Advanced Research on Shallow Foundations

Problematic soils (e.g., loose, erodible, and collapsible) are unstable and pose significant challenges to the geotechnical engineering communities due to their low bearing capacity and high compressibility. Such unstable soils are widespread in the world and greatly hinder civil infrastructure developments such as building foundations, roads, retaining walls, etc. Engineered ground improvement is often necessary to increase soil load-carrying capacity and prevent excessive post-construction deformations. However, existing techniques of ground improvement can be highly expensive (e.g., pile foundations) and some have significant environmental, sometimes toxic effects (e.g., chemical stabilization by Portland cement). In this presentation, an innovative technology for ground improvement, through biologically induced calcite cementation (bio-cementation), will be presented and discussed. This technology should replace most conventional soil stabilization methods, for superior urban and coastal infrastructure developments.

The most important aspects for the design of any foundation system are safety, optimisation and the sustainability. An optimised and safe design of foundation systems for high-rise structures in difficult soil and groundwater conditions is based on a reduction of construction material used, construction time spent, energy consumed and the adequate consideration of the soil-structure interaction. This is also important for the high-rise structures like skyscrapers and bridge piers in weak rock like in Dubai, UAE. Due to the large loads most of these structures are founded in the Dubai Sandstone and Dubai Siltstone. Up to now the rock mechanical parameters for these rock layers have been defined on the very conservative side which led to over-dimensioned foundations in many cases. In a large research program the bearing behaviour of Dubai Sandstone and Dubai Siltstone has been investigated by field and laboratory tests, by in-situ pile load tests and the numerical back-analysis using the Finite-Element-Method (FEM). The comprehensive research investigations show, that the stiffness of Dubai Sandstone and Dubai Siltstone is more than 20 times higher as it is assumed up to now. The paper presents the scope of research, the epoch-making results and the significance for the engineering practice.

The nature of dynamic settlement of a shallow foundation is strongly influenced by the response of soil whereas the response of soil largely depends on the cyclic loading. Therefore, it is necessary to study the soil response due to cyclic loading. The present study focuses on investigating the settlement response of a surface strip foundation resting on soft clay subjected to vertical cyclic loading in the form of a rectangular pulse. A strip foundation-soil system is modeled and the settlement response is estimated using numerical programming tool Open System for Earthquake Engineering Simulation (OpenSees). Combinations of an allowable vertical static load and rectangular pulse with a frequency of one cycle per second are applied to the footing. The magnitude of the allowable static load and the amplitude of cyclic load are varied to observe the settlement response of the foundation. The effect of variation in the magnitude of static allowable load and increase in the amplitude of cyclic load on the behavior of the foundation is observed. Based on the results obtained from the numerical study, the critical number of load cycles (n_cr) after which the settlement becomes negligible for further cyclic loading, is estimated for each case. Furthermore, the effect of frequency of loading on the settlement response of the foundation-soil system for limited cases has been investigated considering two additional frequency of 0.5 Hz and 2 Hz.

Settlement of structures founded on soft soil is a primary concern for geotechnical engineers. Buildings will frequently fail due to reaching the serviceability state due to cracking and or tilting from uneven settlement well before reaching the ultimate limit state. There is ongoing discussion in the geotechnical engineering community on the mechanics of settlement, specifically the point at which secondary consolidation (creep) begins: ought Hypothesis A or Hypothesis B be adopted? This paper addresses the challenges in estimating Hypothesis B, where the strain values in both thick and thin soil samples at the end of primary consolidation are believed to be a function of both the change in void ratio, which is understood well, and creep, which is not. A new empirical-based model, the EOP-Anchor model, is proposed to estimate Hypothesis B strain as a function of settlement time and soil thickness in conjunction with standard laboratory oedometer test data on thin samples.

The progressive technologies, which are aimed at reinforcement of soils, play a special role in complex conditions of construction of foundations for laying aboveground main pipelines. Initially, it is explained by ever-changing loads in the technological chain of the pipeline transport of hydrocarbons as well as the variety of soils in green deposits. First and foremost, these requirements include the limitations associated with values of admissible absolute and relative settlements of foundations of the technological chain structures. Based on the abovementioned, a research for innovative approaches to meet the requirements of the reliability of the “soil-foundation” system for these structures is quite relevant.

