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Strength variability in lime-cement columns and its effect on the reliability of embankments
KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Soil and Rock Mechanics.
2011 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Ground improvement by deep mixing (DM) is a generic term used for a number of methods in which a binding agent, often lime and/or cement, is mechanically mixed with the soil to increase its engineering properties. The inherent variability with respect to the engineering properties of the improved soil is high due to the variations in geology and the complex mixing process. High variability introduces uncertainty in estimating improved soil properties and the performance of the structure.

Current design methodology deals with soil properties deterministically and the uncertainties involved are incorporated in a single value represented by a total factor of safety (FS). The chosen FS is highly dependent on the engineer’s judgment and past experience, in which both of these factors vary between different geotechnical designers. Therefore, current design methodology used in practice for DM does not deal with uncertainties in a rational way. In order to design a geotechnical system with the desired level of confidence, the uncertainties involved must be integrated in the DM design. This can be achieved by using reliability-based design (RBD) methods.

The research work in this thesis is presented as a collection of three papers. In the first paper, a comprehensive statistical analysis of cone penetration test (CPT) data is described. The objective was to make a contribution to empirical knowledge by evaluating the strength variability of lime-cement columns within the group of tested columns. In the second paper, the effect of the spatial variability and statistical uncertainty with regard to the embankment’s reliability was investigated within the framework of RBD. The study in the third paper investigated the strength variability in lime-cement columns based on two test methods, namely CPT and column penetration test (KPS). In this study, the effect of different test methods on the evaluation of the design value was addressed.

The main conclusions from this study can be summarized as follows. First, the probability distribution function (PDF) for the undrained shear strength of lime-cement columns can be modeled in RBD as normal or log-normal distributions. However, the use of log-normal distribution is recommended for RBD analyses. Second, the evaluated scales of fluctuation indicate ranges of 2 to 4 m and 0.2 to 0.8 m in the horizontal and the vertical directions respectively. This means that in order to fulfill the requirements of independent/uncorrelated samples for assessment of the design value, the spacing between samples must exceed the horizontal scale of fluctuation. It is therefore proposed that the spacing between individual samples should be at least 4 meters. Third, the design values evaluated using CPT and KPS were approximately the same. However, the inherent variability evaluated differs due to the larger volume tested with the KPS probe than with the CPT probe. However, this difference was not significant between the two tests. Fourthly, due to the limitation in the deterministic design in terms of dealing with uncertainties, it is recommended that RBD design should be used in parallel with the deterministic design of lime-cement column.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2011. , v, 35 p.
Series
TRITA-JOB. LIC, ISSN 1650-951X ; 2016
Keyword [en]
lime-cement, embankments
National Category
Civil Engineering Geotechnical Engineering
Identifiers
URN: urn:nbn:se:kth:diva-57992OAI: oai:DiVA.org:kth-57992DiVA: diva2:473093
Presentation
2012-02-06, Q2, Osquldasväg 10 NB, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20120110

Available from: 2012-01-10 Created: 2012-01-04 Last updated: 2014-06-18Bibliographically approved
List of papers
1. Strength variability in lime-cement columns based on CPT data
Open this publication in new window or tab >>Strength variability in lime-cement columns based on CPT data
2012 (English)In: Ground Improvement, ISSN 1365-781X, E-ISSN 1751-7621, Vol. 165, no 1, 15--30 p.Article in journal (Refereed) Published
Abstract [en]

Natural and improved soils have relatively high inherent property variability that should be taken into consideration in design. Investigations of the spatial variability in lime-cement columns are important since they provide a rational quantification of the variability parameters needed for a reliability-based design analysis of geotechnical systems. Statistical analyses are used to evaluate the spatial variability parameters, i.e. the mean, the variance, and the scale of fluctuation, which is the distance within which soil properties reveal strong correlation. This paper presents a field test, in which 30 CPT soundings were performed and analyzed statistically in order to address the spatial variability in a group of lime-cement columns, with respect to the cone tip resistance. The objective of this paper is to describe the statistical analyses and to make a contribution to the empirical knowledge about strength variability in a volume of lime-cement columns. Stationarity has been assessed, and the scale of fluctuation has been evaluated in the vertical and horizontal directions. Random field theory was used based on the sample autocorrelation function ACF. The scale of fluctuation was found to be within the range of 0.2-0.7 m and 2-3 m in the vertical and horizontal direction, respectively. A simple design consideration shows that the variance reduction factor has a major influence on the determination of the design value.

