Behavior of single rammed aggregate piers considering installation effects

Although rammed aggregate piers (RAPs) have attracted much attention as a cost-beneficial ground improvement technique, their construction process and the other affecting parameters on the behavior of these geotechnical structures during the construction and loading phases have not been properly investigated and only some predictions for downward and radial expansion resulting from the construction process have been proposed to be used in numerical simulations. The goal of this paper is to numerically simulate the RAP construction process to investigate the effect of construction sequences on RAP behavior. Keeping this aim in mind, a hybrid distinct element–finite difference algorithm is developed to simulate the placing and ramming of the RAP's clean crushed stone layers in detail. Introducing several numerical models, a parametric study was then conducted to compare the behavior of RAPs during the construction process for different pier geometries and soil properties. The results of this research not only help to understand the RAP behavior during the ramming process but can also be used as a guide for predicting the amount of downward and lateral expansion of RAPs after the ramming process.

Subsurface conditions at the site of a large box culvert constructed beneath a deteriorating bridge consisted of highly compressible alluvial clay. Concerned with settlements estimated up to 50 cm, engineers considered completely removing and rebuilding the bridge structure. With estimated settlements still exceeding 10 cm, rammed aggregate piers were selected for installation beneath the box culvert to control differential settlement and prevent downdrag on the existing bridge pier foundations. Despite construction challenges including high water table, very soft soil conditions, and low clearance for machinery beneath the bridge structure, it was determined that the benefits of the box culvert and embankment (i.e. ease of future roadway expansion and continual service of the highway throughout construction) outweighted the cost of replacing the bridge structure. Approximately 250 rammed aggregate piers were installed in a grid pattern with lengths ranging from 2.3 m to 6.7 m depending on embankment fill heights. Performance was monitored with settlement plates and compared to predictions.

Within the scope of this manuscript, the mitigation performance of a composite ground improvement solution, which is composed of 18 m long 40 cm diameter GeoConcrete® Column (GCC) and 50 cm diameter Impact® Rammed Aggregate Pier® (RAP) along with 40 m long 80 diameter piles is assessed by pre- and post-cone penetration testing (CPT). These elements are designed for controlling excessive total and differential settlements, and liquefaction triggering at a paper mill site. In this paper, the site geology, geotechnical model, design aspects of GCC and Impact® RAP patented systems and QA/QC measures are discussed. As a mitigation solution, 18 m and 40 m long elements are designed to be constructed in the soft to medium stiff silty clay with scattered silt and sand interlayers. Improvement expectations from GCC and Impact® RAP elements are partially verified by pre- and post-CPT data, and are listed as: (i) densification of cohesionless silt and sand layers, (ii) shear stress transfer to rigid columns during cyclic (seismic) loading, reducing seismic demand from foundation soils (iii) increased horizontal stresses, leading to increased soil (and column) stiffness and strength, (iv) vertical drainage through aggregate columns to dissipate cyclically – induced excess pore water pressures. The results show that due to ramming and vibration induced-densification, cone tip resistance has increased by a factor of 1.3–1.6 in cohesionless layers.

A parametric study of large piled raft foundations on clay soil

Structural analysis of piled raft foundation in soft soil: An experimental simulation and parametric study with numerical method

A screw pile has higher end bearing capacity than any other straight pipe piles due to its larger helix with respect to central shaft. However, larger helices are not frequently used as it will bend and may reduce the actual bearing capacity of the ground. In the present study, the effect of helix bending deflection on the load settlement behaviour and ultimate bearing capacity is investigated. To achieve the objectives, model scale pile load tests were conducted. The effect of helix bending on the load settlement behaviour at higher stress level was also investigated in this research. The helices with different helix-to-shaft-diameter ratios and thicknesses were used, so that clear difference of deformed and non-deformed screw piles in the load settlement behaviour can be observed. Dry Toyoura sand in dense state was used as a model ground. It is observed from test results that the helix bending deflection starts affecting the load settlement behaviour of the ground if it is more than the critical helix bending deflection. The ratio of critical helix bending deflection to outstand length decreases with increase in helix-to-shaft-diameter ratio, and its relationship is presented in this study. It is also observed that the Roark’s formula for flat circular plate having uniform load over a very small circular area with fixed outer edges showed good agreement with the measured helix bending deflection. In order to estimate the optimum helix thickness, the well-agreed equation is also modified with respect to critical helix bending deflection.

