Mine Dewatering and Depressurisation getting it right......
......and what happens when we don’t Jon Hall (Technical Director – Mine Water Management...
Mine Dewatering and Depressurisation getting it right......
......and what happens when we don’t Jon Hall (Technical Director – Mine Water Management) AIG Groundwater in Mining Conference – Adelaide, rpsgroup.com.au May 2014
Outline of presentation
Overview (high level) of: » » » » »
what is dewatering & depressurisation? why do we (need to) do it? how do we plan and implement it? what happens when we get it wrong? how can we stop it going wrong?
Some examples: » from sites (who shall largely remain nameless) » for more details - see me in the bar later rpsgroup.com.au
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Dewatering/Depressurisation What are they? How are they different? Why do we do it? rpsgroup.com.au
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Dewatering – what is it?
It’s all about draining (de-saturating) the soil/rockmass
Lowering the water table – primary aim
Generally involves “large” abstraction rates
Also results in reducing pore pressure: » ie dewatering will result in depressurisation
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Dewatering – why do we do it? To reduces “wet” pit problems
To reduce “wet” UG mine problems
Can also help reduce pit or UG stability problems: » promoting/inducing drainage/depressurisation of footwalls/hanging walls » but only if conditions are right
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Wet and boggy pit floors/ramps: - trafficability - wet ore/waste - wear and tear - safety
Is all about reducing pore pressure Lowering the potentiometric surface
Does not necessarily require de-saturation: » confined conditions – depressurisation does not result in dewatering » unconfined conditions – depressurisation = dewatering
Generally does not require large drainage rates rpsgroup.com.au
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Some pore pressure 101 – what is pore pressure?
Source: Guidelines for Open Pit Slope Design (Chapter 6)
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Pore pressure profiles – impacts of flow to pit
Source: Guidelines for Open Pit Slope Design (Chapter 6)
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Depressurisation – why do we do it?
Directly improves pit wall (and UG) stability: » Allows us to mine safely – work with existing slopes/designs » Allows us to steepen up pit slopes – improve strip ratio » Allows us to mine deeper – access to more ore » Allows us to increase unsupported areas of UG mining
How does it help? » Refer Rock Mechanics 101
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Some rock mechanics 101
Rock mass may fail in three broad ways
1: Rock (or soil) mass controlled failure » Failure surface develops within rock mass » Dependent on properties of rock mass - intact rock strength » Excess pore pressure can: reduce intact rock strength – eg development of “slip circles” overcome rock strength – eg floor heave or rupture
» Reducing pore pressure – reduces risk of failure rpsgroup.com.au
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Rockmass Failure - slip circle developed due to excess pore pressure
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Rockmass Failure - slip circle developed due to excess pore pressure
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Rockmass Failure - slip circles developed due to excess pore pressure
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Rockmass Failure - pit floor heave/rupture due to excess pore pressure rpsgroup.com.au
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Some rock mechanics 101
2: Structurally controlled failure » Failure occurs along existing defects (structural features) in rock mass » Dependent on strength/orientation of structures (shear strength): Shear strength ≈ resistance to shear stress and resulting strain (failure)
» Reducing pore pressure results in increased shear strength due to: increase in effective normal strength – reduction in bouyancy increase in friction angle and cohesion – reduction in bouyancy and lubrication
» Reducing saturation (ie dewatering) also results in: increase in friction angle and cohesion – reduction in lubrication rpsgroup.com.au
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Structural Failure - reduced shear strength due to high pore pressures
Structural failure - reduced shear strength due to high pore pressures rpsgroup.com.au
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Some rock mechanics 101
3: Partial structural control failure: » Combination of rock mass and structural failure » Often starts as one and then develops into combined failure: Typically starts as rock/soil mass failure But can be other way around
» Post failure inspection often reveals “hidden” structures
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Combined rockmass/structural failure: reduced shear strength and development of slip circles due to pore pressures #1
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Combined rockmass/structural failure #2
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Combined rockmass/structural failure #3
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Combined rockmass/structural failure
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Depressurisation – why do we do it?
Reducing pore pressure can also result in: » Reduced hydraulic gradients and reductions in inflows » Mainly for underground mines
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Underground inflow from cable bolt hole due to excess groundwater pressure
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Underground inflow from vertical drill hole due to excess groundwater pressure
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How do we do it? Broad dewatering/depressurisation approaches
How do we select the appropriate approach?
