Foundation Uplift Research Articles

Centrifuge Tests of Impulsive Vertical Acceleration Generated by Foundation Uplift During Strong Shaking

Dynamic centrifuge tests were performed using soil-footing-superstructure models to investigate the effects of the roughness of the soil-footing interface on the uplift and the resulting vertical acceleration of the footings during strong shaking. Four footing models, each of which had a smooth or rough surface and was embedded or not embedded in soil, were subjected to strong shaking. The horizontal shearing and the vertical compressive forces on the base of each footing, as well as the earth pressure and the wall friction forces on the active/passive sides of each footing, were elaborately measured with newly developed 2D load cells. It was shown that (1) the collision impulse of the uplifted footing against the ground induced an extremely high vertical acceleration of the structure, which was much larger than could be induced by the vertical movement of the gravitational center of the structure; (2) the vertical acceleration caused by the collision impulse increased with the induced rotation angle of the footing; (3) if the footing was not embedded in the ground, the rotation angle and the resulting vertical acceleration tended to be larger for the rough footing than for the smooth footing, probably because the larger horizontal sliding of the smooth footing was able to reduce the footing rotation; and (4) if the footing was embedded, by contrast, the rotation angle and the resulting vertical acceleration tended to be smaller for the rough footing than for the smooth footing, probably because the larger wall friction that developed on the passive side in the rough footing was able to reduce the footing rotation.

Calculation and Control of Seepage of Hardfill Dam with Different Foundation

As a new dam type, Hardfill dam has been arisen recently. The suitable geologic condition of t Hardfill dam construction was analyzed based on considering its characteristics in this paper; it was also studied how to choose the reasonable impervious and drainage system to control the dam body and foundation’s seepage on a given geologic condition to make sure the safety of the project. On the basis of analyzing the foundation geologic conditions of many common dam types, two kinds of typical Hardfill dam foundation were put forward. The seepage field law of dam foundation was analyzed by dam foundation seepage discharge, uplift pressure and seepage gradient of some typical positions after setting reasonable impervious and drainage measures on the two kinds of dam foundations. It is indicated by the calculation results that the adaptability of Hardfill dam foundation is comparatively nice, and the safety of dam foundation can be ensured if appropriate impervious and drainage measures are adopted.

The Bearing Capacity Character of Enlarged Base Shallow Foundation under Uplift Load

By theory analysis and field tests, failure model of soil around foundation and ultimate uplift resistance of enlarged bass shallow foundation were analyzed. The results showed that the deformation and failure process of soil around foundation under uplift load undergone three phases: 1) soil upon enlarged base compressed, 2) appearance and extension of plastic zone of soil around foundation, and 3) soil around foundation general shear failure. With the combination of the Limit Equilibrium Method, and Sliding Curve Theory, a simplified theory model to calculate net ultimate uplift resistance of shallow foundation assuming cir failure surface under uplift load was established. Then three examples were calculated by this method and the good agreement between theory calculation solution and experimental results validated the rationality of the model. The study in this paper provided an important theoretical support and brand-new idea on calculation method of uplift capacity of foundation with enlarged base and failure surface characteristics determination of soil around foundation.

An experimental study on the rocking response of bridge piers with spread footing foundations

AbstractSome spread footing foundations from real retrofitting practices in Taiwan were extended to be uneconomically large due to the restriction of foundation uplift regulated in the design code. Although rocking mode of spread footings induced from foundation uplift is not favorable in current design code, recent studies have shown that the rocking of a spread footing may have a beneficial effect on the dynamic performance of piers by reducing the earthquake forces that can be transmitted to the pier base. This implies that the plastic deformation that occurs at the pier's plastic zone can be decreased and as a result the ductility demand of piers can possibly be reduced. In order to gain a better understanding of the structural behavior related to rocking and to clarify that if the widening and strengthening of the foundations to limit the rocking mechanism of spread footing is necessary for the retrofitting work, a series of preliminary rocking experiments were performed. A total of three circular reinforced concrete columns with spread footing foundations were tested. Using pseudo‐dynamic tests and a cyclic loading test, these columns were subjected to different levels of earthquake accelerations, including a near field ground motion. The results of the tests and the rocking behavior of the footings are discussed in this paper. From the benchmark test, the difference between the response behavior of a rocking base and a fixed base foundation was highlighted. By comparing the experimental responses of the retrofitted column with the responses of the original one, the effect of the rocking mechanism on the ductility demand and strength demand of the columns was also identified. Copyright © 2010 John Wiley & Sons, Ltd.

