Mass property variability of some estuarine sediments

Sedimentary Geology, 10 (1973): 205--213 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

MASS PROPERTY

VARIABILITY

OF SOME ESTUARINE SEDIMENTS

RICHARD W. FAAS Department of Geology, Lafayette College, Easton, Pa. (U.S.A.) (Accepted for publication August 22, 1973) ABSTRACT Faas, R.W., 1973. Mass property variability of some estuarine sediments. Sediment. Geol., 10: 205--213. Bottom sediments from two areas of the York River Estuary of southeastern Virginia were systematically sampled to depths of 40 cm and analyzed for their mass properties, i.e., water content, unit weight, and shearing strength. The samples fell into: (1) a coarsegrained group of clayey sands, silty sands, and sandlilt---clays; and (2) a fine-grained group of silty clays and clayey silts. Analysis of variance between both groups revealed distinct differences in the mass properties. Means, variances, and coefficients of variation for each parameter were determined and made possible a statistical characterization of each sediment type. The data presented may be used to predict expectable values of mass properties for similar sediments, having experienced similar depositional histories, e.g., from neighboring estuaries and Chesapeake Bay. INTRODUCTION A n u m b e r o f r e c e n t studies h a v e i n d i c a t e d t h a t s e d i m e n t a r y p a r a m e t e r s o f m a r i n e s e d i m e n t s e x h i b i t w i d e ranges o f v a r i a t i o n ( R i c h a r d s a n d Keller, 1 9 6 2 ; Keller a n d B e n n e t t , 1 9 6 8 ; Keller, 1 9 6 9 ) . M a n y have b e e n c o n c e r n e d w i t h vertical v a r i a t i o n i n d u c e d b y t i m e a n d s e t t l e m e n t , a n d o n l y r e c e n t l y w o r k e r s have e x a m i n e d areal v a r i a b i l i t y ( B e n n e t t et al., 1 9 7 0 ) . A s t u d y o f t h e m a s s p r o p e r t i e s o f s o m e s e d i m e n t cores f r o m l o w e r C h e s a p e a k e B a y was p e r f o r m e d b y H a r r i s o n et al. ( 1 9 6 4 ) . T h e a u t h o r has studied t w o areas o f fine-grained clastic s e d i m e n t s in t h e r e s t r i c t e d e n v i r o n m e n t o f a small coastal-plain e s t u a r y w h i c h enters Chesap e a k e B a y . . I t was possible t o s a m p l e t h e s e d e p o s i t s in a regular a n d consist e n t f a s h i o n , t h u s a v o i d i n g small s a m p l e bias. A n s w e r s w e r e s o u g h t t o t h e following questions: (1) H o w m u c h v a r i a t i o n in s e d i m e n t a r y p r o p e r t i e s exists w i t h i n a specified area o f an a p p a r e n t l y h o m o g e n e o u s s e d i m e n t a r y d e p o s i t ? (2) H o w m u c h v a r i a t i o n in certain p r o p e r t i e s m a y b e e x p e c t e d t o o c c u r w i t h i n a specified s e d i m e n t class, i.e., c l a y e y silt or silty s a n d ? (3) Is it possible to establish m e a n values a n d p r e d i c t a b l e v a r i a t i o n a b o u t t h o s e m e a n s f o r all m e a s u r a b l e s e d i m e n t a r y p a r a m e t e r s ?

206

LOCATION The research described below was performed in the York River Estuary of southeastern Virginia, which forms a part of the larger estuarine system of Chesapeake Bay {Fig. 1). The York River extends westward from its m o u t h in Chesapeake Bay and forms a relatively straight channel for 28 miles to West Point, Virginia, where it bifurcates into two highly meandering streams, the Mattaponi and the P a m u n k e y rivers. The York is estuarine to West Point where the salinity drops to a few parts per thousand, several miles above the meeting of the Mattaponi and the Pamunkey. Tidal effects extend for approximately 40 miles up the freshwater portions of the system. Two areas of the York River b o t t o m were selected for analysis. Both were judged to be texturally homogeneous through visual observation using SCUBA techniques. One site, Station 10, was located on the south side of the York in 10 m of water. The second site, Station 11, was located 365 m shoreward in 4.5 m of water. At each site, a sampling grid, 15 m square was established on the river b o t t o m . Sixteen cores, 6.35 cm in diameter and averaging 40 cm in length, were extracted at 5 m intervals from the grid by gently pushing the core barrel into the sediment, digging down alongside and capping the b o t t o m before withdrawal. Each was carried to a waiting boat, stored vertically, and analyzed the same day to minimize possible bacterial effects on the properties being measured. In addition to the sediment cores,