PurposeThe purpose of this study is to investigate the effect of presence of buried flexible pipe on the bearing capacity of shallow footing. First, a model study is performed where shallow footing model is tested for its load settlement behavior, with and without the existence of buried PVC pipe lying vertically below the base of the footing.Design/methodology/approachThe experimental set-up consisted of a steel box filled with sand at two different relative density values [RD = 50 per cent (medium dense) and RD = 80 per cent (dense sand)] and vertical load was applied on the model footing through hydraulic jack and reaction frame arrangement connected with a proving ring. Test results are verified numerically using commercially available finite element code PLAXIS 2D. With due verification, a parametric study has been conducted, numerically, by varying the range of input parameters, such as unit weight, angle of internal friction, diameter of buried conduit and the Elastic modulus of the soil to assess the pre cent reduction in the capacity of the foundation soil because of the presence of underlying buried flexible pipe.FindingsResults show that for each footing, there exists a critical depth below which the presence of the buried conduit has negligible influence on the footing performance. When the conduit is located above the critical depth, the bearing capacity of the footing varies with various factors, such as geotechnical parameters of soil and location and diameter of the buried conduit.Originality/valueIt is an original paper performed to assess the presence of buried flexible pipe on the bearing capacity of the shallow footing.

Energy geostructures, particularly energy piles, are foreseen as an alternative to reduce the environmental impact of the growing energy demand for space conditioning. This environmental friendly technology is a type of closed-loop Ground Source Heat Pump (GSHP) in which foundation elements are used to extract or inject thermal energy from/to the soil. Despite their multiple benefits, energy piles have not yet been employed in Mexico, where there is a general lack of knowledge about these systems. In the present paper the feasibility, limitations and challenges of using energy piles in the Mexican context are discussed. In order to understand the geotechnical implications of their functioning, a numerical simulation of the behavior of an energy pile subjected to different combinations of thermo-mechanical loads considering Coatzacoalcos (Veracruz) subsoil conditions is presented. The fully coupled thermo-hydro-mechanical analysis are made using the finite element program PLAXIS 2D Thermal. Vertical displacement and axial load distributions are obtained. The results of the parametric studies show that the magnitude of the thermal-induced stresses is significant, but their effects on the behavior of the foundation depends on the magnitude of the applied mechanical load.

To increase reliability and fatigue life of machines it is necessary to improve calculation methods of machine elements. Elaboration and make use of calculations taking into consideration the real work conditions of machines and influence of various constructive factors upon machine capacity for work is one of the most important ways of acceleration of scientific and technical progress in mechanical engineering. Threaded parts are the most important machine parts which determine reliability of machines as a whole. The prime importance problem, on threaded joints calculations, is to know, how an axial load acting on the threaded joint is distributed on bolt and nut thread portions. Having considered the problem solution in historical development the authors have given the methods of the calculations of the axial load distribution on the bolt and nut threads for four main loading schemes of the threaded joints. The influence of bolt, nut and thread geometrical dimensions and materials are taking into account.