Application and Validation of Practical Tools for Nonlinear Soil-Foundation Interaction Analysis

Practical guidelines for characterization of soil-structure interaction (SSI) effects for shallow foundations are typically based on representing foundation-soil interaction in terms of viscoelastic impedance functions that describe stiffness and damping characteristics. Relatively advanced tools can describe nonlinear soil-foundation behavior, including temporary gap formation, foundation settlement and sliding, and hysteretic energy dissipation. We review two tools that describe such effects for shallow foundations and that are implemented in the computational platform OpenSees: a beam-on-nonlinear-Winkler foundation (BNWF) model and a contact interface model (CIM). We review input parameters and recommend parameter selection protocols. Model performance with the recommended protocols is evaluated through model-to-model comparisons for a hypothetical shear wall building resting on clay and model-data comparisons for several centrifuge test specimens on sand. The models describe generally consistent moment-rotation behavior, although shear-sliding and settlement behaviors deviate depending on the degree of foundation uplift. Pronounced uplift couples the moment and shear responses, often resulting in significant shear sliding and settlements. Such effects can be mitigated through the lateral connection of foundation elements with tie beams.

HYSTERETIC CHARACTERISTICS AND PLASTIC DEFORMATION CAPACITY OF UPLIFT YIELDING BASE-PLATES FOR CONTROLLED ROCKING FRAMES

Previous studies have suggested that rocking vibration accompanied with foundation uplift motion might reduce the seismic damage to buildings subjected to severe earthquake motions. Based on this knowledge, the authors have proposed the uplift rocking system that makes use of the uplift yielding mechanism of specially designed flexing base plates, and showed that the yielding base plates work as hysteretic dampers. In this paper, the authors examine the hysteretic characteristics and cumulative plastic deformation capacity of the yielding base plates for the uplift rocking system using the results of the statically cyclic loading tests. It is shown that the proposed calculation provides the approximate estimation of the mechanical properties of hysteretic behavior of the yielding base plates, and that the haunched base plates improve the cumulative plastic deformation capacity compared with non-haunched base plates.

Macroelement modeling of shallow foundations

The paper presents a new macroelement model for shallow foundations. The model is defined through a non-linear constitutive law written in terms of some generalized force and displacement parameters. The linear part of this constitutive law comes from the dynamic impedances of the foundation. The non-linear part comprises of two mechanisms. One is due to the irreversible elastoplastic soil behavior. It is described with a bounding surface hypoplastic model, adapted for the description of the cyclic soil response. An original feature of the formulation is that the bounding surface is considered independently of the surface of ultimate loads of the system. The second mechanism concerns the detachment that can take place at the soil–footing interface (foundation uplift). It is totally reversible and non-dissipative and can thus be described by a phenomenological non-linear elastic model. The macroelement model is qualitatively validated by application to soil–structure interaction analyses of simple real structures and by comparison with results from more sophisticated methods of analysis.

Large-Scale Experiments on Nonlinear Behavior of Shallow Foundations Subjected to Strong Earthquakes

We conducted a series of 1G large-scale shake table tests and cyclic eccentric loading tests of a shallow foundation model. The experimental parameters were the difference in loading methods (i.e., dynamic and static), input seismic motions (i.e., intensity and number of cycles), soil densities (i.e., dense and medium dense), and the ratio of horizontal and overturning moment loads. The experimental data set contains the accelerations and displacements of the soil and foundation as well as the distributions of normal and shear reaction forces at the foundation base. The experimental results provide crucial data to model the coupling effect among vertical, horizontal, and overturning loads, the accumulation of irreversible displacement, and the foundation uplift, and so is one of the most complete benchmark data sets for the development and validation of numerical models for the nonlinear response of shallow foundations to strong earthquakes.

Uplift of shallow foundations with cement-stabilised backfill

This paper describes the results of a series of centrifuge model tests undertaken to investigate the effects of modifying a loose granular backfill using cement on the uplift performance of shallow anchors. These model tests, which involved a range of cement contents, are supported using a series of laboratory element tests and finite-element analyses. The study indicates that significant increases in uplift stiffness and peak capacity can be achieved by the addition of relatively small quantities of cement. Such increases are, however, limited to relatively low uplift displacements because of the brittle nature of the improved backfill shear strength characteristics.

An investigation into the Winkler modeling of the cyclic response of rigid footings

The response of shallow foundations under lateral cyclic loading is generally nonlinear and involves rocking, sliding, settlement, and may also involve foundation uplift and soil yielding. Unlike pile foundations, the nonlinear Winkler model is not widely used for modeling cyclic soil–footing–structure interaction (SFSI) problems. Nonetheless, it has some important features that make it more attractive than conventional finite element approaches, and is a recommended approach in the FEMA 356 design guideline. This paper explores the strengths and limitations of the FEMA 356 nonlinear Winkler modeling approach, when used in predicting the cyclic response of various recent SFSI experiments. The results of the study show that the model is capable of satisfactorily predicting all the different cyclic response quantities apart from permanent horizontal displacements. This should therefore be noted when using the approach in design.

