Technical Documentation

SPT Calculation
Methodology

A transparent breakdown of the mathematical models and correlations used by Borebase to process Standard Penetration Test (SPT) data.

Input Parameters

SPT analysis begins with the N-value (blow count) and site-specific conditions.

Measured Variables

SPT blow count N (N2 + N3)
Depth below ground surface (m)
Hammer energy ratio (%)

SPT Corrections

Raw SPT N-values are corrected for equipment and procedural factors to obtain standardized N60 values.

Energy Correction

Standardizes the measured N-value to 60% hammer energy efficiency.

sptMethodology.section02.boreholeDiameter	CB
65-115 mm	1.00
150 mm	1.05
200 mm	1.15

sptMethodology.section02.rodLength	CR
< 3m	0.75
3-4m	0.80
4-6m	0.85
6-10m	0.95
> 10m	1.00

Corrected N60 Calculation

Combines all correction factors to obtain the standardized blow count.

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

Stress Calculations

Vertical stresses are calculated considering groundwater effects for overburden correction.

Total Vertical Stress

\sigma_v = \gamma_{dry} \cdot h_{dry} + \gamma_{sat} \cdot h_{sat}

sptMethodology.section03.porePressure

u = \gamma_w \cdot (z - GWT)

Effective Vertical Stress

\sigma'_v = \sigma_v - u

Overburden Correction (N1)

Normalizes SPT values for confining pressure effects using Liao & Whitman (1986) approach.

N_1 = N_{60} \cdot C_N

C_N = \left(\frac{100}{\sigma'_v}\right)^{n}

TIP

The n-factor varies by soil type. CN is typically capped at 2.0 to avoid overcorrection at shallow depths.

Friction Angle

Internal friction angle (φ') is estimated using three empirical correlations. All methods use the overburden-corrected N1 value.

Jamiolkowski et al.

\phi' = 20.6 + 11 \cdot \arcsin\left(\frac{N_1}{54}\right)

Valid for N1 ≤ 54

Hatanaka & Uchida (1996)

\phi' = \sqrt{20 \cdot N_1} + 20

Valid for 3.5 ≤ N1 ≤ 30

Wolff (1989) / Peck et al. (1974)

\phi' = 27.1 + 0.3 \cdot N_1 - 0.00054 \cdot N_1^2

Valid for N1 < 50

TIP

Borebase calculates all three methods simultaneously, allowing engineers to apply judgment based on soil conditions and local experience.

Relative Density

Relative density (Dr) indicates the denseness of granular soils. Borebase calculates two widely-used correlations for comparison.

sptMethodology.section05.method1

sptMethodology.section05.method1Desc

D_r = \sqrt{\frac{N_1}{60}} \times 100 \quad (\%)

sptMethodology.section05.method2

sptMethodology.section05.method2Desc

D_r = \sqrt{\frac{N_1}{27 + 0.28 \cdot \sigma'_v}} \times 100 \quad (\%)

TIP

sptMethodology.section05.note

Engineering Parameters

Additional geotechnical parameters are derived from the corrected SPT values.

CPT Equivalence

Estimates equivalent CPT cone resistance from SPT data.

q_c = N_1 \cdot \frac{c_{factor}}{10} \quad (MPa)

Elastic Modulus

Provides a range of elastic modulus values based on soil type.

E_{min} = N_1 \cdot \frac{E_{s,min}}{1000} \quad (MPa)

E_{max} = N_1 \cdot \frac{E_{s,max}}{1000} \quad (MPa)

Bearing Capacity

Ultimate bearing capacity for preliminary foundation design.

q_{a,ult} = N_1 \cdot q_{factor} \quad (kPa)

INFO

Only calculated for granular soils.

Soil Type Parameters

Borebase uses soil-type-specific factors for various correlations.

Soil Type	n	c	Es min	Es max	qa_ult
Gravel	0.5	6.0	1200	2000	600
Coarse Sand	0.5	5.0	800	1500	450
Medium Sand	0.5	4.5	600	1200	350
Fine Sand	0.5	4.0	500	1000	300
Silty Sand	0.5	3.5	400	800	250
Silt	0.6	2.5	250	500	150
Sandy Clay	0.7	2.0	200	400
Silty Clay	0.8	1.5	150	300
Clay	1.0	1.0	100	200

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