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IC4_II

In situ geotechnical property evaluation of low strength on-shore and near-shore sediments in use assigned areas

Initial questions and objectives of the project

1. Peats are a frequent geotechnically problematic soil in the North Sea and Northern Germany. The first study was to link the Cone penetration test (CPT) data of peat to the geotechnical properties of peat starting with onshore peat samples. (Bremen, Germany).

Current stage of work for the first objective and description of achieved results
Undisturbed peat samples were collected from Blockland, Bremen, Germany and characterized by the geotechnical index tests water content, Atterberg limit, ash content, organic content, fibre content and pH. In a second step the permeability and consolidation characteristic was derived from oedometer tests and the shear strength was determined from direct shear tests. During that work the idea was developed to study the stabilization of peat by sand columns in a series with four different peat sand ratios. The sample with 70 % peat and 30 % sand showed the best strength and consolidation characteristics. A paper entitled “Strength and Compressibility Characteristics of Peat Stabilized with Sand Columns” was submitted to Journal of Geomechanics and Engineering and is currently under review.
A CPT was performed at the location were the undisturbed peat samples were collected and compared with the laboratory results. However to develop a relationship between CPT data and geotechnical properties of peat more profiled would be needed and the work was concentrated on the peat stabilization theme. CPT data and cores from offshore peats from Holocene age were screened but no in depth studies were accomplished so far.


2. Weak decomposed ashes are frequent geotechnical problematic soils off- and onshore at Bay of Plenty, New Zealand. The aim of the second study was to use CPT data form the port of Tauranga and onshore sites to assess geotechnical problems concerning harbour construction, dredging and slope stability.

Current stage of work for the second objective and description of achieved results
The Geotechnical Offshore Seabed Tool (GOST) was shipped from Bremen, Germany to Tauranga Harbour, New Zealand and in-situ tests were started in February 2012 at on/offshore areas of Bay of Plenty. During three weeks of on/offshore operation 27 off/onshore CPT were performed, comprising more than 350 meters CPT profiles. Offshore measurements were performed in the Tauranga Harbor and onshore measurements were performed at the landslides at Pyes Pa and Omokoroa. Additionaly to the normal static CPT, vibratory CPT, remolding CPT and dissipation tests were performed. The vibratory CPT aims to identify liquefiable soils, and the dissipation test gives the in situ permeability, while the remolding CPT is a totally new mode of operation with the aim to identify sensitive soils.
The working barge and tugboat for the 20 days offshore operation equivalent to ~ 150 000 € was provided by the Port of Tauranga, as a consequence four CPT sites where chosen to characterize a mound in the shipping channel. The Sites of the CPTs were chosen in contact with Viggi from IC10 to provide static reference data for his dynamic CPT tests.

A data report was prepared for the Port of Tauranga and a comprehensive report of the operation was published on Intercoast web-page.

One part of the CPT tests was dedicated to the characterization of the subsurface in the dredged and un-dredged part of Tauranga Harbor to compare effect of dynamic loads on harbor sediments. During the piling for a bridge project unexpectedly a pile was “fell” through several meters obviously hitting a somehow unstable layer. One aim was to identify and map out this layer with CPT tests. There are two processes that could have led to such a behavior, the first being liquefaction of coarse grained material, which is caused by a fabric change of the grain skeleton to denser conditions, subsequent pore water pressure rise leading to a total loss of effective stress and hence a loss of shear strength of the material. The other process is sensitivity, a sensitive material has a high strength, but if this strength is one overcome the material has a very low residual shear strength. The ratio of peak to residual strength may be several orders of magnitude. The vibratory and remolding CPT tests were especially aimed to detect this potentially hazardous and geotechnical interesting material.
In the static CPTs data no specific layer with high sensitivity or high liquefaction risk was found even though some ideas of data processing have not yet been applied. A paper concerning the geotechnical character of the harbor subsoil has just been started and will be presented on GEOMATE 2013 November 13-15 in Nagoya Japan.
The vibratory CPT detected several liquefiable layers in the tip resistance data, using the method of Tokomatsu (1984). Vibratory CPT pore pressure data has not been published before, but gives valuable insight in the soils reaction to cyclic loads. Variable reactions were observed for “intermediate” silty soils. A new method to process and interpret vibratory CPT data was developed. The outcome of this work is currently described in a manuscript entitled “Development of piezovibrocone in marine soils”, which one main focus of the current work.
The onshore CPTs were focused on landslides sites in the Bay of Plenty area at Pyes Pa and Omokoroa. Static, vibratory CPT and dissipation tests were performed to characterize the landslides. One major issue in landslide research is identification and characterization of potential and actual slip surfaces. Static CPT results appealed general hydraulic situation of penetrated layers and identified probable slip surface along with the triggers that have be led or may be lead to slope failure.
Based on the work done at Pyes Pa a paper entitled “Utilizing cone penetration test for landslide characterization at Pyes Pa, Bay of Plenty” was submitted to the 6th International conference of submarine mass movements and their consequences, 23rd – 25th September, Kiel, Germany and is currently under review. The Papers of this conference will be published as a ISI listed Springer book.
The trigger leading to the Omokoroa landslide is under discussion and as a consequence the measures needed to prohibit further sliding are also controversial. One proposed trigger mechanism is the toe erosion by waves during the storm happening at the time of the landslide. The measure to inhibit further sliding is than centered on costal protection. The other proposed trigger is the pore water pressure rise caused by the rain preceding the slide event. The measures to inhibit further sliding then have to be centered on drainage to reduce the pore water pressure in the critical zone. A new trigger or preconditioning factor that came up during the study is the reaction of the soil to cyclic loads caused by the waves and wind attacking the headland, for which there exist no idea for countermeasures yet. Static and vibratory CPT identified same layer as the most probable slip surface such that the static weak layer reacts additionally with the most reduction of effective stresses under the effect of cyclic loading. The dissipation test shows a dramatically low permeability at the probable slip surface which specifies the vulnerability of the layer to fail on the occasion of heavy rainfall. The database so far hints to a pore pressure trigger mechanism, maybe in combination with cyclic loading. The work on this site lead to a profound collaboration between Germany and New Zealand. The result of landslide scar scanning Michael Cunningham did for his master thesis will be used as well as his results of the detailed sampling and stratigraphic work at the slide scar.
The field work in 2013 is dedicated to the landslide at Omokoroa, where a 50 m drilling will take place to extract high quality samples for geotechnical tests. The drillhole will be instrumented with pore pressure sensors to further constrain the possible pore water pressure trigger of the slide. The New Zealand side will contribute with a mobile weather station for a local reference and a three axes seismometer to constrain the cyclic loading by wind and waves on this headland site.

