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Shear strenght measurements of the sediments of the Archipelago Sea, a field studyJyrki Hämäläinen1, Joonas J. Virtasalo1 and Antti E.K. Ojala1

1Geological Survey of Finland, P.O. Box 96, FI-02151 Espoo, Finland E-mail: jyrki.hamalainen@gtk.fi

References:KORHONEN, K-H. 1963. Geotekniikka ja pohjarakennus. In: Maa-ja vesirakentajan käsikirja: Maa-ja vesirakennusinsinöörien yhdistyksen vuosikirja, 1963. 600 p.GARDEMEISTER, R. 1975. On engineering-geological properties of fine-grained sediments in Finland. Publication / Technical research centre of Finland. Building technology and community development, ISBN 9789513801854. 91 p.VIRTASALO J. 2001. Laivaliikenteen aiheuttamien ja luonnollisten virtausten vaikutus sedimentaatio-olosuhteisiin Pohjois-Airis-tolla. Masters thesis, Department of Geology, University of Turku. 65 p. + 9 p. append.

Background

In marine geological studies concerning underwater instal-lations, such as cables and pipe lines, the sediment’s geo-technical properties play a key role. Grain size, water con-tent and organic content are easily analyzed from sediment samples obtained from the seabed. However, measuring the shear strength of the sediment is more challenging, as in situ measurement is virtually impossible in deep water and collecting undisturbed sediments and transporting them to laboratory is also very difficult. Shear strength values are needed for example to assess the bearing capacity of the seabed and to choose a suitable method for cable trenching.

To get an overview of the shear strength values of the sedi-ments of the Archipelago Sea, bottom sediment samples were collected on R/V Geomari and the shear strength val-ues were measured onboard using a hand vane (pictures 1 A and B).

Shear strength measurements

Shear strength values of the sediment samples were measured using a Pilcon hand vane with a 33 mm vane. A 330 mm extension rod was used when measuring deeper than 20 cm in the sediment. Ac-cording to manufacturer’s instructions the vane should be pushed at least 20 cm into the sediment. However, the coverage of the sedi-ment with Box corer was mainly about 10 cm and the measure-ments were done in 5 cm depth (see figure 1B). Measurements in Gemax corer were done in 20 cm and 50 cm depths. With piston core samples, the measurements were done at 50 cm intervals, typ-ically at sediment depths of 50 cm, 100 cm, 150 cm and 200 cm, depending on the core length.

Following the shear strength measurements, each core section was opened by cutting them in two halves and a description of the sed-iment characteristics (structure and components) was made. This allowed us to compare sediment characteristics with the existing acoustic records and validate the hand vane measurements. Meas-ured shear strength values with sample information are given in table 1.

Sampling sites

Six sampling sites (WP1-WP6, see map 1) were selected on the basis of existing acoustic records . The aim was to cover various bottom types to get a comprehensive view on the shear strength values of different sediment layers. Sampling site locations with acoustic re-cords are presented in picture 2. Sediment samples were collected using a Box corer and a Gemax corer for surface sediment samples (0-60 cm) and a Kullenberg -type piston corer for 2.5 meter long sediment cores.

TurkuArchipelago

Sea

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AiristoAaslaluoto

Seili

Nauvo

Innamo

WP1 WP2

WP3WP4

WP5WP6

Bedrock Clay/silt Hard bottom

Mud

WP1 UTM34N X: 6677665 Y: 540095 Depth: 33m

WP2 UTM34N X: 6677765 Y: 540868 Depth: 31m

Bedrock

Mud

Clay/silt

WP3 UTM34N X: 6680593 Y: 551138 Depth: 57m

WP4 UTM34N X: 6681130 Y: 552530 Depth: 61m

Bedrock

Hard bottom

Clay/silt

Mud

WP6 UTM34N X: 6683281 Y: 555555 Depth: 37m WP5 UTM34N

X: 6683030 Y: 555544 Depth: 46m

Site Sample IDNorth. UTM34N

East. UTM34N

Water depth (m)

Core type Sediment type

Sediment depth (cm)

Shear strength (kPa)

