Open-Ended vs Closed-Ended Pile Groups: Comparing Behaviour Under Vertical Loading

Steel pipe piles serve as a reliable foundation solution across challenging soil conditions, offering durability, shorter construction periods, and the ability to be driven into ground where other pile types cannot penetrate. A critical design decision engineers must evaluate is whether to use open-ended or closed-ended piles, as their behaviour under vertical loading differs substantially. This article examines a research study from the National Institute of Technology, Kurukshetra, that compared open-ended and closed-ended pile groups in loose sand through controlled model testing. For additional context on related pile foundation challenges, see Negative Skin Friction On Piles and Pile Groups, which explores another critical load factor affecting pile group performance.

Understanding the Mechanics of Open-Ended and Closed-Ended Piles

Fundamental Differences in Driving and Load Transfer

Closed-ended piles feature a solid tip that displaces soil laterally during driving, creating a full displacement effect. Open-ended piles allow soil to enter the hollow interior, forming what is known as a soil plug. This plug plays a decisive role in load-bearing capacity. When an open-ended pile is driven in fully unplugged or coring mode, soil enters the hollow section at the same rate as the pile advances. Under plugged conditions, the soil inside compresses and locks against the inner wall, effectively creating a closed end. The degree of plugging depends on several factors:

• Penetration depth — deeper penetration generally increases plugging, causing open-ended capacity to approach that of closed-ended piles
• Soil density and grain characteristics — denser sands promote more effective plugging and higher skin friction
• Pile diameter relative to grain size — smaller diameter-to-grain ratios increase plugging likelihood
• Driving method and energy — continuous versus intermittent driving affects plug formation and compaction

Installation Effort and Driving Resistance

Field tests by Szechy (1959) established that blow counts required to drive a pile to a given depth in sand are consistently lower for open-ended piles than closed-ended piles under identical conditions. This reduced installation effort translates directly into cost and time savings on construction projects. However, the relationship between driving resistance and ultimate capacity is not straightforward. Research by McCammon and Golder (1970), Smith et al. (1986), and Brucy et al. (1991) showed that the driving mode significantly influences resistance. Fully coring driving produces the lowest resistance, partially plugged driving generates intermediate values, and fully plugged driving approaches the resistance of closed-ended piles because the soil column inside behaves as a solid tip.

Load Capacity Convergence with Depth

Short open-ended piles consistently show lower load capacity than equivalent closed-ended piles. However, as penetration depth increases, capacity converges because greater embedment promotes more complete soil plugging, allowing the internal soil column to transfer load through both shaft friction and end bearing (Klos and Tejchman, 1981; Paikowsky and Whitman, 1990). Settlement behaviour also differs: under identical load and soil conditions, open-ended piles exhibit greater settlement than closed-ended piles, a factor critical for serviceability limit state design.

Experimental Methodology and Material Characterisation

Test Materials and Model Configuration

Sand was collected from the banks of the Yamuna River near Radaur in Yamunanagar District, Haryana. Its engineering properties are summarised in the table below.

Steel model piles were fabricated with a diameter of 2 cm and total length of 50 cm (40 cm embedded). Pile caps were 12 mm thick mild steel plates, with dimensions varying according to pile count. Spacing within each group was set at 2.5 times the pile diameter. A minimum cover of approximately half the spacing was maintained around outer piles. Thirteen tests were conducted covering single piles through 4×5 groups containing 20 piles, including five circular groups. Each configuration was tested with both open-ended and closed-ended piles for direct comparison.

Test Procedure

For single piles, a proving ring was attached to a screw jack for load application. Two Baty dial gauges (least count 0.01 mm, 25 mm travel) were mounted on the pile cap via magnetic bases and cross-angle sections. The average of the two gauge readings was taken as the settlement under each load. Load was applied in small increments and maintained constant until settlement stabilised. The process continued until failure, defined as rapid continuous settlement. The same fundamental procedure was applied to pile groups, with the proving ring positioned at the cap centre and dial gauges at opposite corners. Group efficiency was calculated using the Converse-Labarre formula and compared with experimentally determined values.

