The work combined physical characterization, cyclic triaxial tests and field trials to examine how reclaimed coral sand behaves during deep ground improvement. The foundation came from a roughly 4.5-square-kilometer development near Juraid Island and the Berri Oil Field, about 40 kilometers off Saudi Arabia’s east coast. Its roughly 150,000-square-meter built area was planned for 800 hotel rooms, 11 restaurants, an extreme sports center and a 50-berth marina. The site profile included about 3 meters of medium-dense to dense sand, a 1.5–2-meter layer of moderately weak to moderately strong coral material, 3.5 meters of loose to medium-dense sand and more than 10 meters of dense sand beneath the principal development area.

Coral sand presented a particular densification challenge. More than 100 field tests showed that the reclaimed material was poorly graded, with a non-uniformity coefficient of 25, a curvature coefficient of 0.36 and a median particle diameter of 0.39 millimeter. Its particle-size distribution had a distinct plateau, indicating a shortage of intermediate-size particles and inefficient filling of spaces between coarse and fine grains. The measured maximum and minimum dry densities were 1.93 and 1.27 grams per cubic centimeter, and the average relative density of untreated material was about 0.45. Its fine content below 0.074 millimeter was less than 10%, satisfying the stated criterion for applying vibro-compaction without backfill.

Field density measurements showed why surface rolling alone could not address the foundation’s layered deficiencies. Rolling increased average relative density to approximately 0.87, compared with 0.60 after vibro-compaction, but its influence was concentrated near the surface. Large particles could obstruct the movement and consolidation of finer coral sand, limiting uniform shallow compaction and leaving deep weak interlayers untreated. Vibro-compaction reached greater depth by disturbing the sand skeleton, rearranging particles and compressing pores. The comparison therefore does not show that rolling was less effective at the surface; rather, shallow treatment could not provide the deep coverage required for the reclaimed foundation.

Twenty-four cyclic triaxial tests examined how particle-size distribution, density and drainage affected the sand’s dynamic response. Specimens represented intact gradation or narrow ranges of 0.1–0.25 and 0.25–0.5 millimeters, and were tested under drained and undrained conditions at 300 kilopascals effective confining pressure, a back pressure of 600 kilopascals and a cyclic stress ratio of 0.2. Drained specimens accumulated only 0.1%–0.4% axial deformation and showed no pore-pressure buildup. Undrained specimens developed pore pressure rapidly; after approximately 65 cycles, their dynamic strain rose sharply while stiffness and bearing capacity deteriorated, indicating pronounced liquefaction. The 65-cycle threshold was specific to the laboratory conditions and should not be used directly as a criterion for other soils or projects.

The laboratory results supported a “moderate-liquefaction” construction principle: vibration should temporarily raise pore-water pressure and reduce effective stress enough to unlock particles, but it must avoid prolonged shaking that causes collapse or uneven settlement. Coral sand is highly crushable, and fines generated during cyclic shearing can lower permeability and encourage further pore-pressure accumulation. Under drained conditions, by contrast, broken fragments can rearrange into voids with less deformation. The field vibration durations were therefore not scaled from the laboratory cycle count. Instead, the 180-kilowatt vibroflot was controlled using a target densification current, with staged penetration, bottom dwell and withdrawal. The laboratory observations supplied a mechanistic framework, while field current readings governed the actual treatment.

Four field schemes tested the effects of spacing, centroid treatment and backfill. Test A used an equilateral triangular layout at 3.5-meter spacing with in-situ reclaimed material. Test B reduced the spacing to 3.0 meters, also using in-situ material. Test C retained 3.5-meter spacing, added centroid densification and used coarse-grained reclaimed coral sand. Test D combined 3.5-meter spacing and centroid densification with in-situ backfill. Each scheme covered 600 square meters. After treatment, dynamic penetration tests indicated that Test B increased the mean blow count by 10.8% relative to Test A and reduced weak-interlayer thickness by 13.8%, from 2.9 to 2.5 meters, but its coefficient of variation rose from 0.20 to 0.32. Centroid densification alone in Test D reduced the mean blow count to 7.83 and produced the highest dispersion, with a coefficient of variation of 0.35.

Test C achieved the clearest combined benefit, with a mean dynamic penetration blow count of 14.70, a weak-interlayer thickness of 1.9 meters and a coefficient of variation of 0.12, compared with corresponding values of 8.83, 2.9 meters and 0.20 for Test A. The authors attributed the improvement to the coarse backfill supplying missing intermediate and coarse particles, creating a more continuous skeleton through which vibration energy could spread. Its greater permeability also allowed excess pore pressure to dissipate more readily, helping surrounding loose material reconsolidate. Centroid densification addressed blind spots between primary compaction points, whereas the closer spacing tested in Test B and centroid treatment alone in Test D each produced less favorable overall results. The study did not separately quantify how much improvement came from particle breakage, and recommended future assessment with particle-size comparisons and a relative breakage index.

