Events & Courses

The course will begin with a discussion of geologic causes of ground collapse and the relationships between void space in solid strata, the behaviour of different types of overburden, and groundwater. Several case studies and examples from the eogenetic Florida karst, (the “Sinkhole Capital of the World”) will be reviewed and compared / contrasted with more mature  UK karst conditions in limestone, Chalk, and evaporites. The challenges associated with arresting and repairing geologic sinkholes in an urban environment will be explored. Using Florida examples, we will review compaction grouting, underpinning, and chemical (polymer) grouting as repair techniques for geologic (karst)  sinkholes. We will also examine current UK practices for stabilization of karst-impacted properties.

We will then discuss causes of ground collapse in non-karst geology, including lava tubes, pseudo-karst, and collapsible soils, and explore strategies for stabilization and remediation.

Anthropogenic causes of ground collapse are often linked to mine entries (shafts and adits) and where, mining is shallow, collapse of open workings. Old, hand-excavated wells present similar challenges to mineshafts. The differences and similarities with geologic sinkholes will be addressed along with strategies for repair and remediation. 

Nested voids in artificial fill / made ground are also a common cause of ground collapse in urban areas. The soil mechanics and drainage relationships of voids in made ground will be discussed. Examples of ground collapse caused by piping erosion due to groundwater seepage into basements, dry docks, and other underground structures will be presented. 

Underground services / utilities are also a common cause of “sinkholes”, particularly during extreme weather events, with stormwater drainage systems and pressurised water mains being particularly prone. The relative risk to life and property of collapses causes by stormwater drains is often higher than that from geologic sinkholes.

Armed with the knowledge of the various mechanisms of ground collapse, how should one respond to that “emergency callout”? We shall discuss basic elements of public and first responder safety, drawing upon the presenter’s work with Florida Fire & Rescue Departments following the fatal Seffner sinkhole of 2013 and comparing with current UK practice. Methodologies to assess and rank risk to property and infrastructure will be discussed along with first-choice options to minimize ongoing damage and cost. Rapid geotechnical and geophysical techniques to delineate areas at risk of future collapse or subsidence will be reviewed.

Finally, we will summarise ways in which the geoscientist can be proactive in assessing the risk of collapse before it happens, identifying signs of antecedent ground strain, and designing both intrusive and non-intrusive explorations that will provide geotechnical data both safely and without initiating a collapse.

1. Build a background appreciation of what causes a collapse of the ground surface.

2. Understand the mechanics of void migration to the surface and the relationship between underground void space and overburden.

3. How to be a “first-responder” geoscientist and minimise risk to life and property following a ground collapse.

4. Choosing appropriate geotechnical and geophysical tools to delineate unstable ground.

5. Explore methods of temporary and permanent ground stabilization.

6. Identifying ground collapse before it occurs and choosing remedial strategies.

PART ONE: “The ground has collapsed! What caused that to happen?”

-      Understanding the Ground Collapse hazards associated with your local geology.

-      Formation of natural void space in rocks.

• Karst in limestone, Chalk, and evaporite deposits.
• Lava tubes, pseudo-karst, and collapsible soils.

-      Anthropogenic void space in rocks.

• Open coal, evaporite, and metalliferous mine workings.
• Voids associated with mine entries, tunnels, and wells.

-      The relationship between overburden soil and voided rock.

• Tensile strength and pressure-arching
• Static or dynamic groundwater conditions

-      Voids associated with urban development.

• Underground services / utilities
• Basements, septic tanks, dry docks & waterfront structures
• Piping erosion
• Nested voids in made ground.

Break: 20 minutes

PART TWO: “How do we fix this? Will it happen again?”

-      The Emergency Callout – life safety and damage control.

• People are naturally curious – especially geoscientists!
• Safe cordons, working with local authorities and emergency services.
• Protecting property and minimising economic disruption.

-      Strategies for temporary stabilization.

• Managing utility damage
• Bulk fill.
• Deliberate collapse.

-      Void management

• Drilling & grouting.
• Cementitious compaction grouting
• Chemical / polymer grouting.

-      Structure stabilization and surface repair

• Underpinning
• Polymer injection
• Concrete caps

-      Proactively avoiding further collapse

• Recognising ground strain prior to collapse
• Non-intrusive explorations with geophysics.
• Safety concerns with intrusive ground investigation.