*Important notice: This news reports on an unedited version of an accepted paper and is awaiting final editing. Therefore, the paper should not be regarded as conclusive or treated as established information.
Researchers have developed constitutive models and practical strength-estimation methods for advanced foundation design. To achieve this, they comprehensively investigated the interface shear mechanisms of concrete–cement soil composite piles using ring shear testing. Their findings were published in Scientific Reports.
Study: Interface shear mechanisms in concrete–cement soil composite piles revealed by ring shear testing. Image Credit: ABCDstock/Shutterstock.com
Composite Pile Interface Mechanics
Composite piles combine a precast concrete core with an outer cement soil column and have gained traction as effective foundation solutions in soft ground conditions. These hybrid piles capitalize on the high strength and durability of concrete while leveraging the improved soil-pile interaction offered by cement-soil columns.
The load-carrying and transfer behavior of such piles primarily depends on two critical interfaces: the concrete–cement soil interface and the cement soil–surrounding soil interface. Despite their growing use, understanding the mechanical behavior and shear mechanisms at these interfaces remains limited.
Past investigations primarily utilize direct or simple shear tests that impose constraints on natural failure planes and accurate shear stress measurement. To address these knowledge gaps, the recent study employed ring shear testing to more accurately characterize interface shear properties.
This approach allows for insights into interface shear strength, failure modes, and the development of constitutive models tailored for practical design and analysis of composite piles.
Interface Ring Shear Testing
In this research, ring shear tests were conducted to explore the shear behavior of both the concrete–cement soil interface and the cement soil–soil interface under controlled laboratory conditions.
The apparatus applies circumferential torsional shear to ring-shaped samples, enabling failure to occur naturally along the interface without constraining shear planes, while maintaining a constant shear area for precise stress measurement. Cement soil mixtures with varying cement contents (10–30%) were prepared, with standardized water content and soil gradation.
These mixtures were formed into rings to simulate the outer column, paired either with a concrete core for the concrete–cement soil interface tests or with dense quartz sand representative of surrounding soil for the second interface. Parallel consolidation of undrained triaxial tests determined intrinsic strength parameters of the cement soil samples.
Testing was performed under normal stresses ranging from 0 to 200 kPa and accounted for variables such as cement soil thickness and normal stress level. Multiple replicates ensured repeatability.
Outcomes included shear-stress-displacement responses, from which constitutive models capturing initial elastic behavior, peak and residual shear strengths, and strain-hardening effects were developed. These models incorporate reductions in factors that relate interface strength to intrinsic material properties, enhancing predictive capability without requiring exhaustive interface testing in practice.
Interface Behavior and Modeling
The ring shear tests revealed distinctive shear behaviors for the two interfaces critical to composite pile performance. The concrete–cement soil interface exhibited a characteristic three-stage response: an initial linear elastic loading stage, followed by post-peak softening as shear displacement increased, and finally a residual stabilization phase.
Peak shear strength at this interface increased proportionally with both cement content and applied normal stress. A reduction in cement soil thickness was found to induce premature cracking within the cement soil, thereby limiting the mobilization of interface strength.
Analysis demonstrated that the interface shear strength parameters could be effectively estimated via dimensionless relationships linking interface friction angle and cohesion to those of the cement soil with reduction coefficients (approximately 0.56 for friction angle and 0.28 for cohesion). This led to the proposal of a simplified three-segment linear constitutive model describing the mechanical response of the concrete–cement soil interface.
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Conversely, the cement soil–soil (sand) interface displayed a strain-hardening behavior transitioning into a stable shear resistance stage without a significant peak-softening phase. The shear strength at this interface was primarily governed by normal stress and the properties of the surrounding soil, rather than cement content.
No effective bonding was observed at this interface, resulting in negligible adhesion, and the friction angle was roughly constant at around 15.4°. A nonlinear hyperbolic constitutive model was introduced to capture this behavior, and a method was established to estimate the interface friction angle from the soil’s effective internal friction angle.
Validation against experimental data under maximum normal stress showed that the proposed models predict shear strength within roughly nine to 10% of observed values, providing slightly conservative yet reliable estimates under the laboratory conditions.
Implications for Composite Foundations
This study presents a comprehensive examination of the shear mechanisms governing the two critical interfaces in concrete–cement soil composite piles using ring shear testing.
Key results demonstrate that the concrete–cement soil interface behaves with a three-stage shear response controlled by cement content and normal stress, while the cement soil–soil interface exhibits strain-hardening with friction-dominated strength controlled by surrounding soil properties.
The research thus offers a significant contribution to advancing the understanding of the design and construction of composite foundation piles in soft soils, providing an experimental basis for safer, more economical, and more effective pile systems in geotechnical engineering.
Journal Reference
Junjie M., Liaoyi F., et al. (2026). Interface shear mechanisms in concrete–cement soil composite piles revealed by ring shear testing. Scientific Reports. DOI: 10.1038/s41598-026-70864-0. https://www.nature.com/articles/s41598-026-70864-0.