Two-layer soil resistivity model from 4-pin probe measurements per IEEE 81-2012. Supports Wenner 4-pin (§5.1) and Schlumberger (§5.2) arrays. Enter raw resistance (Ω) or pre-computed apparent resistivity — the calculator handles both. Output includes two-layer parameters ρ₁ / ρ₂ / h, 3-zone trench model for cable ampacity, 2-zone bore model, seasonal correction, and sufficiency flag.
Cross-discipline: grounding studies (IEEE 80), cable burial ampacity (Neher-McGrath / IEC 60287), earth electrode design. Applies to solar grounding grid and cable trench design.
Fitted Model Parameters
—
Ω·m
Upper layer resistivity ρ₁ (Ω·m)
—
Ω·m
Lower layer resistivity ρ₂ (Ω·m)
—
m
Upper layer depth h (m)
RMS fit error:—RMS error (% of mean):—Data points:—
Measured vs. Modeled Resistivity
Residuals (% of measured)
a (m)
Measured ρ
Residual
Sufficiency Assessment
Seasonal / Worst-Case Model
Worst-Case Parameters (1.4× early spring / winter)
—
ρ₁ worst-case (Ω·m)
—
h worst-case (m)
3-Zone Trench Model (Cable Ampacity)
Effective trench thermal resistivity:—Ω·m
Zone
Dimensions
Moisture
ρ (Ω·m)
2-Zone Bore Model
Effective bore thermal resistivity:—Ω·m
Zone
Dimensions
Moisture
ρ (Ω·m)
Export model report
Enter your email to receive a formatted IEEE 81 soil resistivity report with model parameters, trench/bore zone data, measurement table, and clause references.
Worked Example — Two-Layer (ρ₁=100, ρ₂=500, h=3 m)
50
Ω·m
Upper layer resistivity ρ₁ (Ω·m)
300
Ω·m
Lower layer resistivity ρ₂ (Ω·m)
2
m
Upper layer depth h (m)
RMS: 83.879 Ω·m · 7 data points
Need grounding grid design from this soil model?
FrameAI combines the IEEE 81 soil model (ρ₁, ρ₂, h) with IEEE 80-2013 to compute mesh voltage, touch voltage, and conductor sizing — automatically.
The Wenner four-terminal method (IEEE 81 §5.1) places four equally-spaced electrodes in a straight line on the soil surface. Current is injected between the outer two electrodes and the resulting voltage difference is measured between the inner two. The apparent resistivity is computed as ρ_app = 2πaR, where a is the electrode spacing and R is the measured resistance.
What is the Schlumberger method?
The Schlumberger array (IEEE 81 §5.2) uses the same four electrodes but with MN (voltage) much smaller than AB (current). The apparent resistivity is ρ_app = (π/ ln(c/m)) · (c²−m²) · R where c = AB/2 and m = MN/2. This method has better resolution at large spacings and requires less electrode repositioning — preferred for deeper investigation.
What does the sufficiency flag mean?
IEEE 80 §13.2 recommends at least 5 measurement points for a confident two-layer model. The calculator flags < 5 points as INSUFFICIENT and recommends a new test plan. Points below 3 are flagged as INADEQUATE. High RMS fit error (> 25%) and extrapolation beyond the tested depth range are also flagged as warnings.
How do I use the 3-zone trench model for cable ampacity?
The 3-zone model (dry-out zone, recompacted zone, native zone) provides zone-specific thermal resistivities that feed directly into the Neher-McGrath / IEC 60287 cable ampacity calculation. The effective trench thermal resistivity (ρ_eff) is the depth-weighted average of the three zones. Use the worst-case seasonal factor (early spring/winter = 1.4×) for design in seasonally cold climates.
When should I enter fill material and base course data?
Enter fill material resistivity and depth when the cable route passes through a dedicated bedding layer (e.g., sand, controlled low-strength material, imported granular fill). Similarly enter base course and insulative rock data when the cable is in a road or pavement structure. This overrides the native soil resistivity for the trench zone calculations.
How do I account for frost depth?
Enter the frost depth in meters and select the early-spring/winter seasonal factor (1.4×) when the site is in a seasonally frozen climate. The frost factor (up to 1.5×) is applied to the upper layer resistivity in both the trench and bore zone models. The frost justification section explains the engineering basis for the correction.