Calculator Inputs
Example Data Table
| Layer | Thickness (m) | gamma (kN/m^3) | Cumulative depth (m) |
|---|---|---|---|
| Fill / Sand | 3.0 | 18.0 | 3.0 |
| Silty Clay | 7.0 | 19.5 | 10.0 |
| Weathered Rock | 5.0 | 22.5 | 15.0 |
Formula Used
- Total vertical stress: sigma_v(z) = Sum(gamma_i * delta_z_i) up to depth z.
- Hydrostatic pore pressure: u(z) = gamma_w * (z − z_wt), for z > z_wt; otherwise u(z)=0.
- Effective vertical stress: sigma_v'(z) = sigma_v(z) − u(z).
- Optional at-rest horizontal effective stress: sigma_h'(z) = K0 * sigma_v'(z).
How to Use This Calculator
- Select depth, unit weight, and pressure units.
- Enter water table depth and gamma_w for the fluid.
- Add layers with thickness and unit weight values.
- Set the target depth within the layer profile.
- Press Calculate to view results above the form.
- Use CSV or PDF buttons to export the report.
Layered overburden modeling
Overburden stress increases with depth as unit weight accumulates. For a three-layer profile of 3 m at 18.0 kN/m³, 7 m at 19.5 kN/m³, and 5 m at 22.5 kN/m³, the total stress at 10 m is 3×18.0 + 7×19.5 = 190.5 kPa. The average gradient over the first 10 m is 19.05 kPa/m, useful for quick sanity checks. For deep shafts, extend layers to exceed the reporting depth by 20%.
Water table and pore pressure
When groundwater is present, pore pressure reduces effective stress. With a water table at 2 m and γw = 9.81 kN/m³, pore pressure at 10 m is 9.81×(10−2) = 78.48 kPa, so effective vertical stress becomes 190.5 − 78.48 = 112.02 kPa. Lowering the water table to 5 m would reduce u to 49.05 kPa and raise σ′v to 141.45 kPa.
Effective stress for strength and settlement
Many design checks depend on σ′v, not σv. In the example, σ′v at 10 m is about 59% of σv, highlighting how groundwater can dominate compressibility and shear strength evaluations in soft soils. If an oedometer modulus is correlated as 5–15 MPa, the same total stress can translate to very different settlements when pore pressure is ignored.
At-rest lateral stress estimate
For retaining structures and in-situ stress screening, the calculator can estimate σ′h using K0. If K0 = 0.6, σ′h at 10 m is 0.6×112.02 = 67.21 kPa. Using K0 = 1 − sinφ with φ = 30° gives about 0.50, yielding 56.01 kPa, which brackets typical preliminary ranges.
Units and conversion discipline
The tool accepts meters or feet and multiple unit-weight systems, then converts internally to kPa. For reference, 190.5 kPa equals 0.1905 MPa and about 27.63 psi. Consistent conversions let teams compare mine planning, foundation, and wellbore inputs without reformatting spreadsheets or risking transcription errors.
Quality control and reporting
Use the layer summary table to verify cumulative depth and incremental contributions. A common check is that σv equals Σ(γ·t) and increases monotonically. Exporting CSV supports peer review, while PDF output is convenient for design notes, bid packages, and field verification during drilling or excavation. Document the assumed water table date because seasonal shifts of 1–3 m can materially change σ′v.
FAQs
1) What is overburden pressure used for?
It supports preliminary checks for foundations, excavations, shafts, and wellbore design by estimating total and effective stresses with depth for layered ground conditions.
2) Why does the calculator show effective stress?
Effective stress controls most soil strength and compressibility behavior. Groundwater pore pressure reduces σ′v, which can change settlement, stability, and lateral stress estimates.
3) How should I choose unit weight values?
Use lab/field-derived values when available. For early screening, select representative bulk or saturated unit weights from geotechnical reports, then refine with testing and logging.
4) What does K0 represent in the results?
K0 is the at-rest lateral earth pressure coefficient. Multiplying K0 by σ′v gives an estimate of σ′h for preliminary in-situ stress and retaining checks.
5) What if my target depth is deeper than the layers?
The calculator will warn you. Add layers or extend the last layer thickness so the total profile depth exceeds the reporting depth, then recalculate.
6) Are CSV and PDF exports identical to the screen units?
Exports include the same layer model and summary, but numeric fields in the export payload are stored in internal meters and kPa for consistency and traceability.