Use Spt N Values for Settlement Calculations

SPT-N values are a critical parameter in geotechnical engineering for assessing soil strength and settlement potential. This guide explains how to use SPT-N values in settlement calculations, including the standard penetration test procedure, interpretation methods, and practical applications in construction projects.

What Are SPT-N Values?

The Standard Penetration Test (SPT) is a common in-situ test used to determine the relative density and consistency of soil. The SPT-N value represents the number of blows required to drive a standard split-spoon sampler 30 cm into the soil. Higher SPT-N values indicate denser, stronger soil.

SPT-N values are used in settlement calculations to estimate how much a structure will settle under its own weight. The relationship between SPT-N values and soil settlement is based on empirical correlations developed from field observations and laboratory testing.

How to Use SPT-N Values

Using SPT-N values for settlement calculations involves several steps:

Obtain SPT-N values from soil borings at the construction site.
Adjust the SPT-N values for overburden pressure and energy correction factors.
Use the corrected SPT-N values in settlement prediction formulas.
Compare the predicted settlement with allowable settlement limits.

Energy Correction Factor

The energy correction factor (C_N) accounts for differences in hammer energy between the field test and standard conditions:

C_N = (E_field/E_standard)^0.75

Where E_field is the field hammer energy and E_standard is the standard hammer energy (60 ft-lb).

Settlement Calculation Methods

Several methods exist for calculating settlement using SPT-N values:

1. Terzaghi and Peck Method

This empirical method estimates settlement based on soil type and SPT-N values:

Settlement (mm) = C₁ × C₂ × C₃ × (q × D × I_c / SPT-N)

Where:

C₁ = 0.1 for loose sand, 0.2 for medium sand, 0.3 for dense sand
C₂ = 1.0 for normally consolidated soils, 0.8 for overconsolidated soils
C₃ = 1.0 for square footings, 0.8 for circular footings
q = foundation pressure (kPa)
D = depth of embedment (m)
I_c = correction factor for overburden pressure

2. Schmertmann Method

This method uses SPT-N values to estimate settlement for shallow foundations:

Settlement (mm) = (q × B × C_s / SPT-N) × (1 - v_u/v_s)

Where:

q = foundation pressure (kPa)
B = width of foundation (m)
C_s = soil compression index
v_u = ultimate settlement
v_s = settlement at service load

Example Calculation

Let's calculate settlement using the Terzaghi and Peck method for a square footing on medium sand with these parameters:

SPT-N = 25
Foundation pressure = 150 kPa
Depth of embedment = 2 m
Overburden correction factor = 1.2

Settlement = 0.2 × 1.0 × 1.0 × (150 × 2 × 1.2 / 25) = 11.52 mm

This calculation suggests a settlement of 11.52 mm, which should be compared with the allowable settlement limit for the structure.

FAQ

What is the difference between SPT-N and corrected SPT-N?: The raw SPT-N value is adjusted for overburden pressure and hammer energy to get the corrected SPT-N value, which better represents the soil's in-situ strength.
How accurate are settlement predictions using SPT-N values?: SPT-N based settlement predictions are generally accurate within ±30% for typical construction projects, but site-specific factors can affect results.
What are the limitations of using SPT-N values for settlement calculations?: SPT-N values may not account for soil variability, layering, or other geotechnical factors that can affect settlement behavior.
How often should SPT tests be performed for a construction project?: SPT tests should be performed at key locations and at intervals based on soil variability and project requirements, typically every 10-15 meters.
Can SPT-N values be used for all soil types?: SPT-N values are most reliable for cohesionless soils like sand and gravel. For cohesive soils like clay, other tests may be more appropriate.