Field measurements in five treated zones also showed a linear relationship between the dynamic penetration blow count, foundation bearing capacity and deformation modulus. The authors used these data to develop site-specific empirical formulas for estimating the characteristic bearing capacity and deformation modulus from the blow count. Across the five zones, mean blow counts of 6, 7, 8, 9 and 10 corresponded respectively to bearing capacities of 239.55, 278.74, 317.93, 357.12 and 396.31 kilopascals and deformation moduli of 16.60, 21.06, 25.89, 31.06 and 36.55 megapascals. Following the best-performing treatment, blow counts remained above 10 and bearing capacity exceeded 400 kilopascals. The proposed correlations may provide a rapid basis for acceptance testing, but their coefficients require local calibration before use on other marine deposits.

Overall, the study presents a site-calibrated method for strengthening reclaimed coral sand containing weak interlayers. Laboratory tests identified drainage condition as a primary control on liquefaction, while the field comparison showed that effective deep treatment required more than merely increasing the number of vibration points. The strongest tested solution combined centroid densification with coarse-grained backfill and a 3.5-meter triangular point layout, producing both higher penetration resistance and greater uniformity. The evidence does not establish a universal cycle limit, transferability of the exact coefficients or a separate causal effect of grain breakage. It does show that particle-size correction and spatial coverage can work together in a large island-reef project, while also demonstrating why treatment parameters developed for terrestrial sand should be verified against the grading, drainage, crushability and layering of coral-sand foundations.

Subject of Research: Laboratory and field evaluation of vibro-compaction for reclaimed coral-sand foundations with weak interlayers.

Article Title: Applicability and quality evaluation for vibro-compaction reinforcement of coral sand foundations

Article References: Ren, M., Chen, P., Ji, F., & Luo, H. (2026). Applicability and quality evaluation for vibro-compaction reinforcement of coral sand foundations. Frontiers in Earth Science, 14, Article 1933811. https://doi.org/10.3389/feart.2026.1933811

Image Credits: AI Generated

DOI: 10.3389/feart.2026.1933811

Keywords: coral sand, vibro-compaction, ground improvement, weak interlayers, soil liquefaction, dynamic penetration testing, bearing capacity, deformation modulus, island reclamation, particle-size distribution, Applicability, quality

The research, led by Arpita Biswas of the Indian Institute of Technology Kharagpur together with Sankar Kumar Nath of the University of Burdwan, drew on an extraordinary geotechnical and geophysical database of approximately 6,000 investigation sites across the basin. These included Standard Penetration Test boreholes, downhole seismic surveys, multichannel analysis of surface waves, ambient noise measurements and resistivity surveys, supplemented by laboratory tests on soil samples for grain size, moisture content, plasticity and density. The sheer scale of the dataset allowed the team to move beyond the city-level case studies — focused previously on Kolkata, Dhaka and Chittagong — that had dominated liquefaction research in the region.

The geological setting could hardly be more favourable for liquefaction. The basin contains the world’s deepest Neogene-to-recent sedimentary accumulation, exceeding 18 kilometres in thickness, and about 77 percent of the study area is blanketed in Quaternary alluvium — loose silt, sand, clay and gravel deposited by the Ganges, Brahmaputra and Meghna rivers. Statistical analysis of the borehole data revealed that the upper 15 metres of basin sediment is very loose to medium dense, with corrected penetration values averaging between 6 and 35 depending on depth, and a mean fines content of just 36.8 percent, indicating predominantly coarse-grained soils. Grain-size curves for sites in Dhaka, Rajshahi, Kalyani and Bhubaneswar showed that most soil layers fall within the internationally recognised boundaries of ‘most liquefiable’ and ‘potentially liquefiable’ soils.

Groundwater compounds the danger. Post-monsoon water tables across the basin range from just 0.27 to 4.71 metres below the surface, and shallow groundwater dramatically lowers the effective confining stress on sediments, creating ideal conditions for pore-pressure buildup during shaking. Historical precedent is sobering: the 1885 Bengal earthquake of magnitude 6.8, the 1897 Shillong earthquake of magnitude 8.1, the 1918 Srimangal earthquake of magnitude 7.6 and the 1934 Bihar-Nepal earthquake of magnitude 8.1 all triggered documented liquefaction, leaving ground fissures, sand boils, lateral spreading and soil slumping in their wake. Even moderate events such as the 2003 Kolabunia earthquake of magnitude 5.6 produced ground rupture along riverbanks in Rangamati.

