Application of ERT for Quality Assurance in Jet Grouting Columns; A development of an alternative quality control

Abstract

With the increased urbanization major cities are facing, expansion of larger facilities and households are inevitable. Expansion means larger areas of the surface needs to be used where the risk of varying subsurface soil properties increases with larger surface areas. This can result in soil properties that might not be stable enough to support these constructions. A potential solution to solve this problem is the soil improvement method of jet grouting. Jet grouting is a versatile soil improvement method used for various geotechnical aspects in construction projects worldwide. By eroding and mix in-situ soil with fluid binders from a high-pressure jet, improved geotechnical properties of the soil body could be achieved. When achieving the desired stability of the column it is crucial that the geometry and homogeneity correlates with the preliminary design standards. Thereby, quality controls are routinely performed on the column however, existing quality controls vary in methodology efficiency as well as the level of accuracy obtained from the controls. The thesis therefore aspired to find and compare an alternative quality control to the existing quality controls based on Electrical Resistivity Tomography (ERT) practice. ERT is a geophysical measuring technique for determining the electric properties of the subsurface. This technique consists of placing electrodes in contact with a specific medium and inject current via the electrodes to create an electric potential field. The electrodes will then measure the potential differences of the potential field which are translated to resistivity properties of the medium.

In this thesis a composed cable consisted of electrode cables and temperature sensors were inserted into a freshly produced test column. By injecting current the potential differences in the column and the surrounding soil were measured and the apparent resistivity properties of the underground could be determined. The measurements were performed at different times to study the curing process. The apparent resistivity measurements were inverted for two software: Res2DInv and pyGIMLi. Due to the borehole design, the resistivity measuring was performed differently than the traditional surface ERT-measurement, a geometry adapted to the data had to be applied for obtaining suitable inversion models of the column.

The measured data contained several outliers and errors which increased during the curing process of the column. This was most likely due to a corrosive action taking place on the electrode surfaces. The inversion models resulted in relatively large uncertainties as well, however the pyGIMLi inversion models showed better correlation with the intended geometry of the column compared to Res2DInv. Even though uncertainties of the inversion models were found, a resistivity contrast between the treated and untreated soil was identified throughout the curing process which decreased with curing time.

The conclusion of this thesis is that the ERT-method has potential to be applicable to quality assuring jet grouting columns. The quality parameter for determining the geometry and homogeneity of the column would be defined by the boundary of which the resistivity contrast between the treated and untreated soil was located in the inversion models or how the relative resistivity changes during the curing process. However, this quality control was not ideally executed, neither with the set of inversion models developed in this thesis, nor the set of equipment used when measuring as it was less adaptive to the column conditions and the installation procedure. The developed quality control of this thesis was not validated and it could therefore not be compared with the accuracy existing quality controls have. But we believe that the ERT-method could potentially be a more time-effective and accurate quality control compared to existing quality controls with refinement in routine application, measuring equipment and inversion models.

Abstract (Swedish)

Med den ökade urbaniseringen som de flesta städer står inför, är utbyggnad av större anläggningar och hushåll oundviklig. Denna expansion medför ett behov av mer tillgänglig markyta att bygga på, vilket även innebär att platser med varierande markförhållandena behöver utnyttjas. Detta kan resultera i att markegenskaperna inte alltid är passande för stora anläggningar. En lösning till detta problem är att stabilisera marken med den så kallade jetinjekterings-metoden. Jetinjektering är en mångsidig markförbättringsmetod som används för olika geotekniska aspekter i byggprojekt världen över. Genom att erodera marken och blanda cementslurry tillsammans med den befintliga jorden på platsen med hjälp av en högtrycksstråle, bildas en pelare som förbättrar markförhållandet. Vid tillverkningen av en jet-pelare är det viktigt att geometrin och homogeniteten stämmer överens med den förbestämda standarden. Därför genomförs kvalitetskontroller på pelarna för att säkerhetsställa detta. De befintliga kvalitetskontrollerna varierar i metodisk effektivitet samt i nivå av noggrannhet. Syftet med denna rapport är därför att undersöka en alternativ kvalitetskontroll som utförs med hjälp av ERT-mätningar (Electric Resistivity Tomography), samt att jämföra denna metod med de befintliga kvalitetskontrollerna. ERT är en geofysisk undersökningsmetod som mäter de elektriska egenskaperna i marken. Metoden går ut på att placera ut elektroder i kontakt med ett specifikt medium och via dessa elektroder injicera ström som skapar ett elektriskt potentialfält. Elektroderna mäter sedan potentialskillnaden i potentialfältet som omvandlas till resistivitetsegenskaper hos mediet.

I detta projekt fördes en kabel med elektroder och temperatursensorer ner i en nyproducerad kolumn. Genom att injicera ström i kolumnen och den omkringliggande jorden, kunde potentialskillnaden i potentialfältet som skapades bestämmas. Denna process gjordes vid flera tillfällen för att kunna studera härdningsprocessen. Med denna information kunde den skenbara resistiviteten för marken att räknas ut. Den skenbara resistiviteten gjordes om till den verkliga resistiviteten genom inversioner i olika datorprogram. De datorprogram som användes i detta arbete var Res2DInv och pyGIMLi. Eftersom mätningarna i detta arbete inte utfördes på samma sätt som de traditionella ERT-mätningarna på jordytan, definierades en ny geometri som var bättre anpassad till kolumner och på så sätt blev inversionmodellerna mer trovärdiga.

