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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
Chris Gorse
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
Chris Gorse
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
Chris Gorse
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.
Chris Gorse
Mandrel positioned ready to compact and displace
Chris Gorse
Ground displaced
Chris Gorse
Ground compacted void remains
Chris Gorse
Void filled with stone
Chris Gorse
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.
Chris Gorse
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.
Chris Gorse
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.
Chris Gorse
Soil modification, stabilisation and recycling machine
Milling and mixing chamber
Working direction
Unstable soil Stable or modified soil ready for compaction
Chris Gorse
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
Chris Gorse
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
Chris Gorse
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