WP1 MGBC-2013-36 6677673 540101 33 Box sandy silty mud 5 1.1WP1 MGML-2013-1 6677670 540103 33 Piston clayey silt 50 2.1WP1 MGML-2013-1 6677670 540103 33 Piston clayey silt 100 4.2WP1 MGML-2013-1 6677670 540103 33 Piston clayey silt 150 4.1WP2 MGBC-2013-37 6677765 540873 31 Box silty mud 5 1.3WP2 MGML-2013-2 6677780 540873 31 Piston silty mud 50 1.6WP2 MGML-2013-2 6677780 540873 31 Piston silty mud 100 3.1WP2 MGML-2013-2 6677780 540873 31 Piston silty mud 150 3.8WP2 MGML-2013-3 6677780 540873 31 Piston silty clay 185 9.5WP2 MGML-2013-3 6677780 540873 31 Piston silty clay 240 9WP3 MGGN-2013-26 6680593 551145 57 Gemax silty mud 20 0.1WP3 MGGN-2013-26 6680593 551145 57 Gemax silty mud 50 0.2WP3 MGML-2013-7 6680593 551145 57 Piston silty mud 50 0.1WP3 MGML-2013-7 6680593 551145 57 Piston silty mud 100 1.8WP3 MGML-2013-7 6680593 551145 57 Piston silty mud 150 1.8WP3 MGML-2013-7 6680593 551145 57 Piston silty mud 200 2.1WP4 MGBC-2013-40 6681131 552536 60 Box silty mud 4,5 0.2WP4 MGML-2013-6 6681127 552535 60 Piston silty mud 50 0.2WP4 MGML-2013-6 6681127 552535 60 Piston silty clay 100 2.9WP4 MGML-2013-6 6681127 552535 60 Piston silty clay 150 4.5WP4 MGML-2013-6 6681127 552535 60 Piston silty clay 200 7WP5 MGBC-2013-38 6683033 555554 46 Box clayey silt 5 1.8WP5 MGML-2013-4 6683033 555553 46 Piston clayey silt 50 2WP5 MGML-2013-4 6683033 555553 46 Piston clayey silt 100 4.2WP5 MGML-2013-4 6683033 555553 46 Piston clayey silt 150 4.1WP5 MGML-2013-9 6683038 555554 46 Piston clayey silt 50 1.3WP5 MGML-2013-9 6683038 555554 46 Piston clayey silt 100 2.8WP5 MGML-2013-9 6683038 555554 46 Piston clayey silt 150 3.8WP5 MGML-2013-9 6683038 555554 46 Piston clayey silt 200 4WP6 MGBC-2013-39 6683285 555563 37 Box sandy clayey silt N/A N/AWP6 MGML-2013-5 6683282 555560 37 Piston silty clay 50 7.2WP6 MGML-2013-5 6683282 555560 37 Piston silty clay 100 5.2WP6 MGML-2013-5 6683282 555560 37 Piston silty clay 150 5.2WP6 MGML-2013-5 6683282 555560 37 Piston silty clay 180 7.2

Discussion

Measured shear strength values of the sediments fall between 0.1 and 9.5 kPa. This is in line with the values given in Finnish literature. Korhonen (1963) gives values of 3-12 kPa, Gardemeister (1975) re-ports values of 2-5 kPa and Virtasalo (2001) has measured a value of 1.9 kPa for corresponding sediments.

Major factor controlling the shear strength values seems to be the consolidation state of the sediments. In most cases, consolidation state increases downwards in the sediment, generally giving higher shear strength values deeper in the section. It is also known that water and humus content of the sediment influence the geotechni-cal properties of the deposits (e.g. Gardemeister, 1975), and thus sediment type (mud, clay, silty clay, etc.) have an influence on the shear strength results gained.

Possible sources for errors in shear strength measurements are mainly 1) disturbance of the sediment during sampling and 2) relatively narrow diameter of the piston core liner, which leads to sediment starting to revolve in the core liner rather than “break” during turning of the vane. Both of these phenomena are likely to lower the shear strength values compared to in situ measurements

Map 1. Location of sampling sites . Picture 1 A: a Pilcon hand vane, B: hand vane used for shear strength measurement.

AB

Picture 2. Sampling sites with acoustic records.

Table 1. Sample information and shear strength values.