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Interrelationships between porosity and other geotechnical properties of slowly deposited, fine-grained marine surface sediments
Marine Geology, 92 (1990) 105-113 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
105
Interrelationships Between Porosity and other Geotechnical Properties of Slowly Deposited, FineGrained Marine Surface Sediments ANDREAS WETZEL Geologisch-Pal~iontologisches
Institut der Universitdt, Bernoullistrasse 32, CH 4056 Basel (Switzerland) (Received by publisher November 1, 1989)
Abstract Wetzel, A., 1990. Interrelationships between porosity and other geotechnical properties of slowly deposited, finegrained marine surface sediments. Mar. Geol., 92: 105-113. The pore volume, expressed as porosity or void ratio, of fine-grained marine deposits at the sediment surface (0-2 cm) is closely related to other geotechnical properties such as liquid limit, plastic limit, compression index, and specific surface area. Equations have been formulated for the interdependencies between (1) average porosity at the sediment surface (n.) and liquid limit (LL) In. = 100 (0.0378LL + 0.43)/(0.0378LL + 1.43)], (2) liquid limit (LL) and specific surface area (Sg) [LL = 1.01 88 + 46.5 if LL > 50], and (3) compression index (C¢) and void ratio at a stress of 0.01 MPa (eo) [e'o= 3.352 exp Col. From the relationship between geotechnical properties and initial porosity, the state of consolidation of surface sediments and the influence of bioturbation often lead to an increase in porosity (5-10%) and compressibility (up to 10%), and a decrease in the overconsolidation ratio (from 2-3 to about 1) at the sediment surface. The interrelationship between initial porosity and physical properties can be explained by the dependence of the geotechnical properties on the electrostatic forces between the particles and the adsorbed water around them.
Introduction
The depositional interface has been a subject of investigation for a long time, because it is there that sediment, water and organisms interact (e.g., McCave, 1984; Nowell and Hollister, 1985). The sediments are subjected to permanent changes under the influence of organisms and water movement. This leads to the question of whether under such circumstances defined relationships can exist between sediment composition and geotechnical properties such as porosity, compressibility and sonic velocity. Many investigators have suggested that the 0025-3227/90/$03.50
porosity of fine-grained surface sediments correlates with other geotechnical parameters, a correlation which may reflect the composition and depositional environment of the sediments (e.g., Skempton, 1944; Parasnis, 1960; Meade, 1964; Bryant et al., 1981). However, most of these relationships are restricted in their validity because they are often subject to a large degree of error or were deduced from only a small number of observations. Generally applicable equations have therefore not yet been formulated. With the use of a large data base it is the purpose of this paper to quantify some of the relationships between the geotechnical properties of surface sediments.
© 1990 Elsevier Science Publishers B.V.
106
A.WETZEL
Materials and m e t h o d s
Sediment composition Fine-grained, slowly deposited marine sediments of various compositions were investigated (Table 1). The selection of samples with respect to these criteria has some implications regarding their physical properties: (i) "Fine-grained" means that the samples contain at least 35% clay minerals, so that theoretically all coarser grains can be surrounded by argillaceous particles and the cohesive components therefore dominantly affect the behavior of the deposits (Mitchell, 1976). (ii) Only marine sediments were considered, so effects resulting from varying ionic composition of the pore water and its influence on the exchangeable cations can be excluded. (iii) Slow deposition of the samples excludes the effects of sedimentation rate on the initial porosity, although this parameter has not yet been quantified.
Sampling interval The sediment surface is not always sharply developed, and the transition from water through an increasingly concentrated suspension to cohesive sediment may be gradual (e.g., McCave, 1984; Nowell and Hollister, 1985). Normally, only the cohesive sediment is usually at rest, as indicated by its colonization by TABLE 1 Composition of the investigated sediments (in percent) Clay Sand Silt Diatoms Radiolaria Foraminifera Calcareous nannofossils Corg
4 100 0 40 ~~ <602 } 0-60 0 10 : 0 10 < 403 0 20 0-40 0-5
> 401 < 602
~Minimum value for clay-sized material; 2Maximum value for coarse-grained components; ~Maximum value for all biogenic components.
more-or-less immobile organisms (e.g., foraminifera, bacteria, etc. Thiel, 1983). Although the boundary between sediment and water is known in theory (when the biologic definition is accepted), the sediment is sometimes too soft for undisturbed samples to be taken from the surface sediment layer. In order to obtain reproducible geotechnical data the interval between the cohesive surface and the 2 cm depth was sampled as one unit. It is probable that sampling this interval would provide representative samples, because bioturbation homogenizes a layer at least as thick as this (e.g., Seibold and Berger, 1982).
Strategy The geotechnical parameters were determined on the same sample eleven times. Obviously, this number is too low to evaluate whether important relationships exist. Consequently, additional data were taken from the literature to enlarge the data base. Data from the literature were utilized on sediments that are (i) of similar composition (Table 1), and (ii) sampled from the same interval (0-2 cm). The locations of these samples are given in the figure captions.
