Coarse backfill and centroid densification strengthen coral sand foundations

Vibro-compaction combined with a 3.5-meter triangular layout, additional densification at triangle centroids and coarse-grained coral-sand backfill produced the strongest and most uniform improvement in a field trial of a reclaimed island-reef foundation. The laboratory and field study found that this combined method reduced the remaining weak-interlayer thickness to 1.9 meters, raised the mean dynamic penetration blow count to 14.70 and lowered the coefficient of variation to 0.12. The results apply to the reclaimed coral sand tested at the project site; the numerical construction parameters and calibration formulas remain site- and material-specific.

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