Permeation, compaction and compensation grouting

Also called: pressure grouting, chemical grouting, low-mobility grouting, compensation grouting, void filling, tube-à-manchette grouting

How it is done

1. Characterise the ground for grouting: grading, permeability from in-situ tests, voids and fissures (karst, mine workings), groundwater flow, and the structures to be protected.
2. Select grout and method: cement or bentonite-cement for gravel and fissured rock, microfine cement for medium sand, silicate or other chemical grouts for fine sand, stiff low-slump mortar for compaction grouting, and bulk cement-ash grout for large voids.
3. Run a field trial: grout a test panel, record takes and pressures, and check the result by coring, permeability tests or CPTs before fixing hole spacing and stage lengths.
4. Drill on a primary-secondary (split-spacing) pattern; for permeation and compensation work, grout in sleeved pipes (tubes-à-manchette) with ports every 0.3–0.5 m so each level can be injected again.
5. Inject stage by stage within limits of pressure, volume and flow rate, recording all three continuously; end each stage on refusal, on the volume limit or on the first sign of ground heave.
6. For compaction grouting, inject the stiff mortar slowly in 0.3–1 m stages, usually from the bottom up, forming bulbs that displace and densify the surrounding soil, with close level monitoring of the surface and nearby structures.
7. For compensation grouting, inject small volumes between a structure and an excavation or tunnel in response to real-time settlement readings; close out every method with verification holes and a final level survey.

Key numbers

• Groutabilityparticulate grouts penetrate when D15 of the soil exceeds about 25 times D85 of the grout; ordinary cement suits gravel and coarse sand, microfine cement medium sand, chemical grouts fine sand.
• Permeability limits, roughlyordinary cement above about 10⁻³ m/s, microfine cement to about 10⁻⁵ m/s, chemical grouts to about 10⁻⁶ m/s.
• Permeation pressure limited to avoid fracturing: conservative US practice about 23 kPa per metre of depth (1 psi per foot); European practice allows more.
• Compaction groutslump about 25–50 mm, injection about 0.03–0.06 m³ per minute, stages 0.3–1 m, grid 1.5–4 m.
• Permeation takesroughly the pore volume, about 25–40 % of the treated soil volume in sand.

Choosing it

• Choose permeation grouting over jet grouting or DCM when the soil is permeable enough and the aim is a seal or modest strength with little disturbance: it is cheaper and leaves the soil structure intact.
• Prefer jet grouting in silts and fine sands, which particulate grouts cannot penetrate, and where a column of defined geometry is needed.
• Choose compaction grouting to densify loose soil or fill voids beneath existing foundations where vibratory or dynamic methods cannot be used; it is also the standard repair over sinkholes.
• Prefer micropile underpinning when the structure needs new capacity rather than improved ground.

Plant, pace and money

Risks and controls

Origins

Charles Bérigny injected clay and lime grout to repair a sluice at Dieppe in 1802; Hugo Joosten's two-shot silicate process of 1925 opened sands to chemical grouting; compaction grouting was developed in the US in the 1950s, and compensation grouting was proven on London's Jubilee Line Extension in the 1990s, most visibly beneath the Big Ben clock tower.

Examples

Sealing gravel beneath river cofferdams, filling karst voids before piling, re-levelling settled piers and abutments, and protecting historic structures above new tunnels.

Case studies

Related methods

Further reading

EN 12715 (execution of special geotechnical works: grouting) · ASCE/G-I 53-10 Compaction Grouting Consensus Guide · Warner, Practical Handbook of Grouting: Soil, Rock, and Structures · Karol, Chemical Grouting and Soil Stabilization · Mitchell and Jardine, A Guide to Ground Treatment (CIRIA C573)