Comparative Analysis of Pile Group Behaviour

Ultimate Bearing Capacity Comparison

The experimental results revealed clear and consistent differences between open-ended and closed-ended pile groups across all configurations. The table below presents the measured ultimate bearing capacities.

The capacity ratio remained remarkably consistent at approximately 1.35 across all group sizes, indicating the relative performance advantage of closed-ended piles is independent of group configuration. Both pile types showed proportional capacity increases as the number of piles increased, confirming that group action effects scale predictably for both configurations.

Group Efficiency and Geometric Effects

Efficiency was evaluated using the Converse-Labarre formula, one of the most widely used pile group efficiency equations. The experimental results revealed several important patterns:

• Square groups exhibited the highest efficiency among all configurations tested
• Rectangular groups showed lower efficiency than square groups with equivalent pile counts, highlighting the influence of geometry on soil-stress overlap
• Experimental efficiency exceeded theoretical predictions from the Converse-Labarre formula in all cases, confirming it provides conservative estimates for loose sand conditions
• Efficiency values greater than 1.0 were recorded, indicating group action can produce combined capacity exceeding the sum of individual piles, attributed to sand densification during driving

Circular pile groups, tested in configurations of 3, 5, 7, and 11 piles, demonstrated the highest ultimate bearing capacity among all group shapes tested. This makes circular arrangements particularly attractive for symmetric loading conditions and constrained footprints. Experimental failure loads consistently exceeded Indian Standard code method predictions, underscoring the value of site-specific load testing over purely theoretical approaches.

Practical Implications for Foundation Design

Selecting Between Open-Ended and Closed-Ended Piles

Closed-ended piles offer approximately 35% higher ultimate capacity in loose sand, making them the preferred choice when maximising bearing capacity is the primary design driver. However, open-ended piles present compelling advantages that may outweigh this difference:

• Lower installation costs due to reduced driving effort
• Shorter construction periods, particularly in dense or boulder-bearing strata
• Ability to penetrate ground conditions that would damage or deflect closed-ended alternatives
• Cost efficiency for long piles extending through deep loose soil deposits

Since capacity converges with penetration depth, for deep foundations the penalty of open-ended piles diminishes substantially. Engineers should evaluate the plugging potential of the soil profile, as complete plugging functionally transforms an open-ended pile into a closed-ended equivalent at full penetration. For perspectives on how structural systems respond to combined loading, see Framed Building With Shear Walls Subjected to Horizontal.

Group Configuration and Related Material Systems

Square arrangements deliver the highest group efficiency, while circular groups offer excellent bearing capacity for constrained footprints. The finding that experimental efficiency exceeds 1.0 in loose sand suggests designers can potentially achieve cost savings through reduced pile counts when validated by site-specific testing. The performance of pile foundations is also linked to the durability of materials used in their construction. For comparisons of protective and insulation systems used in building construction, see Spray Foam Insulation Complete Technical Guide to Open and Spray Polyurethane Foam Insulation Open Cell Vs Closed. The principle of comparing open versus closed configurations in material systems provides an interesting parallel to the pile engineering question explored in this study.

Key Conclusions from the Research

1. Pile group load capacity increases proportionally with pile count for both open-ended and closed-ended configurations
2. Square groups achieve the highest efficiency in loose sand, followed by circular groups, with rectangular groups showing the lowest
3. Experimental efficiency exceeds Converse-Labarre predictions, confirming the formula provides conservative estimates
4. Group efficiency exceeds 1.0 due to sand densification during pile driving
5. Circular groups demonstrate the highest ultimate bearing capacity among all geometries tested
6. Experimental failure loads exceed IS code method predictions, underscoring the value of site-specific load testing
7. The consistent 35% capacity advantage of closed-ended piles provides a reliable factor for preliminary design sizing