YORK RIVE

R

LOWER k CHESAPEAKE BAY Sl

I0

@

Fig. 1. Location map of lower Chesapeake Bay, showing sampling Stations 10 and 11.

207

• 0

Station 10 Station 11

Clay

Sandy Clay

Clayey

~

Silty Clay

0

0

Clayey Silt

0

Sand

0 o 0 O0 0

0 0

Sand

~

Silty Sand

I

Sandy Silt

/

Silt

Fig. 2. Ternary classification of York River sediments.

5 separate in situ measurements of sediment shearing strength were made at each core locality using a hand-held shear vane apparatus (Faas, 1972). A total of 80 in situ measurements were taken in each grid. In the laboratory, the sediments were analyzed for water content, unit weight, and the Atterberg limits used in the engineering classification of soils (i.e., liquid and plastic limits). Shearing strength measurements, including both undisturbed and remolded tests, were made in the upper and lower 20 cm of each core while still in the core barrel. Particle-size analyses were performed for each sample using the Buoyucos h y d r o m e t e r method. Sediments were grouped into separate textural classes according to Shepard {1954). Fig.2 shows the distribution of all samples on a ternary diagram. ANALYSIS The data were analyzed statistically through "analysis of variance". This technique seeks to isolate and determine the magnitude of variability due to natural and induced treatment effects. It allows tests of significance to be made between different areas treated in the same fashion, i.e., to make comparisons among means and variances of the same properties by the use of the " F " or variance-ratio test. This test assumes no difference in experimen-

208 tal error between " p l o t s " and "samples" of a homogeneous population, all receiving the same treatment. Should significance exist, i.e., should the " p l o t " variance be very much greater than the "sample" variance, then there is a distinct probability that significant differences in properties exist among the plots tested and the population is n o t homogeneous. Sampling proceeded according to the "randomized block" design (Snedecor, 1956). Cores were taken at 5 m intervals at each grid intersection within the total area. Shearing-strength measurements were taken randomly by the diver on the perimeter of a 1.5 m diameter circle which centered on the core locality. The randomized block design affords better control of sampling and better precision in estimating environmental effects. Error induced due to treatment effects is diminished or eliminated. This design is particularly useful where testing proceeds under adverse conditions, e.g., poor visibility, cold temperatures, and psychological stresses. SHEARING STRENGTH Shearing strength is a function of the cohesion and internal friction of the sediment and the effective stress normal to the shear plane, expressed as: rc = c + ~

tan ¢

where c = the cohesion; ~ = the effective stress; and ¢ = the angle of internal friction. Fine-grained, saturated sediments stressed w i t h o u t loss of pore water behave as if t h e y were cohesive materials w i t h o u t any internal friction (4 = 0). Shearing strength is then equal to cohesion ( r c = c ) . An excellent review paper on this subject has been published by Kenney (1968), and soilmechanics texts discuss this more fully (Hough, 1957; Means and Parcher, 1963; Lambe and Whitman, 1969). Table I(a) shows the analysis of variance p e r f o r m e d for 4 plots, 4 sample sites/plot, and 5 determinations/sample site. Testing for differences between determinations from adjacent sample sites was not significant, the F-test showing that in 99 out of 100 times one could predict t h e same mean shearing strength to exist along any linear segment of the grid. On the other hand, the test for differences in mean values of shearing strength between adjacent plots was decidedly significant. The variance ratio of 10.48 might be exceeded by chance only 1 out of each 100 times in similar populations. It appears that r e a l differences do exist in mean shearing strength values between adjacent 15 m strips of York River Station 10 sediments. UNIT WEIGHT Table I(b) shows the analysis of variance for unit weight. In this case, only two samples were measured from each core, hence the reduction in degrees of freedom.