In practice, if it is impossible to reduce the values of loads, it is proposed to control a required value of soil settlement by changing a form of load through foundation geometry.

There was conducted comprehensive research, which included the simulation of compressive stresses of soil depending on a geometry of a pile foundation, and plate model experiments. Based on its results and taking into account physical-mechanical soil properties there have been selected technologies of preparation and rational parameters of consolidation of soil for the structures of oil and gas industry (Innovative patents ## 28833, 28834, 29158, 29424).

Pre-consolidation of subsoil and construction materials allows increasing stability (reliability) of foundations of engineering facilities of main pipelines during a long period of their exploitation.

Moreover, application of the obtained research results for construction of foundations of closely located structures of the technological chain allows reducing their mutual influence on each other at the soil level. It provides the admissible values of absolute and relative soil settlements excluding additional financial costs.

Earthquakes have occurred for millions of years and will continue in the future affecting millions of lives, thousands infrastructure and costing billions of dollars. Seismic soil structure interaction is the process in which the structure rested on the ground and subjected to an earthquake is affected by the response of the soil medium beneath it having its own characteristics. Thus, soil structure interaction response is dictated by the interaction between the structure, foundation and underlying soil or rock. To clarify the contribution of the seismic soil structure interaction, 3D time history finite element models of 15 story concrete frame building were performed using Abaqus under Kocaeli’s Mw = 7.5 strong ground motion. The effects of soil boundary limits, mat thickness and soil linearity were investigated. To demonstrate the importance of modeling correct soil structure interaction problems, the numerical analysis was carried out for three cases: (1) a fixed-base structure, (2) a structure rested on silty sandy soil, and (3) a structure supported by raft foundation and rested on silty sandy soil. The results, presented in terms of the structural lateral deflection and base shear versus time, show that soil structure interaction effects should be evaluated when the maximum lateral deflection is obtained at top of the structure regardless when it occurred as well as at maximum absolute lateral deflection at each level. Moreover, excluding soil structure interaction effects may result in inadequate structural safety for the frame building rested on silty sandy soil and considering linear soil properties may result in non-economical structural design.

In the assessment of slopes, factor of safety values still remain the primary indexes for determining how close or far slopes are from failure. Limit equilibrium and finite elements methods are among the most popular methods of slope analysis. Both methods were used to analyze homogeneous and inhomogeneous slopes. However, Limit equilibrium methods are relatively simple and suffer from many assumptions compared with finite element analysis. Recently, as computational techniques advance, more and more attention has been paid to the slope stability evaluations using finite element methods (FEM). This article demonstrates a finite element approach to analyze the response of homogeneous and layered slopes. A detailed parametric analysis is presented to study the effect of surcharge (it can be an imposition of building load….etc.) on the stability of the slope. In regard with these results it is seen that the factor of safety increases with increase in soil properties, groundwater depth, slope length, distance from the crest of slope. However, it decreases with increase in surcharge intensity, increase of weak soil layer thickness and increase of slope angle and slope height.

Reinforced soil foundation is a soil foundation containing reinforcing elements or inclusions in the form of strips, grids or ties which are placed horizontally. The purpose of this research was to study the load settlement behavior of square footings placed on geogrid reinforced sandy soil foundation using model tests. While the geometric parameters are specific to certain specifications and conditions such as gradation, relative density of soil beneath the foundation etc., the type, aperture size, stiffness and thickness etc. of geogrid may have some effect on the critical parameters of the reinforced soil foundation. Therefore, two types of geogrid have been used in this study to investigate the load settlement behavior for model footings reinforced with uniaxial (UNX100) and biaxial (BIX100) polyester geogrids resting on poorly graded medium sand. Parameters of this study include the depth to the first reinforcement layer, spacing between reinforcement layers, plan area of the reinforcement and the grade of reinforcement. Load tests were conducted on model footings made of mild steel plates (100 mm * 100 mm size) in four phases to evaluate the effects of single and multiple layers of geosynthetic reinforcement placed below shallow spread footings and to quantify the difference in performance between the uniaxial and biaxial Geogrid reinforcement. Based on the current study, values for optimum embedment depth, optimum spacing between geogrid layers, and optimum plan area of reinforcement for both types of geogrid have been proposed. It was observed that inclusion of reinforcement increased the bearing capacity as well as decreased the settlement of the foundation. The presence of geogrid in the soil makes the relationship between the settlement and applied pressure of the reinforced soil almost linear. Biaxial geogrid performs better than uniaxial geogrid, though the uniaxial reinforcement also leads to significant improvement in performance of footings compared to unreinforced footings.

