Experimental and numerical investigations of mechanisms in unsaturated geomaterials


i numerička istraživanja mehanizama
u nesaturiranim geomaterijalima


This project was funded by the Croatian Science Foundation under the number UIP-2017-05-3429.
The project was also co-financed by FCEAG in the amount of 15% of the HRZZ's participation.
Research team members are grateful for support.

Project Summary

Weathering as rock degradation under the direct influence of environmental conditions and human activity in the engineering period of time, is a key process that affects geomorphologic, ecologic and societal processes and events. Soft rocks, such as marl, are very common in the Mediterranean and particularly susceptible to weathering. This is manifested through the decomposition of the binder from clayey rock structure and breaking of the rock structure due to physical and chemical processes. Under such influence, weathering degrades soft rocks to fine-grained material, which is related to environmental sustainability (erosion, climate change issues, slopes, rock falls and other geohazards), stability, bearing capacity and durability issues (temporal change of strength and deformability).

According to recent scientific findings, a significant contributor to the development of weathering is the process of differential suction and induced differential swelling, which occurs in unsaturated conditions. Differential suction comes as a result of capillary effects in voids of the material and it is closely related to slaking of the material. The consequences of differential swelling are development of tensile and shear stresses that cause weathering. Therefore, the aim of this project is to investigate and simulate the mechanisms of weathering from the aspect of suction, and to link the acquired new findings with the significant previous research carried out at the Faculty.

The shear strength and deformability in unsaturated conditions for the given degree of suction, and the characteristic SDSWCC curves (Stress Dependent Soil Water Characteristic Curve) will be determined by sophisticated testing equipment on still unexamined soft rock material from the Croatian coastal area. At the same time all “accompanying” tests, including physical, mechanical, mineralogical and petrological, will be carried out and new and existing databases will be linked. Full experimental programme will be carried out on approximately 30 specimens.

The obtained results will be incorporated into the existing numerical model (Plaxis) and into additionally developed new numerical model based on a discrete approach, which will include the fracture mechanism in geomaterials in unsaturated flow conditions. This will significantly improve the knowledge of weathering mechanisms in geomaterials and enable sustainable project solutions.

A newly formed research group with the necessary resources provides additional value of the project.

Project Team

Team members:

  • Assoc. prof. Nataša Štambuk Cvitanović
  • Mijo Nikolić, PhD
  • Assist. prof. Goran Vlastelica
  • Ana Raič, MEng
  • Prof. Predrag Miščević (associate/consultant)

Assoc. prof. Nataša Štambuk Cvitanović, Ph.D., principal investigator of the project, is an expert and researcher in the field of geotechnical engineering. She is engaged in scientific research work and solving problems related to geotechnical investigations and design using in situ and laboratory parameters, especially soil and rock mechanics, and laboratory/field testing, observations and research.

Mijo Nikolić, Ph. D., a researcher on the project, deals with the development of numerical models (e.g. finite element method with embedded discontinuities) and their application in the rock and structural mechanics (fracture mechanisms under various flow and other conditions).

Assist. prof. Goran Vlastelica, Ph. D., a researcher at the project, deals with geotechnical researches, in particular the problems of soft rock in terms of weathering (degradation), changes of properties in time and stability/durability of cuts in flysch.

Project Goals

  • Literature review and expand of the current knowledge
  • Establish a research group and laboratory for unsaturated soil
  • Provide appropriate initial (introductory) experimental and numerical research with appropriate dissemination and training, and determine the final testing methodology and usability of available software (Plaxis)
  • Carry out main experimental and numerical research of mechanisms in unsaturated geomateriaals (suction, degradation) using the example of soft rock (full test program on marl samples) and develop a new numerical model that can include the fracture mechanism in unsaturated flow conditions
  • Based on the acquired findings, through further training and dissemination improve the knowledge of the behavior of unsaturated geomaterials; link the current and new research (unique database) and establish criteria for using suction as part of laboratory and field research
  • Submit project proposals from the research group to other sources of funding


  • Analysis of additional literature and update/analysis of the existing database (2018)
  • Procuring computers and software (Plaxis) for initial numerical investigations (2018)
  • Training at Faculty of Civil and Geodetic Engineering in Ljubljana on the topic of laboratory and field tests of suction and significance of soil water characteristic curve (SWCC) (2018)
  • Synthesis and dissemination of the findings - paper accepted for publication at the ISSMGE International Congress in Reykjavik in 2019 (2018/2019)
  • Supply, installation and commissioning of advanced research equipment for testing in unsaturated conditions – automatic suction oedometer, manufacturer GDS Instruments (2018)
  • Performing a public tender for recruiting an PhD candidate (2018 and repeated in 2019)
  • Servicing and calibration of existing equipment used in research (2018/2019)
  • Supply of materials needed for testing (2018.)
  • Sampling in the field (exploration pits) for initial testing (2018/2019)
  • Initial laboratory tests (FCEAG-Split, FCE-Rijeka and Croatian Geological Survey-Zagreb) and analysis of results – established methodology for experimental research (2018/2019)
  • Initial numerical research in Plaxis for unsaturated conditions and phase verification in model development - paper accepted for publication in the journal Granular Matter (2018/2019)


  1. Vlastelica, G., Štambuk Cvitanović, N., Miščević, P, Nikolić, M., Raič, A.: The role of unsaturated condition and suction in the weathering mechanisms in flysch marls; paper accepted for publication in the proceedings of the XVII European Conference on Soil Mechanics and Geotechnical Engineering, Reykjavik Iceland, 1 - 6 September 2019.
  2. Rizvi, Z.H., Nikolić, M., Wuttke, F.: Lattice element method for simulations of failure in bio-cemented sands; paper accepted for publication in the journal Granular Matter, Springer Berlin Heidelberg (february 2019).

Photo Gallery

Kick-off meeting 21/03/2018.

Field trip to Slano Blato landslide in Slovenia (18/10/2018); A. Petkovšek (FGG-Ljubljana) i N. Štambuk Cvitanović (FCEAG-Split) near the deep reinforced concrete dowels (shafts) constructed for mitigation of the landslide

Family of Soil Water Characteristic Curves (SWCC) with the marked range of examined marls; wPI value – a product of the percentage of particles below 0.075 mm (decimal value) and plasticity index (%) (adapted from Zapata et al., 2000)

Degradation mechanism: differential swelling caused by suction gradients (from Alonso et al., 2010)

An example of degradation caused on marl sample by suction

Installation of automatic suction oedometer

Cell pedestal with special porous disk for unsaturated tests (HAEPD - High-Air-Entry Porous Disk)

Specimen cell prepared for testing

New automatic test equipment: load frame, sample cell, pressure/volume controllers for water and air, data logger with connected force, axial displacement, pore water pressure and air pressure transducers

Measurement results of volume changes under suction


Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of Croatian Science  Foundation.