Investigation into the Effects of Foundation Uplift on Simplified Seismic Design Procedures

Uplifting of and yielding below shallow foundations supporting rigid lateral force–resisting elements can provide additional nonlinearity into a system's overall force-deformation behavior. While this nonlinearity may be advantageous, potentially reducing seismic demands, displacement compatibility may result in overstress of lateral and/or gravity-resisting elements. Incorporating this balance of benefit versus consequence in structural design is one goal of performance-based earthquake engineering (PBEE). There are a variety of approaches in design codes for estimating seismic demands and incorporating “performance” as a design goal. Such methods generally account for the displacement of an equivalent SDOF system by reducing the design strength, however, not explicitly for the case of foundation uplift. To address this shortcoming, this paper investigates the relationship between the strength ratio R and the displacement ratio C1 using the beam on nonlinear Winkler foundation (BNWF) concept. Numerical models were constructed considering a range of soil-structure natural periods and a range of design R values. Nineteen ground motions with a broad range of characteristics are used to conduct nonlinear time-history analyses. Results from these simulations indicate that current suggestions for C1- R relations are highly unconservative when uplifting foundations are anticipated. Revised C1- R relations for uplifting foundations are presented and an example numerical comparison provided.

Hydromechanical analysis of flow behavior in concrete gravity dam foundations

A key requirement in the evaluation of sliding stability of new and existing concrete gravity dams is the prediction of the distribution of pore pressure and shear strength in foundation joints and discontinuities. This paper presents a methodology for evaluating the hydromechanical behavior of concrete gravity dams founded on jointed rock. The methodology consisted of creating a database of observed dam behavior throughout typical cycles of reservoir filling and simulating this behavior with a distinct element method (DEM) numerical model. Once the model is validated, variations of key parameters including lithology, in situ stress, joint geometry, and joint characteristics can be incorporated in the analysis. A site-specific simulation of a typical reservoir cycle was carried out for Albigna Dam, Switzerland, founded on granitic rock, to assess the nature of the flow regime in the rock foundations and to evaluate the potential for sliding surfaces other than the dam–rock interface to develop. The factor of safety against sliding of various rock wedges of differing geometry present within the dam foundations was also evaluated using the DEM model and conventional analytical procedures. Estimates of crack propagation patterns and corresponding uplift pressures and factors of safety against sliding along the dam–rock interface obtained with the DEM were also compared with those from simplified procedures currently used in engineering practice. It was found that in a jointed rock, foundation uplift estimates after crack development obtained from present design guidelines can be too conservative and result in factors of safety that are too low and do not correspond to the observed behavior.Key words: hydromechanical, jointed rock, flow, dam design.

Neural network analysis of overturning response under near-fault type excitation

Under strong seismic excitation, a rigid block will uplift from its support and undergo rocking oscillations which may lead to (complete) overturning. Numerical and analytical solutions to this highly nonlinear vibration problem are first highlighted in the paper and then utilized to demonstrate how sensitive the overturning behavior is not only to the intensity and frequency content of the base motion, but also to the presence of strong pulses, to their detailed sequence, and even to their asymmetry. Five idealised pulses capable of representing “rupture-directivity” and “fling” affected ground motions near the fault, are utilized to this end: the one-cycle sinus, the one-cycle cosinus, the Ricker wavelet, the truncated (T)-Ricker wavelet, and the rectangular pulse “Overturning-Acceleration Amplification” and “Rotation” spectra are introduced and presented. Artificial neural network modeling is then developed as an alternative numerical solution. The neural network analysis leads to closed-form expressions for predicting the overturning failure or survival of a rigid block, as a function of its geometric properties and the characteristics of the excitation time history. The capability of the developed neural network modeling is validated through comparisons with the numerical solution. The derived analytical expressions could also serve as a tool for assessing the destructiveness of near-fault ground motions, for structures sensitive to rocking with foundation uplift.

ROCKING ISOLATION OF A TYPICAL BRIDGE PIER ON SPREAD FOUNDATION

It has been observed that after some earthquakes a number of structures resting on spread footings responded to seismic excitation by rocking on their foundation and in some cases this enabled them to avoid failure. Through application to a standard bridge supported by direct foundations, this paper discusses the major differences in response when foundation uplift is taken into consideration. Special focus is given on the modifications of rocking response under biaxial and triaxial excitation with respect to uniaxial excitation. It is found that inelastic rocking has a significant isolation effect. It is also shown that this effect increases under biaxial excitation while it is less sensitive to the vertical component of the earthquake. Finally, parametric analyses show that the isolation effect of foundation rocking increases as the size of the footing and the yield strength of the underlying soil decreases.

