Laboratory and Field Tests in Geotechnical Engineering

NCEES FE Civil spec: Geotechnical Engineering, C. Laboratory and field tests. FE Reference Handbook: Civil Engineering chapter, Geotechnical section; Civil Engineering chapter, Hydrology/Water Resources for permeability tests.

What the FE Civil Spec Covers

This section tests whether you know what each common soil test measures, which soils it suits, and how to reduce the raw data to a design parameter. Most questions are conceptual (which test gives the preconsolidation pressure?) with an occasional calculation such as permeability from a falling head test, relative compaction, or a corrected SPT blow count.

Key Concepts: Laboratory Tests

Test	What it gives you
Sieve and hydrometer analysis	Grain-size curve, D10, D30, D60, percent fines
Atterberg limits	LL, PL, PI for fine-grained classification
Proctor (standard or modified)	Maximum dry unit weight and optimum water content
Constant head or falling head	Hydraulic conductivity k
One-dimensional consolidation (oedometer)	Cc, Cr, preconsolidation stress, cv
Direct shear	Friction angle and cohesion on a forced failure plane
Triaxial (UU, CU, CD)	Total or effective strength parameters under controlled drainage
Unconfined compression	Undrained shear strength of clay, su = qu/2

Key Concepts: Field Tests

Standard Penetration Test (SPT): the number of blows of a 140 lb (63.5 kg) hammer falling 30 in. needed to drive a split-spoon sampler the last 12 in. of an 18 in. drive. It also recovers a disturbed sample.
Cone Penetration Test (CPT): continuous tip resistance and sleeve friction; excellent for stratigraphy, no sample.
Field vane shear: undrained shear strength of soft clay.
Sand cone, rubber balloon or nuclear gauge: field density and water content for compaction control.
Plate load test: bearing pressure versus settlement at a footing-like scale.

Core Formulas

Constant head: k = \frac{Q L}{A h t}

Falling head: k = \frac{a L}{A t} ln (\frac{h _{1}}{h _{2}})

R C = \frac{γ _{d, f i e l d}}{γ _{d, ma x}} \times 100%

N_{60} = N \cdot \frac{E _{m}}{60} \cdot C_{B} \cdot C_{S} \cdot C_{R}

In the falling head formula, $a$ is the standpipe area, $A$ and $L$ are the specimen area and length, and $h_{1}$ , $h_{2}$ are the heads at the start and end of time $t$ . In the SPT correction, $E_{m}$ is the hammer efficiency in percent (roughly 60 for a safety hammer and higher for automatic hammers), and the C factors correct for borehole diameter, sampler type and rod length (short rods reduce the delivered energy).

Where It Is in the FE Reference Handbook

The Civil Engineering chapter, Geotechnical section, defines relative compaction, compression indices and strength terms. The constant head and falling head permeability equations are in the same chapter under Hydrology/Water Resources, next to Darcy's law. The handbook writes the falling head form with 2.303 log10, which equals ln. SPT procedure details are not in the handbook, so know the basics.

Worked Example

A falling head test uses a standpipe 1.0 cm in diameter and a soil specimen 10 cm in diameter and 15 cm long. The head drops from 100 cm to 50 cm in 600 s. Find k.

a = \frac{π ( 1.0 ) ^{2}}{4} = 0.785 cm^{2} A = \frac{π ( 10 ) ^{2}}{4} = 78.54 cm^{2}

k = \frac{( 0.785 ) ( 15 )}{( 78.54 ) ( 600 )} ln (\frac{100}{50}) = (2.50 \times 1 0^{- 4}) (0.693)

k = 1.73 \times 1 0^{- 4} cm/s = 1.73 \times 1 0^{- 6} m/s

That value is typical of a silt or very fine silty sand, which is exactly the range where a falling head test is the right choice. Clean sands and gravels, with k around 10⁻³ m/s or higher, would drain the standpipe in seconds and are better tested with a constant head setup. Clays, with k below about 10⁻⁹ m/s, are often evaluated indirectly from consolidation test data because direct tests take so long.

Quick SPT check. A field count of N = 20 with a 75 percent efficient hammer, standard borehole and sampler (C = 1.0) and a rod length correction of 0.95 gives N60 = 20 x (75/60) x 0.95 = 23.75, about 24 blows per foot.

Common Mistakes

Using a natural log versus base 10 log incorrectly in the falling head equation. The formula above uses ln.
Mixing the standpipe area and the specimen area.
Comparing field dry unit weight to the moist unit weight from the Proctor curve instead of the maximum dry unit weight.
Thinking the SPT measures strength directly. It is a blow count that is correlated to strength and density.
Forgetting that unconfined compression strength is twice the undrained shear strength.

Practice Problems

Try these from the free PolyCalc FE Civil problem bank. Each one has a full worked solution.

The standard penetration test (SPT) N-value is based primarily on which measurement? See the worked solution or practice the Soil Mechanics deck
A field compaction test gives a dry unit weight of 17.2 kN/m³. The laboratory maximum dry unit weight from a standard Proctor test is 18.5 kN/m³. The relative compaction is most nearly: See the worked solution or practice the Soil Mechanics deck
A 2 m thick sand layer with hydraulic conductivity 5 × 10⁻⁴ m/s overlies a 1 m thick clay layer with hydraulic conductivity 1 × 10⁻⁷ m/s. For vertical flow through both layers, the equivalent hydraulic conductivity is most nearly: See the worked solution or practice the Soil Mechanics deck
A clay has a preconsolidation stress of 240 kPa and a current effective overburden stress of 120 kPa. Its overconsolidation ratio (OCR) is: See the worked solution or practice the Soil Mechanics deck

Related Calculators and Examples

Frequently Asked Questions

What is the difference between a constant head and a falling head permeability test?

A constant head test keeps the water level fixed and measures the volume of flow; it suits sands and gravels with high permeability. A falling head test lets the water level in a standpipe drop and records the time; it suits silts and less permeable soils where flow volumes are small.

Why is the SPT blow count corrected to N60?

Different hammers deliver different fractions of the theoretical free-fall energy. Correlations for friction angle, relative density and bearing capacity were developed for about 60 percent efficiency, so the field count is scaled to that energy level before it is used.

What is relative compaction?

Relative compaction is the field dry unit weight divided by the maximum dry unit weight from the laboratory Proctor test, expressed as a percent. Specifications commonly require 90 to 95 percent.

Which test gives the preconsolidation pressure of a clay?

The one-dimensional consolidation (oedometer) test. Plotting void ratio against the log of effective stress shows a break in slope near the preconsolidation pressure. The same test gives the compression index, the recompression index and the coefficient of consolidation.

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