Methodologically, the study is a tour de force of integrated hazard modelling. Because instrumental recordings are absent for the great historical earthquakes, the team used the EXSIM finite-fault stochastic simulation technique, which discretises a fault plane into numerous subsources and sums their contributions with realistic rupture propagation and travel-time delays. The resulting bedrock motions were then fed into two-dimensional nonlinear finite element site response analyses in PLAXIS 2D, capturing soil behaviour that simpler one-dimensional equivalent-linear methods miss. The simulated ground motions were validated against Modified Mercalli intensities derived from historical isoseismal maps, yielding a root-mean-square error of 0.91 — a respectable agreement for regional-scale work.

The probabilistic component is equally ambitious. Combining a comprehensive probabilistic seismic hazard model with fourteen ground-motion prediction equations, the researchers computed bedrock peak ground acceleration for a 475-year return period — the conventional design-basis earthquake — ranging from 0.08 to 0.58 g. Multiplying these values by spatially interpolated site amplification factors, which ranged from 1.30 to as much as 4.60 in the soft coastal deposits of the Sundarbans, produced surface-consistent hazard values reaching a remarkable 1.40 g near Sylhet in the northeast, close to the Dauki fault and the Indo-Burman subduction zone. Cities such as Mymensingh and Sylhet fell into the ‘severe’ category, while Dhaka, Kolkata and Khulna were rated ‘moderate’ and Bhubaneswar and Dhanbad ‘low’.

With these ground motions in hand, the team evaluated liquefaction using the stress-based framework of Idriss and Boulanger, computing factors of safety for every soil layer, then aggregating them into the Liquefaction Potential Index, the Liquefaction Risk Index, the probability of liquefaction and the probability of ground failure. The results are stark: under the probabilistic scenario, roughly 42 percent of the basin’s area falls into the ‘severe’ liquefaction potential class, with a further 35 percent rated ‘high’. Dhaka, Sylhet, Mymensingh, Chittagong, Barisal, Khulna and Rangpur all exceed the severe threshold of LPI greater than 15, while Rajshahi and Kolkata sit in the high class. The risk index map places nearly half the region — about 48 percent — in the ‘extremely high’ risk zone. At representative sites including Dhaka, Chittagong, Haldia and Kalna, probabilities of liquefaction exceeded 65 percent between depths of 5 and 20 metres, and cities from Dhaka to Barasat showed probabilities of ground failure above 0.9 — ‘extremely high to absolutely certain’.

Crucially, the modelled hazard aligns with reality. The predicted severity zones show an increasing trend in the number of documented liquefaction occurrences, providing qualitative validation of the approach. The spatial pattern also makes geological sense: the northeast around Sylhet and Mymensingh combines high seismic loading from nearby active faults with young floodplain deposits and shallow groundwater, while the southwest, underlain by older, stiffer sediments near the Indian Shield, fares considerably better. Notably, the study shows that strong shaking alone does not guarantee severe liquefaction — the Chittagong Fold Belt’s uplifted, folded sediments partially offset high seismic demand, whereas the unconsolidated Holocene deltaic plains of Khulna and Barisal remain highly susceptible despite more moderate loading.

Perhaps the most practically valuable contribution is the back-analysis of target penetration resistance. By reversing the standard liquefaction triggering equations, the researchers calculated the improved SPT-N values — denoted N-improved — needed at each site to guarantee no liquefaction under the probabilistic scenario, assuming a target factor of safety of 1.5. Mapped at depth intervals from 5 to 20 metres, these values range from as low as 1 in the stable west to 50 at 20-metre depth in the most demanding northeastern locations. Such maps give engineers, contractors and planners a first-order estimate of the densification or reinforcement required at specific sites, bridging the persistent gap between regional hazard assessment and ground-level engineering decisions.

The authors are candid about limitations: the empirical triggering relationships were calibrated on global case histories rather than Bengal Basin-specific data, uncertainties were not formally propagated through the entire framework, and the maps represent regional trends rather than site-specific design criteria. Ground improvement techniques — vibro-compaction, stone columns, gravel drains, grouting, dewatering and reinforcement — each have their place depending on geology, land use and economics, and the study suggests densification suits the young deltaic south while targeted foundation measures may be more realistic in congested megacities like Dhaka. Still, as a unified, basin-scale portrait of liquefaction threat across one of the world’s most populous regions, the work offers an essential foundation for risk-informed urban planning — and a sobering reminder that beneath the streets of Dhaka and the fields of Sylhet, the ground itself may be the region’s most insidious seismic hazard.