Methods To quantify the interdependencies between various geotechnical properties of finegrained, clayey sediments at the depositional interface, parameters are required that reflect (i) the sediment composition, which determines the electrostatic properties of the particles, and (ii) the ionic composition of the water, which influences the thickness of the adsorbed water and the exchangeable cations (when stress at the sediment surface is ignored). With a given sea water composition, and hence, constant chemical conditions, sediment composition is closely related to porosity, liquid limit and compressibility (expressed as the compression index). Additionally, the specific surface
GEOTECHNICALPROPERTIESOF FINE-GRAINEDSEDIMENTS
107
area w a s also determined. These parameters were used in this study. Porosity The porosity (n) was calculated from the grain density and water content (corrected for salt content) of a sample with a known volume, with the formula: n = (w'ps/w'ps + 1) 100
(%)
(1)
where w is water content and ps is grain density. In some geotechnical studies the void ratio is used in place of porosity. Porosity (n) and void ratio (e) are related to each other by the equations: e
=
(n/lO0)/(1
-
n/100)
n = (e/1 + e) 100
(%)
(2) (3)
Atterberg limits Atterberg (1911) introduced the liquid and plastic limit to provide an empirical but quantitative measure for describing the plasticity of clays. The tests became internationally standardized. Briefly, the liquid limit of fine-grained sediment is the water content at which, in the remolded state (i.e., artificially completely homogenized), it passes from a plastic to an almost liquid condition. The plastic limit is the water content at which the remolded argillaceous material passes from the plastic to a friable or brittle condition (e.g., Terzaghi and Peck, 1967).
sediment compressibility is quantified by the compression index. This is the slope of the straight line part of the void ratio versus the log stress curve (Fig.l), calculated by the Terzaghi equation (Terzaghi and Peck, 1967): e.+l = e x - Cc lg (ax+l/a~)
where the indices refer to the void ratio (e) at a defined stress (a), and C¢ is the compression index. Specific surface area The specific surface area was determined using a StrShlein Areameter II instrument. The test is based on the BET (Brunauer, Emmett and Teller, 1938) method which measures N 2 adsorption on the internal surfaces of a sample at the temperature of fluid nitrogen. Reliability of data The geotechnical data used in this study were taken from various sources. However, their reliability should be affected only to a 0 0.01. I
,
,
1 I
,
2 I
,
,
actual void ratio T e3
1
,
,
3 |
/7 ///
/
e2
,
[
1 //,'/
//
/
e
_
void ratio
I
initial
void ratio
O. 1 ~/ --
~
preconsolidation stress
~
6"2
. / Cc= 1 . 0 4 ~
Compressibility The compressibility of a sediment is determined by means of a standardized compression test (e.g., Terzaghi and Peck, 1967) in which a sediment volume is compressed by applying different known stresses under confined conditions. The test results provide information on (i) previous consolidation (expressed as stress value - - the so-called pre-consolidation stress, a°) which the sediment has experienced and (ii) compressibility. For stresses exceeding the preconsolidation stress ("first loading"), the
(4)
/
1 (MPa) G'
stress
Fig.1. Theoretical curve of void ratio (e) versus the log of effective stress (a) showing typical compression behavior of a pre-consolidated sediment. When stress increases, the void ratio does not change significantly until the preconsolidation stress is reached. Then, during "first loading", there is a linear relationship between void ratio (e) and log effective stress (a); the inclination of the graph is defined as the compression index (Co) by eqn. (4): e3=e2-Cc lg
108
A. WETZEL
w
water
~
content
Fig.2. Relationship between liquid limit, plastic limit and specific surface area for fine-grained marine sediments (31 samples; off northwest Africa, Sulu Sea, central Pacific and Gulf of Mexico). This pattern suggests that the water adsorption capacity of a sediment depends mainly on surface properties of particles when the chemical conditions are similar. Stars indicate surface sediment samples for which initial porosity and compressibility were also determined. All data were determined by the author.
~
(4) 100-
• lk liquid limit
lk
0 ple$tl¢ Ilml|
o
~
e-
•
50.
/
oO
oO °
: *
o£ O o
° 0
0
o
O0
Sp®cific surface a r e a (rn=g -~) ! 0
|
I
50
100
s t a n d a r d i z e d a n d h e n c e a c e r t a i n a c c u r a c y of t h e m e a s u r e m e n t s h a s b e e n achieved. T h e e r r o r d u e to l a b o r a t o r y p r o c e d u r e s for p o r o s i t y is _+2%, for l i q u i d l i m i t + 3 % , a n d for c o m p r e s s i o n i n d e x _ 5 % . T h e e r r o r i n specific s u r f a c e a r e a d e t e r m i n a t i o n is _ 2.5%. H o w e v e r , d i s t u r b a n c e s d u e to s a m p l i n g a n d s u b s a m p l i n g p r o c e d u r e s a r e difficult to e s t i m a t e a n d t h e y m a y i n f l u e n c e t h e r e s u l t s of t h i s study.
Sg
Results m i n o r degree by the fact t h a t t h e y were o b t a i n e d i n m a n y d i f f e r e n t l a b o r a t o r i e s , because the test procedures are internationally
Three relationships between the geotechnic a l p r o p e r t i e s of s u r f a c e s e d i m e n t s a r e con-
(%) porosity at floor at the the sea sea floor
100-
__
-
.---
- e- _ ~• _ - - . . . . . .e-.
