A grout record stating only that a hole took 300 litres says very little on its own. To understand whether that is a low, expected or excessive result, the quantity must be related to the treated ground volume, drill length, grout mix and injection stage. This is how to calculate grout take in a form that supports both design estimates and sound site decisions.
What grout take actually represents
Grout take is the quantity of grout accepted by the ground during an injection operation. It may be recorded as slurry volume, cement mass, dry solids mass, or a normalised value such as litres per metre of borehole. All can be useful, but they answer different questions.
For daily control of a curtain, contact grouting programme or pre-excavation grouting round a tunnel, litres per metre is often the clearest immediate measure. It allows holes of different length to be compared. For procurement, cost control and material planning, cement consumption in kilograms or tonnes is normally more reliable. For assessing the response of the rock mass, take should also be considered against the estimated volume of the grouted zone.
A distinction is needed between theoretical and measured take. Theoretical take is an estimate of groutable void volume, adjusted for loss and uncertainty. Measured take is what passed through the pump and was recorded for a hole or stage. They should not be expected to match exactly. Fissure connectivity, aperture distribution, filtering, grout bleed, pressure, water conditions and local geological structures can all change the result significantly.
Start with the reporting basis
Before calculating quantities, define the basis consistently across the works. Mixing litres of suspension with kilograms of cement, or comparing a 3 m stage with a full 20 m hole, creates apparent anomalies that are only reporting errors.
For an individual hole or stage, the basic normalised take is:
Grout take per metre = injected grout volume / treated drill length
If a 15 m hole accepts 280 litres of grout, the result is:
280 / 15 = 18.7 litres per metre
For a group of holes, use the total injected volume divided by the total treated length. Do not average the individual litres-per-metre values unless all holes have the same treated length.
In staged grouting, retain both the stage result and the whole-hole result. A high take in the first stage may be normal, while an increasing take in later or outer stages may indicate open structures, poor closure of the curtain, or communication with an adjacent hole. The trend is frequently more useful than the final total.
Estimating theoretical grout take from ground volume
A preliminary estimate begins with the volume of ground intended for treatment. For a simple treatment block:
Treated ground volume = length × width × thickness
For a tunnel grout curtain, the geometry is commonly better represented as an annulus around the excavation. For a tunnel radius R, grout cover thickness t and tunnel advance length L:
Treated ground volume = π × [(R + t)² – R²] × L
The next step is not to apply the total rock porosity. Most of that pore space may be inaccessible to a cementitious grout, particularly in sound rock. Instead, use an estimated effective groutable void ratio, often derived from geological mapping, water loss tests, previous grouting records, core logging and local experience.
Theoretical void volume = treated ground volume × effective groutable void ratio
If the annular treatment zone around a tunnel is 565 m³ and the assumed effective groutable void ratio is 0.25%, the estimated void volume is:
565 × 0.0025 = 1.41 m³, or approximately 1,410 litres.
This is not yet the planned pump volume. A practical estimate must allow for uncertainties such as grout migration beyond the nominal envelope, imperfect interception of fractures, washout, returns, drilling disturbance and the fact that the grout system may not remain at its original volume. A project-specific loss or contingency factor can therefore be applied:
Planned grout volume = theoretical void volume × allowance factor
With an allowance factor of 1.5, the example gives about 2,120 litres. The factor should be based on comparable ground and method where possible, rather than adopted as a generic rule. A highly fractured zone can require several times the calculated void volume; tight, massive rock may take much less.
Convert slurry volume into cement consumption
Pump records commonly state litres of grout suspension, whereas material orders and cost reports need cement mass. The conversion depends on water-cement ratio by mass, grout density and any admixtures or fillers.
For a quick theoretical conversion, let the water-cement ratio be w/c = r. Using a cement density of approximately 3,150 kg/m³ and water density of 1,000 kg/m³, the theoretical cement content of one cubic metre of grout is:
Cement per m³ = 1 / [(1 / 3150) + (r / 1000)]
At w/c = 0.8, this gives roughly 890 kg of cement per m³ of fresh suspension. In practice, use the established mix yield or density calibration for the actual plant. Admixtures, retained material in the mixer, foam, entrained air and batching tolerances mean that theoretical yield is not a substitute for a controlled site calibration.
Returning to the 15 m hole that accepted 280 litres at w/c = 0.8:
Cement mass = 0.280 m³ × 890 kg/m³ = about 249 kg
If twelve similar holes are completed, the total slurry volume is 3,360 litres and the cement consumption is approximately 3.0 tonnes. This is useful for planning, but the recorded take should still separate pumped volume, verified return where relevant, and waste from flushing or cleaning. Flushing water is not grout take.
How to calculate grout take by stage and area
Normalising take against drill length is practical, but it does not describe treatment intensity fully. Where hole spacing and stage length vary, consider additional measures.
For a staged fan, calculate litres per metre for each stage and plot the values by position, depth and sequence. This provides a direct view of zones that are accepting disproportionate quantities. For consolidation or slab grouting, litres per square metre of treated plan area can be useful. For a defined rock volume, litres per cubic metre provides a more direct comparison with the design void ratio.
The relevant formula is simply:
Volume-normalised take = injected grout volume / assigned treated ground volume
The assigned volume must be defined carefully. Dividing each borehole’s take by an arbitrary cylinder around that hole may be acceptable for an internal trend assessment, but it can be misleading where holes overlap or where grout is deliberately injected in a primary-secondary sequence. For a curtain, the design panel or tunnel advance is usually a better reporting unit.
Use pressure and take together, not separately
A large take at low pressure can indicate a well-connected fissure system, open joints, voids or a loss path. A small take at high pressure may indicate tight rock, but it can also indicate premature refusal caused by grout filtration, packer leakage, blocked equipment or an unsuitable grout rheology. Neither value is meaningful without the other.
Each stage record should therefore include start and finish pressure, flow rate, mix designation, injected volume, duration, refusal criterion, returns and any observed communication. Geological observations, groundwater conditions and the hole sequence should sit alongside the numerical record. This is the information needed to distinguish normal variability from a developing problem.
Maximum take limits should be treated as control measures, not as proof that the required ground improvement has been achieved. Reaching a limit may require a change in grout mix, stage length, pressure regime or drilling pattern. Equally, a very low take should prompt a check that the intended fractures have actually been intercepted.
A practical calculation workflow
Set up the calculation before drilling begins. Define the expected treatment geometry, estimate an effective groutable void ratio and state the allowance factor with its engineering basis. Establish the grout yield from the actual mix design, then decide which normalised measures will be reported: litres per metre, kilograms of cement per metre, litres per square metre, or litres per cubic metre.
During execution, record quantities at stage level and keep units fixed. A straightforward digital form that calculates cumulative take, take per metre and cement consumption as entries are made reduces transcription errors and makes trends visible while corrective action is still possible. The same calculation logic should work on the desktop during planning and on a tablet or mobile phone at the face.
The most useful grout take calculation is therefore not a single predicted number. It is a transparent baseline, measured consistently and reviewed against pressure, geology and sequence. Used in that way, grout take becomes a practical indicator of ground behaviour rather than merely a record of material consumed.