Subject of Research: Assessment of earthquake-induced soil liquefaction potential and mitigation targets across the Bengal Basin using probabilistic seismic hazard analysis and historical earthquake scenarios

Article Title: Probabilistic seismic hazard and historical scenario-based assessment of liquefaction potential in the Bengal Basin for liquefaction mitigation strategies

Article References: Biswas, A., & Nath, S. K. (2026). Probabilistic seismic hazard and historical scenario-based assessment of liquefaction potential in the Bengal Basin for liquefaction mitigation strategies. Discover Geoscience, 4(1), Article 293. https://doi.org/10.1007/s44288-026-00672-6

Image Credits: AI Generated

DOI: 10.1007/s44288-026-00672-6

Keywords: Bengal Basin, soil liquefaction, seismic hazard, liquefaction potential index, probabilistic seismic hazard analysis, ground motion simulation, site amplification, Standard Penetration Test, ground improvement, earthquake engineering, Ganges-Brahmaputra delta, geotechnical investigation

The research team drew on an extensive drilling campaign of 228 geotechnical boreholes across the study area, located roughly two kilometers south of the El-Hassana Dome and sixteen kilometers west of Cairo. From this grid, nine boreholes were selected for detailed core testing, supplemented by three surface block samples collected from outcrops of the Flint Series. The selection strategy deliberately covered all four stratigraphic units of the Turonian sequence, captured lateral variability across the site, and included both fault-proximal and fault-distal settings, with core recovery exceeding ninety percent to guarantee intact rock properties.

Laboratory testing followed the ASTM D7012 standard, with cylindrical core samples of fifty-four millimeter diameter and a length-to-diameter ratio of approximately two tested under dry conditions. The results reveal a rock mass of low to moderate strength with substantial heterogeneity. Uniaxial compressive strength values ranged from 4.93 to 19.69 megapascals, while Young’s modulus varied between 2.86 and 14.30 gigapascals. Allowable bearing capacities, derived using a conservative bearing capacity factor and the safety factor of three prescribed by the Egyptian Code EGP 202-2005, spanned 1.25 to 4.99 megapascals. These figures place the Abu Roash carbonates firmly in the category of weak to moderately strong rocks, susceptible to degradation under cyclic wetting and urban loading.

What makes the study distinctive is its insistence on direct measurement rather than empirical shortcuts. The team also conducted point load tests and compared them statistically with the direct UCS measurements. The conversion ratio between the two varied wildly across the dataset, from 8.5 in weak, vuggy Flint Series samples to 24.1 in strong Acteonella samples, with a mean of 16.2 and a coefficient of variation of forty-two percent. This scatter demonstrates that applying a universal conversion factor, such as the value of twenty-five commonly used for intact igneous rocks, would significantly overestimate strength in the weak units and compromise foundation safety. All geomechanical interpretations in the study therefore rest exclusively on directly measured UCS values.

The stratigraphic architecture of the formation emerges as the master control on engineering behavior. The Rudistae Limestone, a roughly thirty-five-meter-thick unit of rudist-bearing grainstones deposited in high-energy shoals, benefits from early marine cementation and delivers strengths above fourteen megapascals. The overlying Limestone Series, about 105 meters of alternating limestone, dolomite, and dolomitic limestone, shows moderate properties sensitive to diagenetic overprint. The Acteonella Series, approximately thirty meters of marl-limestone alternations capped by a dense, ledge-forming bivalve-rich limestone, proved to be the strongest foundation layer in the entire sequence, with allowable bearing capacities reaching 4.8 to 5.0 megapascals. At the other extreme, the Flint Series, a fifty-five-meter succession of chalky limestones, marls, and chert bands deposited in deeper, quieter waters with elevated clay content, yielded the weakest results, with UCS values of five to eight megapascals and bearing capacities below 2.1 megapascals.

This mechanical layering is a direct legacy of the rocks’ depositional history along the southern margin of the Tethys Ocean roughly ninety million years ago. High-energy shoal deposits developed grain-supported fabrics and early cementation that locked in strength, while deeper basinal facies accumulated clay and organic matter that later promoted dissolution, vuggy porosity, and weak marl intercalations. Meteoric water infiltration along faults and fractures accelerated karst development in susceptible units. The result is a vertical and lateral heterogeneity in which foundation quality can be anticipated from stratigraphic position alone, the essence of the geomechanical stratigraphy model the authors propose.

