Strip Foundation Research Articles

Model test study on the mechanical characteristics of shallowly buried covered excavation PBA station construction

The shallowly buried covered excavation PBA method is one of the new construction methods that has attracted widespread attention in the past 20 years. Understanding the deformation of strata and supporting structures is key to ensuring the safe construction of PBA stations at present. Relying on the Beijing Line 12 Beitaipingzhuang PBA subway station and based on the 1:40 model test, the production of similar materials for the model and the station excavation construction plan are introduced. According to the actual construction process of the station, the construction process of the two stations of the pile foundation and strip foundation stations are more realistic. The variations in the strata around the station and the station structure during the whole construction process are obtained. The test results show that the ground displacement of the station is most affected by the excavation of the pilot tunnel, accounting for more than two-thirds of the total displacement, and the ground within the range of 0–12 m around the station is greatly affected by the excavation disturbance during the actual construction process. By analyzing and comparing the ground displacements and the internal forces of the vertical bearing structure of the two types of stations, it is concluded that the pile foundation method is more advantageous for controlling ground displacement and the internal force of the structure. These test results improve design theory of the PBA method and are conducive to further promotion.

A procedure for addressing the soil-abutment interaction problem in the safety assessment of bridges

The paper proposes a methodology for assessing the seismic soil-foundation-abutment response in the spirit of the sub-structure approach. The methodology is suitable for conventional bridge abutments characterized by massive structural components and accommodates for various structural and soil configurations, from both stratigraphic and topographic perspectives. The abutment is assumed to be rigid, which is a generally acceptable hypothesis given the typical geometry of conventional bridge abutments with walls, orthogonal wingwalls, and a concrete deep or strip foundation. The methodology aligns with codes, which foresees a linear behaviour for the abutments and suggest a low behaviour factor to account for dissipative capabilities due to the backfills and the radiation phenomena in the soil. The soil is assumed to perform elastically, but the overall soil nonlinearity induced by seismic wave propagation can be incorporated into the method using a linear equivalent representation of soil properties. The methodology is applied to a real case study in order to demonstrate its effectiveness in addressing the soil-structure interaction problem and to provide practical suggestions for the method implementation, including possible simplifications.

Finite element analysis of large deformation for progressive failure of soil based on Cosserat continuum theory

The large deformation accompanied by strain softening and strain localization of the material is usually encountered in geotechnical engineering. Integrating the RITSS method and the Cosserat continuum theory, a large deformation Cosserat-RITSS finite element method is developed to tackle this issue. Based on the second development interfaces of UEL and SDVINI offered by the finite element software ABAQUS, the Cosserat-RITSS method adopting the MUEM interpolation and mapping method is realized automatically and used to simulate the problem of progressive failure and large deformation characterized by strain localization. The accuracy and effectiveness of the proposed method are verified by the comparison of the small deformation and the large deformation analysis of the penetration process of a rigid strip foundation. Further, the Cosserat-RITSS method is used to analyze the large deformation of a retaining wall resisting on the soil with non-associated plastic flow and strain-softening properties. Compared with the classical RITSS method, it is demonstrated that the proposed method has better performancy of simulating large deformation and overcoming the pathological mesh-dependent problem usually suffered in the classical FEA of large deformation accompanied by strain localization.

Performance-based design of strip foundation considering the full effect of ground improvement

Ground improvement is an effective way to improve the bearing capacity of a shallow foundation. However, the benefit of reducing uncertainties in soil parameters for shallow foundation design is rarely recognized. This study investigated the full effect of rapid impact compaction (RIC) on a strip foundation design. The finite difference method coupled Monte Carlo simulation were used to calculate the failure probability and the required width of the strip foundation, where the friction angle of soil was treated as a random variable. The results show that the foundation width reduces by 48.5% when considering the full effect of RIC, and a significant part of the reduction came from the decrease in the uncertainty of friction angle. Although the adopted relationship between the friction angle and tip resistance of cone penetration test affects the designed width of the foundation, the full effect of ground improvement contributed by the uncertainty reduction of soil parameters is still significant. The implication of the present study provides a basis for the performance-based bearing capacity design of shallow foundations.

