GEOTECHNICAL PROPERTIES OF SHELF SEDIMENTS FROM AEGEAN SEA

V. LYKOUSIS and K. PECHLIVANOGLOU Depdrtment o f Oceanography, Hydrographic s e r v i c e o f t h e H e l l e n i c Navy, Cholargos, Athens Abstract The g e o t e c h n i c a l p r o p e r t i e s o f 14 s e l e c t e d sediment s h o r t p i s t o n cores, a r e i n v e s t i g a t e d , f r o m c h a r a c t e r i s t i c sedimentary environments f r o m s h e l f s e d i m e n t s o f Aegean Sea. Sediment t e x t u r e , m i n e r a l o g y , c a l cium carbonate c o n t e n t , o r g a n i c m a t t e r and t h e near bottom enveronment o f d e p o s i t i o n (bottom c u r r e n t s , n e a r bottom suspended sediment, s e d i r e n t a t i o n r a t e s ) seem t o have a s i g n i f i c a n t e f f e c t on g e o t e c h n i c a l p r o p e r t i e s o f t h e s u r f a c e s h e l f sediments f r o m Aegean sea.

Les p r o p r i e t e s g6otechniques de 14 p e t i t e s c a r o t t e s , s e l e c t i o n n 6 e s p a r des environnements s e d i m e n t a i r e s c a r a c t e r i s t i q u e s dy p l a t e a u c o n t i n e n t a l de l a Mer Egee s o n t examinees. C a r a c t e r i s t i q u e s t e l s l a t e x t u r e s e ~ i m e n t a i r e , l e s e l ements m i n e r a l o g i q u e s , l a t e n e u r en CaCO, ( c a l c i u m c a r b o n a t i f ) e t en m a t e r i e l organique e t l e s c o n d i t i o n s de 1 ' e n v i r o n e me , t de l a d e p o s i t i o n ( c o u r a n t du fond, sediments p r k s du f o n d de l a nier e t v i t e s s e du s e d i m e n t a t i o n ) ,semblent a v o j r une remarquable s i g n i f i c a t i o n p o u r l e s p r o p r i e t e s g6otechniques des sediments du p l a t e a u cont i n e n t a l de l a Mer Egee. ....................... F o u r t e e n s h o r t p i s t o n cov.es were c o l l e c t e d by t h e r e s e a r c h v e s s e l s o f Hydrographic s e r v i c e , f r o m f i v e r e p r e s e n t a t i v e p r o v i n c e s . i n t h e Aegean sea s h e l f . Three c o r e s were c o l l ectecl from Alexandroupol is b a y 1ocated i n t h e n o r t h e a s t e r n p a r t o f t h e Aegean sea. R i v e r Evros, t h e b i g e s t r i v e r i n t h e Aegean area, d i s c h a r g e s i n t o t h e bay. R e l a t i v e l y h i g h v a l u e s o f mean b u l k d e n s i t y (1.65-1.8991 /cm3) ,and low v a l u e s o f w a t e r c o n t e n t (25-62%) and p o r o s i t y (44-62%) observed i n t h e s u r f a c e s e d i n e n t s . Average vane shear s t r e n g t h were ranged f r o m 41g/cm2 t o 62g/cm2. The h i g h e r b u l k d e n s i t i e s and t h e l o w e r w a t e r cont e n t s and shear s t r e n g t h s appear t o be r e l a t e d w i t h t h e l o w c a l c a r e o u s sandy s i l t y and sandy muddy sediments, t h e m i n e r a l s p r e s e n t i n t h e c o a r s e r f r a c t i o n (micas, q u a r t z , f e l d s p a r ) , t h e s t r o n g n e a r bottom c u r r e n t s (-4Ocm/sec) and p r o b a b l y t h e r e l a t i v e l y l o w o r g a n i c m a t t e r . The l o w e r mean b u l k d e n s i t i e s and t h e h i g h e r w a t e r c o n t e n t s , p o r o s i t i e s and shear s t r e n g t h s a r e found t o be a s s o s i a t e d w i t h t h e muddy sediments, t h e weak e r bottom c u r r e n t s (-25cm/sec), t h e l . ) ~CaCO, c o n t e n t and l o w o r g a n i c m a t t e r . According t o t h e i r p l a s t i c i t y c h a r a c t e r i s t i c s t h e s e sediments are c l a s s i f i e d t o t h e "inorganic clays o f high p l a s t i s i t y " . Four c o r e s were c o l l e c t e d frorc K a l ~ a l abay a s h a l l o w m a r i n e enbayment. i n t h e n o r t h e r n Aegean sea which f l a n k e d by t h e Ncstos r i v e r d e l t a p l a i n t o t h e e a s t e r n s i d e . Mean b u l k d e n s i t y o f t h e s u r f a c e sediments v a r i e s f r o m 1.48g/cm3 t o 1.57g/cm3, w a t e r c o n t e n t s f r o m 61% t o 79%, por o c i t i e s . f r o m 6 4 m 69%and shear s t r e n g t h s f o r m 34g/cm2 t o 83g/cn;? T h e h i g h e r b u l k d e n s i t i e s , s h e a r s t r e n g t h s and t h e l o w e r w a t e r con t e n t s & porosit.i:es ap,!ear t o be a s s o s i a t e d w i t h t h e r e l a t i v e l y c o a r s e r muddy 'and s i l t y sed i ~ ~ r . n ttowards s the d e l t a p l a i n , t h e mineralogy o f t h e coarser (quartz,

