Editorial Note - Vol. 15., No. 2, 2021

Welcome to Issue 2 of DFI’s Journal Vol. 15. We are happy to introduce six publications which span a wide mix of manuscript types and technical content. While most published papers in the DFI Journal have historically been research papers and technical case histories, this issue introduces a forum paper and a book review in addition to valuable research publications. Forum papers, a new type of paper to the DFI Journal, encompass a much shorter manuscript style, and can include commentaries, opinions, research highlights (e.g., of work currently underway), and responses to previously published papers. Forum papers do not require a response from the author if they discuss another author’s manuscript. However, should the forum paper be closer to a discussion, and should the author of the discussed manuscript desire to provide a response, the submission will be converted into a formal “discussion.” Forum papers and book reviews first undergo a review within the editorial board, and are only subject to external peer review if the content is found to be technically controversial and/or should the area of expertise discussed in the manuscript lie outside the capacities of the editorial board. Hence the Forum Paper is a unique outlet with much technical and editorial flexibility and will, along with the newly introduced book review, enrich the palette of products offered by the DFI Journal.

Evaluation of Negative Skin Friction on Bridge Abutment Piles

Piles are used to transfer loads of structures to deeper and stronger soil layers through skin friction and/or end bearing. Surcharge loads, site grading, or dewatering may induce downward movement of soil adjacent to piles installed in a compressible medium. This movement creates negative skin friction stresses acting downward at the pile-soil interface, which applies additional loads “drag forces” to the pile causing a maximum axial load in the pile shaft at the “neutral plane”. To evaluate the development of drag forces, a comprehensive field monitoring program was conducted over four years for three instrumented abutment H-piles as part of a three-span bridge project. The soil settlement and changes in pore water pressure in the soil adjacent to the piles due to the construction of an approach embankment were monitored using multiple-point extensometers and vibrating wire piezometers. The piles’ elastic settlement and strains were measured using single-point extensometers and vibrating wire strain gauges. The field measurements are presented and discussed in terms of responses time histories and load distribution along one pile shaft. In addition, the calculated forces from vibrating wire strain gauges are compared with the unified design method prediction considering the total stress method (α-method) for cohesive soils. The results show that the maximum drag force was developed after the complete dissipation of excess pore water pressure and that the location of neutral plane varied during the embankment construction stages. Employing the total stress method in the unified design method provided a reasonable prediction of the drag force and the neutral plane’s location.

Commentary to: Prediction, Testing and Analysis of a 50 m Long Pile in Soft Marine Clay-Journal of the Deep Foundations Institute, 13(2) 1-7

This paper presents an excellent illustration of the best practice for installation and testing of a long displacement pile in soft clay to minimize installation disturbance. This loading test is also a good case study for the application of the SHANSEP-based approach described by Saye et al. (2013) and corresponding LRFD calibration of the SHANSEP-based approach by Stuedlein et al. (2020).

Model Uncertainties in Foundation Design

The book focuses on providing a foundation designer information on the model factor and its statistics for conventional foundation types such as shallow foundations, driven piles, and drilled shafts as well as special foundations such as spudcans and helical piles. Besides foundations, the book also provides information for other geostructures such as mechanically stabilized earth (MSE) walls, soil nail walls, pipes and anchors, slopes, and braced excavations.

Optimum Open Cell Sheet Pile (OCSP) Quay Wall System in Port Said, Egypt

The principal intent of this paper was to propose a new quay wall system, whose validity was verified; to play the same role of the existing diaphragm quay wall at East Port Said Port in Egypt as an optimum quay wall. Firstly, the background information of the proposed Open Cell Sheet Pile (OCSP) was introduced. Then, a parametric study was conducted including 24 cases using soil constitutive Hardening Soil Model (HSM) by the PLAXIS 3D Finite Element Software. The behavior of the proposed system was presented graphically, and the effectiveness of the OCSP system was found satisfactory with the possibility to increase the seabed depth in front of the wall up to four meters. The system also was consisted of a bored piles group to carry the heavy loads generated by the gantry crane. Finally, the construction cost of the OCSP was compared to the existing quay wall structure system for evaluating the system design optimization.

Lateral Spreading and Stability of Embankments Supported on Fractured Unreinforced High-Modulus Columns

Construction of column-supported embankments (CSEs) with unreinforced high-modulus elements is now common practice to accelerate fill placement. These brittle columns are susceptible to column fracturing and CSE designs often limit the degree of lateral spreading such that tensile rupture will not occur, which stems from salient concerns that fracturing may trigger uncontrolled lateral spreading and/or the cessation of intended vertical load transfer. However, tensile rupture is unlikely to coincide with full mobilization of available passive resistance at the toe. Thus, it is disputed in industry whether some degree of column fracturing is tolerable. The objective of this study is to elucidate the influence of column fracturing on lateral spreading and stability of CSEs. A collective examination of available performance data is accompanied by a parametric 3D finite element study of hypothetical embankments, which considers the cessation of column bending resistance due to tensile rupture at discrete crack locations. A factor of safety, which reflects development of a passive failure wedge at the embankment toe, is used as a proxy for lateral stability. Factors of safety are linked to the magnitude of lateral spreading to address whether adequate confinement can be provided by foundation soils when fracturing occurs in unreinforced high-modulus columns that support embankments.