NB: “Dewatering” = dewatering and depressurisation for rest of presentation rpsgroup.com.au
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Dewatering approaches
Active (or advanced): » » » »
dewatering ahead of mining pumping from bores active pumping from deep sumps in pit base drainage adits/galleries ahead of mining
Passive (or reactive): » natural pore pressure dissappation through seepage » shallow in-pit sumps or catch drains on berms
Hybrid (bit of both): » drainage adits/galleries after mining commences » drain holes in pit walls and UG floors/backs rpsgroup.com.au
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Inpit dewatering bores
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Horizontal drain holes
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Pit sumps and natural seepage/drainage
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Extensive inpit dewatering (bores & sumps)
Extensive footwall and hanging wall depressurisation
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Passive seepage from whole hanging wall (and some drain holes) – pit wall remains largely pressurised
Active (high rate) dewatering of limestone footwall by perimeter bores – pit wall drained and no residual seepage except at toe of pit or residual pore pressures.
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Depressurisation hole drilling ahead of decline (cover drilling)
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Key factors in selecting dewatering methods
Mine plans: »
how deep
»
how quick
»
what shape
Hydro-geotechnical conditions: »
pit wall properties
»
regional properties rpsgroup.com.au
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Mine Plans
These have many drivers: » mining (achieving what is possible/practical/safe) » processing (meeting blend requirements) » commercial (meeting performance projections) » marketing (meeting customer requirements - blend & schedules)
They can (and do change) change: » regularly & often at short notice
We normally have direct input to only one of these (mining) rpsgroup.com.au
volumes that need to be pumped residual moisture (how will drained material behave) when to start pumping (or drainage)
Recharge and throughflow: »
pumping rates & bore/drain separation when to start pumping (or drainage)
maintenance pumping or drainage
We can’t change these (mostly) – we need to live with them rpsgroup.com.au
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Impacts of mining on hydro-geotech conditions
Blasting: » induces “damage” in sub-blast zone (increased K)
Lithostatic unloading: » » » »
as a result of overburden removal: decrease in total stress leads to dilation/expansion (and increased K) initial drop in pore pressure as voids open up subsequent re-bound as voids refill
Hydrostatic unloading: » as a result of dewatering of overburden » rare…..(but has been planned at one “local” and very large pit) rpsgroup.com.au
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Impacts of unloading
Source: Guidelines for Open Pit Slope Design (Chapter 6)
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Matching methods to conditions ……some open pit examples
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HIGH RATE PUMPING
PRE-PUMPING WATER LEVEL
REGIONALLY EXTENSIVE DRAWDOWNS PUMPING WATER LEVEL
Extensive high K aquifer
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PRE-PUMPING WATER LEVEL
DEWATERING WATER LEVELS DRAINS DRAWDOWNS RESTRICTED TO IMMEDIATE PIT AREA SUMPS
Extensive low K aquifer
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PRE-PUMPING WATER LEVELS HIGH PERMEABILITY
PUMPING PUMPING WATER LEVEL LOW PERMEABILITY
Multiple aquifers: high K over low K (eg alluvium over tight basement) rpsgroup.com.au
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PRE-PUMPING WATER LEVELS WATER LEVEL – UPPER AQUIFER
LOW PERMEABILITY DRAINS
HIGH PERMEABILITY
PUMPING WATER LEVEL – LOWER AQUIFER
Multiple aquifers: low K over high K (eg saprolite over transition zone aquifer) rpsgroup.com.au
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PRE-PUMPING WATER LEVELS DEWATERING WATER LEVEL DRAINAGE PUMPING
PUMPING WATER LEVEL
AQUITARD
Orebody aquifer: bounded by low K FW/HW (eg paleochannels, shear hosted gold, Pilbara Fe deposits) rpsgroup.com.au
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Mixed aquifers
Source: Guidelines for Open Pit Slope Design (Chapter 6)
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Source: Guidelines for Open Pit Slope Design (Chapter 6)
HDH drainage
Structurally segmented aquifers rpsgroup.com.au
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What can (and often does) actually happen How it can go wrong What we can learn when it does rpsgroup.com.au
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Not dewatering at all (or starting too late)
Not recognising the need (or as an afterthought only) » project being developed by “non miners” » inappropriate/incomplete mine planning investigations
Dewatering plans lagging behind accelerated mine plans: » mine plans accelerated to meet changing project requirements » no accounting for dewatering/pressurisation lead times
Adopt passive approach (where active/hybrid is really required): » assume that pit walls will drain as pit develops rpsgroup.com.au
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Ksap=10-1m/d
Ksap=10-2m/d
Time for natural drainage to pit rpsgroup.com.au
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Ksap=10-3m/d
Ksap=10-4m/d
Time for natural drainage to pit rpsgroup.com.au
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Ksap=10-2m/d
Ksap=10-4m/d
Time for natural drainage to pit (enhanced by horizontal drain holes) rpsgroup.com.au
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Monitoring and data interpretation
Not monitoring at all: » therefore no way to check/validate and/or improve performance
Not interpreting monitoring data correctly: » not recognising the warning signs » not monitoring or recognising the right indicators: eg the measure of depressurisation is pore pressure reduction…NOT drainage volumes rpsgroup.com.au
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Drain holes made only very minor water...BUT… drainage was very effective in reducing pore pressures.