139430 539623 1569049

F~

F~ 6 = 2 . 3 5 / 1 . 3 4 = 1.75

3 12 48

F~4 = 2 8 . 3 / 2 . 7 = 1 0 . 4 8 " *

2.35 2.32 1.34 F~=44970/36688=1.22

7.05 27.80 21.52

ss

F~8 = 4 6 4 7 7 / 3 6 6 8 8 = 1 . 2 6

3 12 16

df

F]~ =2.32/1.34=1.72

28.3 4.1 2.7

85 59 174

ms

(c) W a t e r c o n t e n t

=4.1/2.7=1.52

3 12 64

ss

df

ms

df

ss

(b) U n i t w e i g h t

(a) S h e a r i n g s t r e n g t h

S y m b o l s : d f = degrees o f f r e e d o m ; ss = s u m of squares; m s = m e a n square; a n d ** = 0.1 significance

Plots Samples Determinations

Source o f variance

TABLE I Analysis of variance

46477 44970 36688

ms

~O

L'O

210

F-testing proved non-significant, indicating the magnitude of variability of unit weight was no greater t h r o u g h o u t an entire plot than between duplicate analyses made on the same core. WATER CONTENT

T h e third parameter measured was water content, defined as the ratio, in percent, of the weight of water in a given sediment mass to the weight of the oven-dry particles (105°C for 24 h). No correction for salt c o n t e n t was made. Table I(c) gives the analysis of variance. Four determinations were made on each core. No significant differences are seen between the mean square terms for plots, cores, and determinations. F-testing indicated t hat variation between the plots was no greater than that between determinations on the same core. This indicates textural h o m o g e n e i t y and strengthens the reliability of prediction, using means and variances. VARIABILITY

In order mo r e c o m p l e t e l y to characterize the Station 10 sediments, mean values of each parameter were calculated for each plot with a grand mean d e ter min ed for the 378 m 2 area. Coefficients of variation, defined by Snedecor (1956) as the standard deviation divided by the mean, were calculated for each plot and averaged t h r o u g h o u t the entire grid. Table II shows these data. S o me variation is present within the whole grid, but with the exception of shearing strength, no significant differences between any of the measured parameters exist at Station 10. The greatest coefficient of variation is f o u n d with shearing strength. Based on the above, we can say t hat the upper 40 cm o f the York River Station 1 0 silty clays have a mean shearing strength of 40.8 g/cm 2, a mean unit weight of 1.33 g/cm 3, and a mean water c o n t e n t of T A B L E II Statistical c h a r a c t e r i z a t i o n o f S t a t i o n 10 s e d i m e n t s Plot

1 2 3 4 Total grid

Shearing s t r e n g t h (g/era 2)

34.5 42.9 44.3 41.5 40.8

Unit w e i g h t (g/cm 3 )

Water c o n t e n t (%)

S

s/~

~

s

S/x

x

S

S/~

5.6 7.0 6.3 7.7 7.7

16.3 16.3 14.3 18.6 18.9

1.34 1.35 1.31 1.31 1.33

0.040 0.041 0.048 0.033 0.043

3.0 3.0 3.6 2.5 3.2

195 172 190 189 186

10.7 22.9 14.4 15.9 18.4

5.5 13.3 7.6 8.4 9.7

S y m b o l s : x = m e a n ; S = s t a n d a r d deviation;

S/x

=

c o e f f i c i e n t o f variation.

211

186% (based upon dry weight). Any point within that 40 cm thick sediment unit may be expected to vary 18.9% about the mean shekring strength, 3.2% about the mean unit weight, and 9.7% about the mean water content. STATION 11 Fig.2 shows the variation in size classes that was f o u n d from the 378 m e area of Station 11. F r o m a total of 27 samples analyzed, 8 were classed as "silty sand", 4 as " c l a y e y sand", 1 as " s a n d " , 1 as " s a n d y silt", and 13 fell into the intermediate classification of "sand--silt--clay". Vertical variation also existed and Fig.3 indicates the change in mass properties with depth found in core 11--4D. In general, in Station 11 the finer-grained sediments were found deeper than 15 cm, giving rise to a two-layer situation. Inasmuch as Station 11 samples represent two obviously different populations, i.e., a fine-grained sediment, and a coarse-grained sediment, analysis of variance could not be performed. However, means, standard deviations, and coefficients of variation of shearing strength, unit weight, and water content for three of the sediment types are presented in Table III.

