The governing standard is ASTM D8169/D8169M-26 (Standard Test Methods for Deep Foundations Under Bi-Directional Static Axial Compressive Load), which is separate from ASTM D1143 used for top-down compression testing from the pile head. This article explains how the test works, what data it provides, how it differs from Kentledge and Reaction Pile setups, when it is suitable, and its limitations, as general knowledge for project owners and contractors.

The first figure shows the principle with the cell embedded near the pile toe. Note the arrow directions for the upper and lower sections, and the chart on the right showing that the same force moves the two sections in opposite directions.

Principle of the Bi-Directional Static Load Test (O-cell) in a bored pile, with a chart of load versus two-way movement The left figure is a section through a bored pile in soft clay, stiff clay and dense sand layers, with an O-cell pressure cell and bearing plates embedded near the pile toe. When hydraulic pressure is applied from a pump at ground surface through a hose, the cell pushes the upper pile section up, resisted by skin friction, and pushes the lower section down, resisted by end bearing. Telltale rods and gauges on a reference beam measure the movement of the upper and lower plates. The right figure is a chart of O-cell load versus movement: the upper-section curve moves up and the lower-section curve moves down. The strip at the bottom summarizes that no kentledge is needed, skin friction and end bearing are separated in a single test, and the two opposing forces are always equal, so the test stops when either side reaches its limit. O-cell principle: push from inside the pile, 2 ways Not to scale Soft clay Stiff clay Dense sand layer Hydraulic pump Upper section pushed up ↑ Resisted by skin friction (Qs) Telltale + gauges Upper / lower plate movement O-cell (pressure cell) Hydraulic jack + steel plates Lower section pushed down ↓ Resisted by end bearing (Qb) Load–movement curve, 2 directions ↑ Upward ↓ Downward O-cell load → Upper (Qs) Lower (Qb) Both forces always equal Weaker side limits the test 1 No kentledge needed No anchor piles / reaction beam 2 Separates Qs and Qb in a single test 3 Reference standards ASTM D8169/D8169M-26
O-cell principle with the cell embedded near the pile toe — the upper section is pushed up and resisted by skin friction (Qs); the lower section is pushed down and resisted by end bearing (Qb). The two opposing forces are always equal, so the test stops when either side reaches its limit first. (Diagram: SPN Soil Engineering)

How Does a Bi-Directional Load Test Work?

A Bi-Directional Load Test uses a hydraulic jack embedded in the pile to push two pile sections apart. The O-cell assembly is attached to the reinforcement cage at the design depth and then encased in concrete. Once the concrete has gained strength, the cell is pressurized to expand, pushing the upper section up and the lower section down simultaneously. The reactions in the two directions balance each other within the pile, with no external kentledge required.

  • Upper section (above the cell) is pushed up → measures the skin friction of the length above the cell; the self-weight of this pile section must be deducted during interpretation
  • Lower section (below the cell) is pushed down → measures the combined resistance of lower skin friction and end bearing; if the cell is near the pile toe, this value approaches the end bearing
  • Instruments measure the jack pressure and the movement of the upper and lower plates → giving two-way load-movement curves

The key advantage in principle is that the pile acts as both the loaded element and the reaction, so very high loads can be tested in a limited space. The upward and downward forces are always equal, so the test ends when either section reaches its limit first.

The data obtained directly from the O-cell is only the resistance of the sections above and below the jack. To separate skin friction by soil layer, or to separate end bearing from lower skin friction, additional strain gauges must be installed at several levels in the pile — the same principle as the Instrumented Static Load Test

The 2D (side section) and 3D figures below show the case with the O-cell assembly embedded at the balance point. Note the positions of the upper and lower steel plates, the jacks, the telltale rods and the resistance arrows. The upper section is resisted by skin friction along the upper half; the lower section is resisted by skin friction along the lower half plus end bearing.