Den uppmätta data som erhölls efter mätningarna innehöll ett antal extremvärden och fel som ökade under härdningsprocessen av kolumnen. Detta berodde antagligen främst på korrosion på elektrodernas yta. Inversionsmodellen resulterade i relativt stora osäkerheter, men pyGIMLi-inversionsmodellerna hade bättre korrelation med den avsedda geometrin för kolumnen jämfört med Res2DInv. Trots en del osäkerheter gick det att urskilja en resistivitetskontrast mellan den obehandlade jorden och den behandlade jorden under härdningsprocessen, vilket dock minskade över tid.

Slutsatsen i detta examensarbete är att ERT-mätning har potential att användas som kvalitetskontroll för jet-pelare. Kvaliteten gällande geometrin och homogeniteten i kolumnen definieras utifrån var resistivitetskontrasten i inversionsmodellen är lokaliserad eller hur den relativa resistivitetsökningen ser ut under härdningsprocessen. I detta arbete utfördes inte kvalitetskontrollen på ett optimalt sätt då inversionsmodellerna utvecklade i detta arbete inte var idealiska med den samlade mätningsdatan, samt att utrustningen som användes vid mätningarna inte var anpassade för förhållandena och installationsprocessen. Kvalitetskontrollen i detta arbete har inte validerats och kunde därför inte jämföras med de befintliga kvalitetskontrollernas noggrannhet. Med förbättrade rutintillämpningar, utrustning och inversionsmodeller kan ERT-metoden potentiellt vara en mer tidseffektiv och noggrann kvalitetskontroll jämfört med befintliga kvalitetskontroller.

Popular Abstract

With the increased urbanization major cities are facing, additional structures must be constructed on the ground surface which need sufficient soil strength to be built upon. But with more land used for constructing on, the risk of variable soil conditions increases, and sufficient soil strength becomes more uncertain. One solution to this problem is jet grouting, which is a versatile soil improvement method that radially injects cement slurry in a pre-drilled borehole to erode and mix in-situ soil that forms a stabilized underground column. The most important aspects of achieving desirable properties of the column are the homogeneity of the grout mixture and the diameter of the column, but this can be difficult to inspect without a quality control that intrudes on the surrounding soil. Therefore, can a less intrusive quality control be used for quality assuring jet grouting columns specifically within Electrical Resistivity Tomography (ERT) -practice, and is it comparable with existing quality controls?

The underground soil can consist of different layers which inherit certain resistances towards eroding and mixing. This can affect how far the injected grout will penetrate radially into each soil layer, causing the diameter of the column to be varied. Discontinuous column diameter can severely compromise the intended column properties. By routinely performing quality controls on jet grout columns, the column diameter and the homogeneity of the grout mixture can be estimated. However, existing quality controls differ in accuracy and credibility which is why this thesis’ purpose was to investigate if an alternative quality control could be developed using ERT.

ERT is a geophysical measuring method that measures the resistivity distribution of the underground by injecting current through electrodes in contact with a material. By applying this method into jet grout columns, the expectation of finding a resistivity contrast between the treated volume and the surrounding soil as well as the resistivity distribution of the treated soil would determine the homogeneity and geometry of the column.

In-field measurements were performed on a test column at a construction site which comprised of inserting a composed cable into the centreline of a freshly produced column. The composed cable consisted of a multi-electrode cable with a spacing of 0.5 meters and 4 temperature sensors mounted on the electrode cable to ensure both ERT- and temperature measurements were performed simultaneously at different depths throughout the curing process. An ABEM Terrameter LS2 instrument was used, measuring in dipole-dipole and multigradient configurations.

The inversion procedure was carried out in two different computer software: pyGIMLi and Res2DInv. A borehole geometry was implemented to the measured data in both software for a better interpretation of the resistivity distribution. This was made due to the original geometry was based on traditional ERT-measuring, where the electrodes would assume to be on the ground surface. 3D-borehole inversions were made in pyGIMLi while cross-borehole inversions were executed in Res2DInv.
The results indicated that a resistivity contrast between the treated volume and the surrounding soil was found for both inversion software which decreased with curing time. The pyGIMLi-models seemed to better correlate with the measured data compared to Res2DInv, mainly due to Res2DInv assuming a 2D-geometry. Many outliers and errors were also found in the data, that could be explained by corrosion taking place on the electrodes’ surfaces. Thereby, a different measuring setup or equipment suited for jet grout conditions could have provided better data quality.

Furthermore, this quality control was not verified and was unable to be compared with existing quality controls. Many existing quality controls are performed when curing of the column is finished, while the ERT-method can give results directly after column installation. That is why the ERT-method has the potential to be a time-effective quality control with refinement in inversion modelling and routine application.

• author: Nilsagård, Edvin and Knutsson, Rebecka
• supervisor: Tina Martin; Peter Jonsson
• organization: Engineering Geology; Civil Engineering (M.Sc.Eng.)
• alternative title: Tillämpning av ERT för kvalitetssäkring av jet-pelare
• course: VTGM05 20221
• year: 2022
• type: H3 - Professional qualifications (4 Years - )
• subject: Technology and Engineering
• keywords: ert, jet grouting, quality control, pygimli, res2dinv, resistivity
• publication/series: ISRN
• other publication id: LUTVDG/(TVTG—5175)/1-113/(2022)
• language: English
• additional info: Examiner: Matteo Rossi
• id: 9100679
• date added to LUP: 2022-10-03 13:12:43
• date last changed: 2022-10-03 13:12:43