Chris Gorse

Advanced Construction Technology

By Professor Chris Gorse & Ian Dickinson – licensed under the Creative Commons Attribution – Non-Commercial – Share Alike License

http://creativecommons.org/licenses/by-nc-sa/2.5/

Ground and soil stabilisation

Chris Gorse and Ian Dickinson

These slides should be read in conjunction with Emmitt, S. and Gorse, C. (2010) Barry’s Advanced Construction

of Buildings. Oxford, Blackwell Publishing

Chris Gorse

General problems of ground instability include:

• Landslip• Surface flooding and soil erosion• Natural caves and fissures• Mining and quarrying• Landfill• Natural geological variation – faults,

changes in geology – differential settlement

Chris Gorse

Improving the ground

• There are a number of different methods that can be used to increase the strength and stability of the ground.

Chris Gorse

Ground stabilisation

• Dynamic compaction• Vibro compaction - Vibro displacement • Vibro flotation - high pressure water jets

(improves penetration of hard substrates)• Pressure grouting • Surcharging• Geotechnic membranes• Soil modification and stabilisation

Chris Gorse

Dynamic compaction

• This involves dropping heavy weights onto the ground.

• The weight causes the ground to compact.

Chris Gorse

Dynamic compaction

• Ground is consolidated by repeatedly dropping dead weights and specially designed tampers

• Weights include: Flat bottomed and cone tampers• Traditional weights are flat bottomed with cable• Modern systems use cones with guide rails• Dynamic compaction is suitable for granular soils,

made-up and fill sites• Using dynamic compaction bearing capacities of

50 to 150kN/m2 can be achieved

Chris Gorse

Dynamic compaction

Typical weight (mass) 7-11 tonnes

Tamer drops and exerts known impact energy on strata

Pass 2 Pass 2 Zone compacted 2nd Pass

Zone compacted 1st Pass

Pass 1 Pass 1

Zone compacted 3rd Pass

Sound strata

Pass 1 and pass 2

Pass 3

50 – 150 kN/m2 Typical bearing capacity

Required treatment depth

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Typical cone type tampers (adapted from www.roger-

bullivant.co.uk) Long cone

Flower pot cone

Multiple point cone

Used for densifying deep layers of strata

Consolidates strata closer to the surface

Typical weight (mass) 7-11 tonnes

2.5m

Traditional weight

10 – 20 tonnes Energy does not penetrate the ground as much as the cone weights

Chris Gorse

Dynamic compaction rig

Chris Gorse

Vibro compaction or displacement

• Vibrating rods are forced into the ground causing the surrounding ground to compact and consolidate.

Chris Gorse

Vibro compaction or vibro displacement

• Vibrating mandrels (poker, shaft or rod) penetrates, displaces and compacts the ground.

• Void Created is filled with stone and recompacted• Produces stone columns in the ground, compacts

surrounding strata enhancing the ground bearing capacity and limiting settlement

• Typical applications include support of foundations, slabs, hard standings, pavements, tanks or embankments.

Chris Gorse

Vibro compaction - continued

• Used in soft soils, man made and other strata, can be reinforced to achieve improved specification

• On slopes it can limit the risk of slip failure. • Ground bearing capacities, for low to medium rise

buildings and industrial developments, is in the region of 100kN/m2 to 200kN/m2.

• Improved ground conditions may allow heavier loads to be supported.

• Used in granular and cohesive soils

Chris Gorse

Benefits of vibro-compaction

• Buildings can be supported on conventional foundations (normally reinforced and shallow foundations).

• Work can commence immediately following the vibro displacement. Foundations can be installed straight away.

• The soil is displaced. No soil is produced.• Contaminants remain in the ground – reduces disposal

and remediation fees.• Economical, when compared with piling or deep

excavation works.• Can be used to regenerate brownfield sites• Can use reclaimed aggregates and soils.