2D model (side section) of a Bi-Directional Static Load Test (O-cell) installation in a bored pile A bored pile with an O-cell assembly embedded at the balance point, consisting of upper and lower steel plates with hydraulic jacks between them. When pressurized, the upper pile section is pushed up, resisted by upper-half skin friction, and the lower section is pushed down, resisted by lower-half skin friction and end bearing. Telltale rods run from the upper and lower plates up to the pile head. LVWDT sensors measure the plate separation. A reference beam with dial gauges measures movement at the pile head, and hydraulic lines run to the pump and data logger at ground level. Side section (2D) Not to scale Ref. beam + gauges Measures pile-head movement Telltale rods Down to upper / lower plate Upper-half skin friction Resists upward move Balance point Upper ≈ lower resist. Lower-half skin friction Resists downward move End bearing End Bearing Upper part ↑ Measures upper skin friction Upper/lower steel plates Fixed to rebar cage O-cell (hydraulic jack) LVWDT Measures plate separation Lower part ↓ Measures end bearing + lower skin Hydraulic pump + data logger Pressure · 2-way movement Hydraulic line From pump down to jack
2D model (side section) — the O-cell assembly is embedded at the balance point, designed so the upper-half resistance is close to the lower-half resistance. Telltale rods measure the movement of the upper and lower plates separately and the LVWDT measures plate separation, giving two-way load–movement curves. The reference beam at the pile head checks pile-head movement. (Figure: SPN Soil Engineering)
3D model of a Bi-Directional Static Load Test (O-cell) with a soil cutaway and a transparent pile A 3D view with one corner of soil cut away to show the full bored pile. The O-cell assembly is embedded in the lower part of the pile. When pressurized, the upper section is pushed up and the lower section is pushed down. Telltale rods and hydraulic lines run up to the pile head. A reference beam spans the pile head on supports resting on the ground at both sides. The hydraulic pump and data logger stand on the ground. Detail box A shows an enlarged O-cell assembly: upper steel plate, lower steel plate, 3 jacks and the LVWDT. Not to scale A Detail A — O-cell assembly (enlarged) Upper steel plate Hydraulic jack ×3 LVWDT: plate gap Lower steel plate Telltale rods and hydraulic lines fixed to plates Ref. beam + gauges Supports outside zone of influence Telltale rods Down to upper / lower plate Upper part ↑ Upper-half skin friction resists Lower part ↓ End bearing + lower-half skin Bored pile Transparent view Hydraulic pump + data logger Pressure · 2-way movement Hydraulic line Pressurize jacks Balance point Upper ≈ lower resist.
3D model — the O-cell assembly divides the pile into two sections. Jack pressure pushes the upper section up and the lower section down simultaneously, with no kentledge on the pile head. Box A shows the upper and lower steel plates, 3 jacks and the LVWDT that measures plate separation. (Figure: SPN Soil Engineering)

How Does the O-cell Differ from Kentledge and Reaction Piles?

The O-cell differs from conventional methods in that the load comes from inside the pile rather than being applied from the top. A conventional Static Load Test uses one of two main systems: Kentledge (actual dead weight) and Reaction Pile (anchor piles holding down a beam). Both systems must provide a reaction above the test pile at least equal to the maximum test load. Read more about these two systems in Kentledge vs Reaction Pile: What Is the Difference?

Comparison of the source of test load in 3 systems: Kentledge, Reaction Pile and Bi-Directional O-cell Three panels from left to right. Panel one, Kentledge: dead-weight blocks sit on a platform above the main beam, which rests on cribbing stacks on both sides; a jack between the test pile head and the main beam pushes the pile down with the kentledge as the reaction. Panel two, Reaction Pile: the main beam is tied to anchor piles on both sides with tie bars; the jack pushes the test pile down and the anchor piles take uplift as the reaction. Panel three, O-cell: a pressure cell is embedded in the bored pile at the balance point; when pressurized, the upper section is pushed up and the lower section down simultaneously, and the two forces balance within the pile, leaving only a reference beam and gauges on the pile head. Below each panel is a summary of maximum test load, site area, advance preparation and reference standard: Kentledge and Reaction Pile to ASTM D1143, O-cell to ASTM D8169/D8169M-26. Where the test load comes from: 3 systems Not to scale Kentledge Dead weight pressing from top Reaction Pile Anchor piles + steel beam Bi-Directional (O-cell) Load from inside the pile P down Kentledge ≥ max. test load P down Anchor piles take uplift Balance point Pump No external reaction needed Pile head = ref. beam + gauges Max. test load Limited by kentledge Site area Must be large Advance prep. No cell to embed Reference standards ASTM D1143 Max. test load Limited by anchor piles Site area Needs nearby piles Advance prep. No cell to embed Reference standards ASTM D1143 Max. test load Very high Site area Small footprint Advance prep. Cell cast in before concreting Reference standards ASTM D8169/D8169M-26
Three different load sources — Kentledge uses actual dead weight pressing from the top, Reaction Pile pulls a steel beam through anchor piles, while the O-cell generates the load directly inside the pile, so no external reaction is needed. (Diagram: SPN Soil Engineering)

