development of fast setting pumped foamed cement r.m. smart – minova rsa… · 2009. 10. 15. ·...

SAIMM, SANIRE and ISRM

6th

International Symposium on Ground Support in Mining and Civil Engineering Construction

R M Smart and A T Harrison

59

DEVELOPMENT OF FAST SETTING PUMPED FOAMED CEMENT

PACKS

R.M. Smart – Minova RSA, Technical Director

A.T. Harrison – Minova RSA, Consultant

ABSTRACT

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.


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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.

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/m2

, 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

top ring

reinforcement loops

mesh

outer containment bag

bottom ring

cartwheels


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- 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.

Fig 2 Foamed Cement Placing Machine

Pack Support Requirements

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.


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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: 5 - 0.5 1.25 80t

2: - 5 0.5 1.25 227t

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).


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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.


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Fig 5 Pack load vs. cement foam density

Further Development

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.


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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.


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development of fast setting pumped foamed cement r.m. smart – minova rsa… · 2009. 10. 15. ·...

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