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Ground Improvement by Dry Deep Mixing Lime-Cement Column Panels as Excavation Support
KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Soil and Rock Mechanics.ORCID iD: 0000-0003-2288-6925
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Many urban areas near the coastal regions of Sweden are characterized by post-glacial clay deposits with very low undrained shear strength and high compressibility. Column type ground improvement by the Deep Mixing, DM, method using a binder mixture of lime and cement is commonly used in areas with poor soil conditions due to its cost-effectiveness, predominantly for settlement reduction and to improve the stability of embankments. With increasing urbanization and infrastructural development in these areas there is great interest in the industry in extending the practice of the Deep Mixing method to include other applications such as deep excavation and temporary and permanent improvement of natural slopes. Swedish experience related to use of the DM method for excavation support is limited and the current design recommendations for DM columns installed in the passive zone are very restrictive regarding the allowable mobilized column strength, resulting in a design which is often not cost-effective.

In order to increase the use of the method to include applications where DM columns are subjected to unloading and lateral loading conditions, the mobilized strength and stiffness properties of the columns as well as the soil-column interaction need to be reliably predicted. The main objective of this study is to present a consistent method to adequately predict the behavior of lime-cement columns installed as excavation support in the passive zone of the structure and to investigate the strength and stiffness properties of lime-cement improved clay under different unloading and laterally loading conditions together with the soil-column interaction under these conditions.

In order to investigate the field behavior of lime-cement column panels as excavation support, two experimental full-scale tests were performed. In each of these tests, a braced steel sheet pile wall supported by panels of overlapping lime-cement columns was first excavated to a pre-determined depth and thereafter loaded to failure by stepwise increasing a load applied behind the sheet pile wall. The tests provided a case record of deformations, stresses, and pore pressure responses, and failure mechanisms of the structures focusing on the improved soil. These tests showed that column-type ground improvement installed as panels of overlapping columns in the passive zone of a sheet pile wall significantly increases stability and reduces both excavation- and loading-induced structural forces and vertical and horizontal displacements in the soil.

This thesis also presents the results of a laboratory study involving undrained and drained isotropic consolidated triaxial compression, extension and tension tests on laboratory improved clay with a binder of lime-cement similar to that used in the experimental field tests. Based on undrained triaxial test results, a relationship between the undrained strength, effective consolidation stress, and overconsolidation ratio is presented for different stress paths to failure. From the drained triaxial tests it was found that a failure surface comprising of two failure functions, one for tension failure and one for shear failure, similar to that observed for cemented sand, is consistent with the experimental data. Finally, a 3D FE-study of the experimental field tests considering the laboratory observed stress-strain behavior and mobilized strength of lime-cement improved clay was conducted. The results of these analyses are promising and failure load, deformations and structural forces in the retaining structure were predicted reasonably well.

Summarizing the most important findings and conclusions from this study:

-          Lime-cement columns panels installed in the passive zone acting as excavation support for a sheet pile wall will significantly increase the stability of the structure.

-          Lime-cement column panels installed as excavation support are effective in reducing excavation induced displacements that can be of major concern for deep excavations conducted in areas with soft clay layers.

-          The undrained strength of lime-cement improved clay at low consolidation stresses, corresponding to approximately 10 m of depth in field conditions, is dependent of the stress path to failure and it was found to be significantly lower for unloading stress paths compared to lateral loading stress paths, i.e. stress induced anisotropy. 

-          The Young’s Modulus of lime-cement improved clay evaluated from undrained triaxial extension tests was significantly higher, 2.7 to 4.1 times, compared to the corresponding Young’s Modulus evaluated from the undrained triaxial compression tests. Also, significantly more brittle stress-strain behaviour was observed for triaxial extension tests compared to triaxial compression tests, regardless of applied stress path to failure and type of test, i.e. undrained/drained. 

-          Results of the Finite Element analysis of the conducted experimental tests show that the current Swedish Design Guide for lime-cement columns installed in the passive zone overestimates the material undrained strength when based on results from Unconfined Compression tests, but also significantly underestimates the material drained strength. Since the Swedish Design Guide specifies that the lowest of the undrained/drained column strength should be chosen in the design, the consequence is often a too conservative design as the strength increase in the improved clay is not properly considered.

