Bentonite slurry holds a bored pile or diaphragm wall excavation open until concrete displaces it, at 4 to 6 % bentonite by weight in many applications (Akhtar 2023). A polymer support fluid holds the excavation at 0.05 to 0.2 % polymer by weight of the mix water.
A support fluid shares its base with every drilling fluid: water, a clay or polymer viscosifier and a few treatment chemicals. It differs in what the crew measures, because the project specification caps density, Marsh viscosity, pH and sand content before concrete goes in.
The four key figures of a support fluid are listed below.
- Mineral slurry under AASHTO (2016) / FHWA (2010): 64.3 to 72 lb/ft3 (8.60 to 9.63 lb/gal; 1,030 to 1,153 kg/m3), Marsh viscosity 28 to 50 s/qt, pH 8 to 11.
- Sand content before concreting under the same US limits: at most 4.0 % by volume in mineral slurry and 1.0 % by volume in polymer slurry.
- European bored piles: EN 1536:2010+A1:2015 clause 6.2 names bentonite suspensions and polymer solutions as support fluids.
- US practice: 17 state DOTs allow both mineral and polymer slurry (FDOT research report BDV25-977-19).
Standards and regulations are checked against their issuing bodies, product values against the named data sheet and figures against the dated source named in the sentence; see our methodology and fact-checking.
Which Ground and Site Conditions Shape a Support Fluid?#
Groundwater chemistry and site space shape a support fluid more than depth does: salt or cement-contaminated water changes the density and pH limits, and a cramped site can rule out the tanks a bentonite system needs. The excavations are bored piles (drilled shafts in US specifications) and diaphragm wall panels.
Groundwater and salt water#
FDOT (2018) sets separate density ranges for mineral slurry in fresh and in salt water, 64 to 73 lb/ft3 (8.56 to 9.76 lb/gal) against 66 to 75 lb/ft3 (8.82 to 10.03 lb/gal).
Polymer fluids cannot be made heavier by adding minerals (Larisch 2022), so the density of a polymer fluid follows the mix water and the ground it picks up.
Site space and plant#
On a 79 x 131 ft (24 x 40 m) site in Glasgow, 62 bored piles of 29.5 in (750 mm) diameter could only be built with a polymer support fluid (Jefferis and Lam 2013). Bentonite needs hydration and storage tanks before it goes into the excavation, and a site without room for them favors a polymer solution.
What Goes into a Bentonite Slurry for Piling?#
A bentonite slurry for piling is bentonite hydrated in water, and 2.5 to 5 % bentonite gives a slurry density of 1.014 to 1.028 g/cm3 (8.46 to 8.58 lb/gal) (Akhtar 2023 review).
The clay is the same drilling-grade bentonite that viscosifies water-based fluids elsewhere, and it builds the gel and the filter cake that support the bore wall. Soda ash raises the pH of mix water that arrives low, and hard or salty water weakens the clay before it hydrates.
Re-used slurry is thinned with organic thinners or polyphosphates when it flocculates, and its fluid loss is cut with carboxymethyl cellulose. The make-up dose sits in the additive table below.
How Is a Polymer Support Fluid Made Up?#
A polymer support fluid is a solution of partially hydrolyzed polyacrylamide (PHPA) at 0.05 to 0.2 % polymer by weight of the mix water (Akhtar 2023; Jefferis and Lam 2013).
At the Stratford trial the polymer fluid read 69 to 70 s/qt on the Marsh funnel against 34 s/qt for the bentonite slurry (Lam and Jefferis, summarized in FDOT research report BDV25-977-19). Viscosity alone does not rank the two fluids, and the full comparison of polymer vs bentonite fluids weighs them across applications.
Piles built under PHPA showed a much stiffer load-settlement response than a pile built under bentonite (Soils and Foundations, 2015). Density stays at the mix-water value, because minerals cannot be added to raise it.
Which Additives Do Support Fluids Use?#
Support fluids draw on four additive classes: viscosifiers (bentonite, PHPA), fluid-loss additives (CMC), pH and alkalinity control (soda ash, sodium bicarbonate) and thinners (organic thinners, polyphosphates), plus an oxidizing breaker for spent polymer.
The table lists each additive with its role in a support fluid and the dose where the sources give one.
Send one specification to independent suppliers of drilling fluid materials. request supplier quotes
| Class | Material | Role in a support fluid | Dose (basis) |
|---|---|---|---|
| Viscosifiers | Bentonite | Gel and filter cake of the slurry | 4 to 6 % by weight in many applications (Akhtar 2023) |
| Viscosifiers | PHPA | Polymer support fluid | 0.05 to 0.2 % by weight of mix water: 0.175 to 0.70 lb/bbl (0.50 to 2.0 kg/m3; 0.42 to 1.67 lb/100 gal) |
| Fluid loss | Carboxymethyl cellulose (CMC) | Cuts the fluid loss of re-used bentonite slurry | Product directions |
| pH and alkalinity control | Soda ash | Raises the pH of mix water and slurry | Product directions |
| pH and alkalinity control | Sodium bicarbonate | Lowers the pH after cement contamination | Product directions |
| Thinners | Organic thinners and polyphosphates | Thin flocculated re-used slurry | Product directions |
| Oxidizing breaker | Sodium hypochlorite | Breaks spent polymer before settlement | Product directions |
Buyers who want to compare additive suppliers find the market overview there, and the polymer drilling fluids page describes the polymer systems of other applications.
