Fundamentals of the Bi-directional Expansive Cell Test
The bi-directional static load test is based on the installation of one or more expansive hydrostatic cells within the deep foundation element before concrete placement. Unlike conventional top-down load tests, which require heavy surface reaction systems anchored by tie-downs or tension piles, the bi-directional method uses the structural element itself and the surrounding soil as mutual reaction.
Applying pressure to the cell generates forces of equal magnitude in opposite directions. The upper half of the pile is pushed upwards, while the lower half is pressed downwards. In Brazil, the procedure for static load tests on deep foundations is governed by ABNT NBR 16903:2020, while general guidelines for design and performance control are found in ABNT NBR 6122:2022. Internationally, ASTM D8169/D8169M-26 details the specific execution procedures and instrumentation requirements for deep foundation elements under bi-directional axial static loading.
Physical Separation Between Upward and Downward Response
The main mechanical distinction of the bi-directional test compared to conventional top-down static loading lies in the separation of resistance components. In a top-down load test without instrumentation along the shaft, the obtained curve represents only the global behavior of the soil-pile system, mixing elastic deformation of the material, lateral friction along the entire length, and end-bearing resistance.
In the bi-directional test, the cell creates a programmed discontinuity plane. This allows for independent measurement of the displacement of the segment located above the device and the displacement of the segment positioned below it. This configuration transforms the element into two simultaneous tests occurring in opposite directions.
Quantities Measured Directly During the Test
For the separation between responses to be technically valid, the instrumentation must isolate the fundamental quantities of the test:
- Hydraulic pressure applied in the expansive cell circuit, converted into axial force based on prior laboratory calibration of the actuators.
- Upward displacement of the cell top, measured by internal reference rods, technically known as tell-tales, anchored immediately above the device.
- Downward displacement of the cell base, measured by reference rods that cross the interior of the cell and anchor just below its bottom plate.
- Displacement of the pile head at ground surface, monitored with electronic transducers or deflectometers supported on independent reference beams.
- Actual opening of the expansive cell stroke, which corresponds to the mathematical sum of the upward and downward displacements measured at the device's plates.
Mobilization of Lateral Friction in the Upper Segment
The upper segment, when pushed upwards by the cell's expansion, mobilizes the lateral resistance of the interface between concrete and soil in the upward direction. For small displacement levels, generally between 2 and 5 millimeters, rapid mobilization of lateral friction is observed in this section, depending on the soil type and the roughness of the pile shaft.
Geotechnical interpretation requires attention to the shear direction. In conventional top-down loading, the pile moves downward and transfers friction from top to bottom. In the bi-directional test, the upper shaft is pulled upwards. In cohesive soils or conventional granular materials, experimental research indicates that upward and downward lateral friction exhibit similar magnitudes, provided the self-weight of the upper pile segment, which acts in favor of resistance during upward movement, is corrected.
Behavior of the Lower Segment and End-Bearing Resistance
The response obtained in the lower segment depends directly on the installation elevation of the expansive cell:
Cell Positioned at the Pile Tip
When the cell is installed at the base of the shaft, resting directly on the bottom of the excavation, the component that reacts against the lower stroke is exclusively end-bearing resistance. In this configuration, the downward movement recorded by the instrumentation purely represents the stress versus displacement curve of the foundation tip. This makes it possible to evaluate the stiffness of the soil beneath the base and verify the occurrence of softening or accumulation of sediments resulting from the excavation process.
Cell Positioned at an Intermediate Elevation
If the cell is installed at a certain height above the base, with the objective of balancing the expected upward friction and downward resistance loads, the lower segment simultaneously mobilizes the lateral friction of the remaining shaft and the end-bearing resistance. To separate these two components in the lower section, it becomes indispensable to install complementary instrumentation, such as levels of electrical strain gauges, along the lower shaft and immediately above the pile tip.