Chris Gorse

Vibrofloatation

• Vibro floatation uses a similar process to vibro compaction

• Water jets at the tip of the poker

• Water jets help the vibrator penetrate hard layers of ground

• Major disadvantage is that the system is messy and imprecise, thus rarely used

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Vibro displacement - Typical sequence

2. As the mandrel drives into the ground the soil is displaced (surrounding granular soil is compacted.

1. A grid is marked out and the vibrating mandrel (poker) is inserted to the required depth

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Vibro displacement - Typical sequence

3. Having reached the engineered depth the mandrel is withdraw and hardcore is placed up to the first level. The hardcore is built up in layers of 0.3 to 0.6m. The mandrel is inserted into the hardcore, it penetrates and compacts each layer before the next load of hardcore is placed

Rigs weighs 14 – 55 tonnes

4. By compacting in layers and reintroducing the cone mandrel a dense stone column is constructed.

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Mandrel positioned ready to compact and displace

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Ground displaced

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Ground compacted void remains

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Void filled with stone

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Hardcore is repeatedly displaced and compacted

Chris Gorse

Chris Gorse

Grouting

• Grouting may be used to fill the voids in the ground increasing the strength of the ground.

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Pressure grouting

• In permeable soils, pressure grouting may be used to fill the voids.

• Holes drilled using mechanically driven augers.• As the auger is withdrawn cement slurry is forced

down a central tube into the bore under pressure. • Pressures of up to 70,000 N/mm2 can be exerted

by the grout on the surrounding soil. • Slurry contains cementious additives, e.g.

pulverised fuel ash (pfa), microsilica, chemical grout, cement or a mixture.

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Soil modification and stabilization

• Machines are available that can break-up the ground, mix the ground with new cementious material and improve the ground quality.

Chris Gorse

Soil modification and recycling

• Additives used in soil stabilisation increase the strength better, improve compacted and maximise bearing capacity and minimise settlement.

• The technique can be used to provide stabilised or modified materials for earthworks, or may be used to provide permanent load transfer platforms or hard standings.

• Can be used to treat and neutralise certain contaminants or encapsulate the contaminants, removing the need for expensive removal and disposal.

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Soil modification, stabilisation and recycling machine

Milling and mixing chamber

Working direction

Unstable soil Stable or modified soil ready for compaction

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Schematic of soil modification and mixing chamber

The milling and mixing rotor breaks down soil and mixes the soil and additives

Hopper and cellular wheel sluice spread lime or cement or other additive

Variable milling and mixing chamber.

Soil mixture with reduced water content – ready for compaction

Working direction

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Soil modification and stabilization rig

www. roger-bullivant.co.uk

Chris Gorse

Soil modification and stabilization plant

www. roger-bullivant.co.uk

Chris Gorse

www. roger-bullivant.co.uk

Chris Gorse

Soil modification and stabilization plant

www. roger-bullivant.co.uk

Chris Gorse

Surcharging

• This involves placing heavy loads on the ground for long periods of time.

• Over time the ground will compact.• Surcharging is time consuming and ties up the

land• Can be used if long lead-in time available• Can be used on roads• May be used on investment land (land bank). The

increase in strength will increase the value of the land.

Chris Gorse

Surcharging

• Excavated material, quarried stone or other heavy loads.

• Settlement and compaction period 6 months to a few years.

• For economics the surcharging acts as a temporary storage facility

Chris Gorse

Geotechnical membranes

• Geotechnical membranes provide a sheet of reinforcing material that can be added to the ground. This increases the stability and tensile strength of the ground.

Chris Gorse

Geotecnic membrane

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Geotechnical membranes

• Natural

• Plastic manmade

• Built up in layers compacted between ground hardcore

• Sheets, fibres and strips

Chris Gorse

Further information supporting these slides can be found in the following publication and websites.

Emmitt, S. and Gorse, C. (2010) Barry’s Introduction to Construction of Buildings. Oxford, Blackwell Publishing

Emmitt, S. and Gorse, C. (2010) Barry’s Advanced Construction of Buildings. Oxford, Blackwell Publishing

Virtual Site (2010) Virtual Site at Leeds Met University http://www.leedsmet.ac.uk/teaching/vsite

Virtual Site Gallery (2010) Virtual Site Gallery at Leeds Met University http://www.leedsmet.ac.uk/teaching/vsite/gallery