← Scroll to see the full table →

TopicKentledgeReaction PileBi-Directional (O-cell)
Load sourceDead weight from aboveAnchor piles + steel beamPressure cell inside the pile
Max. test loadLimited by kentledgeLimited by anchor pilesVery high; limited by cell capacity and the side with lower resistance
Site areaRequires a large areaRequires surrounding pilesSmall footprint
Cell installationNot requiredNot requiredMust be embedded before concreting
Best forModerate-capacity pilesModerate–high capacityVery high-capacity bored piles
Reference standardsASTM D1143ASTM D1143ASTM D8169/D8169M-26

The key point is that Kentledge requires a large amount of dead weight to be brought to site, while Reaction Pile requires anchor piles to be prepared around the test pile. When piles are designed for very high loads, providing either type of reaction becomes harder and the cost rises with the test load. The O-cell was developed to overcome this limitation.

When Should a Bi-Directional Test Be Used Instead of Conventional Methods?

A Bi-Directional Test is suitable when bored piles carry such high loads that Kentledge or Reaction Pile setups become uneconomical or impractical. Situations where this method is commonly considered include:

  1. Large bored piles carrying several thousand tonnes — beyond what a typical kentledge system can conveniently handle
  2. Limited site space — no room to place kentledge or install anchor piles
  3. Over-water or offshore work — transporting kentledge is very difficult
  4. Separate upper and lower resistance is required — the O-cell measures these two sections separately and directly; with the cell near the pile toe, the result approaches end bearing, while separation by soil layer requires additional strain gauges

The choice of test method must also consider the pile type, soil profile, budget and project specifications, and should be decided by the responsible engineer.

What Are the Limitations of the O-cell?

The main limitations of the O-cell are that the decision must be made before concreting, the cell level must be chosen so that the resistance on both sides is balanced, and the results must be interpreted by an experienced engineer. Details are as follows:

  • Requires advance planning — the cell must be embedded in the pile before concreting; it cannot be retrofitted to a completed pile
  • Cell position matters — the two opposing forces are always equal; if one side has much lower resistance, it reaches its limit first and the test must stop, so the other side is not tested to full capacity
  • Specialized interpretation — the two-way load-movement curves must be combined into an equivalent top-load curve, which requires an experienced engineer
  • Reusing the test pile must be decided from the outset — whether it is possible depends on the design and on grouting the gap at the cell level after testing

Because of the high level of technical detail, the choice between the O-cell and other methods must be analyzed case by case. For large bored piles, concrete integrity testing should be planned alongside, such as Cross Hole Sonic Logging (CSL), which likewise requires tubes to be installed before concreting.

Summary

A Bi-Directional Static Load Test (O-cell) tests high-capacity bored piles using a pressure cell inside the pile that pushes up and down, instead of loading from the top. Its strengths are the ability to test very high loads in a limited space and to obtain the resistance above and below the cell separately in a single test. It suits projects where Kentledge or Reaction Pile setups are difficult, at the cost of planning the cell embedment in advance, more complex interpretation, and the need for additional strain gauges if layer-by-layer data is required.

SPN Soil Engineering does not provide O-cell testing, but offers pile testing services by Static Load Test, PDA and Seismic Integrity Test, and can help assess which test method suits your project.