Cohesion

Unit Wt.

(g/cm 2 )

(g/cm 3 )

,o

~oo

,40

|

i

I

L'.",

,.e

Water C o n t e n t (% }

,~li

1

60

80

i

i

--0

cm

-10 20-

._

i

\

Porosity (7%0) Void Ratio 50

60

i

i

[~.0

i

I

2.0 I

I

-20

Sensitivity

3.0 | 1.0 i

2.0 i

3.0 j i

cm

-30 20-

30-

Fig.3. Mass p r o p e r t i e s , core 11--4D.

J

-40

212 TABLE III Statistical characterization of Station 11 sediments Type

Silty sand Sand--silt--clay Clayey sand

Shearing strength (g/cm ~ )

Unit weight (g/cm 3 )

x

S

x

S

S/x

x

S

S/x

63.3 91.4 86.5

17.6 27.8 19.7 21.5 29.5 34.2

1.72 1.60 1.77

0.075 0.023 0.017

4.3 1.4 7.3

56 77 57

4.7 10.2 2.6

8.4 13.2 4.6

S/x

Symbols: x = mean; S -- standard deviation; S / x

Water content (%)

= coefficient

of variation.

C o m p a r i n g T a b l e III w i t h T a b l e II, it is seen t h a t d i f f e r e n c e s in the average value o f t h e mass p r o p e r t i e s exist b e t w e e n s e d i m e n t t y p e s , w i t h greater variation in shearing s t r e n g t h associated with t h e coarser sediments. Significant changes o c c u r in m e a n w a t e r c o n t e n t b e t w e e n t h e S t a t i o n 1 0 and S t a t i o n 1 1 sediments. This is d u e t o t h e r e d u c t i o n o f available p o r e space u p o n the a d d i t i o n o f t h e coarser sand-sized grains. Rogers and Head ( 1 9 6 1 ) indicate p o r o s i t y decreases w i t h increasing m e d i a n d i a m e t e r s in p o o r l y s o r t e d s e d i m e n t s similar t o t h o s e o f S t a t i o n 1 1 . H o w e v e r , b o t h s t a n d a r d deviations and c o e f f i c i e n t s o f variation r e m a i n c o m p a r a b l e b e t w e e n t h e t w o groups. CONCLUSIONS This w o r k has p r e s e n t e d m e a n values a n d variances o f shearing strength, u n i t weight and w a t e r c o n t e n t f o r t w o 378 m 2 p a t c h e s o f t h e Y o r k River b o t t o m . A f t e r analysis, it was seen t h a t several s e d i m e n t classes were repres e n t e d in each o f t h e h o m o g e n e o u s - a p p e a r i n g b o t t o m s . O f t h e p a r a m e t e r s a n a l y z e d , shearing s t r e n g t h s h o w e d t h e greatest variability, ranging f r o m 18.9% in t h e fine-grained s e d i m e n t s t o as m u c h as 34.2% in coarser-grained sediments. Values f o r w a t e r c o n t e n t had a c o e f f i c i e n t o f variation o f 9.7% for the fine-grained s e d i m e n t s w h i c h is m a t c h e d b y an average o f 8.8% in t h e coarser-grained sediments. U n i t w e i g h t has an average c o e f f i c i e n t o f variation o f 3.2% in t h e fine-grained s e d i m e n t s , a n d varies b e t w e e n 1.4% and 7.3% in t h e coarser-grained s e d i m e n t s . In general, a l t h o u g h p a r a m e t e r m e a n s d i f f e r e d in t h e s e d i m e n t s a n a l y z e d , the variability o f each p r o p e r t y w i t h i n t h e s e d i m e n t was a b o u t t h e same. N o single s e d i m e n t t y p e b e h a v e d in an a b e r r a n t or erratic fashion. T h e d a t a have p r o v i d e d answers t o t h e t h r e e original q u e s t i o n s p o s e d , i.e., h o w m u c h variation in s e d i m e n t p r o p e r t i e s exists in an a p p a r e n t l y h o m o g e n e o u s s e d i m e n t and in a specific s e d i m e n t t y p e . It also s h o w e d t h a t it is possible t o c h a r a c t e r i z e a s e d i m e n t in t e r m s o f t h e m e a n s and variances o f measurable sedimentary parameters. T h e d a t a p r e s e n t e d h e r e m a y serve t o establish average values and ranges