Stone or masonry foundations of historic buildings and objects of cultural heritage used to be built using the lime-based mortar and such practice was quite common mainly due to the availability of the local materials. The main disadvantage of such foundations is the missing or crumbling of the mortar between stones under the impact of freezing-thawing annual cycles and the groundwater aggressivity affects. The recovery of their bearing capacity usually requires special operations such as high pressure cement grout injections. However the quality control of the injections is a challenging task mainly because the degree of the cracks and the cavities filling may be estimated only by inspection of foundation surface after the excavation of pits.

In order to increase the reliability of injections the authors developed a novel method. They add fine electrical conductive substance directly to the grout and place electrodes on the foundation surface prior the reconstruction. That allows to control the injection process by the measurement of the electrical conductivity of the foundation substance. Moreover, the application of ferro-materials as additive allows to increase the degree of filling of the cracks and the cavities due to the magnetic field effect.

The main aim of the current study is to analyze the properties of cement grout with ferro-additives in various concentrations. The electrical conductivity, rheological performance and strength of the grout were determined by different laboratory experiments.

The results of the experiments clearly showed the high perspective of this method and confirmed that the magnetic field effect may considerably simplify the strengthening of old stone foundations.

A bacterial solution that generated microbial-induced calcite precipitation (MICP) was injected into kaolinite specimens through the electroosmotic technique in this study. Suitable electroosmotic injection times were selected to improve kaolinite specimens with the bacterial solution. After being treated for 360 h with electroosmotic injection of the bacterial solution, the cone resistance of the kaolinite reached a maximum of 2,000 kPa in the area close to the anode (approximately 10 times the original CPT cone resistance (200 kPa)) and 1,000 kPa in the area close to the cathode (approximately 5 times of the original CPT cone resistance). Based on the results, the electroosmotic injection method can overcome the difficulty of injecting a bacterial solution into low-permeability soft clay and can improve its strength.

Consolidation analyses of embankments are often conducted on the assumption of the plane strain condition. This poses a problem if the effects of vertical drains (used for accelerating consolidation) are to be modelled. The conditions for the consolidation around vertical drains are close to axial symmetry. “Matching” procedure (Hird et al. 1992) is often used for estimating the plane strain equivalent hydraulic conductivity and/or the drain spacing for modelling simplicity. “Matching” is achieved by comparing the average degree of consolidation between axisymmetric and plane strain unitcells (a single drain surrounded by area of influence). As the process is based on average degree of consolidation, the excess pore water pressure at corresponding points (plane strain and axisymmetric) in an unitcell can be different. The discrepancy can be maximum at the boundary of the unitcell even though the settlement may match with good accuracy. In this paper, a relationship is developed to correct the calculated excess pwp in a plane strain analysis. A correction factor is proposed and the variation of the correction factor with different variables (hydraulic conductivity, level of imposed stress, mechanical properties of the soil) are discussed. A series of finite element consolidation analyses using elastic, elastoplastic and elastic viscoplastic constitutive models have been used for this purpose.

The modulus of subgrade reaction (Ks), is a commonly used parameter for a mat foundation analysis. But, it is not one of the basic soil properties and depends on many parameters including soil stiffness, foundation stiffness, foundation shape, size etc. Researchers and engineers agree that the general practice of using a constant modulus of subgrade reaction, is over simplified and may produce erroneous results. In fact, assumed contact pressure distribution may vary significantly from the actual one and shows a considerable spatial variation [8]. However, due to the non-availability of any standard method, engineers tend to avoid a proper distribution of the modulus of subgrade reaction. A proper distribution cannot be assumed or pre-calculated, rather it must be a result of a soil-structure interaction analysis.

This paper proposes an automated method to accurately distribute subgrade modulus. It is an iterative solution, and relies on the convergence of displacement profiles of two successive analysis. Any of the standard equations can be used to calculate soil stress and the corresponding settlement at any point of the mat. First iteration of Ks at a given point is then estimated as the ratio of soil stress to the settlement. The solution will then iterate and calculate new sets of Ks after each successful analysis by dividing the base pressure with the displacement. A few case studies have been presented to verify the effectiveness of the proposed method. As expected, resulting distribution of the subgrade modulus shows a spatial variation. Also, the vertical displacement diagram of a mat under uniformly distributed load, takes the shape of a bowl. Another case study is done to understand the effect of foundation stiffness. It shows changes in contact pressure due to the change in foundation thickness. A flow-chart has been provided to help create a computer program.