Evaluating the feasibility of inverted T foundation on expansive soils

This paper presents a comprehensive design study conducted in Saudi Arabia, focusing on the performance evaluation of an inverted T foundation system in a building constructed on expansive soils. The study aimed to investigate the causes of damage and evaluate the performance of a proposed inverted T foundation. A single story market building in a semi-arid region with expansive soil was constructed utilizing a 40 cm-thick mat foundation as a precautionary measure against soil swelling. However, the building experienced instability and damage shortly after completion. This study explored the replacement of the existing mat foundation with an inverted T foundation. The research involved assessing the ability of the inverted T foundation to withstand swelling pressures and its impact on the structural members of the building. Design guidelines and tools were developed to support the design and analysis of the inverted T foundation. Economic feasibility was also evaluated. The study compared the effects of swelling pressure on two types of foundations: a mat foundation and a rigid strip foundation. The results showed that the inverted T foundation demonstrated less upward movement and was found more effective in mitigating the detrimental effects of soil expansion compared to the mat foundation. Design guidelines and tools, including schedules, and charts were developed to support the design and analysis of the inverted T foundation. The findings have significant implications for the design and construction of buildings on expansive soils, offering insights into the effectiveness of the inverted T foundation as an alternative solution. The research contributes to the knowledge of foundation design on expansive soils in general and provides practical guidance for engineers and practitioners in similar geological contexts.

Collapse Pattern in Gypseous Soil using Particle Image Velocimetry

Abstract Gypseous soil is prevalent in arid and semi-arid areas, is from collapsible soil, which contains the mineral gypsum, and has variable properties, including moisture-induced volume changes and solubility. Construction on these soils necessitates meticulous assessment and unique designs due to the possibility of foundation damage from soil collapse. The stability and durability of structures situated on gypseous soils necessitate close collaboration with specialists and careful, methodical preparation. It had not been done to find the pattern of failure in the micromechanical behavior of gypseous sandy soil through particle image velocity (PIV) analysis. This adopted recently in geotechnical engineering to track the motion of soil grains and using tracer particles by applying digital particle image analysis. It has also been used to study the displacement distribution in some cases of granular materials. Therefore, the goal of this study is to find out how gypseous sand medium moves when in contact with a rigid strip foundation that is under static stress and plane strain conditions. The experimental model would focus on two common types of wetting, namely water table rise and dry conditions. The PIV showed that the collapse pattern under the footing is of the type of punching shear failure. The predominant mechanism of soil deformation was the vertical compression of the gypseous granular soil. The results showed that understanding gypseous sandy grain displacement and failure patterns at the local scale is crucial for enhancing the design of foundations under static stress conditions.

Effect of soil stiffness on end bearing resistance of foundations in clay from large deformation numerical modelling

Accurately predicting the installation resistance is of great benefit to a rational and economical design of foundations. This research performs a large-deformation finite-element analysis to investigate the effect of soil stiffness on the end bearing resistance in uniform clay. Different types of geotechnical structures and foundations, e.g., cone penetrometer, strip foundation, bucket foundation, and deep piled foundation are considered. Results show that soil stiffness has a negligible impact during shallow penetration but becomes pronounced at deep penetration. It is found that for strip footings, effect of soil stiffness is minimal, whereas the end bearing resistances of CPT and deep piled foundation with a partial plug are highly dependent on the soil stiffness. For a rough pile foundation, the resistance factor Nc,pf increases by approximately 30% as soil rigidity Ir increases from 50 to 500. For a bucket foundation, the effect of soil stiffness on the end bearing resistance falls between that of “shallow” and “deep” foundations. Based on the numerical results, empirical expressions are proposed to predict the end bearing resistance factor accounting for the effect of soil stiffness.

Protective effects of measures for railway subgrade adjacent to excavation of a parallel pit

The Hongqiao Airport–Pudong Airport urban railway construction project in Shanghai, includes a 156.1-m-long deep strip foundation pit that runs closely parallel to the subgrade of the Shanghai–Hangzhou High-speed Railway, operating at 300 km/h. With a maximum excavation depth of 25.5 m and a minimum distance of only 10.6 m between the outer edge of the pit and the subgrade slope foot, controlling the deformation of the foundation pit in soft soil and ensuring the safe operation of the adjacent high-speed railway present significant challenges. Various protective measures were implemented to mitigate subgrade deformation caused by pit excavation, including partition excavations, 1.5-m-thick diaphragm walls, a servo steel strut system, isolation piles between the subgrade and diaphragm walls, and water-resistant barriers. The foundation pit, soil, and railway subgrade deformations during excavation were measured and analysed. The results demonstrate that the protective measures limited lateral diaphragm wall deformation to within 0.14% of the excavation depth. The maximum horizontal and vertical displacements of the subgrade caused by excavation were 3.2 mm and 1.5 mm, respectively. As a result, trains safely passed through this section at a limited speed of 120 km/h. The findings presented provide useful reference information for similar projects.

