SPT Analysis

SPT Data Tab
Deep Dive

Import, view, and analyze Standard Penetration Test data. Automatic N60 corrections, friction angle calculations using multiple methods, and engineering parameter estimation.

Interface Overview

The SPT Data tab provides comprehensive data management with industry-standard corrections:

Data point count, depth range, and action buttons (Settings, Export, Import, Add Point).
Equipment parameters (borehole diameter, sampler type, hammer energy) and column visibility toggles.
Quick statistics: data points count, depth range, average N, maximum N.
Full data grid with blow counts, corrections, and engineering parameters.

Equipment Parameters

Before analyzing SPT data, configure the equipment parameters that affect correction factors:

Borehole Parameters

Parameter	Options	Correction
65-115 mm	Standard	1.00
150 mm	Larger	1.05
200 mm	Large	1.15

Sampler Type

Type	Description	Correction
Standard sampler	With liner	1.10
Sampler without liner	Dense sand, clay	1.00
Sampler without liner	Loose sand	1.00

Rod Length Correction (CR)

Rod Length Correction (CR)	Correction
< 3 m	0.75
3 ≤ L < 4 m	0.80
4 ≤ L < 6 m	0.85
6 ≤ L < 10 m	0.95
≥ 10 m	1.00

Tip

Click the Settings button to access equipment parameters. Changes are saved automatically and recalculate all derived values.

Open the Metadata tab to review the SPT type and equipment settings.

In edit mode, check the SPT type, hammer weight, drop height, rod length, borehole diameter and whether a plastic liner is used. Save the settings for this test.

Understanding Data Columns

Data columns are organized into four categories:

Blow Counts (Editable)

Column	Unit	Description
Depth	m	Test depth from surface
N1c	-	Calibration blows (first 15 cm, seating)
N2	-	Second 15 cm blow count
N3	-	Third 15 cm blow count
N	-	Total N value (N2 + N3)
ER	%	Energy ratio (default 60%)

Soil Parameters (Editable)

Column	Unit	Description
γd	kN/m³	Dry unit weight of soil
γsat	kN/m³	Saturated unit weight of soil
Soil Type	-	Select from stratigraphy layers

Corrected Values (Read-Only)

Column	Unit	Description
N60	-	Energy-corrected N value (N × ER/60)
Hd	m	Height of dry soil column
Hs	m	Height of saturated soil column
σv	kPa	Total vertical stress
u	kPa	Pore water pressure
σ'v	kPa	Effective vertical stress

Engineering Parameters (Read-Only)

Column	Unit	Description
N1	-	Overburden-corrected N60
qc	MPa	CPT equivalent cone resistance
φJ	°	Friction angle (Jamiolkowski method)
φH	°	Friction angle (Hatanaka-Uchida)
φW	°	Friction angle (Wolff method)
DrKM	%	Relative density (Kulhawy & Mayne)
DrS	%	Relative density (Skempton)
Es	MPa	Minimum elastic modulus
Esmax	MPa	Maximum elastic modulus
qa	kPa	Ultimate bearing capacity

N60 Energy Correction

The N60 value normalizes SPT blow counts to 60% energy efficiency:

N_{60} = N \cdot \dfrac{ER}{60} \cdot C_B \cdot C_S \cdot C_R

Factor	Description
ER	Energy ratio of hammer (typically 45-80%)
CB	Borehole diameter correction
CS	Sampler type correction
CR	Rod length correction

Info

N60 is the standard reference value used in most correlations. A field N value with 45% energy needs to be multiplied by 0.75 (45/60) to get the equivalent N60.

Friction Angle Calculations

Borebase calculates friction angle using three established methods:

Jamiolkowski et al. (1988)

\varphi = 20.6 + 11.0 \cdot \arcsin\!\left(\dfrac{N_1}{54}\right)

Best for normally consolidated sands. Uses overburden-corrected N value.

Hatanaka & Uchida (1996)

\varphi = \sqrt{20 \cdot N_1} + 20

Widely used correlation for granular soils. Simple and reliable.

