Mechanical Behaviour of Structured Chalk and Its Implications for Offshore Pile Installation
Author
Molla, Kassahun Abera
Term
1. term
Education
Publication year
2026
Submitted on
2026-06-04
Abstract
This master’s thesis examines the mechanical behaviour of Rørdal chalk and its implications for offshore monopile foundations in the North Sea. The work is based on laboratory testing of intact block samples from the Aalborg Portland quarry, combined with a literature review of previous studies, including the ALPACA research programme and other investigations of offshore chalk formations. The chalk is first characterised through index tests, including measurements of bulk and dry density, particle density, porosity, void ratio and water content, to describe its structure and provide a geotechnical classification. The main experimental programme consists of monotonic triaxial tests (drained and undrained), in which stress–strain response, structural evolution and breakdown, stiffness and strength degradation, volumetric behaviour and pore pressure development are analysed using effective stress paths and shear strength mobilisation. Particular emphasis is placed on destructuration, i.e. the transition from bonded, structured chalk to a more friction-dominated, remoulded state at large strains. Although no direct pile installation tests were conducted, the observed material behaviour is interpreted in the context of soil–pile interaction under monotonic loading conditions representative of monopile installation in chalk. The thesis thus provides new triaxial experimental data for Rørdal chalk and offers additional insight into how degradation of strength and stiffness, volumetric collapse and pore pressure response may influence drivability, shaft resistance and the design of offshore pile foundations in chalk-dominated ground conditions.
Dette speciale undersøger den mekaniske adfærd af Rørdal-kalk med fokus på betydningen for offshore monopælfundamenter i Nordsøen. Udgangspunktet er laboratorieforsøg på intakt blokprøvet kalk fra Aalborg Portland, kombineret med en litteraturgennemgang af tidligere studier, herunder ALPACA-programmet og andre undersøgelser af kalkformationer offshore. Chalken karakteriseres først ved indeksforsøg, herunder bestemmelse af tør- og våd densitet, partikel- og rumvægt, porøsitet, void ratio og vandindhold, for at beskrive materialets struktur og klassificere det geoteknisk. Hoveddelen af arbejdet omfatter monotone triaxiale forsøg (drænede og udruinede), hvor spændings–tøjningsrespons, strukturopbygning og -nedbrydning, styrke- og stivhedsdegradering, volumetrisk respons samt poretrykudvikling analyseres via effektive spændingsbaner og mobilisering af forskydningsstyrke. Særlig vægt lægges på destructuration, dvs. overgangen fra en cementeret, struktureret kalk til en mere friktionsdomineret, remoldet tilstand ved store deformationer. Selvom der ikke er udført direkte pæleinstallationsforsøg, fortolkes resultaterne i lyset af jord–pæl-interaktion under monotone belastninger, som er repræsentative for installation af monopæle i kalk. Dermed bidrager studiet med nye triaxiale forsøgsdata for Rørdal-kalk og giver yderligere indsigt i, hvordan styrke- og stivhedsreduktion, volumetrisk sammenbrud og poretrykrespons kan påvirke drivbarhed, skaftbæreevne og design af offshore pælefundamenter i kalkrige undergrundsforhold.
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