The Chartership Forum

Foundation Engineering / Pile capacity from SPT and CPT; drilled shafts

Drilled shafts

Where the capacity is decided on site, not on the drawing.

Lesson review: expert verification remains open

This lesson has received a full-context editorial review and a check of its technical explanations. Selected arithmetic has been checked separately. This is not independent engineering sign-off.

Checks needed for project use

  • A geotechnical reviewer must confirm drainage and stress basis, bearing-factor formulation, correlations and settlement influence factors. Partial factors and project-specific ground conditions are outside these teaching checks.
  • Verify CPT averaging/correlation method and drilled-shaft construction quality controls. Driving set alone does not prove static resistance; under-ream applicability requires construction review.

These are educational models and exercises. Check their assumptions before using the results. The full technical content and every diagram have not yet been independently verified.

Concrete and steel calculations retain their stated first-generation Eurocode teaching basis. Use the standards and amendments specified for your project; do not mix generations without an appropriate design basis.

Read the lesson register and outstanding review notes

Why this matters

Both driven and bored piles need construction records and an appropriate verification plan. Driving resistance is influenced by the hammer, wave propagation and soil response and is not a direct static-capacity measurement. Bored-pile performance depends on bore stability, shaft condition, base cleanliness and concrete placement.

Four things decide a drilled shaft's real capacity, and none of them appears in the equation.

Base cleanliness. Spoil, slurry or collapsed material left at the bottom sits between the concrete and the bearing stratum. A base designed to mobilise several megapascals cannot do so through 300 mm of soft debris, and the effect is not partial — the base can be effectively absent.

Softening of the shaft. An open hole in clay swells towards the void, and water from the concrete or from the support fluid softens the interface further. The longer the hole stands open, the lower the α actually achieved.

Support fluid. Bentonite or polymer keeps the hole open, but a filter cake left on the walls reduces the concrete's contact with the soil. Managing the fluid's condition before concreting is a specified activity, not a courtesy.

Concrete placement. Concrete has to displace the fluid upwards from the bottom without mixing with it. A poorly managed tremie leaves inclusions, and necking or contamination in the shaft is both a structural and a geotechnical defect.

Under-reaming is the one thing a bored pile can do that a driven pile cannot: enlarge its own base.

Because base resistance scales with the square of the base diameter, the gain is dramatic. Taking Module 14's pile — 0.6 m shaft, base in clay at su = 90 kPa — and under-reaming the base to 1.5 m multiplies the base area by (1.5/0.6)² = 6.25, raising base resistance from 229 kN to 1431 kN. Total capacity goes from 1502 kN to 2704 kN, an increase of 80%, for a modification affecting only the last metre of the pile.

The conditions are strict: the base must be in a stiff cohesive material that will stand unsupported while the under-ream is cut, and the base must be cleanable afterwards. Groundwater, bore stability, the construction system and available verification methods determine whether an under-ream can be formed reliably. Direct visual inspection is not always possible; specify a suitable quality-control and verification plan.

Check yourself

Why does debris left at the base of a bored pile matter so much more than a similar thickness of soft material along its shaft?

Practice

A pile with a 0.6 m base has a base resistance of 229 kN. Under-reaming the base to 1.5 m in the same clay gives what base resistance, in kN?

Try it

Under-reams, and the deep bearing factor

Two ways a pile base can be worth far more than it first appears — and one way it can be worth far less.

1.50 m
90 kPa
150

Conditions

  • An under-ream needs a stiff cohesive stratum that stands unsupported and a base that can be cleaned and inspected.
  • The deep Nq is illustrative here. This course does not supply a chart, because the right value depends on installation method.
  • The two Nq values are compared to show sensitivity. Neither is selected as a design value for this pile.
1.50 m base
Base area factor
6.25×
Straight-shafted base
229 kN
Under-reamed base
1431 kN
Shaft resistance
1679 kN
Total, straight
1909 kN
Total, under-reamed
3111 kN
Gain
63 %

Base-factor comparison: which Nq

Shallow Nq at φ′ = 39.4°
59.1
Base on the shallow factor
1740 kN
Base on the deep factor
4418 kN
Base with a 15 MPa limit
2945 kN
Removed by the limit
33 %

Enlarging the base from 0.60 m to 1.50 m multiplies the base area by 6.25 — and base resistance with it. All of that gain depends on a base that can actually be cleaned and inspected.

Summary

  • The SPT chain to a pile base has more judgement in it than measurement
  • The deep Nq and the limiting base pressure decided more than the blow count did
  • The CPT suits piles because the cone is itself a small pile
  • A bored pile's capacity is set by base cleanliness, standing time, support fluid and concreting
  • Under-reaming 0.6 m to 1.5 m gave 6.25 times the base resistance — where the base can be inspected