Rapp. Comm. int. Mer Mgdit., 29, 2 (1985).

f e l ~ l s p a r , micas) and t h e f . i n e r (>60"/,1lite) fraction, the r e l a t i v e l y w t a t near bottom c u r r e n t s ( 1 0-1 5cm/sec), t h e l o w CaCO, c o n t e n t ( < I 0%), and t h e low o r g a n i c m a t t e r . Based on t h e i r p l a s t i c i t y c h a r a c t e r i s t i c s a r e c l a s s i f i e d t o " t h e i n o r g a n i c s i l t s o f h i g h c o m p r e s s i b i l i t y " . The l o w e r b u l k d e n s i t i e s , shear s t r e n g t h s and t h e h i g h e r w a t e r c o n t e n t s , p o r o s i t i e s were found w i t h i n t h e f i n e r muddy sediments where weak near bottom c u r r e n t s (5-1 Ocm/sec) p r e v a i 1. These sediments a r e c h a r a c t e r i sed as " i n o r g a n i c c l a y s a f h i g h p l a s t i c i t y " . The g u l f o f P e t a l i o n i s shaped bettween A t t i k i and S. Evia i s l a n d and i s covered m a i n l y f r o m calcareous sand and s i l t ( c a l c a r e o u s algae, c c r a l s ) . The two cores c o l 1e c t e d from th.i s sedimentary environment,' d i s p l a y h i g h mean b u l k d e n s i t i e s (l;82g/cm3 -1 .93g/cni3 ) b u t v e r y low w a t e r contents (27-35%) and p o r o s i t i e s ( 4 3 4 0 % ) . These v a l u e s a r e r e l a t e d t o t h e sediment t e x t u r e 'and t h e l o w s e d i m e n t a t i o n r a t e s ( 1 i m i t t e d . sediment input) i n the g u l f o f Petalion. A s i n g l e c o r e c o l l e c t e d from south Evoikos g u l f , a semi-enclosed s h e l f - s e a , l o c a t e d n o r t h o f t h e g u l f o f P e t a l i o n . The mean values o f b u l K dendi t y (1.53g/cm), w a t e r c o n t e n t (80%), p o r o s i t y (682) and shear s t r e n g t h (25g/cm2) r e f l e c t s t h e v e r y f i n e bottom sediments (>60% c l a y ) , t h e h i g h amounts o f near bottom suspended sediment, t h e weak bottom c u r r e n t s (3-8cm/sec) an t h e low CaCO, c o n t e n t (<15%). The sediments i n S. Evoikos a r e c h a r a c t e r i z e d as " i n o r g a n i c c l a y s o f h i g h p l a s t i c i t y " . The remained f o u r cores were c o l l e c t e d f r o m k e r a t s i n i - P s i t a l i a , a sludge area, i n t h e inernlost p a r t o f Saronikos g u l f . The c e n t r a l sewage canal o f Athens c i t y o u t f l o w s i n t h e sampling area. The low mean bu l l: densi t i e ; (1,. 25-1 .44glcni3 ) , t h e t ~gh i w a t e r c o n t e n t s (96-1 36%) and pc!:,!)si t i e s (78-82%) nad t h e v a r i a b i 1 it y i n shear s t r e n g t h s (36-63g/cm2 ) i z (1.ttl-.ibated t o t h e v e r y h i g h o r g a n i c m a t t e r , t h e low CaCO, c o n t e n t ( ~ 5 4 )and t h e r e l a t i v e l y weak near bottom p r e v a i 1 i n g c u r r e n t s (5-1 5cm/ /5w:).

Sed.ilneritologica1 pr.ocess:?s r e l a t e d t o t h e sediment t e x t u r e , minera-

l c g y CaCO, c o n t e n t o r g a n i c m a t t e r , n e a r bottom environment o f d e p o s i t i o n (iol.ron~ c u r r e t i t s , near-bottom suspended sediment, appears t o be respons i b1e f o r much o f t h e v a r i a t i o n o f g e o t e c h n i c a l p r o p e r t i e s observed i n tt,$: s h e l f s e d i ~ i l e n t sf r o m Aegean sea.

....................... REFERENCES Lykousis, V., 1984. Modern s e d i m e n t a t i o n i n t h e g u l f o f Kavala. 1 s t He1 l e n i c Symposium o f Oceanography and F i s h e r i e s I.O.K.A.E. Lykousis, V. and Kouroumpali, F., 1984. Bottom sediments i n 5. Evoikos G u l f and G u l f o f . P e t a l i o n . 1 s t H e l l e n i c Symposium o f Oceanography and F i s h e r i e s I.O.K.A.E. Pechl i v a n o g l ou, K. , 1984. Modern s e d i r s n t a t i o n i n A1 exandroupol i s bay. 1 s t H e l l e n i c Symposium o f Oceanography and F i s h e r i e s I.O.K. A.E. Papageorgiou, E. and Balopoulos , E., 1983. C u r r e n t o b s e r v a t i o n s between K e r a t s i n i and P s i t a l i a I (Saronikos G u l f ) . Tech. Rep., I.O.K. A.E. Terzaghi, K. d n u Peck, R.B., 1948. S o i l Mechanics i n E n g i n e e r i n g P r b a c l i ce. Wiley, New York, N.Y., 566pp.