Experimental and Numerical Evaluation Performance of Strip Foundation Reinforced with Steel Slag Columns Compared to Skirted Foundation

Experimental and Numerical Evaluation Performance of Strip Foundation Reinforced with Steel Slag Columns Compared to Skirted Foundation

METHODS OF INJECTION STRENGTHENING OF FOUNDATIONS IN CONSTRUCTION

The article discusses the problems of strengthening strip foundations of buildings and structures. In accordance with the main reasons for strengthening foundations and artificially improving the construction properties of the soil foundation, the reserves and reserves of the bearing capacity of the soil foundation were assessed by analyzing the results of the technical condition of the building and analyzing its physical condition and mechanical characteristics. The exact dimensions of the foundation structures, their condition and the loads acting on them were determined, and the geological and hydrogeological conditions of the surrounding area were studied. A classification of methods for strengthening foundations and artificially improving the construction properties of the soil foundation is given; it is proven that strengthening methods depend on the design features of the building, types of foundations, operating loads, engineering-geological and hydrogeological conditions. In addition to the need to strengthen, as well as the capabilities and technical equipment of organizations performing this type of work, there are many other methods. Pile reinforcement methods are one of the most effective types of foundations. Research has proven that piles are not used to strengthen foundations, since the resulting dynamic impacts can lead to additional settlement of the building being restored. The study noted the basic requirements for reliable strengthening of the foundation and artificial improvement of the construction properties of the soil foundation, especially when water-saturated salty clay soils are formed at the base of the building.

Numerical Study on the Ultimate Bearing Capacity of the Strip Foundations on the Inclined Multi-Layered Soil Mass with a Weak Layer

Numerical Study on the Ultimate Bearing Capacity of the Strip Foundations on the Inclined Multi-Layered Soil Mass with a Weak Layer

Stochastic analysis for bearing capacity computation of twin strip footings on geogrid-reinforced sands

ABSTRACT The interaction of closely-spaced footings on soils is of concern for recent decades. The inherent variability of soil makes the topic more challenging. This study investigates the behaviour of twin strip foundations on both unreinforced and geogrid-reinforced spatially random sands. The Random Finite Difference is performed by coupling Matlab and FLAC2D in each Monte Carlo simulation. Two types of sands, dense and loose, are assumed as the spatially correlated log-normal random fields and the friction angle is considered a random parameter. This study discusses how much the bearing capacity of twin footings is impressed by the heterogeneity of random sands. The unreinforced soil results show that when the uncertainty of φ is high, the homogeneous soil assumption could overestimate the interference effect by about 18% and 9% in the dense and loose sands, respectively. While soil reinforcement reduces the difference between results obtained from probabilistic analysis with those calculated with deterministic analysis. Moreover, the isotropic random fields with rapid fluctuation of φ yield the greatest interference influence in the unreinforced and reinforced dense sands. The combined impact of interference and reinforcement is greater in loose sand than in dense sand, regardless of whether the soil is heterogeneous or homogeneous. HIGHLIGHT Random heterogeneous variability of φ on the interaction of the twin foundations on both unreinforced and geogrid-reinforced sand was investigated. For twin footings on unreinforced dense sand, Scr/B changes from 1.25 to somewhere between 1.25 and 1.5 by increasing COV φ , while on the unreinforced loose sand, Scr/B = 1 remains constant, irrespective of COV φ values. For high variability of φ, the assumption of soil without variability results in overestimation of twin footings interference effect by about 18% and 9%, which means the risk acceptance yields irrevocable damages. Increasing the number of geogrid layers, N, declines the risk of estimating the analysis with homogeneous soil, especially for high variability of φ. By all means, the footing interference and geogrid effects become lower when N is added. The interference effect is usually higher for twin footings on unreinforced sand than on the same reinforced sand. Overall, the integrated influence of interference and reinforcement in loose sand is more than that in dense sand The combined influence of interference and the first geogrid layer is the most in the isotropic random field (θ x = θ y = 1 m) for both sand since the rapid variation of φ in both directions increases the influence and reinforcement effects. The deterministic analysis underestimates the interaction of twin foundations on the reinforced loose sand compared to the anisotropic random fields.

