📋 Case Study

Coal Mine Spoil Geochemical Capping at Hunter Valley Reclamation Project

Spoil with pyritic shale interbeds generating ARD despite initial alkaline overburden; inconsistent capping led to localized seeps

🏗️ Project Overview

Hunter Valley coal mine rehabilitation (NSW), 2017–2022

🎯 Challenge

Spoil with pyritic shale interbeds generating ARD despite initial alkaline overburden; inconsistent capping led to localized seeps

🔧 Design Approach

Geochemically zoned capping: alkaline overburden (ANC >150 kg CaCO₃/t) directly over pyritic zones; GIS-integrated waste characterization guided placement; validated with NAG-pH and humidity cell testing

📐 Design Diagram

_max = 1.8 m Humidity Cell pH < 4.0 at 22 wks GIS Waste Mapping Alkaline Cap Pyritic Zone NAG-pH Test Geochemically Zoned Capping Design

AI-generated project design illustration

📐 Key Calculations

Maximum Pyritic Zone Thickness

t_max = ANC / (NAG × density)

Result: 1.8 m

Defined maximum uncapped lift height

Humidity Cell ARD Onset

Time-to-pH<4.0

Result: 22 weeks

Benchmarked capping efficacy

📊 Results

Zero ARD seeps detected post-capping; 98% of monitored points maintained pH >6.0 for 4+ years; cost reduced 23% vs. uniform clay cap

💡 Lessons Learned

•NAG-pH correlation stronger than ABA for humid climates
•GIS-based zonation reduced sampling density by 40% without loss of confidence
•Humidity cell validation was mandatory for regulator sign-off
•Overburden ANC variability required real-time on-site titration
•Cap integrity monitoring via soil moisture sensors detected incipient failure 8 months pre-seep

✅ Key Takeaways

1NAG-pH correlation stronger than ABA for humid climates
2GIS-based zonation reduced sampling density by 40% without loss of confidence
3Humidity cell validation was mandatory for regulator sign-off
4Overburden ANC variability required real-time on-site titration
5Cap integrity monitoring via soil moisture sensors detected incipient failure 8 months pre-seep