Monitoring (and interpreting) the right data rpsgroup.com.au
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Inflexible mine management Demanding performance (quite rightly) but not allowing solutions: » “I need the pit dry, but I don’t want any in-pit bores” » “I don’t want drain holes above the ramp, they make the ramp boggy”
» “We need to increase production, but also to reduce your operating costs” » “ We’ve had a dewatering plan since Day 1, why do you want to change it” » “I’ve been mining this type of orebody all my life…and we’ve never needed
to do this (dewatering/depressurisation) before”
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“Undoing” dewatering – the BIGGEST SIN
Achieving dewatering targets takes: » a lot of effort – to get systems in place (refer inflexible management) » time & resources
The good work can be (and often is) undone by: » unexpected events – but contingency plans should be in place » poor overall mine water management practices*
* these are usually adopted “unconsciously” of the impact on dewatering rpsgroup.com.au
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Discharge to pit floor or berms can lead to……. rpsgroup.com.au
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Pit wall collapse due to re-saturation
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Re-saturation of unstable hanging wall by seepage from dam
Operating “leaky” dams on benches or crests rpsgroup.com.au
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Re-saturation of fault zone by seepage from dam
Operating “leaky” dams at pit crest rpsgroup.com.au
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Leaking transfer station on berm……..can lead to rpsgroup.com.au
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Shallow water level maintained by seepage from transfer station
Piezo Pzgt 90 Water Level (mRL)
200.00 190.00
Water Level (mRL)
180.00 170.00 160.00 150.00 140.00 130.00 120.00
Water Level (mRL)
Groundwater levels not affected by seepage
12-04-2006
12-03-2006
12-02-2006
12-01-2006
Pit Water Level Rising
12-12-2005
E < Q (+I)
12-11-2005
12-10-2005
12-09-2005
12-08-2005
12-07-2005
Pit Water Level Rising
12-06-2005
E < Q (+I)
12-05-2005
12-04-2005
12-03-2005
110.00 E = Q (+I)
Pit Water Level Rising
Date
Pit wall pore pressures affected by seepage rpsgroup.com.au
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Operating “leaking” diversions near crests can lead to…… rpsgroup.com.au
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Pit wall failure due to re-saturation of pit wall rpsgroup.com.au
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Inadequate maintenance
Depressurisation infrastructure needs maintenance: » bores, drainholes, drains, sumps, adits » pumps » reticulation – water & power
A “no brainer” – you would think, but often neglected: » out of site out of mind
» budget constraints/ inflexible management » inexperience of operators – eg “it’s pumping water so it must be OK rpsgroup.com.au
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So, how do we get it right? a summary of the key factors
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Must start early: » » »
Dewatering/depressurisation take time investigations take time procurement and installation take time
Manage external factors » » » »
surface water diversions mine water storage ponds/dams old drill-holes recirculation within the pit
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Lined drainage channel along haul ramp
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Lined drainage channel along haul ramp
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Lined and rock filled drainage channel under haul ramp