Abstract [sv]

Många bebyggda områden runt Sveriges kuster karaktäriseras av postglaciala leror med mycket låg skjuvhållfasthet och hög kompressabilitet. Jordförstärkning med kalkcementpelare (Dry Deep Mixing) används ofta i områden med dåliga grundförhållanden på grund av metodens kostnadseffektivitet, dock i huvudsak för reducering av sättningar och för att öka stabiliteten vid byggnation av bankar. Till följd av ökad byggnation och infrastrukturutveckling finns det ett starkt intresse att öka användningen av kostnadseffektiva metoder såsom djupstabilisering med kalkcementpelare till att i större omfattning förstärka djupa schakter och temporära och permanents slänter där pelarna riskerar bli lateralt belastade eller dragbelastade till följd av en avlastning. Svenska erfarenheter av förstärkning av djupa schakter med kalkcementpelare är begränsade och dagens dimensioneringsmetodik för pelare installerade i passiv zon är mycket restriktiv gällande tillåten hållfasthet i pelarna vilket ofta resulterar i en oekonomisk design.

För att öka metodens användbarhet till att i större grad omfatta ovan nämnda användningsområden behöver materialets hållfasthets och styvhetsegenskaper kunna tillförlitlig beskrivas för de aktuella belastningarna. Huvudsyftet i denna avhandling är att presentera en metod för att adekvat beskriva hållfastheten i kalkcementpelare installerade i passiv zon och att undersöka hållfasthet och styvhetsegenskaper hos lateralt och dragbelastade kalkcementförstärkt lera samt samverkan pelare-jord under dessa belastningsförutsättningar och.

För att undersöka materialens beteende och samverkan pelare-jord för stabilisering av djupa schakter genomfördes inom ramen för denna forskningsstudie två stycken fältförsök. I vart och ett av dessa försök, installerades en stålspont strävad mot en mothållspont som förstärktes med överlappande kalkcement pelarskivor installerade i passiv zon mellan de två sponterna. Avschaktning utfördes till en i förväg bestämd nivå innan konstruktionen drevs till brott genom att stegvis öka belastningen bakom sponten på aktiv sida. Genom dessa tester kunde en omfattande dokumentation insamlas avseende deformationer, spänningar, portrycksrespons och brottmekanism i konstruktionen med fokus på den stabiliserade jorden. Dessa tester visade att kalkcementpelarförstärkning utförd som skivor av överlappande pelare i passiv zonen av en spont signifikant ökade konstruktionens säkerhet mot brott, minskade schakt- och belastningsinducerad laster i stödkonstruktionen och minskade såväl vertikala som horisontella deformationer.

I denna avhandling presenteras också resultat av en omfattande laboratoriestudie som innefattar odränerade och dränerade isotropisk konsoliderade aktiva och passiva triaxialförsök. De passiva triaxialförsöken utfördes under olika spänningsvägar till brott som ska spegla både lateral belastning och avlastning av materialet. Baserad på utförda odränerade triaxialförsök, en relation mellan materialets hållfasthet, konsolideringsspänning och överkonsolideringsgrad presenteras för olika spänningsvägar till brott motsvarande aktiv belastning, lateral belastning och avlastning. Från de dränerade triaxialförsöken konstaterades att beskrivning av materialets brottyta med hjälp av två brottfunktioner, en för dragbrott och en för skjuvbrott, liknande dem som har rapporterats för cementerat sand, stämmer väl överrens med test resultaten. Slutligen, en 3D – Finita Element studie av båda fältförsöken presenteras där den i laboratoriestudien observerade materialbeteendet gällande spännings-töjningssamband och mobiliserbar hållfasthet beaktas. Resultaten av denna studie är lovande och predikterad belastning vid brott, deformationer och storlek på belastningar i spontkonstruktionen stämmer relativt väl överrens med de i fält observerade.

De viktigaste upptäckterna och slutsatserna från denna studie kan summeras enligt:

-          Kalkcementpelarskivor installerade i passiv zon som support för en spontkonstruktion kommer att avsevärt öka säkerheten mot stabilitetsbrott i jorden.

-          Kalkcementpelarskivor installerade i passiv zon som support för en spontkonstruktion är en effektiv metod att minska schaktinducerade deformationer som kan vara av stor betydelse vid djupa schakter i områden med lös lera.