Tectonics adds a second layer of complexity. Northeast-southwest trending faults associated with the Syrian Arc deformation, which reactivated older structures from the Late Cretaceous through the Miocene, degrade foundation quality through increased fracture density, reducing rock mass strength by thirty to fifty percent compared with intact core values near fault zones. The effect is vividly illustrated by sample B10, collected within fifty meters of a mapped fault in the Flint Series. Despite a moderate UCS of 12.6 megapascals, this sample exhibited an elevated Poisson’s ratio of 0.35 and the lowest Material Index in the dataset at minus 0.40, signaling fracture-induced loss of elastic integrity and heightened lateral strain under load. The authors recommend that foundations within one hundred meters of mapped faults require denser borehole spacing, in-situ plate load testing, and safety factors of at least four.

Statistical analysis of the dataset reinforced the coherence of the framework. UCS and Young’s modulus showed a strong positive linear correlation with an R-squared of 0.89, expressed in a regression equation that allows preliminary stiffness estimation from strength data during early site assessments. Poisson’s ratio, by contrast, was remarkably consistent, averaging 0.298 with a coefficient of variation of just 8.1 percent, which empirically supports the isotropic, linear-elastic assumptions used to derive the bulk and shear moduli and competence indices. Coefficients of variation exceeding fifty percent for UCS, modulus, and bearing capacity, however, underscore the necessity of dense, site-specific testing in critical foundation zones, particularly within the Flint Series and marly intervals.

The practical recommendations flowing from the model are concrete. Zones underlain by the Acteonella ledge and competent parts of the Limestone Series, with allowable bearing capacities of 2.5 megapascals or more, can support standard shallow isolated footings without ground improvement. In contrast, weak zones such as those encountered in borehole BH-4 and block samples B9 and B12, where bearing capacities fall below 1.8 megapascals, require mandatory ground improvement. For moderately vuggy intervals, cementitious or chemical pressure grouting can raise effective capacity to at least 2.5 megapascals. For critical structures over the weakest ground, micropiles of 150 to 300 millimeter diameter can be socketed through the Flint Series to bear directly on the underlying, continuous Acteonella ledge, bypassing the weak strata entirely. Large-footprint structures on marginal ground may employ reinforced raft foundations to mitigate differential settlement, and all development must incorporate surface water management to protect the dissolution-prone marl and chalk facies from chemical weathering.

The authors are candid about the limitations of their dataset. With only twelve samples tested in detail, the statistical resolution of intra-unit heterogeneity is limited, and localized weak zones may exist between tested boreholes. The derived parameters describe intact rock, whereas bedding planes, joints, and karst cavities will reduce rock mass strength in the field, so the reported bearing capacities should be treated as upper-bound estimates pending rock mass classification and in-situ validation through plate load and pressuremeter testing. Future work integrating geophysical methods such as cross-hole seismic tomography could map karst cavities and fault damage zones between boreholes. The broader significance, however, extends well beyond New Giza: the Abu Roash Formation underlies 6th of October City and Sheikh Zayed City, and similar Cretaceous and Eocene carbonates lie beneath New Cairo and the Alexandria coastal zone. By adapting this stratigraphy-driven approach, municipal planners across Egypt’s New Urban Communities could develop predictive geotechnical zoning maps, reducing risk and cost as the country’s Vision 2030 urban expansion accelerates over ground laid down in the age of the dinosaurs.

Subject of Research: Geomechanical characterization of Turonian carbonate rocks of the Abu Roash Formation for foundation design in New Giza, Egypt

Article Title: Geomechanical stratigraphy defines foundation suitability of Turonian carbonates in New Giza Egypt

Article References: Ali, E.-H. M., Mahdy, A., El Sayed, A. M. A., Mousa, S. E.-D. A., & El-Mashad, M. E.-D. M. (2026). Geomechanical stratigraphy defines foundation suitability of Turonian carbonates in New Giza Egypt. Discover Geoscience, 4(1), Article 369. https://doi.org/10.1007/s44288-026-00739-4

Image Credits: AI Generated

DOI: 10.1007/s44288-026-00739-4

Keywords: geomechanical stratigraphy, Turonian carbonates, Abu Roash Formation, foundation engineering, uniaxial compressive strength, bearing capacity, New Giza, Egyptian geology, rock mechanics, karst, ground improvement, EGP 202-2005

Composite piles of the concrete–cement soil variety are built by treating soft or weak ground with cement, forming an improved soil mass, and then casting or driving a concrete element through or within that treated zone. The concept is attractive because it combines the stiffness and strength of concrete with the improved bearing and reduced compressibility of cement-stabilized soil. The load path, however, depends on how well the two materials transfer forces to one another. If the concrete–cemented soil interface slips under load, the composite action that makes the system economical is lost, and the pile may behave as two separate elements rather than one integrated foundation unit. Understanding the shear behavior of that interface is therefore not an academic detail but a central question in geotechnical design.