Conventional analytical and numerical methods for the mechanical properties of helical threads are relied on many assumptions and approximations and thus hardly yield satisfied results. A parameterized 3D finite element model of bolted joints with real helical thread geometry is established and meshed with refined hexahedral elements. The Von Mises plasticity criterion, kinematic hardening rule of materials and interfacial contacts are employed to make it possible for the suggested model be able to approach real assembly conditions. Then, the mechanical properties of bolted joints with different thread pitches, thread numbers and modular ratios are investigated, including the contact pressure distribution at joint interfaces, the axial load distribution and stress concentration in screw threads during the loading and unloading process. Simulation results indicate that the load distribution in screw threads produced by the suggested model agrees well the results from CHEN’s photoelastic tests. In addition, an interesting phenomenon is found that tightening the bolt with a large preload first and then adjusting the clamping force by unloading can make the load distribution more uniform and reduce the maximum residual equivalent stress in thread roots by up to 40%. This research provides a simple and practical approach to constructing the 3D finite element model and predicting the mechanical properties of helical thread connection.

Novel three-dimensional simplified numerical simulation method of a compact precooler for hypersonic precooled air-breathing engine

A Modified Symmetric and Antisymmetric Decomposition-Based Three-Dimensional Numerical Manifold Method for Finite Elastic–Plastic Deformations

SUMMARY This paper customizes a contact detection and enforcing scheme to fit the three-dimensional (3-D) numerical manifold method (NMM). A hierarchical contact system is established for efficient contact detection. The mathematical mesh, a unique component in the NMM, is utilized for global searching of possible contact blocks and elements, followed by the local searching to identify primitive hierarchies. All the potential contact pairs are then transformed into one of the two essential entrance modes: point-to-plane and crossing-lines modes, among which real contact pairs are detected through a unified formula. The penalty method is selected to enforce the contact constraints, and a general contact solution procedure in the 3-D NMM is established. Because of the implicit framework, an open-close iteration is performed within each time step to determine the correct number of contact pairs among multi-bodies and to achieve complete convergence of imposed contact force at corresponding position. The proposed contact algorithm extensively utilizes most of the original components of the NMM, namely, the mathematical mesh/cells and the manifold elements, as well as the external components associated with contacts, such as the contact body, the contact facet and the contact vertex. In particular, the utilization of two mutually approaching mathematical cells is efficient in detecting contacting territory, which makes this method particularly effective for both convex and non-convex bodies. The validity and accuracy of the proposed contact algorithm are verified and demonstrated through three benchmark problems. Copyright © 2013 John Wiley & Sons, Ltd.

Strain rockburst is a severe failure phenomenon caused by the release of elastic strain energy in intact rocks under high-stress conditions. They frequently occur in deep tunnels, causing significant economic losses, casualties, and construction delays. Understanding the factors influencing this disaster is of significance for tunnel construction. This paper first proposes a novel three-dimensional (3D) discrete element numerical analysis method for rockburst numerical analysis considering the full stress state energy based on the bonded block model and the mechanics, brittleness, integrity, and energy storage of the surrounding rock. This numerical method is first validated via laboratory tests and engineering-scale applications and then is applied to study the effects of compressive and tensile strengths of rock mass, tunnel depth, and lateral pressure coefficient on strain rockburst. Meanwhile, sensitivity analyses of these influencing factors are conducted using numerical results and systematic analysis methods, and the influence degree of each factor on the rockburst tendency is explored and ranked. The results reveal that laboratory tests and actual engineering conditions are consistent with numerical simulation results, which validates the rationality and applicability of the novel rockburst analysis method proposed in this paper. With the increase in compressive strength, the stress concentration degree, energy accumulation level, maximum stress difference, and maximum elastic strain energy within the rock mass all increase, leading to a stronger rockburst tendency. Tunnel depth and the lateral stress coefficient are positively correlated with rockburst tendency. As the lateral pressure coefficient and tunnel depth increase, rockburst tendency exponentially increases, while the maximum stress difference and maximum elastic strain energy within the rock mass also increase. The influence degree of each factor is ranked from highest to lowest as follows: tensile strength, lateral pressure coefficient, compressive strength, and tunnel depth. The research results provide theoretical support and technical guidance for the effective prediction, prevention, and control of rock burst disasters in deep tunnels.

Numerical Analysis of Horizontal Moisture Barriers in Pavements Constructed on Expansive Soils