How load test instrumentation transforms the test into geotechnical diagnosis
See how load test instrumentation separates side friction and end bearing, generating a complete geotechnical diagnosis of the pile.
Por Rodrigo Gontijo
Engenheiro Civil Geotécnico
CREA/CONFEA 1416959840
My relationship with geotechnics began long before university. I am the son of a geotechnical engineer, founder of Gontijo Fundações, and I was born when the company already had ten years of operation. I grew up in an environment that breathed geotechnics, where my learning began even before entering the Civil Engineering course at UFMG. Today, 27 years after graduating, I can say that my history combines five decades of coexistence and learning in this universe.
This training gained new perspectives with international experience. For five years, I lived in Guyana and worked intensely in the civil construction market in the Caribbean and Central America, especially in the construction of resorts in several Caribbean countries. Following the evolution of construction in Brazil and abroad broadened my vision of engineering challenges and the importance of understanding the particular conditions of each project.
This trajectory reinforced a conviction: the better we understand the behavior of a foundation, the better the design and execution decisions are. Proving that a pile supports a certain load is essential information. Investigating how this load is distributed along the shaft and what portion reaches the toe allows for a deeper analysis and evaluation of improvement opportunities with technical support.
It is in this perspective that the instrumented static load test in depth, the subject of this article, is inserted. By expanding knowledge about the interaction between the pile and the soil, instrumentation brings calculation hypotheses closer to the behavior observed in the field. My purpose is to show how this knowledge can guide diagnoses, improve designs, and contribute to safer and more efficient foundations.
Beyond the conventional acceptance criterion
In traditional foundation engineering practice, a conventional static load test provides a global response of the structural element. The test records the load applied at the top and the corresponding vertical displacement, generating the load versus settlement curve. This procedure perfectly meets the regulatory performance verification criteria established by ABNT NBR 6122:2019 with Amendment 1:2022 and follows the executive guidelines of ABNT NBR 16903:2020. However, when restricted only to surface measurements, the final result often boils down to a binary validation: the pile supported the design load with acceptable displacements or reached a limit state of excessive deformation.
This classic approach treats the embedded element as a black box. It does not reveal how stresses were transferred along the different stratigraphic layers of the subsoil, nor does it allow quantifying the portion of load absorbed by side friction in relation to the resistance mobilized by the toe. It is in this scenario that in-depth load test instrumentation changes the level of analysis. By incorporating sensors along the shaft and at the base of the pile, the test ceases to be just a conformity verification and starts to operate as a high-precision analytical tool for geotechnical and structural diagnosis.
Top instrumentation versus in-depth instrumentation
To understand the technical gain provided by this method, it is fundamental to establish the distinction between monitoring systems installed on the surface and devices fixed along the body of the foundation element.
Top instrumentation
Top instrumentation is mandatory according to ABNT NBR 16903:2020. It consists of a load cell or calibrated pressure gauge for measuring the axial force applied by the hydraulic jack, associated with mechanical dial gauges or displacement transducers (LVDT) supported on undeformable reference beams. These sensors accurately measure the total top settlement during loading stages and the residual displacement after complete unloading. However, these data only record the integrated response of the pile and soil mass assembly, without discriminating the localized behavior of any segment of the pile.
Instrumentation along the shaft and toe
In-depth instrumentation involves installing sensors distributed in the reinforcement or in the concrete mass of the pile before or during its concreting. The primary objective of these instruments is to measure internal quantities, such as specific strains and relative shortenings between predefined sections. Among the main devices used in in-depth instrumentation are:
Rod extensometers, also known as tell-tales: rigid steel or fiberglass rods installed inside protective tubes detached from the concrete, anchored at specific depths of the shaft or at the toe itself. By measuring the displacement of the top of the rod relative to the top of the pile, the average elastic shortening of the monitored section is obtained.
Strain gauges and electrical resistance extensometers installed in the reinforcement bars: measure the microstrains of the steel under loading.
Vibrating wire strain gauges: robust sensors stable to thermal variations and electrical noise, widely used to measure the average axial microstrains of the reinforced concrete cross-section at each instrumented level.
Base pressure cells: devices positioned immediately under the pile toe to directly record the increase in soil-concrete contact stress.
Fundamentals of load transfer and physical analysis
The analysis of data collected by an instrumented pile is based on classical elasticity theory and solid mechanics. From the measurement of microstrain in a given cross-section during a load stage, the remaining axial force at that level is calculated by the constitutive relationship of the element:
The internal force in the section is the product of the measured axial strain, the cross-sectional area, and the tangent or secant modulus of elasticity of the composite concrete and steel material. The correct determination of the concrete's modulus of elasticity over the curing time and under different stress levels is one of the most critical points of this analysis, requiring laboratory tests on molded specimens during concreting or calibrations on unconfined sections at the top of the pile itself.
