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Strength variability in lime-cement columns based on CPT data
KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Soil and Rock Mechanics.
KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Soil and Rock Mechanics.
KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Soil and Rock Mechanics.ORCID iD: 0000-0001-9615-4861
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.

Place, publisher, year, edition, pages
2012. Vol. 165, no 1, 15--30 p.
Keyword [en]
ground improvement, deep mixing, lime-cement columns, statistical analyses, spatial variability, cone penetration test
National Category
Civil Engineering Geotechnical Engineering
Identifiers
URN: urn:nbn:se:kth:diva-59086DOI: 10.1680/grim.2012.165.1.15Scopus ID: 2-s2.0-84858857990OAI: oai:DiVA.org:kth-59086DiVA: diva2:475008
Note

QC 20150622

Available from: 2012-01-10 Created: 2012-01-10 Last updated: 2017-12-08Bibliographically approved
In thesis
1. Strength variability in lime-cement columns and its effect on the reliability of embankments
Open this publication in new window or tab >>Strength variability in lime-cement columns and its effect on the reliability of embankments
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
lime-cement, embankments
National Category
Civil Engineering Geotechnical Engineering
Identifiers
urn:nbn:se:kth:diva-57992 (URN)
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
2. Characterization of strenght vaiability for reliability-based design of lime-cement columns
Open this publication in new window or tab >>Characterization of strenght vaiability for reliability-based design of lime-cement columns
2012 (English)Licentiate thesis, comprehensive summary (Other academic)
Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2012. xii, 29 p.
Series
TRITA-JOB. LIC, ISSN 1650-951X ; 2017
Keyword
geotechnical engineering, deep mixing, strength variability
National Category
Civil Engineering Geotechnical Engineering
Identifiers
urn:nbn:se:kth:diva-98816 (URN)
Presentation
2012-06-05, B1, KTH, Brinellvägen 23, Stockholm, 13:00
Opponent
Supervisors
Note

QC 20120703

Available from: 2012-07-03 Created: 2012-07-03 Last updated: 2014-06-18Bibliographically approved
3. Aspects of probabalistic serviceability limit state design of dry deep mixing
Open this publication in new window or tab >>Aspects of probabalistic serviceability limit state design of dry deep mixing
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

An expanding population and increased need for infrastructure increasingly necessitate construction on surfaces with poor soil conditions. To facilitate the construction of buildings, roads and railroads in areas with poor soil conditions, these areas are often improved by means of foundation engineering. Constructions that are fairly limited in scope are often founded on shallow or deep foundations. However, these methods are relatively expensive and thus not applicable for large-scale constructions like roads and railroads. A cost-effective way to deal with poor soil conditions is to use ground improvement. This thesis deals with a ground improvement method called deep mixing (DD)using lime-cement columns.

Lime-cement columns are manufactured by pushing a mechanical mixing tool to the desired depth, with the tool then rotated and retracted while a lime-cement binder is distributed into soil, forming lime-cement columns. Because of the complex mixing process and inherent soil variability, soil improved by DD shows high variability with respect to strength and deformation properties. Due to this high variability, it is difficult to predict the properties in advance; it is therefore important to verify the properties after installation. In Sweden, this is normally done using the column penetration test (KKK) method.

Current design praxis considers evaluated mean values in the design, and the effect of variability and uncertainties is dealt with by using a sufficiently high total factor of safety. A more rational approach for dealing with the effect of variability and uncertainties on the reliability of a mechanical system is to include them as parameters in the design model. This can be done by using reliability-based design (RRR). A major incentive for using 𝑅𝑅𝑅 is that lower variability in design properties produces higher design values. This is important since it encourages contractors to improve their manufacturing methodologies because 𝑅𝑅𝑅 allows more homogenous columns to be assigned higher design values. Reliability-based design is also in line with Eurocode 7, which states that the selection of the characteristic values for geotechnical parameters shall take the variability of the measured property values into account.

The first part of this doctoral thesis deals with test methods and quantification of the strength variability of soil improved by lime-cement columns. Tip resistances from three different test sites using three different penetration test methods – the cone penetration test, the column penetration test and the total-sounding test – are analysed and quantified in terms of means, variances and scale of fluctuations. The second part introduces RRR in serviceability limit state (SSS) design, using First Order Reliability Methods (FFFF) and Monte-Carlo simulations.

Summarizing the most important findings and conclusions from this study:

  •  The scale of fluctuation was estimated to be 0.2-0.7 m and 0-3 m in the vertical and horizontal direction, respectively.
  •  The relation between cone tip resistances measured using the cone penetration test and column penetration test does not correspond to the cone factors proposed in previous studies and in the Swedish Design Guidelines.
  •  The agreement between the column penetration test and total-sounding test was found to be “good enough”. It is therefore suggested that the total-sounding test be used as a complement to the column penetration test in evaluating the average strength properties of a group of medium- and high-strength lime-cement columns.
  • Reliability-based design is a rational approach to incorporate strength and deformation parameter variability with an SSS design.
Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2015. xii, 48 p.
Series
TRITA-JOB PHD, ISSN 1650-9501 ; 1019
National Category
Geotechnical Engineering
Research subject
Civil and Architectural Engineering
Identifiers
urn:nbn:se:kth:diva-168596 (URN)
Public defence
2015-06-10, Sal B1, Brinellvägen 23, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20150605

Available from: 2015-06-05 Created: 2015-06-05 Last updated: 2016-07-05Bibliographically approved

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