The same drilling fluid additives act in oil and gas wells, water wells and horizontal bores, with other doses and limits.
Which Standards and Specifications Govern Support Fluids?#
Support fluids in Europe follow EN 1536:2010+A1:2015 for bored piles, whose clause 6.2 covers bentonite suspensions (6.2.1) and polymer solutions (6.2.2), while US drilled shafts follow state DOT specifications built on AASHTO (2016) and FHWA (2010) slurry limits.
The table compares the instruments by jurisdiction and date.
| Jurisdiction | Instrument and date | What it covers for the fluid |
|---|---|---|
| Europe | EN 1536:2010+A1:2015 (bored piles) | Clause 6.2 support fluids, 6.2.1 bentonite suspensions, 6.2.2 polymer solutions |
| Europe | EN 1538 (diaphragm walls); prEN 1538 CEN enquiry 2026-09-03 to 2026-10-29, CEN/TC 288 | Revision under enquiry |
| Industry | EFFC/DFI Guide to Support Fluids for Deep Foundations, 1st edition, March 2019 | Property limits, including sand before concreting |
| Industry | EFFC/DFI Guide to Tremie Concrete, 3rd edition, December 2024 | Fluid meets the Support Fluids Guide before concreting |
| UK | FPS bentonite support fluids guide, January 2006 | Bentonite only, polymers out of scope |
| US | AASHTO (2016) / FHWA (2010) drilled-shaft slurry limits | Density, Marsh viscosity, pH and sand content |
| US, Florida | FDOT specification (2018) | Fresh-water and salt-water ranges |
| US | DOT survey in FDOT research report BDV25-977-19 | State-by-state slurry allowance |
The EFFC/DFI guide on support fluids, written by a task group of 41 experts, is the industry reference.
Of the US states surveyed, 10 have no drilled-shaft specification, 11 allow slurry only with the engineer's approval and 17 allow both mineral and polymer slurry. The wider set of drilling fluid standards covers the oilfield instruments, and this page describes rules without giving legal advice.
What Property Limits Apply Before Concreting?#
US drilled-shaft specifications cap sand at 4.0 % by volume in mineral slurry and 1.0 % in polymer slurry, with density, Marsh viscosity and pH ranges for each (AASHTO 2016 / FHWA 2010).
The table compares the limits of AASHTO (2016) / FHWA (2010) and FDOT (2018) for the two fluid types, with density in lb/ft3 (lb/gal; kg/m3).
| Property | Mineral slurry, AASHTO 2016 / FHWA 2010 | Mineral slurry, FDOT 2018 | Polymer slurry, AASHTO 2016 / FHWA 2010 | Polymer slurry, FDOT 2018 | Test |
|---|---|---|---|---|---|
| Density | 64.3 to 72 (8.60 to 9.63; 1,030 to 1,153) | Fresh 64 to 73 (8.56 to 9.76; 1,025 to 1,169), salt 66 to 75 (8.82 to 10.03; 1,057 to 1,201) | At most 64 (8.56; 1,025) | Fresh 62 to 64 (8.29 to 8.56; 993 to 1,025), salt 64 to 66 (8.56 to 8.82; 1,025 to 1,057) | Mud balance |
| Marsh viscosity (s/qt) | 28 to 50 | 30 to 40 | 32 to 135 | Manufacturer's range | Marsh funnel viscosity |
| pH | 8 to 11 | 8 to 11 | 8 to 11.5 | Manufacturer's range | pH of drilling fluids |
| Sand content (volume %) | At most 4.0 | At most 4.0 | At most 1.0 | At most 0.5 | Sand content |
Source: FDOT research report BDV25-977-19. Density values are converted from lb/ft3 at 7.48052 US gal per ft3 and 16.01846 kg/m3 per lb/ft3.
FDOT (2018) tightens polymer sand to 0.5 % and leaves polymer Marsh viscosity and pH to the manufacturer's range. The EFFC/DFI guide (2019) asks for less than 2 to 4 % sand in bentonite and less than 1 % in polymer before concreting (Larisch 2022). US values are never mixed with the values of EN 1538.
How the site crew tests a support fluid#
The crew checks four properties on fresh, in-use and pre-concreting samples: density on a mud balance, Marsh funnel time, pH and sand content.
