Sami O. Akbas

Axial Compression of Footings in Cohesionless Soils. I: Load-Settlement Behavior

The results of 167 full-scale field load tests were used to examine several issues related to the load-displacement behavior of footings in cohesionless soils under axial compression loading, including (1) method to interpret the “failure load” from the load-settlement curves; (2) correlations among interpreted loads and settlements; and (3) generalized load-settlement behavior. The L1 - L2 method was found to be more appropriate than the “tangent intersection” and “10% of the footing width” methods for interpreting the failure load. The interpreted loads and displacements indicate that footing load-settlement behavior is less elastic and more nonlinear than that of drilled foundations. The results show that the footing behavior will be beyond the elastic limit for designs where a traditional factor of safety between 2 and 3 is used. A normalized curve was developed by approximating the load-settlement curve for each load test in the database by hyperbolic fitting, and the uncertainty in this curve was qu...

Axial Compression of Footings in Cohesionless Soils. II: Bearing Capacity

An extensive database of full-scale field load tests was used to examine the bearing capacity for footings in cohesionless soils. Each load test curve was evaluated consistently to determine the interpreted failure load (i.e., bearing capacity) using the L1 - L2 method. This test value then was compared with the theoretical bearing capacity, computed primarily using the basic Vesic model. The comparisons show that, for footing widths B>1 m , the field results agree very well with the Vesic predictions. However, for B<1 m , the results indicated a relationship between B and the predicted-to-measured bearing capacity ratio. Accordingly, a simple modification was made to the bearing capacity equation, and the resulting predictions are very good.

Reliability-Based Design Approach for Differential Settlement of Footings on Cohesionless Soils

A probabilistic method is presented to estimate the differential settlements of footings on cohesionless soils, considering the uncertainties in both the load and capacity sides of the design equation. A random field approach is employed to characterize the inherent soil variability. This method is first compared to typical limit values from the literature to denote critical combinations of design parameters that can lead to exceedance of tolerable differential settlements. Then, reliability-based design equations are developed for the serviceability limit state (SLS) design of footings on cohesionless soils. The key parameters controlling the SLS are the allowable angular distortion, site variability, and footing spacing. The results are given in a straightforward design format and indicate that currently suggested deformation factors (resistance factors for SLS) equal to 1.0 are likely to be unconservative for most design situations.

Methods for the Characterization of the Vulnerability of Elements at Risk

Risk assessment is the process of determining the likelihood or threat of a damage, injury, liability, loss, or other negative occurrence that is caused by external or internal vulnerabilities and that may be neutralized through preventive action. More precisely, risk assessment is the systematic prospective analysis aimed at defining, as quantitatively as possible, the potential loss of life, personal injury, economic loss, and property damage resulting from natural and/or anthropogenic hazards, by assessing the exposure and vulnerability of people and property to those hazards. The risk assessment procedure, developed in the Mountain Risks project, is based on the following five steps: (1) Identification and analysis of the specific types of hazards that could affect a territory and its community; (2) Definition of the spatial and temporal likelihood of the damaging events considered in the analysis as well as their magnitude; (3) Inventory of the assets and study of the social and economic features of the study areas; (4) Assessment of vulnerability, evaluating all the hazard consequences for each dimension composing the systems at risk (physical/functional, economic, socio-cultural, ecological/environmental and; political/institutional); (5) Evaluation of the prospective cost of damage or costs avoided through mitigation strategies. Vulnerability assessment plays a crucial role both in ‘translating’ the assessed level of hazard into an estimated level of risk and in providing leading information in mitigation planning processes and emergency management strategies. Under this perspective, it is really difficult, or even impossible, to address risk assessment without assessing vulnerability first and it appears unquestionable that a multi-disciplinary approach is required in vulnerability assessment studies. In this section, the different components (dimensions) of vulnerability are analyzed, both theoretically and practically, and then different methodological approaches, applications and solutions are provided.

Reliability-Based Design of Shallow Foundations in Cohesionless Soils Under Compression Loading: Serviceability Limit State

The reliability-based design (RBD) of shallow foundations in cohesionless soils under drained compression loading is examined for the serviceability limit state (SLS). This SLS is defined when the compression capacity at the allowable settlement limit imposed by the structure is equal to the applied load. The uncertainties in both the load and the capacity sides are considered. A Monte Carlo simulation-based parametric study indicates that the most influential parameters on the reliability index are the statistics of the effective stress friction angle and the ratio of the 50-year snow load to total load, where snow is present. These results were used to select the appropriate domains for the calibration of the simplified RBD equations that can be presented for general use. The target reliability index for calibration was selected after examining the range of reliability levels implicit in existing deterministic foundation designs. The resulting resistance factors are tabulated in a form that can be applied readily in foundation engineering practice.