Evidence in Continuous Flight Auger (CFA) Piles in Brazil
The use of expansive cells in continuous flight auger (CFA) piles has gained prominence in the Brazilian geotechnical community through academic research and papers presented at the Seminar on Special Foundation Engineering and Geotechnics (SEFE). CFA piles present operational challenges for the bi-directional test, as the cell assembly needs to be introduced into the pile after concrete injection, coupled to the reinforcement cage.
Studies published in the SEFE proceedings demonstrated that the bi-directional load test in CFA piles allows for the observation of particular behaviors of Brazilian tropical soils:
- Rapid plastification of lateral friction: sandy and collapsible clayey soil profiles frequently mobilize the totality of unit lateral friction with millimeter displacements, confirming the high initial stiffness of the shaft-ground contact.
- Slow mobilization of end-bearing: end-bearing resistance in excavated and CFA piles demands significantly larger displacements, often on the order of 10% to 15% of the pile diameter, to reach values close to geotechnical failure.
- Effect of base cleaning: records from bi-directional tests in CFA piles revealed that the presence of excavation debris or stress relief at the bottom of the bore hole can generate initial sections of settlement with low stiffness at the base before the definitive embedment of the tip.
Comparison Between Static Load Test Methods
To illustrate the practical and operational differences in identifying resistance components, the table below summarizes the contrasts between the traditional test and the bi-directional method.
| Comparison Parameter | Conventional Static Load Test (NBR 16903) | Bi-directional Load Test (ASTM D8169 / NBR 16903) |
|---|---|---|
| External reaction system | Requires large steel beams and tension piles or reaction weights. | Non-existent at the surface. Reaction is internal between sections of the pile itself. |
| Measurement of friction and end-bearing | Global at the surface. Requires extensive internal instrumentation to individualize components. | Physically individualizes the upper and lower segments by the cell's expansion. |
| Maximum load limitation | Limited by the design capacity of the beam system and reaction anchors. | Limited by the resistance of the section with the lowest capacity (upper or lower shaft). |
| Construction site interference | Occupies a large surface area during assembly and application of load increments. | Free surface area, allowing simultaneous operations on site around the test. |
| Construction of top-down curve | Obtained directly by reading settlement instruments at the element's head. | Requires an analytical synthesis method to compose the equivalent top-down load-settlement curve. |
Construction of the Equivalent Load versus Settlement Curve
Since the bi-directional test applies forces in opposite directions within the shaft, it does not directly generate the load versus settlement curve that would be obtained if the pile received downward loading at its head. To provide the foundation designer with the response applicable to the calculation models of ABNT NBR 6122:2022, a conversion procedure is adopted.
The analytical method, originally based on formulations proposed by Schmertmann, reconstructs the equivalent top-down response following systematic steps:
- Subtraction of the pile's self-weight from the upward force value measured in the upper segment and addition of the corresponding self-weight to the lower segment.
- Determination of the elastic displacement of the shaft under compression, since, in actual top-down loading, the upper section will be compressed and not in tension.
- Compatibility of displacements: a top-down settlement value is selected, and the corresponding load mobilized simultaneously by lateral friction and end-bearing for that same deformation level is calculated.
- Sum of the compatible resistance components to plot point by point the conventional equivalent load versus settlement curve.
The analytical procedure assumes that downward lateral friction is similar to upward friction, a hypothesis considered valid for the vast majority of sedimentary and residual formations, with due corrections for confinement and tip effects in layers very close to the base.
How Geoteste Operates in High-Precision Bi-directional Tests
The correct interpretation of lateral friction and end-bearing resistance in bi-directional tests requires absolute rigor from the test design phase. Geoteste acts in all stages of this process, performing the dimensioning of the ideal elevation for installing expansive cells based on the stratigraphic profile of the ground, assembling hydraulic lines, and calibrating instruments according to ABNT NBR 16903:2020 and ASTM D8169/D8169M-26 standards.
With automated data acquisition systems and high-resolution displacement sensors coupled to tell-tales and shaft extensometers, the Geoteste engineering team ensures reports with high-fidelity load versus displacement curves, in addition to modeling the equivalent top-down curve for validation of the premises of ABNT NBR 6122:2022. Contact our specialists to plan the performance tests for your project's foundations.