Previous studies of soil–structure interfaces have relied heavily on direct shear boxes and simple shear apparatus. These devices have well-known limitations: the shear plane is forced to a fixed location, stress distributions across the sample are non-uniform, and large relative displacements cannot be achieved without losing contact area. Ring shear testing addresses several of these shortcomings. In a ring shear apparatus, an annular specimen is confined in a circular chamber, and one portion of the sample is rotated relative to the other under a controlled normal stress. Because the geometry is continuous, shear displacement can proceed indefinitely without changing the nominal contact area, which makes the device particularly well suited for studying residual interface behavior—the strength that remains after very large relative movements.

In the reported experiments, the research team prepared interface specimens pairing concrete surfaces with cement-treated soil formulations, varying the cement content, curing conditions, and applied normal stresses to capture a representative range of field conditions. The ring shear tests tracked shear resistance as relative displacement accumulated, allowing the investigators to distinguish between peak strength, the transient maximum mobilized early in loading, and residual strength, the lower value that governs behavior after the interface has experienced substantial sliding. This distinction matters enormously in practice: a foundation that relies on peak interface strength may be safe for small, monotonic movements but could lose significant capacity if settlement, lateral spreading, or cyclic loading forces large relative displacements.

The results reveal a clear and consistent pattern in how these interfaces mobilize strength. At small displacements, adhesion and interlocking between the concrete surface and the cemented soil matrix resist relative movement. As displacement grows, the cemented bonds at the contact are progressively sheared, and the resistance transitions toward a steady residual value controlled largely by friction along a polished shear zone and by the granularity of the crushed cemented particles that accumulate there. The tests show that the ratio of residual to peak strength depends on the cement treatment level and on the roughness of the concrete surface, with rougher interfaces retaining more of their capacity after large deformation. This finding provides quantitative support for the long-standing intuition among foundation engineers that surface texture is a first-order design variable, not a secondary consideration.

Normal stress emerges as another controlling factor. Higher confining pressures increase both peak and residual shear resistance, but the shape of the shear-displacement curve changes with stress level, reflecting the competing roles of particle crushing, dilation, and reorientation within the interface zone. Under low normal stress, the interface reaches its residual condition relatively quickly, whereas under high normal stress the cemented soil continues to evolve, with crushing of bonded clusters generating fresh particle surfaces and altering the frictional response over extended displacement. These observations help explain why field-scale interface behavior can be difficult to predict from small laboratory tests alone: the operative mechanisms are stress- and displacement-dependent, and any design model must account for the full loading history rather than a single strength value.

What makes the study particularly valuable to practitioners is the way it translates laboratory observations into parameters usable in interface models. Modern numerical analyses of pile foundations often employ constitutive laws for soil–structure interfaces in which shear stiffness, peak strength, post-peak softening, and residual friction must all be specified. The ring shear data provide calibrated values for these parameters across a range of cement treatments and stress states, allowing analysts to represent the concrete–cemented soil contact with far greater fidelity than was previously possible. In pile load simulations, the difference between assuming full composite action and modeling realistic interface softening can be substantial, particularly for long piles and for designs in which load must be transferred deep into the ground.

The implications extend to construction quality control as well. Because interface strength is sensitive to concrete surface condition and to the degree and uniformity of cement treatment, the study’s findings reinforce the importance of specifying and verifying surface preparation and treatment quality in the field. A smooth or contaminated concrete surface, or a poorly mixed cement-stabilized soil zone, can reduce interface resistance to values near the residual floor identified in the tests. Conversely, deliberate roughening of the concrete and careful control of cement content can preserve a substantial fraction of peak strength even after significant relative displacement. In seismic regions and in ground subject to consolidation-induced downdrag, where large interface movements are plausible, these controls translate directly into reliability.

From a scientific standpoint, the work also contributes to a broader understanding of cemented geomaterial interfaces. Cement-treated soil occupies an intermediate position between uncemented granular materials and intact rock, and its behavior under shear is governed by the progressive destruction of bonds, a process that has been studied extensively in cemented sands and in rock joints but less thoroughly at concrete contacts. The ring shear results document this bond-breaking process at the pile interface in detail, showing how residual strength emerges from a self-organized shear zone of crushed and reoriented particles. The methodology—continuous large-displacement shearing under controlled stress—offers a template that other researchers can apply to related problems, including the interfaces of ground-improvement columns, soil-mixing walls, and caissons founded in stabilized ground.