-          Den odränerade hållfastheten hos kalkcement stabiliserad lera vid låga konsolideringsspänningar, motsvarande cirka 10 m djup under markytan, är starkt beroende av valt spänningsväg till brott, dvs. materialet uppvisar spänningsinducerad anisotropi

-          Elasticitetsmodulen utvärderad från odränerade passiva triaxialförsök var i storleksordningen 2.7 till 4.1 gånger högre jämfört med motsvarande elasticitetsmodul utvärderad från aktiva odränerade triaxial försök. Också, en signifikant mer spröd spännings-töjningsbeteende erhölls från alla passiva triaxial försök, både odränerade och dränerade, oberoende av valt spänningsväg till brott.

-          Finita Element analyser av de utförda fältförsöken visar att den Svenska Dimensioneringsanvisningen för kalkcementpelare installerade i passiv zon dels överskattar materialets odränerade hållfasthet när denna baserar på resultat av Enaxliga Tryckförsök, men också signifikant underskattar den dränerade hållfastheten som kan mobiliseras. Eftersom den Svenska Dimensioneringsanvisningen anger att den lägsta av den odränerade och dränerade hållfastheten i pelaren ska väljas i varje situation blir konsekvensen en alltför konservativ design.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2018. , p. 70
Series
TRITA-ABE-DLT ; 1834
National Category
Geotechnical Engineering
Identifiers
URN: urn:nbn:se:kth:diva-235856ISBN: 978-91-7729-969-1 (print)OAI: oai:DiVA.org:kth-235856DiVA, id: diva2:1254063
Public defence
2018-10-30, F3, Lindstedtsvägen 26, Stockholm, 13:00 (English)
Opponent
Supervisors
Note

QC 20181008

Available from: 2018-10-08 Created: 2018-10-08 Last updated: 2018-10-08Bibliographically approved
List of papers
1. Two- and three-dimensional analyses of excavation support with rows of dry deep mixing columns
Open this publication in new window or tab >>Two- and three-dimensional analyses of excavation support with rows of dry deep mixing columns
2015 (English)In: Computers and geotechnics, ISSN 0266-352X, E-ISSN 1873-7633, Vol. 66, p. 16-30Article in journal (Refereed) Published
Abstract [en]

In this study, a 2D model of an excavation with a tied back sheet pile wall in interaction with perpendicular rows of deep dry mixed overlapping columns was compared to a 3D model. A method to take into consideration the effect of the overlap zones between columns in a 2D model, where the improved soil was modeled as a composite material, was investigated and the results between the 2D and 3D analyses were compared with focus on predicted failure load, failure mechanism and deformations. The results of this numerical study show that both the area improvement ratio of the improved soil and the quality of the overlap zone has a significant influence on how well a 2D model that incorporates the overlap zone between columns, performs compared to the 3D model.

Place, publisher, year, edition, pages
Elsevier, 2015
National Category
Geotechnical Engineering Civil Engineering
Identifiers
urn:nbn:se:kth:diva-160133 (URN)10.1016/j.compgeo.2015.01.011 (DOI)000351981000002 ()2-s2.0-84922719676 (Scopus ID)
Note

QC 20150416

Available from: 2015-02-16 Created: 2015-02-16 Last updated: 2018-10-08Bibliographically approved
2. Behavior of braced excavation supported by panels of deep mixing columns
Open this publication in new window or tab >>Behavior of braced excavation supported by panels of deep mixing columns
2016 (English)In: Canadian geotechnical journal (Print), ISSN 0008-3674, E-ISSN 1208-6010, Vol. 53, no 10, p. 1671-1687Article in journal (Refereed) Published
Abstract [en]

This paper describes the instrumentation, execution and performance of two full-scale tests where a braced steel sheet pile wall interacting with rows of overlapping dry deep mixing columns was excavated and then loaded to failure. The purpose of these tests was to provide knowledge of the behavior of deep mixing column rows located in passive zone and interacting with a retaining structure. Both tests were extensively instrumented on the active as well as on the passive side of the retaining structure. In both conducted tests a stability failure of the retaining structure occurred, resulting in heave at the bottom of the excavation and large settlements of the ground surface behind the sheet pile wall. For a spacing between LC-panels of 3.0 m a very brittle failure developed suddenly in the clay between the panels with small deformations prior to failure. In the second test, with a spacing of 1.5 m between LC-panels, the failure developed in the LC-panels as well as in the clay between the panels. Even if a similar failure mechanism developed, measured horizontal displacements, horizontal stresses, and pore pressure response prior to failure differed between the tests.