Wolff (1989)

\varphi = 27.1 + 0.3\,N_1 - 0.00054\,N_1^2

Uses the overburden-corrected N1 value to estimate the friction angle.

Tip

Compare results from all three methods. If they vary significantly, investigate soil conditions or test quality. For design, consider using the most conservative value.

Relative Density Estimation

Two methods for estimating relative density of granular soils:

Kulhawy & Mayne (1990)

D_r = \sqrt{\dfrac{N_1}{60}} \cdot 100

Uses overburden-corrected N1 value. Suitable for clean sands.

Skempton (1986)

D_r = \sqrt{\dfrac{N_1}{27 + 0.28\,\sigma'_v}} \cdot 100

Accounts for vertical effective stress. Coefficients a and b depend on soil type.

Very Loose
Loose
Medium Dense
Dense
Very Dense

Elastic Modulus Estimation

The elastic modulus range is estimated from N1 values:

E_s = N_1 \cdot \dfrac{E_{s,\min}}{1000}\quad\ldots\quad N_1 \cdot \dfrac{E_{s,\max}}{1000}

Warning

These correlations are approximate. For important projects, confirm with laboratory tests (triaxial, pressuremeter) or use more refined correlations for specific soil types.

Importing SPT Data

Import SPT data from CSV or Excel files:

Click Import in the Data tab toolbar to open the import wizard.

Drop your CSV or Excel file into the upload area, or click Select File to choose it.

The wizard attempts to detect the columns automatically. Review the detected fields and any warnings before continuing.

Set the first data row so that header rows are skipped. Check the depth unit, water level and, for CSV files, the delimiter and decimal separator.

Map the depth column and either N2 plus N3 or N Total. Map optional fields such as energy ratio, unit weights and soil type when they are present.

Review the data preview and warnings. Check whether Replace Existing Data is selected, as this option replaces the current measurements.

Click Import after checking the displayed point count. Wait for the result, then review any reported errors.

Tip

Your file should have a header row. Common column names like "Depth", "N1", "N2", "N3" are recognized automatically.

Soil Type Selection

Link SPT data points to stratigraphy layers for automatic unit weight assignment:

Click the Soil Type dropdown for any data row.
Select from available stratigraphy layers defined in the Strata tab.
Unit weights (γ dry, γ sat) are automatically populated from the layer properties.
All stress calculations update automatically.

Warning

If the unit weight is less than 9.81 kN/m³, a warning icon appears. This indicates a potentially unrealistic value that should be verified.

Adding a Data Point

You can add individual SPT measurements manually in the Data tab.

Click Add Point in the toolbar to open the entry form.

Enter the depth and the available blow counts (N2, N3 or N Total). Add unit weights and a soil type if available, then click Add.

Settings

Click Settings in the Data tab toolbar to open calculation and display settings.

Under Calculation Parameters, set the water level and click Apply & Recalculate to update the calculated values.

Under Visible Columns, expand Raw, Calculated or Engineering and select the columns to show in the table.

Select Export only visible columns to include only the displayed columns in CSV and Excel exports. Clear it to export all available columns.

Exporting SPT Data

Export your SPT data with all calculations for reports:

Click Export in the Data tab toolbar.

Choose Export CSV or Export Excel to download the data in the selected format.

All visible columns in comma-separated format. Open in Excel, Python, or any data tool.
Professional formatting with styled headers, proper column widths, and formulas preserved.

Tip

Use column visibility toggles to customize which parameters appear in the export. Hide columns you don't need before exporting for cleaner reports.

Best Practices

Data Entry

Always enter the calibration blows (N1c) even though they're not used in N calculation.
Verify energy ratio with your testing equipment - don't assume 60%.
Link data points to stratigraphy layers for consistent unit weights.

Quality Control

Watch for refusal values (N > 50) which may indicate hitting bedrock or very dense layers.
Check for unit weight warnings (yellow triangle icons).
Compare friction angles from different methods - large discrepancies may indicate issues.

Interpretation

SPT correlations work best for sands and gravels. Use caution with clays.
Consider using the conservative (lower) friction angle for design.
Elastic modulus ranges are wide - use laboratory tests for important projects.

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