Marsh viscosity is the time for 946 mL (1 US quart) to drain through the funnel, with fresh water at 26 s/qt, and sand content is the volume percent of particles above 75 µm (No. 200 sieve). Procedures for each instrument are set out under drilling fluid testing.
How Is a Support Fluid Mixed and Hydrated?#
A bentonite support fluid is mixed in a fixed order: pH correction of the mix water with soda ash, then bentonite, then at least 12 h of hydration in storage before the slurry goes into the excavation.
The six steps of a support fluid make-up are listed below.
- Raise the mix-water pH with soda ash where it is low.
- Add the bentonite at the project dose.
- Hydrate the slurry in storage before use.
- Dose the polymer by weight of the mix water for a polymer fluid.
- Check density, Marsh viscosity, pH and sand content against the project limits.
- Clean the fluid to the sand limit before concreting.
Mixers, desanders and hydrocyclones are the plant behind steps 2 and 6.
How Are Spent Bentonite and Polymer Fluids Disposed Of?#
Spent bentonite slurry counts as non-hazardous waste but is highly polluting to the aquatic environment, where the clay builds up on fish gills, so it never goes to surface water (Jefferis and Lam 2013).
Spent polymer fluid is broken with an oxidizing breaker such as sodium hypochlorite and left to settle. After breakdown and settlement, the supernatant can go to sewer with the consent of the sewerage undertaker (Jefferis and Lam 2013).
General routes for drilling waste are covered under spent mud treatment and disposal.
What Goes Wrong with Support Fluids?#
Two problems decide whether a support fluid can stay in use: contamination by cement or salt on re-use, and sand that settles in the excavation before concreting.
The table compares four problems with their cause and the fluid-side response.
| Problem | Cause | Fluid-side response | Page |
|---|---|---|---|
| Cement contamination | Concrete contact on re-use | Sodium bicarbonate to lower pH, thinner for flocculation | Cement contamination treatments |
| Sand above the limit | Sand settling from the fluid | Clean the fluid to the limit before concreting | Solids and sand content |
| Salt in the mix water | Saline groundwater or site water | Work to the salt-water density range | This page |
| Release to a watercourse | Spill of spent slurry | Contain it and never discharge to surface water | Drilling fluids and the environment |
Cement contamination on re-use#
Concrete contact raises the pH of re-used bentonite slurry, and sodium bicarbonate brings it down, while soda ash is the treatment when the pH is low.
In drilling fluids, a filtrate pH above 11.5 signals cement contamination (AADE paper 2167). Treatment factors for the problem are set out under cement contamination.
Sand left in the excavation before concreting#
Sand that settles out of the fluid ends up under or around the concrete, which is why every specification caps sand content before concreting. The caps by fluid are in the limits table above.
The polymer cap is lower, and piles built under PHPA showed the stiffer load-settlement response.
How Are Support Fluid Materials and Services Sourced?#
Support fluid materials are bought against the project specification, which names the limits of EN 1536:2010+A1:2015, the EFFC/DFI guide (2019) or the state DOT, so buyers send that specification with every material request.
Contractors who want fluid testing and treatment run by a specialist scope mud engineering services. An RFQ names the items of the additive table: bentonite, PHPA, soda ash, sodium bicarbonate and CMC.
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How Do Support Fluids Differ from Drilling Fluids in Other Applications?#
A support fluid differs from the drilling fluids of other applications because it does not circulate cuttings to a shaker: it stands in an open excavation, is displaced by concrete and must meet a sand limit before concreting.
The other drilling fluids by application answer to their own ground, depth and rules. Tremie concreting and pile design are construction work and are not covered, and soil-bentonite cutoff walls are a construction use of the slurry. The table compares seven applications with a support fluid.
| Application | How the fluid differs from a support fluid | Page |
|---|---|---|
| HDD | Circulates cuttings out of a horizontal bore | HDD drilling fluids |
| Water wells | Fluid removed by well development | Water well drilling mud |
| Mud rotary | Vertical direct circulation | Mud rotary drilling |
| Geothermal | Heat degrades polymers | Geothermal drilling fluids |
| Mining | Clay-free polymer fluids for core | Diamond core drilling fluids |
| Offshore | Discharge rules for fluids and cuttings | Offshore drilling fluids |
| HPHT | Pressure and temperature limits of additives | HPHT drilling fluids |
What is the pH value of bentonite slurry?#
Bentonite slurry for drilled shafts runs at pH 8 to 11 under AASHTO (2016) / FHWA (2010) and FDOT (2018), and freshly mixed bentonite slurry for drilling reads pH 8.5 to 9.5 (The Driller).
Is bentonite used only for piling?#
No: bentonite is the base viscosifier of water-based drilling fluids for HDD bores, water wells and oil and gas wells, and piling is one use among them.
The grades, doses and handling of bentonite drilling fluids are set out on the material page, and consumer uses of bentonite are not covered.