Dr Sanchari Mondal

This experimental study investigates the response of vertical and battered minipiles to two-way symmetrical low-frequency (0.1 Hz) cyclic lateral loading. Laboratory (1- g ) tests were performed on scaled-down minipiles in very dense cohesionless soil, for batter angles of 0°, 25° and 45°. The cyclic loading is classified into two categories: multi-amplitude and long-term single amplitude, where force-controlled load was applied at a constant frequency. The minipiles were instrumented with optic fibres, and strain profiles were obtained at each loading stage, in both compression and tension stroke. The results are presented in terms of hysteresis loops, variation of normalised stiffness, minipile strain and bending moments under cyclic loading. In the multi-amplitude loading category, backbone curves show a stiffer force–displacement response in tension stroke than in compression stroke. For the single-amplitude category, the area of the hysteresis loop is largest for 45° battered minipiles with the lowest accumulated deformation. The normalised stiffness at the end of 50 cycles is highest for 25° minipiles with a value slightly greater than one. The strain profiles along the minipiles show stabilisation of measured strain before the number of cycle reaches 50, for all three battered conditions. A multi-surface hardening constitutive model is used to explain the effect of shearing and cyclic loading, with increasing loading amplitude on 25° battered minipiles. These test results are indicative of better performance capability of 25° battered minipiles, in terms of secant stiffness, compared to the vertical and 45° battered cases.

The ultimate lateral load capacity of battered minipiles is investigated in fine-grained soil via field testing equipped with optic fibre sensing and consequently complemented with finite element modelling. The numerical model developed in COMSOL Multiphysics was validated against (i) field-obtained force-displacement data of minipiles battered at 25 and (ii) strain profile along a vertical minipile shaft. The numerical model was then further used to predict force-displacement curves for minipiles with batter angles of 15, 30 and 45 using the full-scale pile and field soil parameters. The model helps in understanding how the combination of normal and shear stresses along the minipile shaft leads to the optimum batter angle of 30 when subjected to lateral loading. Furthermore, a parametric study was performed to comprehend the effect of apparent cohesion and minipile length on the optimum batter angle. When the apparent cohesion of the soil was reduced, the optimum angle changed from +30 degrees to +45 degrees in the case of positive battered minipiles. A change in optimum batter angle was observed when the minipile length was increased as well. These varying parameters influenced the optimum batter angle of laterally loaded minipiles, and the numerically obtained normal and shear stress profile justified this behaviour.

The field of bio-inspired geotechnics has been growing in response to the demand for foundations that are sustainable and yet have improved load-bearing capacities. This study aims to address the gap in a specialised adaptation of root system architecture for designing resilient foundations. The lateral load behaviour of one such novel grouped battered minipile configuration is evaluated in this study based on full-scale field testing and numerical modelling to report the unknown increase of load capacity caused by shape modification. First, three single minipiles battered at 0 & DEG; and 25 & DEG; were subjected to static lateral loading in fine-grained soil. The strain profiles along the individual minipile shafts were obtained using optic fibre sensors. Consecutively, full-scale lateral load tests on two types of minipile groups were also performed; one group had a configuration of two 25 & DEG; battered minipiles perpendicular to the direction of loading mimicking a tree-root system, and another conventional group had two positive and negative battered minipiles. A numerical model was developed to investigate the effect of pile spacing and obtain soil pressures, bending moments and axial forces of the battered minipile groups. Results show that increased bearing area and higher engagement of soil volume for the novel minipile group with two perpendicular battered minipiles were larger than the conventional minipile group; thus, the former offered higher lateral resistance. The deflection pattern, bending moment and p-y curves showed a shadowing effect in stiff clay for battered minipile groups at a pile head spacing of three times the minipile diameter.

Minipiles are generally hollow driven piles, less than 50 mm in diameter and commonly around 2 m in length, without any grouting. Three types of minipile group configuration consisting of individual minipiles battered at 25° with the vertical are investigated in this study to evaluate their lateral load capacities. When the lateral load is applied in the direction of the batter, they are said to be positive battered minipiles, otherwise, negative. The minipile groups under study includes a combination of positive and negative battered minipiles as well as minipiles battered in the direction perpendicular to the direction of the transverse load. The geometries are developed to mimic a tree root system, where roots move in different directions to engage a large volume of soil. The behaviour of these systems is investigated using 1g physical modelling, and it is found that the minipile group with the diagonally outward orientation of 25° battered minipiles have the highest lateral resistance. The minipile group with two positive and two negatives battered minipile performs slightly better than the group with one positive, one negative and two outwardly perpendicular battered minipiles highlighting the role of orientation of the system in its performance. In addition, optic fibres are used to record the strain profile along the minipile shafts in the 1g small scale physical model with results indicating higher strain in the leading piles compared to that in the trailing counterpart at the same lateral load.

