement blocks is being made at MINOVA RSA. A system is being developed in which the cement material for the packs will be delivered underground in 25kg bags to a foam generating machine which will deliver the foamed cement through a delivery hose directly to the packs. With any grout pack, containment design is critical to performance of the pack, this is particularly the case with this pumped cement pack and this aspect is still undergoing development. top ring reinforcement loops mesh outer containment bag bottom ring cartwheels Fig 1 Foamed cement pack containment Foamed Cement Grout Packs The foamed cement grout pack comprises the following components as in Fig 1 above. - Containment / reinforcement 1) Bag : woven laminated seamless polypropylene, 125g/m 2, 0.75m diameter x 1.2m high. 2) Mesh : 3.15mm x 10cm galvanized mild steel diamond mesh. 3) Reinforcement : 25mm x 5mm flat bar mild steel hoops or cartwheels. 4) Top / bottom rings : 8mm square section mild steel Page 160

3 - Grout A fast setting cement with an aerating additive is used, supplied in 25kg bags, when fed into the machine, a pumpable fast setting foamed cement is produced. Foamed Cement Placing Machine This is shown in Fig 2. The placing machine uses a fixed rate of feed, either one, two, or three 25kg bags of cement product per minute. A flow valve and meter allows variable water : cement ratio to be used thus allowing a range of cement foam densities and hence foamed cement strength to be achieved. Pack Support Requirements Fig 2 Foamed Cement Placing Machine These are summarized in Fig 3. At 10% closure (deflection) a load bearing capacity of 150 tonnes minimum is required, with yield characteristics on closure lying within the shaded area. In some applications a minimum load capacity at 10% closure of 100 tonnes may be acceptable. Page 161

4 Fig 3 Pack support requirements Results to date Effect of hoop reinforcement cartwheel pack This is summarized in the following table and in Fig. 4. Test No. Hoop Containment Water : Powder Ratio Relative Density Load at 50% Displacement Standard Reinforced 1: t 2: t Using a grout with a density of 1.25 in a pack comprising five unreinforced hoops, 2 hours after filling the packs, significant step-wise load shedding occurs between 180mm and 350mm of closure. This is due to the consecutive rupturing of the 3 central steel hoops with the loss of some 30 tonnes of load each time, (line 1 in fig 4). Page 162

5 Fig 4 Effect of hoop reinforcement In contrast, in the pack with five cartwheel hoops, no such load shedding occurs, (line 2 in Fig 4), allowing a more acceptable support performance to be achieved, some 150 tonnes load at 10% and 40% closure (480mm). The reason for the improved performance is that when rupture of the circular hoops takes place, always at the central hoop first, tensile stress is lost on only one quadrant of the hoop, the remaining quadrants of the hoop being held in place by the cartwheel reinforcement which is embedded in the grout. Effect of Foamed Grout Density By adjusting the relative density of the grout by control of the water : cement ratio, and testing at 2 hours after placement, variable pack performance in terms of load bearing capacity can be achieved as illustrated in Fig. 5. Page 163

6 Further Development Fig 5 Pack load vs. cement foam density The concept of a pumped foamed cement pack is still under development, in particular the following: - Containment : this is still somewhat cumbersome and heavy, and lighter materials are under investigation. - Reinforcement : means of combining the cartwheel concept into the containment presents problems of design and these are being addressed as part of the containment development. Page 164

7 CONCLUSION From work carried out to date, a foamed cement pack offers a viable alternative to conventional timber packs provided identified limitations can be overcome effectively. Page 165

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