Combined bearing capacity of footings on geogrid-reinforced granular fill over soft clay

The current study investigates the ultimate bearing capacity of obliquely/eccentrically loaded shallow strip foundations resting on a geogrid-reinforced granular fill with limited thickness over a very soft to soft clay layer. To this end, the lower bound theorems of the finite element limit analysis (FELA) and second-order cone programming (SOCP) are effectively exploited to simulate the underlying clay deposit, geogrid layer, and granular fill along with the inclined/eccentric loading exerted on the overlying footing. The accuracy of the adopted formulations is rigorously examined through several comparisons with the results of a well-established analytical approach in the literature. A comprehensive parametric study is then conducted to properly examine the influences of the geosynthetic layer characteristics and soft clay properties on the overall bearing capacity and failure envelope of the strip footing subjected to wide ranges of inclined and eccentric loadings. The results show that placement of a geogrid-reinforced granular fill layer over the soft clayey soil significantly increases the bearing capacity of the shallow foundation. The ultimate tensile strength of the reinforcement layer and the cohesion of the underlying very soft to soft clay deposit are also observed to have considerable effects on the size of failure envelopes.

Subway station construction parallelly below existing double-cell tunnel without clearance

ABSTRACT A case of the Olympic Park Station of Beijing Subway Line 15 constructed parallelly below the Datun Road Tunnel without clearance using the pile-beam-arch (PBA) method was presented. Cross section, geological condition, and construction process of the subway station are illustrated. The strip foundation PBA approach was first adopted and the settlements of the existing tunnel due to new subway station construction are systematically monitored. Whereas this method could not satisfy the settlement requirements of the existing Datun Road Tunnel. The maximum settlement of the existing tunnel reaches 32.0 mm. Meanwhile, the maximum transverse differential settlement of the existing tunnel reaches 10.1 mm. The improved pile foundation PBA approach was proposed. The numerical simulation was performed to verify the feasibility. The results indicate that the pile foundation PBA approach is more efficient in existing tunnel settlement control than the strip foundation PBA approach. About 80% of the total settlement of the existing tunnel caused by subway station construction using the pile foundation PBA method is produced during the excavation of the drifts and the arches. The settlements of the original tunnel induced by the construction of the concourse and the platform account for about 20% of the total settlement. The increments are negligible after the completion of the concourse construction.

Effect of Twin Circular Underground Voids on Bearing Capacity of Strip Foundations

Underground voids can significantly alter the stress and strain conditions of the surrounding soil mass, consequently affecting the stability of buildings located on the surface. Numerous numerical and experimental methods have been employed to investigate the effects of voids on nearby structures. In this research, a mixed lower bound-finite element method is utilized to study the stability of strip foundations situated above twin continuous voids. The soil behavior is modeled using the Mohr-Coulomb yield function along with an associated flow rule. Three-noded triangular stress elements are used to mesh the stress field, considering stress discontinuities at the common edges of adjacent elements. The study assumes that the two voids are identical in terms of radius, distance from the footing centerline, and depth from the ground level. The primary geometrical parameters considered in the analysis are the location and radius of the voids, as well as the strength parameters of the soil. By investigating the changes in bearing capacity concerning these factors, practical charts are proposed to aid in understanding the stability of the foundation. The results of the current study reveal the existence of an unsafe zone beneath the foundations, where the impact of the twin voids on bearing capacity reduction can be significant. Understanding these effects is crucial for design and construction of structures in areas prone to underground voids and also to decide where to bore infrastructure tunnels in order to have minimum effect on stability of the above buildings.

Organisational and technological reliability of pile foundation construction flow in permafrost

The construction of pile foundations in permafrost conditions is a durable and cost-effective design that shows high performance of piles in contrast to slab, block and strip foundations. In the Russian experience of construction in permafrost conditions, the most common methods of piling are the following: sinking method, driven method, drill-and-pile method, and drill-and-pile method. Descending method, where the piles are driven into thawed and liquefied soil. Pile driving method, in which piles are driven into frozen ground using percussion mechanisms. Drill-and-pile method, which combines the use of sinking and drilling piles. Drill-and-pile method, in which a borehole is drilled first and then the piles are driven into the frozen ground. Organisational and technological flow reliability involves the efficient use of resources, compliance with standards, organisation of work processes and safety. This allows the objectives to be achieved efficiently and ensures reliable process operation. The main indicators are: system uptime without failure TO, delays in information transfer HO, equipment reliability PO, material quality FO, operator errors DO. In fact, the control of these indicators affects the organisational and technological reliability of the flow and the work production system as a whole.

