Development of Guidelines for Development of Guidelines for Construction and Development on Construction and Development on Dispersive Soils.Dispersive Soils.

Presented By Marcus Hardie• TIAR, UTAS, DPIW, CSIRO.

Background to Dispersive Soils• What is a dispersive soil• Why are dispersive soil important• Potentially dispersive soils in Tasmania• Case Studies of problems associated with

dispersive soils.• Management Approach• Purpose of Workshop.

Problems withProblems with SodicSodic / Dispersive soils./ Dispersive soils.

Erosion• Gully erosion• Tunnel erosion• Dam collapse

Water Quality• Extreme turbidity (never settles) • Sediment deposits

Construction / Development• Collapse of Fill• Pipe & Cable Collapse• Breaching Septic Trenches• Dam Failure

What is a What is a Dispersive SoilDispersive Soil

Dispersive Soils• Disperse into basic particles sand, silt and

clay in fresh water.• Appear to ‘dissolve’ in water.• Results in cloudy ring in fresh water• Responsible for tunnel erosion.• Dispersive soils are usually sodic.• Structurally unstable, require special

consideration for development.

SodicSodic Soils & Dispersion.Soils & Dispersion.Sodic soils are prone to dispersion, collapse of clay structure in the soil.

Dispersion: Process where individual clay platelets separate from clay structures.

Normal: Clay Platelets are bunched together.

Dried Aggregates in Rainwater

What is aWhat is a SodicSodic SoilSoil

• ESP Greater than 6

Exchangeable Sodium (ESP) = Exchangeable Na x 100 Percentage Cation Exchange Capacity

Definition of sodic soilsNon-sodic - ESP <6SSodicodic -- ESP ESP 66--1414Strongly sodic - ESP ≥15

National Snapshot.National Snapshot.

• 200 million ha affected by sodicity• 5 times more sodic affected land, than saline land.• Affected area 90 million ha.• Estimated yield loss for wheat alone $ 6.75 Billion(National Audit soil sodicity)

Sodosols

Approx. 30% of

Australia is sodic

Dispersive Soils in TasmaniaDispersive Soils in Tasmania

• Never been specifically Mapped !!!• Occur on a range of Rock Types.• Occur in a range of soil types.

Geology Includes• Tertiary Clays• Triassic Sandstones• Permian Mudstones

Vertosol

Difficult to Infer from Existing Soil and Geological Maps

Kurosol

Sodosols

Potentially Potentially Dispersive Dispersive Soils in Soils in TasmaniaTasmania

(Soil Classification Approach)

DRAFT

Potentially Potentially Dispersive Dispersive Soils in Soils in TasmaniaTasmania

(Soil Classification Approach)

Tunnel erosion is found in all council areas in southern Tasmania.

Potentially Potentially Dispersive Dispersive Soils in Soils in TasmaniaTasmania

(Land Systems Approach)

Activities that increase likelihood of Activities that increase likelihood of tunnel erosion / soil dispersion.tunnel erosion / soil dispersion.

• Overgrazing• Removing topsoil• Excavation of dispersive soil• Poor compaction of sodic clay• Concentration of runoff• Septic trenches• Drains and Culverts

Prevention Strategies• Awareness & education• Avoid ponding water• Topsoil• Burial• Compaction• Gypsum• Pasture / land management

Over GrazingOver Grazing

Honeywood Estate: Brighton.

Over GrazingOver Grazing

Poor Compaction & Dam FailurePoor Compaction & Dam Failure

Tunbridge: Blackman Crk. Dam

Penna: Permian Mudstone.

Poor Compaction & Dam FailurePoor Compaction & Dam Failure

Honeywood Estate: Brighton.

Removal of TopsoilRemoval of Topsoil

Brighton

Installation of Installation of Cables / PipesCables / Pipes

Chain of Lagoons: Granite

Dunalley : Dolerite / T. Sandstone

Drains and CulvertsDrains and Culverts

Honeywood Estate: Brighton

Drains and CulvertsDrains and Culverts

WoodbridgeBruny Island

Drains and CulvertsDrains and Culverts

Honeywood Estate: Brighton

Excavation, Cut & FillExcavation, Cut & Fill

Richmond: Vineyard Factory

Excavation, Cut & FillExcavation, Cut & Fill Woodbridge.

Clarence: House construction on existing tunnel system.

Tasman Hwy: Near Copping

Septic SystemSeptic System

Honeywood: Triassic Sandstone

10 + Years After 10 + Years After DisturbanceDisturbance

Woodbridge: Permian Mudstone

Sedimentation & Hyper TurbiditySedimentation & Hyper Turbidity

Honeywood: Triassic Sandstone

Just Seems to HappenJust Seems to Happen

Woodbridge: Permian Mudstone

Tunnel Driven Gully ErosionTunnel Driven Gully Erosion

Mt. Rumney

Repair of Tunnel ErosionRepair of Tunnel Erosion

Dunalley• 380 meters long• Excavation• Repacking• Gypsum• Sand blocks• Fencing /sowing• 6 people 5 weeks• $75,000

Repair of Tunnel ErosionRepair of Tunnel Erosion

Before

Penna.• 10m (75m) Long• Excavation• Refilling• Gypsum• Sandblocks• (Poor compaction)• $ ?? 75 Meter Long excavation

TakeTake--Home Messages.Home Messages.• Current mapping is unreliable.

• Tunnel erosion occurs in all municipalities in southern Tasmania.

• Hazard is potentially severe but isolated.

• Be alert not alarmed.

• Focus on Awareness and education.

• Applied research is limited.

Way ForwardWay Forward

• Awareness and Prevention based

approach.

• Engagement Councils & Service

Industries.

• Development of ‘Guidelines for

Construction and Development of

Dispersive Soils’.

• Publication, engagement and extension

Questions for the Discussion Groups.Questions for the Discussion Groups.

1) How would you rate understanding of dispersive soils in

your organisation?

2) How well has your understanding of dispersive soils

developed?

3) What are the risks to your council from dispersive soils?

4) How well is your organisation positioned to address the

risks of dispersive soils?

5) What do you need from a set of guidelines?

Questions for the Discussion Groups.Questions for the Discussion Groups.

6) What approach should the guidelines take: technical,

non technical, both, how big etc, what would you

expect?

7) Who in your organisation should use the guidelines?

8) How would you use the guidelines in your role in your

organisation?

9) Which non-local government industries would you

expect to use the guidelines?