Foundation quality inspection


Osterberg self-balancing test (or O-cell test, bi-directional static load test) is a new static load test technology for pile invented and commercialized by an American Professor Jorj Osterberg.


Bi-directional static load test is actual static load test.

Bi-directional static load test uses pile’s skin friction and end bearing for reaction system.

The hydraulic jack assembly is cast in the pile instead of at the top of the pile as normal static load test. Hydraulic jacks are welded between two bearing steel plates and attached to steel cage, positioning of hydraulic jack assembly is determined based on soil data. The main aim of positioning the assembly will be to equalize the bi-directional forces in the pile (skin friction of above segment and skin friction of below segment with end bearing)

Bi-directional static load test measures movement in the top and bottom of jack, not at pile top as normal static load test.

In single-level bidirectional static load test:

  • Dividing the pile into two segments (top and bottom), using each other as reaction.
  • Installing bi-directional jacks to exert equal and opposite force in the pile
  • Positioning of jack level is based on the point of equal resistance in the pile (can be known from pile’s cumulative resistance)
  • Upper and lower shaft is loaded separately (pile can be mobilized more effectively)

In multi-level bidirectional static load test:

  • If pile dimension is small, the jack assembly cannot be accommodated, using multi-level bidirectional static load test can breakup the overall test load for each segment to carry.
  • Multi-level jacks, normally two levels, can be installed to better mobilized each pile segments. Pile is divided into 3 segments (two levels).
  • Multi-level bidirectional static load test test was conducted in 3 steps:
  • 1st step: Segment 3 will be mobilized using Segment 1 & 2 as reaction.
  • 2nd step: Segment 1 & 2 will be mobilized using each other’s reaction, with Level 2 jack released.
  • 3rd step (optional): If Segment 2 has reached ultimate before Segment 1, Level 2 jack will be locked to enable Segment 2 & 3 as reaction to continue to mobilize Segment 1.
  • Very suitable for ultimate test pile, because pile is divided into smaller segment with proportionally smaller resistance
  • Each segment can be more effectively mobilized to better determine the resistance characteristic

Each segment is then mobilized separately, and the multiple results from the same pile are combined to access the overall pile performance



  • ASTM D1143/ D1143M-07 “Standard Test Methods for Deep Foundations Under Static Axial Compressive Load”
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