The performance of a new battered driven minipile group system under uplift loading in a noncohesive material was investigated by field testing and physical and three-dimensional (3D) numerical modeling. A series of uplift tests were conducted on footings with four and six minipiles and on a single minipile for comparison at field and laboratory scales to assess the impact of system efficiency and installation angle, and minipile configuration. A 3D numerical model was also developed and validated based on the field results for further investigation. The results indicate that a group of minipiles carry higher loads, compared with the same number of single minipiles. This is attributed to the system efficiency and the fixity of the minipile heads. Numerical and physical modeling showed that the configuration of the minipiles significantly impacts the uplift capacity. Of three different configurations examined in this study, it was understood that when minipiles are installed diagonally, Case 2, they engage a larger volume of soil with minimal negative group effect, achieving a higher uplift capacity. The installation angle is found to play a critical role when it comes to the uplift capacity. The uplift capacity of this new minipile group increased with installation angle from 0 degrees (vertical minipiles) up to 25 degrees but decreased with higher installation angles, in agreement with findings reported in the literature. However, all battered minipile groups showed higher uplift capacities, compared with footings with vertical minipiles. Two types of minipile groups were also tested using a 1g physical modeling facility, subjected to uplift, to investigate the observations further. Two minipiles of each group were instrumented with optic fibers to record the strain profile, along with the minipile shaft corresponding to each loading stage. The load-carrying capacity obtained from the physical model corroborated the findings from the numerical simulations.

Driven minipile foundations are becoming common due to merits such as lightweight, easy installation and suitability for reinforcing in-situ structures with limited access. However, their interaction with the surrounding soil especially in battered conditions is not fully understood. To develop a better understanding of the interaction of minipiles with the surrounding soil, they were instrumented with Fiber Bragg grating (FBG) sensors. Open-ended steel minipiles of 1600 mm length were then driven at a site with cohesive soil vertically and at positive and negative 25˚ angle with the vertical and were tested for three in-situ lateral static loading cases. The near-continuous strain profile along the minipile shaft was reported using FBG. This helped in identifying the deformation pattern and the impact of load magnitude on this profile. The lateral load capacity was found to be maximum for positive 25˚, which decreased for 0˚, followed by negative 25˚ batter angle.

The lateral load-carrying mechanism of vertically installed and battered minipiles is evaluated using 1g-physical and numerical modelling. Single minipiles with batter angles of 0 degrees, +/- 25 degrees and +/- 45 degrees are tested under lateral load in medium dense and dense sand. The minipiles are instrumented with fibre Bragg grated optic fibres to obtain a strain profile (two-dimensional) along the minipile shaft. A calibrated numerical model is further adopted to produce p-y curves for battered minipiles at various node deflections. The ratio of soil reaction of battered minipiles to vertically installed minipiles is observed to change with both deflection and depth of the minipile. An analytical solution is developed based on the decomposition of lateral load into skin friction and passive pressure for battered minipiles. A reduction factor is proposed that considers a decrease in passive pressure when the minipile is loaded in the opposite direction of the batter. The analytical solution is capable of accounting for soil properties, pile rigidity and the angle of inclination of battered minipiles. The analytical method is subsequently verified for cohesive soils using full-scale field results. The ratio of the ultimate lateral load of battered minipiles to vertical minipiles presented in the literature corroborated the findings of this study.

Battered minipile groups mimicking tree root networks have been gaining popularity as a footing solution for light structural applications in residential, commercial and infrastructure sectors, recently. Battered minipile group configurations are recently in the limelight due to advantages such as ease of installation and environmentally friendly nature. The lateral load resistance of battered minipile groups is investigated in this paper through a combination of physical and numerical modelling. Two-unconventional battered minipile groups with configurations representing the root network of trees with the capacity of engaging a larger volume of soil compared to conventional battered minipile group configurations are studied. A conventional battered minipile group is also included in the study to draw a direct comparison with the new minipile group configurations introduced in this paper. The conventional battered minipile group has two positively and two negatively 25 degrees battered minipiles. The second type of group has one 25 degrees perpendicularly battered minipile in the leading and trailing row each. Another unique orientation of the battered minipile group is also introduced in this study which has four diagonally outward 25 degrees battered minipiles. The third type of minipile group with four diagonally outward battered minipiles offered the highest lateral resistance among the three groups. This better performance capability was attributed to the engagement of a larger volume of soil in resisting lateral load applied at the minipile head. Through this study, the industrial application of the unconventional minipile group configuration with better performance capability in terms of lateral load resistance can be advocated more confidently.