The focus on the second phase of the PhD project will be to prepare manuscript of the Omokoroa slide and get it published in a high ranked high impact journal.

Members

Proponents:Prof. Dr. Tobias MörzUniversity of Bremen
Dr. Stefan Kreiter
:Dr. Vicki MoonUniversity of Waikato
PhD Candidate:Mohammad Ehsan JoratUniversity of Bremen

Publications

Mohammad Ehsan Jorat, Stefan Kreiter, Tobias Mörz, Vicki Moon and Willem de Lange. Utilizing cone penetration test for landslide characterization at Pyes Pa, Bay of Plenty, New Zealand. 6th International conference of submarine mass movements and their consequences, 23rd – 25th September, Kiel, Germany (Under-review).

Mohammad Ehsan Jorat, Stefan Kreiter, Tobias Mörz, Vicki Moon and Willem de Lange, “Strength and Compressibility Characteristics of Peat Stabilized with Sand Columns”, Journal of Geomechanics and Engineering (Under-review).

Mohammad Ehsan Jorat, Prof. Aminaton Marto, Eshagh Namazi, Asso. Prof. Mohd For Mohd Amin (2011). “Engineering Characteristics of Kaolin Mixed with Various Percentages of Bottom Ash”, Electronic Journal of Geotechnical Engineering, Vol. 16 [2011], Bund. H, Page 841-850.

Eshagh Namazi, Dr. Hisham Mohammad, Mohammad Ehsan Jorat, Mohsen Hajihassani (2011). “Investigation on the Effect of Twin Tunnel Excavations Beneath a Road Under path”, Electronic Journal of Geotechnical Engineering, Vol. 16 [2011], Bund. A, Page 441-450.

Mohammad Ehsan Jorat, Prof. Dr. Aminaton Marto, Iman Farshchi (2010). “Strength Characteristics of Kaolin mixed with Bottom Ash”, International Conference on Geotechnical and Transportation Engineering, GEOTROPIKA 2010, 1-3 December, Kota Kinabalu, Sabah, Malaysia, Page 21-26 (Round Stabilization Chapter).

Mohammad Ehsan Jorat, Iman Farshchi, Prof. Dr. Aminaton Marto, Fauziah Kasim. (2010). “Finite Element Simulation Analysis of Steel Frame with Concrete Panel Retaining Wall”, International Conference on Geotechnical and Transportation Engineering, GEOTROPIKA 2010, 1-3 December, Kota Kinabalu, Sabah, Malaysia, Page 52-55 (Numerical Modeling Chapter).

Iman Farshchi, Dr. Nazri Bin Ali, Mohammad Ehsan Jorat. (2010) “Mechanical and Hydrolical Effects of a Tree on Slope Stabilization”, International Conference on Geotechnical and Transportation Engineering, GEOTROPIKA 2010, 1-3 December, Kota Kinabalu, Sabah, Malaysia. Page 43-46 (Numerical Modeling Section).

Iman Farshchi, Mohammad Ehsan Jorat, Prof. Dr. Aminaton Marto. (2010) “Steel Frame with Concrete Panel : An Innovation in the Construction of Retaining Wall”, International Conference on Engineering and Technology Education, World Engineering Congress 2010 (WEC 2010), 2–5 August 2010, Kuching, Sarawak, Malaysia, Pages 532-537.

Iman Farshchi, Dr. Nazri Bin Ali, Mohammad Ehsan Jorat. (2010) “Soil-Root Interaction and Effect on Slope Stabilization”, International Conference on Civil Engineering, MICCE 2010, 9-11 March 2010, Makassar, Indonesia, Pages 767-773.

Mohammad Ehsan Jorat, Iman Farshchi. (2010). “Geotechnical Properties of Kaolin Mixed With Bottom Ash”, International Conference on Civil Engineering, MICCE 2010, 9-11 March 2010, Makassar, Indonesia, Pages 1387-1397.

Miscellaneous

Research Stays at the University of Waikato, Hamilton:

17.02.2013 - current time and 01.02.2012 - 31.07.2012