As cities expand into softer ground and engineers push foundations deeper, composite systems that blend concrete with treated soil will play an increasing role in keeping construction both safe and economical. The ring shear testing program described in Scientific Reports gives the profession something it has long lacked: a quantitative, displacement-resolved description of the shear mechanisms at the heart of these systems. By quantifying how peak strength is developed, how it softens, and what residual capacity remains, the study turns a formerly assumed property into a measured one. For the engineers who must certify that a bridge pier or a high-rise will stand safely on treated ground for generations, that difference between assumption and measurement is precisely where modern foundation engineering is won.

Subject of Research: Interface shear mechanisms between concrete and cement-treated soil in composite foundation piles investigated using ring shear testing.

Article Title: Interface shear mechanisms in concrete–cement soil composite piles revealed by ring shear testing

Article References: Junjie, M., Liaoyi, F., & Xiaonan, G. (2026). Interface shear mechanisms in concrete–cement soil composite piles revealed by ring shear testing. Scientific Reports. https://doi.org/10.1038/s41598-026-70864-0

Image Credits: AI Generated

DOI: 10.1038/s41598-026-70864-0

Keywords: ring shear testing, concrete–cement soil composite piles, interface shear strength, cement-treated soil, residual strength, deep foundations, soil–structure interaction, ground improvement, pile design, geotechnical engineering, Interface, shear

The PCCS system, short for precast concrete piles reinforced by cemented soil, marries two established ground-improvement techniques. A deep-mixing column, created by blending cement into soft soil in place, forms a stiff outer sleeve. Into that sleeve, engineers insert a slender precast concrete pile that acts as a rigid inner core. The two components work in complementary fashion: the concrete core supplies axial stiffness and bearing capacity, while the cemented-soil column enlarges the effective diameter of the pile, expands the contact area with the surrounding ground, and improves lateral confinement and interfacial bonding. Depending on whether the inner core is shorter, equal to, or longer than the outer column, the system is classified as short-core, equal-core, or long-core. For deep and thick soft-soil deposits—the kind encountered along the Beijing–Shanghai corridor—only the long-core variant makes sense, because the extended concrete core can reach down toward a competent bearing stratum that a shallow cemented column alone could never touch.

The field test section sits between chainages K950+054 and K950+158 of the expressway expansion. The original design called for conventional precast concrete piles of 28 to 35 meters, but the project team adopted the PCCS alternative for its combination of high bearing capacity and cost savings. Each PCCS unit consisted of a 32-meter-long, 0.4-meter-diameter precast concrete core of C60-grade concrete topped with a 1.6-meter square pile cap, surrounded by a 12-meter-long, 0.8-meter-diameter cemented-soil outer core. The piles were arranged in a square grid at 3.5-meter center-to-center spacing, and a 0.3-meter gravel cushion laced with a geocell—a honeycomb-like polymer reinforcement—was laid over the pile heads to distribute load and limit differential movement at the ground surface.

To dissect the load-transfer mechanics, the research team built two fully coupled fluid–solid numerical models in ABAQUS: a full half-width cross-section of the embankment and an idealized unit cell centered on a single pile. The deep saturated soft soil was represented with the modified Cam-Clay constitutive model, its compression and swelling indices calibrated against laboratory consolidation and triaxial tests, while the embankment fill, cushion, and cemented soil used the Mohr-Coulomb elastoplastic model. A bond–slip friction model captured the interface between the concrete core and its cemented sleeve, with a bond strength of 200 kilopascals and a tangential friction coefficient of 0.65 drawn from prior experimental work. Mesh sensitivity checks confirmed that predictions of settlement and pile–soil stress ratio changed by less than 2 percent between medium and fine meshes, giving the team confidence in the computational framework.

Validation against field instrumentation proved convincing. At 320 days after construction, measured pile-top stress fell between the two models, with the unit model deviating by at most 12 percent from observations, and normalized settlement profiles along the embankment height agreed within 4.2 percent. The simulations revealed a clear temporal choreography: during embankment filling, both pile and soil stresses climbed steadily, but once construction ended their paths diverged. The pile-top stress kept rising as the soft ground consolidated and drained, while the stress in the surrounding soil peaked and then gradually declined—signature behavior of soil arching, the process by which load sheds from yielding soil onto stiffer inclusions through intergranular shear. In the final stabilized state, the surrounding soil carried only about 33 kilopascals, roughly 55 percent of the embankment’s average applied pressure, while the pile top reached 266.2 kilopascals, yielding a pile–soil stress ratio of 8.1. In load-sharing terms, the PCCS carried 68 percent of the total embankment weight and the soil just 32 percent.