Model Uncertainties in Terzaghi and Peck Methods for Estimating Settlement of Footings on Sand

The Terzaghi and Peck (1948) method was the first for predicting the settlement of footings on sand using standard penetration test blow counts (N values). Over the following 26 years, various modifications to this basic method were suggested. Herein, an extensive database of 426 settlement case histories is used to assess the model factors for this family of methods. The uncertainty of the model factors is characterized using the coefficient of variation. Considering the model factor as a random variable, the common log-normality assumption is investigated critically. The impact of model uncertainty on the reliability of settlement estimations is discussed, and criteria are given to assess their accuracy and conservatism. For settlement prediction, the uncertainty from the model factor is most influential. The results indicate a low accuracy for all of the methods considered.

Sığ Rijit Tabaka Üzerinde Yer Alan Kohezyonsuz Zeminlerdeki Yüzeysel Temellerin Taşıma Gücü Hakkında Sayısal Modelleme Esaslı Parametrik Bir Çalışma

Sig derinlikte rijit tabaka iceren kohezyonsuz zeminlerdeki yuzeysel temellerin tasima gucu, temel genisligi ve sekli, rijit tabaka derinligi ve zemin icsel surtunme acisi degiskenlerinin etkisinde, sonlu elemanlar yontemi ile parametrik olarak irdelenmistir. Kullanilan sonlu elemanlar yonteminin ne derecede gercekci sonuclar ortaya koydugunun belirlenebilmesi icin Texas A&M Universitesi’nde, genislikleri 1 m ve 3 m arasinda degisen sig temeller uzerinde gerceklestirilen yukleme deneylerinin sonuclarindan faydalanilmistir. Dort farkli icsel surtunme acisi ( 3 2°, 35°, 40°, 45°) ve temel genisligi ( B=1m, 1.5m, 2.5m, 3m) ile kare/dairesel ve serit temeller icin rijit tabakanin tasima gucu uzerindeki etkisinin ortadan kalkacagi kritik derinligi etkileyen faktorler arastirilmistir. Modellemeler, beklenildigi uzere artan derinlikle rijit tabakanin etkisinin azalmakta oldugunu gostermekle birlikte, soz konusu etkinin daha once acik sekilde etkileri ortaya konulmamis olan icsel surtunme acisi, temel genisligi ve sekli ile iliskileri ortaya konularak, tasarim abaklari olusturulmustur.

Predicting groutability of granular soils using adaptive neuro-fuzzy inference system

In this paper, the applicability of adaptive neuro-fuzzy inference system (ANFIS) for the prediction of groutability of granular soils with cement-based grouts is investigated. A database of 117 grouting case records with relevant geotechnical information was used to develop the ANFIS model. The proposed model uses the water–cement ratio of the grout, the relative density and fines content of the soil, the grouting pressure, and the ratio between the particle size of the soil corresponding to 15% finer and that of grout corresponding to 85% finer as input parameters. The accuracy of the proposed ANFIS model in terms of the corresponding coefficient of correlation (R) and root mean square error (RMSE) values is found to be quite satisfactory. Furthermore, a comparative analysis with existing groutability prediction methods indicates that the ANFIS model demonstrates superior performance.

Estimation of Resistance Factors for Reliability-Based Design of Shallow Foundations in Cohesionless Soils Under Earthquake Loading

In areas with significant seismic activity, earthquake-induced loading effects may constitute the major portion of the design loads on foundations. However, very few attempts have been made to apply reliability-based design concepts on footings under seismic loads. Therefore, this study aims at contributing to the reliability-based design methodology by estimating the resistance factors for shallow foundations in cohesionless soils under earthquake loads for the ultimate limit state (ULS), which is defined as the condition in which the seismic bearing capacity is equal to the design load including dynamic effects. The uncertainties in both the load and the capacity are considered. To evaluate the variation of the reliability index ( ) with respect to each significant design parameter, a parametric study was conducted using First Order Reliability Method (FORM) and Markov Chain Monte Carlo simulations. These results were used to select the appropriate domains within the most influential design parameters for the calibration of the simplified RBD equations that can be presented for general use. The target reliability index for calibration, which turned out to be significantly smaller than that for static loading conditions, is selected after examining the range of reliability levels implicit in existing deterministic foundation designs. The resulting resistance factors are presented in the form of design charts that can be readily applied in foundation engineering practice.

Drained Soil Modulus from Load Tests for Shallow Foundation Design

Elastic theory is used to back-calculate the drained secant soil modulus from loads and settlements measured in full-scale load tests on footings in axial compression. The moduli are evaluated at L 1 (the end of the initial linear region) and at a settlement of 1% of the foundation width. These drained soil moduli are normalized by standard penetration test results at each site, and the normalized values are correlated effectively with the soil stress history, stress state, and stress level. The resulting correlations will be useful for footing design.