| Item | Value | Unit | Notes |
|---|---|---|---|
| Slab length | 12.000 | m | Measured edge-to-edge. |
| Slab width | 8.000 | m | Measured edge-to-edge. |
| Average void depth | 0.100 | m | Average of probe points. |
| Void coverage | 35.00 | % | Mapped from sounding grid. |
| Shape factor | 1.050 | — | Irregularity adjustment. |
| Contingency | 7.00 | % | Uncertainty allowance. |
| Void volume with contingency | 3.150 | m³ | Target estimate for reporting. |
Your results will vary with coverage and depth variability across the slab.
- Area = L × W
- Void‑covered area = Area × (Coverage ÷ 100)
- Gross void volume = Void‑covered area × Average void depth
- Adjusted void volume = Gross void volume × Shape factor
- Void volume with contingency = Adjusted void volume × (1 + Contingency ÷ 100)
- Plan grout volume = With contingency × (1 + Overfill ÷ 100)
- Plan granular volume = With contingency × (1 + Compaction ÷ 100)
Use shape factor to reflect irregular cavities, honeycombing, or interconnected channels.
- Measure slab length and width along the plan edges.
- Probe or scan to estimate average void depth beneath the slab.
- Estimate void coverage by mapping affected zones as a percentage.
- Set shape factor: 1.00 for uniform gaps, higher for irregular voids.
- Add contingency for uncertainty; add overfill or compaction as needed.
- Click Calculate to view results under the header.
- Use the CSV or PDF options to share a clean report.
Voids typically develop from poorly compacted subgrade, washout from groundwater movement, shrinkage of granular layers, or loss of fines through piping. Even small gaps reduce bearing contact, concentrate stresses, and can trigger cracking, rocking, or joint faulting. Quantifying volume helps select the right repair approach, batching plan, and pump strategy.
Field inputs that control accuracy
The most important variables are average void depth and percent coverage. Depth should come from multiple probe points, GPR interpretations, or lift measurements from localized coring. Coverage should reflect mapped zones, not the full slab, because repairs are often staged by bays. Shape factor accounts for irregular cavities and interconnected channels.
Planning grout or fill placement quantities
The calculator reports a baseline void volume plus a contingency allowance for uncertainty and minor losses. When planning cementitious grout, the overfill percentage adds margin for bleed, migration paths, hose priming, and unavoidable spillage. For granular placement, the compaction allowance estimates the loose volume required to achieve the final compacted state.
Quality control and constructability checks
Compare the planned volume with expected delivery limits per shift, pump rates, and access constraints. Track injected or placed quantities by zone, and re-check elevations for slab lift or settlement. If the required volume is unexpectedly high, confirm depth readings and verify that coverage is not overstated.
Example data and interpretation
Example: Length 12 m, width 8 m, average void depth 0.10 m, coverage 35%, shape factor 1.05, contingency 7%, overfill 5%, compaction 0%.
- Slab area = 96.000 m²; void-covered area = 33.600 m²
- Gross void = 3.360 m³; adjusted = 3.528 m³
- With contingency ≈ 3.775 m³; grout plan (5% overfill) ≈ 3.964 m³
1) How do I estimate void coverage reliably?
Map affected zones using sounding, GPR interpretation, or observed settlement patterns. Convert the mapped area to a percentage of total slab area. Avoid guessing full-area coverage unless evidence supports it.
2) What shape factor should I use?
Use 1.00 for uniform gaps. Use 1.05–1.15 for irregular cavities or variable depths. Use higher values only when field evidence shows interconnected channels or significant uncertainty.
3) Why include contingency if I already measured depth?
Depth and coverage vary across the slab, and measurements sample only portions. Contingency accounts for unseen pockets, minor losses, and practical batching constraints, helping prevent under-ordering.
4) When should I use grout overfill?
Use it when pumping grout or flowable fill where bleed, migration, or pump priming consumes volume. Typical allowances are 3–10%, adjusted for access, hose length, and observed losses.
5) What does compaction allowance represent?
Granular materials are placed loose and compacted to a smaller final volume. The allowance estimates extra loose volume needed to achieve the compacted void fill. Use field compaction targets to refine it.
6) Can I use this for multiple void zones?
Yes. Run separate calculations for each zone using its own coverage and depth, then sum the planned volumes. This improves staging, delivery coordination, and reporting compared with one blended estimate.
7) Does slab thickness change the void volume result?
Thickness is recorded for documentation but does not change void volume. Void volume is governed by plan area, affected coverage, and average void depth. Thickness matters for structural evaluation and repair detailing.