GEOTECHNICAL INVESTIGATIONS, WITH

PARTICULAR REFERENCE TO BUILDINGS

Dr Johan Lourens February 2015

Abbreviated Experience

• Worked as geotechnical specialist for 37 years for BKS (Pty) Ltd, nowadays Aecom Africa.

• 15 years as head of the Geotechnical Division.

• 9 years on my own (“own” a misnomer – networking with engineering geologists and structural engineers.)

Overview of Presentation

• What is meant by: “Geotechnical investigation” ?

• Why is a geotechnical investigation necessary?

• Brief overview of geology • Problem soils • What to expect in the geotechnical report

What is meant by a “geotechnical investigation”? Several ways to go about it

TLB back actor investigation

• Test pits up to ± 2,7m deep. • Get into test pit, profile in

situ and take disturbed & undisturbed samples.

• Profile: describe soil i.t.o. moisture content, colour, consistency, origin, etc.

• Soil samples to the laboratory testing

Large diameter (750mm) auger drill

• Profiler descends into hole to max. depth of ± 35m, to profile in situ and retrieve samples.

Profiler ready to descend into auger hole

Core drill to retrieve soil & rock samples

• High water tables, unstable fills, etc. - in situ profiling unsafe.

• Need rock cores for high rise building pile foundations.

• In situ testing essential.

• Expensive.

Percussion drill

Always utilised on dolomite areas

• Drill driven by an air compressor

• Disturbed samples, soil & rock chips.

• Record penetration rate of drill (material consistency) and air loss (void indication).

• Engineering geologist to inspect and record chips.

Why a geotechnical investigation?

To avoid: • Typical expansive or collapsible

soil problem

Settlement on dolomite area

Dolomite sinkhole

Geological Maps

A brief overview

Extract from East Rand Geological Map

• Plot site on map. • First identification of

rocks/soils to be expected.

• Granite (collapsing sand) • Lava (deeply weathered,

expansive) • Quartzite (shallow rock,

sand) • Dolomite (sinkholes)

Problem soils/rocks with reference to buildings

• Expansive soils – swell & shrink below building • Collapsible soils – sudden settlement • Soft clays – long term settlement • Dolomite – sinkholes & dolines (saucer shaped

depression) • Dispersive soils (dams) • Pedogenic materials (cemented materials; variable conditions)

Expansive soils

• Heave and shrinkage – a recurrent problem. Heave varies from a few mm to 100mm+.

• Residual soil (residual: decomposed rock – dig and parent rock will eventually be reached), derived from basic igneous rocks: basalt, andesite, norite, dolerite, etc.

• Transported soil (i.e. transported by gravity and water), typically fine colluvium and alluvium.

Collapsible soils

• Settlement due to collapsible grain structure; a “honey comb” structure, i.e. relatively large voids in the soil, visible with a magnifying glass.

• Soil appears “hard” when dry.

• Occurs after wetting: soil softens and large voids collapse.

• Often a once only occurrence.

Soft clays

• Settlement can take months or even years to approach the max. values.

• Predominantly in coastal areas • Lagoon environments of main

rivers • Flood plains of the tributaries and

the main rivers • Rivers changed their courses over

thousands of years • Sediments are not now necessarily

related to the present courses

Dolomite Pinnacles • Dolomite: carbonate rock goes into solution in acidic percolating groundwater • Chert residuum sagging between pinnacles • Chert: Insoluble residue left from weathering of dolomite

Dolomite Pinnacles Viewed from above

Dolomite sinkholes

• Solution of dolomite along network of joints, fractures, faults by percolating acid water.

• Pinnacles, slots and chambers form. Widening of joints. Equilibrium situation.

• Lowered water table – leaking pipe or concentrated rain water

• Soil in slot erodes • Successive arches

form, until eventual sinkhole

Dolomite dolines

Wad = weathering product of dolomite; black & highly compressible

• Ancient doline filled with transported material – not visible on surface. Equilibrium.

• Water table is lowered. Consolidation of wad (compressible material) and settlement of surface starts. Cracks at periphery of doline.

• Progressive consolidation of wad. Progressive subsidence at surface.

• Wad completely consolidated – doline development complete.

What to expect in a Geotechnical Report

• Factual Part: factual information collected, i.e. introduction, scope of the work, ground conditions encountered, site plan, trial hole positions, profiles and test results.

• Interpretative part: Calculations and analyses. Problem soils/rocks identified, i.e. swelling, shrinking and collapsible soils, subsidence, landslides, unfavourable rock dips, etc. Discussions on the various foundation/construction options. Recommendations on the most economic foundation system to use.

References

• Engineering Geology of Southern Africa. A.B.A. Brink. Volumes 1 – 4. Building Publications Pretoria.

• Identification of Problematic Soils in Southern Africa. Technical Notes for Civil and Structural Engineers. Department of Public Works. June 2007.

That’s it, thank you!