10-waste dump geotech pitsupervisors

GEOMECHANICS AND SLOPE STABILITYCrest (dump point):
Passive loading
Active loading
(Windrow inadequate)
Too much in the one place
(mix materials up)
Loader undercutting toe
CREST FAILURES
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Truck should be at least 4m from the edge of the dump.
Define this distance using a windrow of sufficient height – i.e. a 1.5m high windrow.
This assumes a 37 degree windrow slope angle. Windrows with steep sides and short base length may be inadequate.
4m standoff assumes a stationary load.
Truck hitting windrow increases load on crest.
Need more than 4m = not practical.
Procedures to prohibit using windrow as backstop.
4m
CREST FAILURES
< 4m
Managing the risk – Tip Head Procedures:
Standard 1.5m high windrow is inadequate to stop a moving truck.
For a loaded truck moving at 20km/hr a 6m high windrow with a base length of 15m is required. Not practical.
DO NOT RELY ON THE WINDROW TO STOP YOUR TRUCK!
Waste Dump & Stockpile Safety - Geotechnical Considerations
WINDROW BREACH
Approximate size windrow required to ensure truck stops (reversing at 20km/hr)
Standard Windrow
Examples of windrow breaches:
Hamersley Tom Price, Australia – a 1.5m high windrow on main haul road was hit at an angle by a laden haul truck (nominal capacity 170t) travelling uphill at 10km/hr. The truck rode through the safety bund and down the slope, resulting in a fatality.
Damang Mine, Ghana – a 1.5m windrow on a main haul road was hit square-on by a laden 100t truck, whilst travelling uphill on a ramp with a 10% grade. The truck hit and rode over windrow into a drain on other side
WINDROW BREACH
Saturation of dump by water.
Bad dump geometry (loss of lateral confinement on “bullnoses” or “doglegs”).
CIRCULAR FAILURE
Managing the risks
Dumps (max lift heights) are designed based on the type of material planned to be tipped.
Weak materials mean lower dump lift height. Maximum height for; Very powdery fines = 20m, Weathered shale = 30m, Blocky BIF = 90m +.
Dump lift heights can be re engineered by terracing.
Tip appropriate material for dump design.
Survey dump levels regularly and control dump geometry.
CIRCULAR FAILURE
Managing the risks – dump geometry
Dump face should be gently curved and the tip head as straight as possible. Rate of advance should be constant across dump face.
Avoid developing “bullnoses” or “doglegs”.
Narrow dumps are less stable than wide dumps – if a narrow dump is required – use better quality material or develop a lower dump.
CIRCULAR FAILURE
Tip Head
Tip Head
MORE STABLE
LESS STABLE
Breaking through soft ground
Sliding on soft ground
BHP Billiton Iron Ore
Lower dump heights in areas with poor foundations (reduced loading).
Tip better quality material in areas with poor foundations.
Dedicated stability analysis may be required (contact Snowden).
Define the area of the dump likely to be affected - awareness.
Plan an increased inspection frequency.
Detailed stability analysis is required before tipping into water (e.g flooded pit).
FOUNDATION FAILURE
Dump fails on weak plane.
PLANAR FAILURE
Mix strong and weak material.
Large amounts of weak material should go to a dedicated dump.
Weak material can be paddock dumped across the dump to form a flat layer.
Avoid forming a continuous layer.
Increased risk:
Saturated dump is weaker than a dry dump. Dump design assumes dry dumps.
Rainfall events can cause the dump to become saturated resulting in failure.
Stronger, coarse rockfill such as BIF drains well, shales, clays and fines may not drain as quickly – more caution required.
Water running down cracks can cause failure and washouts.
Managing the risk:
Direct drainage away from the tip head. Dump gradient should dip away from the tip-head.
Keep water away from cracks. Do not let water pond.
When wet – more frequent inspections, may have to tip short and doze.
WATER INDUCED FAILURE
EROSION
Erosion:
Large washouts. Keep away from shoulders
Scalloped dump faces
Stand off next lift or tip short and push with dozer
No Tipping
No Tipping
EROSION
inspect dump faces after rainfall.
Face washouts may not be obvious when the top of the dump is inspected
Washout undercuts crest potential dump point failure.
Standoff next lift10m or more depending on size of washouts.
Potential slip surface
Profile of washout
EROSION
Routine part of pre-start checks by pit supervision.
Operator awareness of the issues.
Step up inspections in wet weather.
Effective lighting on night shift.
Signs of instability:
Piping holes and sinkholes.
INSPECTIONS & MONITORING
Cracks and Sinkholes:
Settlement cracks are not always serious but should be filled in. Avoid letting water drain into cracks
“Steps” formed across cracks may lead to failure. Tip short and doze to re-establish gradient and windrow.
Sinkholes – risk of tyre / suspension damage or rollover.
Can appear well behind the tip head.
Windrow construction (rock type & height) to site procedures vital.
For weak materials build bigger windrows.
Don’t back into the windrow.
Correct material allocation is the primary control of dump failure:
Max dump lift height 20-30m for fines and weak shales.
No restrictions for coarse blocky BIF.
The risk of dump failure also increases if:
Coarse and fine material is mixed incorrectly (planar failure).
Dump is wet (eg heavy rainfall or water tank overflow).
Dump is tipped onto sloping or soft ground (e.g. a swamp, tailings, mud, water etc.
Report any signs of instability to your Supervisor.

10-waste dump geotech pitsupervisors

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