Module 16 · Lesson 16.2
Drilled shafts
Where the capacity is decided on site, not on the drawing.
Why this matters
A driven pile proves itself as it goes in: the set gives a direct, if crude, measure of resistance. A bored pile proves nothing. It is a hole full of concrete, and whether it performs as calculated depends on things that happen underground, out of sight, in the hours before the concrete arrives.
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. Under-reaming in sand or below the water table is not normally practicable, and an under-ream whose base cannot be inspected buys little.
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.
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 shallow Nq is shown only so the size of that error is visible.
- 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 %
The other base error: 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
This is educational material. It uses simplified examples to teach principles, and must not be relied on for real design or safety-critical decisions. Module overview and checkpoint