Keyword
ground improvement, deep mixing, lime-cement columns, statistical analyses, spatial variability, cone penetration test
National Category
Civil Engineering Geotechnical Engineering
Identifiers
urn:nbn:se:kth:diva-59086 (URN)10.1680/grim.2012.165.1.15 (DOI)2-s2.0-84858857990 (Scopus ID)
Note

QC 20150622

Available from: 2012-01-10 Created: 2012-01-10 Last updated: 2017-12-08Bibliographically approved
2. Effect of spatial variability of the strength properties in lime-cement columns on embankment stability
Open this publication in new window or tab >>Effect of spatial variability of the strength properties in lime-cement columns on embankment stability
2012 (English)In: Geotechnical Special Publication, ISSN 0895-0563, Vol. 228, 231-242 p.Article in journal (Refereed) Published
Abstract [en]

Spatial variability with respect to the strength in lime-cement columns is an unavoidable source of uncertainty that should be considered in design. Current design method for the assessment of embankment stability, based on the deterministic factor of safety, cannot address the effect of spatial variability. Reliability-based design methodology is a powerful tool that can be used to integrate the variability into the analysis. In this paper, the spatial variability with respect to the undrained shear strength in the soil and in the columns was evaluated based on CPT test. The first order second moment (FOSM) reliability method was applied to address the impact of the spatial variability of the strength in the soil and in the columns on the reliability of an embankment founded on improved soil by lime-cement columns. The paper also presents a technique to evaluate the variance reduction factor over the failure surface. The results propose that the undrained shear strength in the soil and in the columns can be modelled following normal or lognormal distribution. The analysed example show that the reliability increased significantly when the spatial variability was considered

National Category
Civil Engineering Geotechnical Engineering
Identifiers
urn:nbn:se:kth:diva-59082 (URN)10.1061/9780784412350.0009 (DOI)2-s2.0-84888341110 (Scopus ID)
Conference
International Conference on Grouting and Deep Mixing
Note

QC 20150629

Available from: 2012-01-10 Created: 2012-01-10 Last updated: 2017-12-08Bibliographically approved
3. Variability of strength and deformation properties in lime-cement columns evaluated from CPT and KPS measurements
Open this publication in new window or tab >>Variability of strength and deformation properties in lime-cement columns evaluated from CPT and KPS measurements
2013 (English)In: Georisk: Assessment and Management of Risk for Engineered Systems and Geohazards, ISSN 1749-9518, E-ISSN 1749-9526, Vol. 7, no 1, 21-36 p.Article in journal (Refereed) Published
Abstract [en]

The strength variability of soil improved by deep mixing with lime-cement columns is generally high. Eurocode 7 states that selection of characteristic values for geotechnical parameters shall take the variability of measured property values into account. This variability can be considered in the design by using reliability-based design. With reliability-based design, three statistical parameters are needed to evaluate the design value; mean, variance and scale of fluctuation. In this paper, the shear strength of soil improved by lime-cement columns was evaluated using two different penetration methods, the cone penetration test and the column penetration test. The strength was quantified statistically by the mean, variance and scale of fluctuation, while each test method was analyzed and discussed with a focus on its influence on the design value. Based on the analyses, the column penetration test is suggested as a test method for soil improved by lime-cement columns.

Keyword
ground improvement, deep mixing, lime-cement columns, CPT, column penetration test
National Category
Civil Engineering Geotechnical Engineering
Identifiers
urn:nbn:se:kth:diva-59088 (URN)10.1080/17499518.2013.763571 (DOI)2-s2.0-84874191684 (Scopus ID)
Note

QC 20140618. Tidigare titel: "Strength variability in lime-cement columns evaluated using CPT and KPS"

Available from: 2012-01-10 Created: 2012-01-10 Last updated: 2017-12-08Bibliographically approved

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