Geological, geomorphological and geophysical investigations of areas prone to clay slides: Examples from Buvika, Mid Norw

Large areas prone to clay slides are present in Canada, Sweden and Norway. Traditionally, mainly geotechnical approaches have been used to solve clay-slide issues. However, there is great potential to utilise interdisciplinary studies in order to prevent or reduce possible slide damage. The present study combines geology, morphology and geophysical investigations, with geotechnical data. The purpose is to better understand landscape development, formation of quick clay and sliding in clay. Thick, marine clay deposits in valleys along the Norwegian coast are occasionally subjected to large landslides involving quick clay. The development of quick clay is closely related to the deglaciation history of the coastal parts of Norway. During and after the last ice age, up to several hundred metres of glaciomarine and marine deposits accumulated in Norwegian fjords. These sediments were subsequently exposed on land due to glacioisostatic rebound. Leaching of salt from the marine clay by groundwater resulted in the development of quick-clay layers or pockets, which completely liquefy when remoulded. The study area of this thesis is the small valley Buvika, located in the Trondheimsfjorden area, Mid Norway. Buvika is characterised by its undulating terrain with numerous slide scars and ravines. There are thick occurrences of quick clay in the subsurface. The existing geotechnical and geophysical data, combined with sedimentology, structural geology, morphology, geophysical results, and 14C-datings, have given input to the understanding of the landscape development of the study area. From this, a deglaciation history has been deduced, indicating at least one, and possibly two, minor glacier re-advances in Late Allerød/Early Younger Dryas time. This implies that there have been more and larger ice-front oscillations in the study area than earlier documented. The lowered relative sea level led to incision by rivers accompanied by numerous slides involving quick clay. The erosion pattern of a valley filled with glaciomarine and marine deposits can be quite complex, but careful analyses have helped outlining the interplay between river and ravine incision, groundwater erosion and sliding. The study of sediments and structures in large excavated sections have resulted in the detection of slide material from old flake-type slides, where only a thin layer of quick clay acted as a slide plane. Younger slide scars cutting into theses older slide deposits show further quick-clay development. Mapping of the morphology in Buvika has identified numerous slide scars and ravines. A relative chronology of slide events has been established based upon the slide scars’ position in the terrain and/or results from 14C-datings of terrestrial organic material. Most of the historical slide scars are located in the northern part of the valley. Detailed mapping of the quick-clay extent is of great interest for planning and protection purposes, as the position of quick clay within slopes has a major impact on the landslide risk. In this study, the resistivity method is found to be potentially well suited for outlining quick-clay occurrences since quick clay has a slightly higher electrical resistivity (10-80 Ωm) than intact unleached clay (1-10 Ωm). This is due to a higher salt content in the latter. These relationships are supported by pore water salt content measurements. The resistivity profiles that were acquired show good correlation with other geophysical data and geotechnical drillings. However, the resistivity method must be combined with other investigations, since both leached, non-quick clay and silty, non-sensitive material may give resistivity values of the same range as quick clay. The stratigraphy of an area strongly influences the landscape development. It determines the morphology, such as ravine development, and size, shape and distribution of slides. To the east of Buvika, thick and frequent layers of sand and gravel in the dominant clay deposits drain the slopes, leading to development of deeply incising ravines. To the south and north, thinner layers of coarse material in the clay lead to pore-pressure build-ups and quick-clay development, resulting in numerous slide scars

Utstrekning og utløpsdistanse for kvikkleireskred basert på katalog over skredhendelser i Norge

Rapporten gir en totaloversikt over kvikkleireskred i Norge med hensyn til skredutbredelse og utløpsdistanse. Rapporten baserer seg på en detaljert morfologisk studie av over 40 skred i marine avsetninger i Norge og hvor det også finnes gode geotekniske data. Mye av dette er allerede publisert i artikkelform på engelsk, men den nåværende norske rapporten vil gi plass til en mer fullstendig oversikt med komplette kart og referansematerial

Kvalitetskontroll, analyse og forslag til oppdatering av historiske kvikkleireskred og andre leirskred registrert i Nasjonal skredhendelsesdatabase (NSDB)

Rapporten tar for seg ulike problemstillinger relatert til datakvalitet av registrerte kvikkleireskred og andre leirskredhendelser i Nasjonal skredhendelsesdatabase (NSDB). Ca. 500 skredhendelser ble kontrollert mht. skredtype, dato og lokalitet. Rapporten presenter en metode for kontroll av kvalitet og viser hvilke andre kilder (bl.a. tekniske rapporter, flybilder, ulike typer kart, LiDARdata, nyheter fra aviser) som kan brukes til å forbedre datakvaliteten. I tillegg peker den på hvilken topografisk og geologisk informasjon man må ta hensyn til for bedre å definere lokalitet

Nasjonal database for grunnundersøkelser (NADAG) – forundersøkelse

Etatene Statens vegvesen (SVV), Norges vassdrags- og energidirektorat (NVE) og Jernbaneverket (JBV) har, gjennom etatssatsningsprosjektet Naturfare - Infrastruktur, Flom og Skred (NIFS) et delprosjekt (DP6) som omhandler kvikkleire. Fra NGU er det bestilt en rapport med hensikt å undersøke muligheter for å etablere en nasjonal database for grunnundersøkelser. Rapporten har tittel «Nasjonal database for grunnundersøkelser (NADAG) – forundersøkelse». Rapporten beskriver behov for og nytteverdien av en nasjonal grunnundersøkelsesdatabase. Den inneholder en modellskisse for hvordan databasen kan bygges opp og forslag til databaseløsning og nett-tjenester, teknisk modell og behovskartlegging

Relationship between electrical resistivity and basic geotechnical parameters for marine clays

Recently, considerable efforts have been made in the attempt to map quick clay areas using electrical resistivity measurements. However there is a lack of understanding regarding which soil parameters control the measured resistivity values. To address this issue, inverted resistivity values from 15 marine clay sites in Norway have been compared with basic geotechnical index properties. It was found that the resistivity value is strongly controlled by the salt content of the pore fluid. Resistivity decreases rapidly with increasing salt content. There is also a relatively clear trend of decreasing resistivity with increasing clay content and plasticity index. Resistivity values become very low (≈5 Ω·m) for high clay content (>50%), medium- to high-plasticity (Ip ≈ 20%) materials with salt content values greater than about 8 g/L (or corresponding remoulded shear strength values greater than 4 kPa). For the range of values studied, there is poor correlation between resistivity and bulk density and between resistivity and water content. The data studied suggest that the range of resistivity values corresponding to quick clay is 10 to 100 Ω·m, which is consistent with other published limits. A comparison is made between two-dimensional electrical resistivity tomography (ERT) and resistivity cone penetration test (RCPTU) data for two of the sites and the two sets of data show similar trends and values irrespective of scale effect.Deposited by bulk impor