Settlement performance was equally impressive. Immediately after filling, the pile and the adjacent soil had settled 6.9 and 22.5 millimeters respectively; as consolidation proceeded, the soil continued to compress, stabilizing after 204 days at a final measured settlement of 47.5 millimeters. The pile top, by contrast, essentially stopped moving about 86 days after construction, finishing at just 15.2 millimeters. The growing gap between pile and soil settlement—eventually stabilizing at 31.3 millimeters after 264 days—is precisely what drives soil arching above the pile caps. Numerical stress contours captured the result vividly: as the embankment rose, principal stress concentrated above the pile tops in a distinctive mushroom-shaped pattern, and stress vectors tilted outward to form a coherent arch. Once the arch was fully developed, the upper embankment fill became nearly differential-settlement-free, meaning the pavement above rode on a smoothly supported platform despite the uneven foundation below.

The study quantified this arching with a critical height of approximately 1.15 times the net pile spacing—the distance between pile-cap edges. Below that plane, differential settlement and stress redistribution are concentrated; above it, settlement curves converge and the embankment behaves as a uniform body. The analysis also distinguished between an inner arch, spanning adjacent piles, and a higher outer arch spanning the diagonals of the pile grid. At an embankment height of 5 meters, the outer arch reached a maximum of 1.55 times the net spacing above the embankment base, reflecting the wider diagonal pile spacing. The horizontal earth pressure coefficient, a direct measure of arching intensity, stayed near the at-rest value through most of the embankment height but surged near the base, where at 5 meters of fill it reached values 3.2 and 2.3 times those measured under 3-meter and 4-meter embankments respectively.

Parametric sweeps explored how the embankment fill’s own properties modulate the arch. Increasing the fill’s elastic modulus modestly reduced peak stress and displacement—each 10-megapascal increment trimmed normalized stress by roughly 4 to 5 percent and displacement by about 3 percent. Cohesion proved more influential: every 5-kilopascal gain cut maximum normalized displacement by 7 to 11 percent, indicating that a more cohesive fill helps the arch develop and hold its shape. The internal friction angle, surprisingly, mattered little, with stress and displacement fluctuating within 3 percent for each 5-degree change. The geocell embedded in the cushion added its own membrane effect, deforming symmetrically along the embankment axis and developing peak tensile force near the shoulder pile, where it helped distribute lateral load and maintain stability at the slope.

Horizontal ground movements told a consistent story. Displacements decreased with depth but showed a localized bulge between 4 and 12 meters during the first 120 days after construction, and intensified from the surface down to 16 meters as time passed. At the slope toe, the computed maximum horizontal displacement was 4.4 millimeters at the end of construction against 2.5 millimeters measured in the field; by 365 days the computed value reached 14.9 millimeters, exceeding the field measurement by only 15.5 percent. Axial-force profiles along the pile depth revealed a neutral point at 16 meters for the central pile, below which negative skin friction gave way to positive resistance, while piles beneath the slope, carrying less overburden, showed neutral points at 9.0 and 4.0 meters for their inner and outer cores respectively.

The authors are candid about the model’s limits: pile installation disturbance, long-term creep of the soft clay, cemented-soil degradation, and progressive interface damage were not explicitly simulated, and these simplifications likely explain why measured soil settlement slightly exceeded predictions. Still, the agreement achieved across stress, settlement, and displacement metrics demonstrates that the long-core PCCS is a genuinely effective answer to deep soft-soil embankments—delivering rigid-pile settlement control at a fraction of the cost of conventional piling, and giving engineers a quantified, field-validated framework for designing the next generation of highways across the world’s soft deltaic plains.

Subject of Research: Load transfer mechanism and settlement performance of long-core PCCS composite foundations supporting embankments over deep soft soil

Article Title: Load transfer mechanism and performance evaluation of the PCCS composite foundation with long-core under embankment load

Article References: Zhang, C., Zhang, D., Liu, S., Wang, Z., Liu, L., & Zhu, M. (2026). Load transfer mechanism and performance evaluation of the PCCS composite foundation with long-core under embankment load. Case Studies in Construction Materials, 25, Article e06506. https://doi.org/10.1016/j.cscm.2026.e06506

Image Credits: AI Generated

DOI: 10.1016/j.cscm.2026.e06506

Keywords: PCCS composite foundation, precast concrete pile, cemented soil, soil arching, embankment, soft soil, pile-soil stress ratio, settlement control, geocell reinforcement, modified Cam-Clay model, Beijing-Shanghai Expressway, ground improvement