Bentonite and polymer slurry stabilisation

How bridges get built · Bored piles and deep foundations · Bentonite and polymer slurry stabilisation

Also called: drilling fluid, bentonite support, polymer slurry

Bentonite and polymer slurry stabilisation — concept diagram — Concept diagram (TheBridgeEng)

How it is done

Choose the fluid for the ground and groundwater: bentonite for most soils, polymer where disposal is restricted or the water is saline; test the make-up water and agree acceptance limits with the designer.
Mix bentonite at about 3–6 % (or polymer at 0.1–0.3 %) in a high-shear mixer, and let bentonite hydrate for the time the supplier specifies before use.
Fill the hole before the tool passes below the water table and keep the fluid level at least 1.5–2 m above the groundwater or tide level at all times, allowing for tide and waves at sea.
Test density, Marsh viscosity, sand content, pH and (for bentonite) filtrate and cake at the start of each shift, during drilling and from the base before concreting.
Desand or exchange the fluid at the base until it meets the pre-concreting limits. Hold point before the cage and tremie.
Recover and recycle the fluid displaced by the concrete; treat spent fluid and dispose of it through licensed routes.

Key numbers

Bentonite dosageabout 3–6 %; polymer 0.1–0.3 %
Bentonite before concreting (EN 1536/1538 type limits): density < 1.15 t/m³, Marsh viscosity 32–50 s, sand < 4 %, pH 7–11
Polymer before concretingdensity about 1.00–1.03 t/m³, sand ≤ 1 %, viscosity within the supplier's range
Headat least 1.5–2 m above groundwater or tide
Plantone slurry plant typically serves 2–4 rigs

Where it fits

All uncased bored piles and barrettes below the water table; polymer in urban sites with disposal limits.

Where it does not

Bentonite in saline groundwater without conditioning; polymer in very loose gravels.

Choosing it

Choose polymer over bentonite where groundwater is saline or contaminated (bentonite flocculates), disposal is costly, or shaft friction matters: comparative tests by Brown found shafts drilled under bentonite had markedly lower side resistance than those under polymer or casing.
Choose bentonite in coarse, permeable soils where a filter cake is needed to hold the head.
Prefer full-length casing where fluid loss, fractured rock or cavities make the head impossible to hold (Benicia–Martinez).
Prefer dry drilling with short casing in stiff clays above the water table; no fluid is cheaper than any fluid.

Plant, pace and money

PlantMixers, tanks, desanders, pumps, testing kit.

ProductivityA slurry plant serves two to four rigs.

CostUSD 20-60 per m3 of slurry; disposal can exceed mixing.

Risks and controls

What goes wrong

Hole collapse from a dropped head, thick filter cake reducing shaft friction, slurry inclusions in the concrete.

Quality assurance

Slurry tests before concreting recorded per pile, level watch, sand content below 4 % (bentonite) or 1 % (polymer).

Origins

Christian Veder pioneered bentonite support for diaphragm walls in Italy in the early 1950s, and the same fluid was soon used for bored piles; synthetic polymer fluids came into piling from the 1980s–90s, and EFFC and DFI issued a joint support-fluid guide in the late 2010s.

Examples

Standard on bored piles worldwide; polymers dominate in Hong Kong and Singapore.

Case studies

Burj KhalifaUAE · 2010A building, but a regional reference: the tower's 1.5 m piles, about 47 m long, were drilled under polymer slurry in highly saline groundwater, and the foundation was given cathodic protection.

New Benicia–Martinez BridgeUSA · 2007Slurry-supported reverse-circulation sockets caved in fractured rock, and the contractor changed to full-depth casing: a case of a fluid that could not hold the ground.