Using shear modulus to predict the bearing capacity of strip foundations on sand

This paper reports results of a study to predict load-settlement behaviour of strip foundations on a dry uncemented sand using only soil stiffness data, under both working loads and extreme loads. This involved predicting the strength of a foundation system without using any strength parameters. A non-linear stepwise load-settlement prediction algorithm was utilised, which used the small-strain shear modulus of layers below the footing, a suitable stiffness modulus reduction curve and elasticity theory. The algorithm was adapted to predict the ultimate bearing capacity, as well as to construct astress-settlement curve and determine the bearing pressure experienced beneath a foundation at any particular settlement. The method was evaluated using two different sets of modulus reduction curves, for small-scale model foundations on loose, medium dense and dense sand. Model tests were conducted at 1g. Particle Image Velocimetry (PIV) was used to observe strains within the soil during loading. These strains were used to calculate the shear modulus of discrete layers throughout loading and back-estimate the layer small-strain shear moduli. Additionally, the mode of shear failure for each test at ultimate bearing capacity was observed using PIV. Modulus reduction curves that initially degrade shear modulus rapidly beyond the elastic threshold strain (0.001%) gave superior predictions under the given conditions. Using such curves, ultimate bearing capacity predictions produced errors of 0.4% to 10.8%. Using the same input parameters, stresssettlement predictions were of satisfactory accuracy over the entire settlement range.

Static load test of unreinforced Franchi (B-4350) soil reinforcement pile

Micro-pile drilling technology is often used today for the reconstruction and repair of buildings whose foundations are heavily loaded. Once installed, the piles take up part of this load. They can also be used to improve the load-bearing capacity of building foundations before the upper floors are built on top of them or before additional underground floors are constructed. Construction of buildings and structures in areas with complex soils is a special case. Buildings constructed in such conditions as a result of underground mining operations, soil movement into the excavated space and formation of a displacement trough on the ground surface are subjected to uneven settlement and horizontal deformations during operation. This paper presents the results of field tests of unreinforced Franki micro-pile (B-4350) unreinforced pile for soil reinforcement. Based on the obtained data, it can be concluded that the use of micro-piles to reinforce the foundations of reconstructed buildings provides a reduction in construction costs, labor costs and construction time, and as a consequence demonstrates their undeniable effectiveness. Application of micro-piles at reinforcement of strip foundations, in comparison with the traditional reinforcement design gives reduction of the estimated cost of construction (by 8 %), mainly due to reduction of the cost of operation of machines (by 46 %) and reduction of wages (by 24 %), though with some increase in the cost of materials (by 28 %), which somewhat levels out the general tendencies of cost reduction.

ОСОБЛИВОСТІ РОБОТИ КОРОТКИХ ПАЛЬ ПРИ ПІДСИЛЕННІ ФУНДАМЕНТІВ МІЛКОГО ЗАКЛАДАННЯ ЗА РЕЗУЛЬТАТАМИ ФІЗИЧНОГО МОДЕЛЮВАННЯ

In this work, the results of physical modeling of the strengthening of the foundation of shallow laying by transplanting on piles on small-scale models are analyzed. The redistribution of the load between the elements of the strip foundation reinforced with ready-made (precast) piles of shallow laying was studied. The operation of the existing strip foundation, its reinforcement with piles, the formation of the grid of the newly created pile foundation was simulated, while the relative length of the piles and their step along the row, the nature of the placement of the reinforcement piles (removable or arranged through the body of the foundation), and the density of the sand base were varied. The operation of models of single piles of different lengths and grid without piles as a foundation for shallow laying was also investigated. The nature of inclusion in the work of piles and the newly created grid is determined. The dependences of the load-bearing capacity of the elements of the pile foundation, the degree of realization of this load-bearing capacity when working together in the soil environment, and the relative length and pitch of the piles, and the density of the base soil are given. The specific values of the load-bearing capacity of the reinforcement piles per unit of spent material are determined.
 The obtained results were analyzed regarding the difference between the inclusion of short and long piles in the work of reinforcement with a comparison of all options.

Bearing capacity factor for strip foundations on unsaturated clay

ABSTRACT The matric suction in unsaturated soil depends on the type of soil and various flux conditions i.e., infiltration, evaporation and no flow. The shear strength of unsaturated soil changes with change in matric suction leading to varying resistance of soil under various flow conditions. In this study, the bearing capacity of strip foundations resting over unsaturated clay considering variation of matric suction with depth for different surface flux conditions and position of water table, has been obtained using finite element lower bound limit analysis. Modified Mohr-Coulomb yield criteria based on unified effective stress approach has been employed to incorporate the contribution of matric suction stress to the failure condition. A non-dimensional bearing capacity factor was introduced to estimate the bearing capacity of strip foundations and presented as a function of different influencing parameters such as foundation width, water table depth, soil properties, various flow conditions and their flow rate, and surcharge pressure. The influence of variation of unit weight of soil in the unsaturated zone, air entry and pore size distribution parameters, residual degree of saturation of soil, specific gravity of soil solids, and foundation-soil interface roughness on the bearing capacity has also been examined and found to be insignificant.