Knowing the axial force passing through each instrumented level, the difference between the normal forces measured at two successive levels corresponds directly to the load transferred to the soil mass by side friction in that depth interval. Dividing this value by the lateral area of the pile between these same levels, the average unit side friction acting in each specific geological layer traversed by the foundation is obtained.
Advanced geotechnical diagnosis
The availability of detailed strain data along the depth enables geotechnical diagnoses that would be impossible to obtain through uninstrumented tests. The main evaluations provided by the method include:
Real separation between side friction and toe resistance
In soils with varied stratigraphy, it is common for theoretical prediction to overestimate or underestimate the contribution of the foundation base. In large-diameter bored piles, for example, the toe often requires significant relative displacements, often on the order of ten to fifteen percent of the pile diameter, to mobilize its ultimate capacity, while side friction peaks with millimeter displacements. Instrumentation accurately reveals the percentage of load absorbed by the shaft and the fraction transferred to the toe for each load stage.
Empirical derivation of load transfer curves
The data allow plotting local side friction versus shaft displacement curves, called t-z curves, as well as the toe stress versus base displacement curve, called q-w curve. These functions precisely describe the soil-pile stiffness for each geotechnical horizon, serving as an empirical basis calibrated for numerical back-analysis and soil-structure interaction modeling.
Evaluation of negative side friction
In scenarios where surface overloads occur or consolidation of soft soil layers traversed by the pile, instrumentation allows identifying the neutral plane position. The neutral plane defines the exact point of depth where the relative deformations between the soil and the shaft reverse, transforming downward drag stresses into upward geotechnical support stresses.
Identification of structural stiffness anomalies
Unexpected variations in the strain curve along the shaft can signal section constrictions, soil inclusion during concreting, or abnormal variations in the shaft's deformability modulus. It is worth noting that the instrumented static load test is not a specific structural integrity test, a function performed by methods such as the low-strain integrity test disciplined in terms of control requirements by ABNT NBR 6122:2019 with Amendment 1:2022. However, it provides clear mechanical evidence of the structural response of the shaft under axial compression or tension loading.
Technical comparison of data obtained in the test
Analysis Parameter | Standard Static Load Test | Instrumented Static Load Test |
|---|---|---|
Global bearing capacity at the top | Directly measured | Directly measured |
Total and residual top settlement | Directly measured | Directly measured |
Load distribution per soil layer | Not determined | Calculated from measured deformations |
Mobilized toe resistance | Estimated by theoretical hypotheses | Measured and calculated by extensometry |
Unit side friction along the shaft | Not determined | Determined in each instrumented interval |
t-z and q-w load transfer curves | Inaccessible | Constructed directly from field data |
Distributed elastic shortening of the shaft | Theoretically estimated at the top | Directly measured by internal sensors |
Advantages for engineering projects and executive optimization
The adoption of load test instrumentation in preliminary phases or at the beginning of construction generates substantial technical and economic benefits. When carried out on performance test piles, as provided by ABNT NBR 6122:2019 with Amendment 1:2022, it provides real parameters to recalibrate semi-empirical design formulations.
If the test proves that the side friction in a certain layer of resistant soil is significantly higher than that adopted in conservative calculation hypotheses, the geotechnical designer gains technical support to optimize the embedment lengths of the production piles for the rest of the project. On the other hand, if it is identified that the pile toe is not mobilizing due to the presence of disintegrated soil or insufficient cleaning of the excavation bottom, executive interventions can be implemented immediately before large-scale execution, mitigating risks of pathologies and excessive differential settlements.
Specialized instrumentation solutions with Geoteste
Geoteste operates with rigor in performing static load tests according to the prescriptions of ABNT NBR 16903:2020 and the guidelines of ABNT NBR 6122:2019 with Amendment 1:2022. Our team carries out the planning and complete installation of in-depth instrumentation systems, including mechanical extensometers, vibrating wire sensors, and pressure cells, ensuring precision in the acquisition and processing of deformation information along the shaft.
Whether for preliminary investigation campaigns with sacrificial piles or for performance verification in definitive, high-responsibility elements, count on the experience of our technical team to transform field tests into consistent data for the design and validation of your foundations. Contact our specialists and define the ideal instrumentation strategy for your project.