The Climate Change Conference in Paris in 2015 supported by all present countries decided to reduce the CO₂ emissions in order to make a substantial contribution to reducing global warming. In this context in 2016 a new fully biological ground injection medium on the basis of the bionics was developed by Stump Spezialtiefbau GmbH Germany. In the following, the most important material parameters of the developed organic soft gel as well as the results of related numerical simulations are presented.

In the present study, finite element analysis was selected to develop numerical models to study the failure mechanism of sandy soil and the wedge angle. The sandy soil is simulated by a semi-infinite element isotropic homogeneous elastic material. The analysis program consists of five footings with different L/B ratio (Ratio between length and width of footing) ranged between 1 to 8. Each footing was analyzed with different foundation depth. In addition, also different angle of internal friction of sandy soil was taken. It was concluded that, the obtained failure mechanism in the present study is identical with the conventional failure mechanism. In addition, it was concluded also that the wedge angle values obtained in the present study decrease with increasing depth of footing and increasing angle of internal friction of the soil as well as increasing L/B ratio while the wedge angle value of sandy soil obtained from previous theoretical approaches is constant [Ψ = 45 + (Ø/2)].

Geotechnical modeling of complex soil problems requires an advanced 3D discretization and a sophisticated soil models. The aim of the present study is to accurately analysis the geotechnical and induced structural failure mechanisms observed in the field and experimental studies. In order to provide a contribution to quantify the stress and deformation analysis and settlement of soft clay subsoil, a geotechnical analytical model is presented in details. The study presents an attempt of a PLAXIS 3D FE analysis model to simulate the subsoil problems, deformation and stress analysis of the multi-layer masonry structures of Abu Serga church in Cairo, Egypt which is loaded in plane. Advanced soil plastic modeling has been used throughout the different phases of the numerical analysis. The numerical and experimental analysis allowed us to define the pathology of this unique architectural heritage and to estimate the subsoil deformation as well as the stress-strain analysis of its superstructure, also to define the ultimate load that they can survive under their present conditions. The results are discussed with respect to the stress-strain and volumetric behavior of the soil. Finally, the paper presents the design and implementation of the complete intervention retrofitting project of Abu Serga church.

The Nevyansk Tower is a unique monument of history and culture in the Ural region of Russia, which was constructed at the beginning of the 18^th Century. At the article along with the description of the history of construction of this monument are reported the information on the known and many unknown facts related to it, data on the tilting of the Tower. Also it shows the reconstruction of the history of inclination of the Tower and the prediction of its development for 100 and 300 years. In the case of possibility of a dangerous progress of its tilt there must provide a detailed engineering investigation of the Nevyansk Tower and develop the projects for stabilization of tower position similarly to what has been made on the worldwide famous monument of Pisa tower.

Hybrid retaining walls comprising a lower soil-nail wall (SNW) and a mechanically stabilized earth wall (MSEW) on top are typically needed in combined cut and fill wall sections such as in road widening. However, available literature and design guidelines on MSE walls in such hybrid systems are limited. The purpose of this research is to numerically-investigate the behavior of MSE walls in hybrid retaining wall systems during and after construction. Two-dimensional finite element models of different wall configurations are analyzed using PLAXIS 2D software. The base-line model is verified against a published case history of a monitored MSE wall with good agreement. The effect of the nail length to total height (L_N/H_T), SNW facing slope (ω), reinforcements vertical spacing (V_L), reinforcements stiffness (J), and MSEW facing slope (ψ) on the tensile forces in the MSEW are studied. The results of tensile force distribution of MSE wall are presented at this paper. The calculated tensile forces from the numerical modeling are compared with calculated values from MSEW design methods using the Simplified method, Structure stiffness method, and K-stiffness method. Furthermore, the tensile forces of numerical modeling are higher than the tensile forces of the K-stiffness method (empirical method) and equal or less than the tensile forces of Simplified Method and Structure stiffness method (Limit equilibrium method).