The Cement-Bentonite Grout Mix Designer calculates exact batch quantities of cement, water, and bentonite to meet a target specification. It answers the question drillers face on every grouting job: "what goes in each batch to meet spec?"
Choose from evidence-based presets for common applications — instrument backfill (per Mikkelsen & Green 2003), neat cement grout, soil nail grout, and borehole sealing — or define a custom mix. The tool uses the specific gravity method (cement SG = 3.15, bentonite SG = 2.6) to calculate grout density for QC verification with a mud balance on site.
Select an application type to auto-populate the water:cement ratio and bentonite percentage. For locked presets (instrument backfill, neat cement, soil nail), these values are fixed to the reference specification. For borehole sealing and custom mixes, you can adjust them freely.
Enter your mixer or batch volume in litres. The tool instantly calculates cement, water, and bentonite quantities per batch, along with bag counts. Optionally enter a total grout volume to see the number of batches and total material requirements.
Application presets Instrument backfill (soft soil) — w:c 6.6:1, 40% bentonite BWOC. Designed to match the stiffness and permeability of soft surrounding soil. Reference: Mikkelsen & Green (2003).
Instrument backfill (medium-hard soil) — w:c 2.5:1, 30% bentonite BWOC. Stiffer mix for firmer ground conditions. Reference: Mikkelsen & Green (2003).
Neat cement grout (micropiles) — w:c 0.45:1, 0% bentonite. Standard structural grout for micropile and minipile installation. Reference: BS EN 14199 (typical range 0.40–0.50).
Soil nail grout — w:c 0.5:1, 5% bentonite BWOC. Typical specification for soil nailing applications.
Borehole sealing — w:c 1.5:1, 50% bentonite BWOC (editable). A typical starting point for borehole abandonment and sealing. Adjust to match your site specification.
The calculation uses the specific gravity method to determine grout density and yield. For each kilogram of cement, the volume contributions are: cement (SG 3.15) = 1/3150 m³, water = w:c ratio / 1000 m³, bentonite (SG 2.6) = bentonite fraction / 2600 m³.
Grout density is calculated as total mass divided by total volume. Yield (litres per kg cement) determines how much grout each kg of cement produces. Batch quantities are derived by dividing the mixer volume by the yield.
The primary reference for instrument backfill mixes is Mikkelsen, P.E. & Green, G.E. (2003) "Piezometers in Fully Grouted Boreholes", which recommends weight ratios of 6.6:1:0.4 (water:cement:bentonite) for soft soils and 2.5:1:0.3 for medium-hard soils. See also Mikkelsen (2002) "Cement-Bentonite Grout Backfill for Borehole Instruments", Geotechnical News.
Limitations Calculated density assumes standard specific gravities for OPC cement (3.15) and sodium bentonite (2.6). Actual values vary by manufacturer and source — bentonite SG can range from 2.2 to 2.8 depending on grade. Always verify grout density on site using a mud balance.
Bag counts are rounded up to whole bags. Actual material usage should account for wastage, spillage, and mixer residue. Allow bentonite to fully hydrate before adding cement where required by specification.
This tool does not account for admixtures, retarders, or other additives. Grout set time, bleed, and strength are not calculated. For structural applications, always refer to the project specification and relevant standards.
Revision history 5 March 2026: Initial release with 6 application presets, specific gravity method calculation, and batch/total quantity outputs.
Disclaimer This tool is provided for educational and general information purposes only. It is not a substitute for professional engineering advice, design or verification.
Diggy and its contributors are not licensed engineering consultants and no results generated by this tool should be used directly for construction, design or safety-critical decisions.
All values and outputs are based on published empirical correlations and should be independently checked and confirmed by a qualified geotechnical engineer before use.
By using this tool, you accept full responsibility for how you interpret and apply the information provided.
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