Investigation of the working principle of bored piles constructed within jet grout columns

In soft, loose soils with a high groundwater table, safely transferring structural loads to the ground while maintaining economic feasibility in construction represents a fundamental challenge in geotechnical engineering. The construction of heavy and high-quality structures on such soils requires addressing complex geotechnical problems, which become even more critical in seismically active regions. In engineering practice, soil improvement techniques are primarily employed to enhance inadequate ground conditions while considering cost-effectiveness. The main objective of these methods is to control settlement behavior and ensure bearing capacity by densifying and stiffening the soil mass. However, in practice, soil improvement techniques-though generally sufficient in terms of bearing capacity-may not always satisfy settlement criteria alone. Consequently, for heavy and critical structures, deep foundation systems, particularly bored piles, are often preferred as safer but more costly alternatives. Population growth, technological advances, and industrial land demands have led to a reduction in building footprint areas and an increase in structure heights. This trend increases both the load per unit area and the stresses beneath foundations. This study was conducted at a silo construction site in the Körfez district of Kocaeli Province, Turkey, characterized by very soft alluvial soils with a high groundwater table. Since shallow foundation systems could not meet the design loads, bored piles were implemented as the deep foundation solution. The site was divided into two sections, Site-1 and Site-2, both exhibiting similar soil conditions and construction challenges. At Site-1, located in the Yeniköy plain, bored piles with a diameter of 80 cm and a length of 24 m were directly constructed. Pile capacity and length were designed considering both static and seismic loads in accordance with the Turkish Earthquake Code of 2007. During pile construction, maintaining borehole stability was challenging due to weak soil profiles and the high groundwater table, which directly affected project cost and schedule. At Site-2, silo diameters were reduced and heights increased to achieve greater storage volume with a smaller foundation system, improving area efficiency. Due to increased sub-foundation stresses, seismic moments and uplift forces necessitated longer piles than normal. Since Site-2 was designed in 2021, the 2018 Turkish Building Earthquake Code was applied. Pile design calculations indicated diameters of 100 cm and lengths exceeding 50 m. One key reason for such long piles was the very soft clay layer within the upper 16 m of soil, where the undrained shear strength (cu) was approximately 15 kPa, providing insufficient shaft friction. To improve the mechanical properties of this upper 16 m of soil, jet grouting was implemented. The design included four jet grout columns, each 80 cm in diameter and 16 m in depth, around every bored pile. The jet grout columns were constructed by injecting cement slurry into the soil under 300-400 bar pressure with a water-cement ratio of 1:2. After setting, bored piles were installed within the improved soil mass. This enhancement reduced the previously predicted pile lengths of over 50 m to a maximum of 42 m, achieving significant time and cost savings. Both sites contained bored piles of varying diameters and lengths. In this study, one representative pile from each site was analyzed: at Site-1, an 80 cm diameter, 24 m long pile with a vertical load capacity of 95 tons, and at Site-2, a 100 cm diameter, 31 m long pile with a vertical load capacity of 182 tons. Static pile load tests, conducted in accordance with ASTM D1143 (2020), revealed that at Site-1, the pile designed for 95 tons could carry up to 142.5 tons, resulting in a total settlement of 8.83 mm, comprising 5.17 mm residual and 3.66 mm elastic settlement. At Site-2, the pile was loaded to 350 tons (1.92 times the service load), exhibiting only 2.35 mm total settlement, including 1.50 mm residual and 0.85 mm elastic settlement. These findings confirm, in line with previous studies, that borehole stability significantly influences bored pile performance. Lam et al. (2015) compared bentonite and polymer drilling fluids, concluding that polymer-based piles performed nearly twice as well due to improved borehole stability. Zhou et al. (2021) highlighted the impact of excavation and injection methods on pile capacity, while Hongbo et al. (2007) demonstrated the influence of construction methods on shaft friction and overall pile performance. In this study, the behavior of jet grout-improved soils was evaluated through experimental observations as well as theoretical and empirical approaches. The composite soil concept was adopted, assuming that the ground comprises both natural soil and jet grout columns. Predicting the degree of improvement and corresponding changes in soil parameters before jet grouting remains challenging. Each composite soil system may exhibit different characteristics depending on soil type, improvement method, cement/water ratio, groundwater table, and saturation level. Considering these variables, predicting pile capacity a priori is complex. To address this, a method from the literature was applied: the area ratio (R) of jet grout columns within the soil volume was determined, and the undrained shear strengths of the components (cu,jet and cu,natural) were weighted accordingly to compute the composite undrained shear strength (cu,composite) of the improved soil. This approach allowed realistic modeling of pile-soil interaction and quantified the effect of soil improvement. For example, in-situ tests indicated cu,natural = 15 kPa for the first 16 m. Using the composite soil approach, cu,composite was calculated as 134.46 kPa. Applying this value in pile capacity calculations allowed achieving 182 tons with a pile length of 31 m instead of 40 m-a 9 m reduction per pile. Considering 55 bored piles per raft foundation, the total pile length reduction amounted to 495 m for a single silo, corresponding to approximately 22.5% savings in time and cost. Furthermore, geophysical measurements before and after jet grouting revealed substantial increases in shear wave velocity (Vs30), from 213 m/s to 343 m/s. Using the empirical Vs–cu correlation (Mayne & Rix, 1995), these increases were numerically linked to improvements in undrained shear strength (cu), providing an alternative method for estimating post-treatment soil strength. The coefficient R in this correlationmust be calibrated, and while cu can be estimated from geophysical data, it should not be directly used for pile design. Instead, this relationship is useful for assessing soil improvement quality by comparing estimated cu values with field and laboratory test results. Results obtained from both the composite soil formulation and the Vs–cu empirical relationship were consistent, reinforcing the reliability of the methodologies. This approach provides a valuable engineering tool, particularly when direct cu measurements (such as CPT or PLT) are unavailable, enabling prediction of soil improvement effects. In conclusion, this thesis provides a comparative evaluation of bored pile performance in both unimproved and jet grout-improved weak soils, quantitatively demonstrating the effects of improvement on borehole stability, pile performance, settlement, and pile length. Bored piles constructed within jet grout-improved composite soils offer a safe, economical, and practical alternative for heavy structures on geotechnically unfavorable soils.