A Small-Scale Experimental Investigation on the Benefits of a Hybrid Pile/Footing System on Sand

This paper explores the capacity effects of a square steel plate welded at the ground surface on a driven open-ended steel pile (i.e. the plate would touch the ground surface after the pile achieves the required penetration). A series of strain-controlled, 1-g small-scale laboratory tests were undertaken on piles with and without a square steel plate attached. The piles were driven in dry, loosely packed, uniform sand. Two plates were used, one with a breadth equal to two times the diameter of the pile (2D) and the other with a breadth equal to three times the diameter of the piles (3D). A 20% increase in capacity was recorded for the 2D plate, and a 110% increase in capacity was recorded for the 3D plate when compared to the pile without an attached steel plate. The back-analysis of the results allowed the derivation of a new expression to calculate the capacity of bearing plates and plot its load-settlement profile, which accounted for the effects of sands compaction and dilation. By extrapolating the findings of these tests to a hypothetical scenario, a model design problem was described where the length of a pile can be reduced by 20% to 60% (depending on the load) by using a plate attached to the pile. The results of this study can help designers to minimise penetration depth; thus, achieving a more economical and sustainable design.

Results of an Instrumented Static Loading Test and Its Application to Design Compilation of an International Survey

Results of a static loading test were used together with soil exploration records in a survey comprising analysis of the test records and estimating settlement of piled foundation to support a pipe rack. The test pile was a strain-gage instrumented, 400-mm diameter, precast, prestressed concrete pile driven into a clay and silt deposit to 25 m embedment. Two main issues were expected to be addressed by the survey participants: First, realization that the strain records were affected by presence of residual force in the pile and, second, calculation of the settlement of the piled foundation expected from the foundation load. A total of 52 submissions were received from 20 different countries. Only 12 of the submissions realized the presence of residual force. Most submissions reported a calculated settlement of the piled foundations ranging from 10 mm through 50 mm; however, 11 reported values between 60 and 200 mm. Surprisingly, only 20 submissions reported ground surface settlement close to the 200-mm value resulting from text-book analysis based on the available information. The subsequent construction of the piled foundations coincided with placing a fill across the site and lowering of the groundwater table, thus, causing a general subsidence.

Resistance Factors at Serviceability Limit State Using the Texas Cone Penetrometer as the Predictive Model

This study presents the development and calibration of resistance factors for the serviceability limit state (SLS) condition (φSLS) used in the load and resistance factor design (LRFD) of deep foundations. The performance function was established based on load corresponding to tolerable displacement (Qδtol) and design load (Qd). A dataset of published full-scale load tests including projects from Texas, Missouri, Arkansas, Louisiana, and New Mexico was compiled and consisted of 60 load test cases comprising 33 driven piles and 27 drilled shafts. Resistance factors for SLS conditions were calibrated for tolerable displacements using both the Monte Carlo simulation (MCS) and the First Order Second Moment (FOSM) approaches. From the calibration study, resistance factors at SLS conditions were obtained ranging from 0.33 to 0.62 using FOSM method and 0.37 to 0.67 using the MCS for driven piles. In the case of drilled shafts, SLS resistance factors ranged from 0.37 to 0.77 following the FOSM method and 0.41 to 0.86 based on MCS.

Skin Friction Directionality in Monotonically- and Cyclically-Loaded Bio-inspired Piles in Sand

Piles can be subjected to axial loading in opposite directions during their installation and service life. For instance, piles for offshore jacket structures and load testing reaction systems are subjected to compressive loading during installation and tensile or cyclic loading during service life. This creates a design dilemma: while a large skin friction can lead to refusal at shallower depths than required during driving, it also promotes a large pile axial capacity. This paper describes the load-transfer behavior of piles with surfaces inspired by the belly scales of snakes that mobilize a direction-dependent skin friction. The investigation presented herein consists of a series of twelve centrifuge pile load tests on bio-inspired and smooth reference piles in dense and loose deposits of Ottawa F65 sand. Test results indicate that greater skin friction forces are mobilized when the bio-inspired piles are displaced in the cranial direction (i.e. soil moving against asperities) relative to the caudal direction (i.e. soil moving along asperities). This is observed during pushing and driving installation, where greater skin friction forces were mobilized during installation by pushing in the cranial direction and driving in the cranial direction required more blows per meter. Similarly, the skin friction mobilized during pullout tests was between 82% and 198% greater in the cranial direction than in the caudal direction, and the skin friction mobilized during pullout by the bio-inspired pile in the cranial direction was between 560% to 845% greater than that mobilized by the reference untextured pile. During cyclic loading, degradation of the skin friction magnitude and pile secant stiffness was observed in both cranial and caudal directions; however, the mobilized magnitudes were generally greater in the cranial direction. Discussion is provided on the potential benefits that the bio-inspired surface texture could realize on the overall performance of axially-loaded piles.