Place, publisher, year, edition, pages
NRC Research Press, 2016
Keywords
dry deep mixing, field test, excavation support, failure mechanism, pore pressure response
National Category
Geotechnical Engineering
Identifiers
urn:nbn:se:kth:diva-188805 (URN)10.1139/cgj-2016-0137 (DOI)000385647600008 ()2-s2.0-84989849507 (Scopus ID)
Note

QC 20160712

Available from: 2016-06-17 Created: 2016-06-17 Last updated: 2019-10-11Bibliographically approved
3. Triaxial Extension and Tension behavior of Lime-Cement improved clay
Open this publication in new window or tab >>Triaxial Extension and Tension behavior of Lime-Cement improved clay
(English)Manuscript (preprint) (Other academic)
Abstract [en]

The performance of Deep Mixing columns installed on the passive side of excavation supporting structures was investigated by means of two instrumented full-scale tests conducted in Enköping, in eastern Sweden, during 2014. In these tests, a braced steel sheet pile wall interacting with panels of overlapping Dry Deep Mixing columns was excavated and loaded to failure. This paper present the results of a series of undrained and drained isotropic consolidated triaxial extension, tension and compression laboratory tests on lime-cement improved clay with a binder of lime-cement similar to that used in the experimental field tests. The different stress paths to failure, with the purpose to reflect the stress path to failure from the experimental field tests, were obtained by varying the direction of the major and minor principle stresses in a conventional triaxial test cell. The undrained tests revealed that at low consolidation stresses, corresponding to depth of approximately 0-10 m below the ground surface, significant stress induced anisotropy was observed depending on the direction of the major and minor principle stresses. Based on undrained triaxial test results, a relationship between the undrained strength, effective consolidation stress and OCR is presented for different stress paths to failure.

The experimental data of the drained tests show that a failure surface comprising of a shear failure function based on Mohr-Coulomb failure criterion and a tensile failure function based on the tensile strength and the confining stress can be applied for lime-cement stabilized clay.

Keywords
Dry Deep Mixing, Triaxial tests, stress paths, mobilized strength
National Category
Geotechnical Engineering
Research subject
Land and Water Resources Engineering
Identifiers
urn:nbn:se:kth:diva-235851 (URN)
Note

QC 20181015

Available from: 2018-10-08 Created: 2018-10-08 Last updated: 2018-10-15Bibliographically approved
4. Numerical analyses of an experimental excavation supported by panels of lime-cement columns
Open this publication in new window or tab >>Numerical analyses of an experimental excavation supported by panels of lime-cement columns
Show others...
(English)Manuscript (preprint) (Other academic)
Abstract [en]

The influence of ground improvement with panels of overlapping lime-cement columns on the behavior of a braced excavation loaded to failure has been investigated using 3D numerical analyses and compared with a reported experimental failure test. The dependency of the mobilized column strength on the stress path to failure was considered by assessing the strength criteria based on results from isotropically consolidated undrained triaxial extension tests. For comparison, the problem is also analyzed with isotropic column material parameters determined from unconfined compression tests. The analyses reveal that stress-induced strength anisotropy of lime-cement improved clay needs to be considered when the stress path for the actual field conditions differs from that in conventional laboratory testing. The failure loads, and also the location of the failure surface predicted by the FE-analyses, were in good agreement with the field tests. On the other hand, analyses conducted with column strength parameters, evaluated from unconfined compression tests, significantly over-predicted the failure load of the experimental tests. In addition to strength parameters, the modulus of deformation that is consistent with the actual encountered stress path is also needed to predict reasonable results.

Keywords
deep mixing columns, finite element analyses, strength anisotropy, deep excavation
National Category
Geotechnical Engineering
Research subject
Civil and Architectural Engineering
Identifiers
urn:nbn:se:kth:diva-235853 (URN)
Note

QC 20181016

Available from: 2018-10-08 Created: 2018-10-08 Last updated: 2018-10-16Bibliographically approved

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