Hypothesis Concerning Distribution of Normal Forces of Frost Heaving across the Lower Surface of Solidly Frozen Soil Layers beneath Foundations

Existing schemes of interaction between heaving soils and immovable foundations are analyzed. Individual deficiencies are noted for use of results of these investigations in determining the uplift of shallow foundations on heaving soils, and the possibility of determining additional forces in foundations subject to nonuniform heaving of soil. A hypothesis regarding the distribution of normal forces induced by frost heaving against the lower surface of solidly frozen layers of soil beneath foundations with allowance for their strength and observance of equilibrium between the loads on the foundations and the normal forces induced by frost heaving, and also an engineering method for their determination are proposed.

Seismic response of bridge piers on elasto-plastic Winkler foundation allowed to uplift

When the bridge piers with shallow foundation are subject to intensive earthquake excitations, uplift of foundations will occur and the foundation soil will partly become plastic. It is very difficult to use an accurate method to simulate the uplifting and yielding of supporting soil. An improved Winkler foundation model, which could be used to consider the uplift and yield, was employed in the analysis. The 1940 El Centro earthquake record is inputted to a rigid pier with shallow foundation so that the non-linear history response is obtained. From the non-linear analysis, it is concluded that the non-linear effect is very remarkable when uplifting and yielding of supporting soil are considered. Compared with the linear analysis, the stiffness of bridge pier–soil system degrades in each cycle after considering uplifting and yielding. It is shown that the non-linear analysis can get larger rotational angles and smaller bending moments compared with the linear analysis.

Large displacement behaviour of a structural model with foundation uplift under impulsive and earthquake excitations

AbstractThis paper considers the dynamical behaviour of a structural model with foundation uplift. The equations of motion of the system considered are derived for large displacements thus allowing for the eventual overturning of the system. The transition conditions between successive phases of motion, derived in terms of the specific Lagrangian co‐ordinates used in the formulation of the equations of motion, present innovative aspects which resolve some previously inexplicable behaviour in the structural response reported in the literature. The dynamical behaviour of the model is considered under impulsive and long‐duration ground motions. The minimum horizontal acceleration impulses for the uplift and the overturning of the system are evaluated in analytical form. The sensitivity of the model to uplifting and to overturning under impulsive excitations is established as a function of few significant structural parameters. Numerical applications have been performed changing either the structural parameters or the loading parameter, in order to analyse several dynamical behaviours and also to validate the analytical results. For earthquake ground motions the results, reported in the form of response spectra, show that linearized models generally underestimate, sometimes significantly, the structural response. Copyright © 2003 John Wiley & Sons, Ltd.

Soil-structure-water interaction of intake-outlet towers allowed to uplift

A procedure that can be used for preliminary seismic analysis of intake-outlet towers including soil-structure-water interaction is developed. The formulation also considers the effect of partial soil-foundation separation, a phenomenon of considerable interest for the design of such systems that has not been addressed in the literature. The hydrodynamic pressure of the water is accounted for through added masses given in concise closed-form expressions for easy use in analysis and design. The nonlinear equations of motion, based on foundation-soil bond conditions, are solved numerically. The effects of soil-structure-water interaction are evaluated for a representative intake-outlet tower. Parametric studies are also conducted in the presence and absence of surrounding and contained water for typical cases of soil conditions and tower height-foundation width ratios. The results indicate that hydrodynamic effects are significant and cause an increase in deflections, moments and shears and a decrease in foundation rotation. The study shows that for short towers, foundation uplift is unlikely to occur. On the contrary for slender towers, uplift is more likely to appear, especially for foundations supported by stiff soil, causing significant decrease in moments and deflections.

Seismic analysis of towers including foundation uplift: Chaojin Xu & C. C. Spyrakos, Engineering Structures, 18(4), 1996, pp 271–278

Seismic analysis of towers including foundation uplift: Chaojin Xu & C. C. Spyrakos, Engineering Structures, 18(4), 1996, pp 271–278

Seismic analysis of towers including foundation uplift

A seismic analysis procedure including soil-structure interaction and partial foundation uplift for tower structures is developed. The nonlinear equations of motion are derived with the aid of Lagrange's equation. Parametric studies are performed to evaluate the effects of factors such as: soil stiffness, ratio of tower height to foundation width, and partial separation of the foundation from the soil. The studies indicate that the effects of soil stiffness on a short tower are greater than on a slender one. The height-width ratio affects the seismic response of the tower significantly, especially for a tower at a rock-like location. The allowance of foundation uplift may substantially reduce the seismic response of moment and foundation rotation in the case of hard soil and a slender tower, or may greatly increase the seismic response of shear in the case of hard soil and a short tower. The study concludes that uplift is not always beneficial and its effects could be significant for structures under strong seismic motions.