213 o f variation f o r the s e d i m e n t mass p r o p e r t i e s o f the Y o r k River. It is o b v i o u s t h a t variations in clay mineral c o n t e n t , g e o c h e m i s t r y o f pore waters, a n d d e p o s i t i o n a l rates m a y n o t allow these d a t a t o be generally applicable. H o w ever, it is e x p e c t e d t h a t similar values m a y be o b t a i n e d f r o m t h e same s e d i m e n t t y p e s o f n e i g h b o r i n g coastal-plain rivers a n d a d j a c e n t C h e s a p e a k e Bay. ACKNOWLEDGEMENTS T h e research f o r this p a p e r was p e r f o r m e d d u r i n g t h e s u m m e r s o f 1 9 6 8 a n d 1 9 6 9 while t h e a u t h o r was a p a r t i c i p a n t in an N S F - R P C T p r o j e c t at t h e Virginia I n s t i t u t e o f Marine Science, G l o u c e s t e r P o i n t , Virginia. T h a n k s are expressed t o b o t h o r g a n i z a t i o n s for providing f u n d s a n d facilities f o r t h e research. T h e a u t h o r p a r t i c u l a r l y wishes t o t h a n k M a y n a r d Nichols f o r his help and advice d u r i n g t h e p r o j e c t and for his critical review o f the m a n u script. G e o r g e Keller and R o b e r t Meade also reviewed this material a n d m a d e critical c o m m e n t s . T h e i r suggestions are greatly a p p r e c i a t e d . REFERENCES Bennet, R.H., Keller, G.H. and Busby, R.F., 1970. Mass property variability in three closely spaced deep-sea sediment cores. J. Sediment. Petrol., 40 (3): 1038--1043. Faas, R.W., 1972. Engineering properties of some York River sediments: In: B.W. Nelson {Editor), Environmental Framework o f Coastal Plain Estuaries; A Symposium o f Estuaries. Geol. Soc. Am. Mere., 133: 337--348. Harrison, W., Lynch, M.P. and Altschaeffl, A.G., 1964. Sediments of lower Chesapeake Bay, with emphasis on mass properties. J. Sediment. Petrol, 34 (4): 727--755. Hough, B.K., 1957. Basic Soils Engineering. The Ronald Press, New York, N.Y., 513 pp. Keller, G.H., 1969. Engineering properties of some sea floor deposits. J. Soil Mech. Found. Div., Nov. 1969, SM6: 1379--1392. Keller, G.H. and Bennett, R.H., 1968. Mass physical properties of submarine sediments in the Atlantic and Pacific basins. Proc. Int. Geol. Congr., 23rd, Prague 1968, Sect. 8: 33--50. Kenney, T.C., 1968. A review of recent research on strength and consolidation of soft sensitive clays. Can. Geotech. J., 5 (2): 97--119. Lambe, T.W. and Whitman, R.V., 1969. Soil Mechanics. John Wiley, New York, N.Y., 553 pp. Means, R.E. and Parcher, J.V., 1963. Physical Properties o f Soils. Charles E. Merrill Books, Columbus, Ohio, 464 pp. Richards, A.F. and Keller, G.H., 1962. Water conte~t variability in a silty clay core from off Nova Scotia. Limnol. Oceanogr., 7 (3): 426--427. Rogers, J.W. and Head, W.B., 1961. Relationships between porosity, median size, and sorting coefficients of synthetic sands. J. Sediment. Petrol., 31 (3): 467--470. Shepard, F.P., 1954. Nomenclature based on sand~ilt--clay ratios. J. Sediment. Petrol., 24 (3): 151--158. Snedecor, G.W., 1956. Statistical Methods. The Iowa State College Press, Ames, Iowa, 5th ed., 534 pp.