,
50
0
. . . .
0
!
50
,
,
,
,
I
100
. . . .
!
150
•
'
•
'
I
200
water content .atthe !i~ui.d limit '
250
•
(%)
Fig.3. Relationship between the porosity and liquid limit of fine-grained marine surface sediments (sample interval 0 2 cm). This graph indicates equilibrium conditions between the pore volume formed during deposition and the electrostatic particles properties, in combination with the ionic composition of the pore water (the latter two are reflected by the liquid limit). The dashed line is the regression line for all 91 data points, while the continuous line marks the observed, upper limit. Stars indicate samples for which compressibility and specific surface area were also determined by the author (off northwest Africa, Sulu Sea, central Pacific and Gulf of Mexico). Other data were taken from Richards (1962, Atlantic), Keller (1971; north Atlantic), Dietrich (1976; Baltic Sea), Lambert et al. (1980, 1986; east Atlantic), and National Geophysical Data Center (Boulder, CO) reports MGG 03005010 (off New Jersey), MGG 03005011 (Caribbean), MGG 03005012 (off New Jersey), MGG 03195001 (Caribbean) and MGG 09005001 (Gulf of Mexico).
109
GEOTECHNICALPROPERTIESOF FINE-GRAINEDSEDIMENTS
e'~ void ratio at stress ~o = 0 . 0 1 MPa
/ • •
,
/: 6 - - ..
5
el
•
....~. •
•
4
•
4 oe
8 •
I
I
"~'
compression Index
0 0
1
2
3
Cc
Fig.4. Relationship between compression index (C¢) and void ratio (e'0) at a standardized effective stress (a'o = 0.01 MPa); derived from 384 compression test data. The continuous line is the regression line for all data. Stars mark surface sediment samples for which initial porosity, liquid limit and specific surface area were also determined. In addition to 112 determinations by the a u t h o r (off northwest Africa, Sulu Sea, central Pacific and Gulf of Mexico), data were t a k e n from B r y a n t et al. (1974; Gulf of Mexico), T r a b a n t et al. (1975; Aleutan Trench), Shepard and B r y a n t (1980; J a p a n Trench), Shepard et al. (1982; Middle America Trench), Geotechnical Consortium (1984; southeastern Atlantic), Schultheiss and Gunn (1985; n o r t h Atlantic), Taylor and B r y a n t (1985; Middle America Trench), Marine Geotechnical Consortium (1985; northwest Pacific) and Gandais and Viguier (1986; Caribbean).
sidered: (i) Atterberg limits and specific surface area, (ii) porosity and liquid limit, and (iii) void ratio and compression index.
Atterberg limits and specific surface area are related to each other (Fig.2) by the equations: L L = 1.01 Sg ÷ 46.5,
L L > 50
(5)
P L = 0.43 Sg + 13.5,
P L > 20
(6)
S p e c i f i c s u r f a c e area a n d A t t e r b e r g l i m i t s
The Atterberg limits depend mainly on (i) the type and amount of the clay fraction, (ii) exchangeable cations and (iii) pore water chemistry (e.g., Mitchell, 1976). Under given chemical conditions, factors (ii) and (iii) are assumed to be constant, and parameter (i) can be approximated by the specific surface area (Rabitti et al., 1983).
where L L is the liquid limit, P L the plastic limit and Sg the specific surface area. The water adsorption capacity of marine argillaceous material reflected by the Atterberg limits and by the specific surface area is important because it may govern the porosity formed during deposition.
110
A. WETZEL
Porosity and liquid limit The porosity of fine-grained surface sediments were found to be closely related to the liquid limit (LL) (Fig.3). The relationship follows the equation n a = 100 (0.0378LL + 0.43)/(0.0378LL + 1.43) (7a) n u = 100 (0.0438LL + 0.5)/(0.0438 + 1.5)
(7b)
where na is the trend line for the porosity for all data points, n u is the upper limit of the porosity data and LL is the liquid limit. The regression coefficient for equ. (7a) is 0.87. Equations (7a) and (7b) are valid only for a liquid limit higher than 20; below this value there are no data available. Regression line n~ also fits the porosity - - liquid limit relationship which was found by Skempton (1970) in samples taken from nine drill holes. Therefore, the observed regression may reflect "average" conditions for marine deposits, conditions which also average over any changes caused by bioturbation. The scattering of the data points around the regression line is ascribed to (i) variations in sediment composition, especially to the different intratest porosities of microfossils, and (ii) to a varying degree of bioturbation (see below). The close relationship between liquid limit and the porosity of surface sediments implies that in the marine environment the particles reach a state of equilibrium during accumulation (e.g., Bennett et al., 1981). Consequently, the porosity of the surface deposits reflects the sediment composition (including exchangeable cations) as well as the ionic composition of the pore water. In this context it is interesting to note that a relatively sharp upper limit of porosity exists at a given liquid limit.
Compression index and void ratio The compressibility, expressed as a compression index, is related to the void ratio (e'o) at a standardized stress a:=0.01 MPa (Fig.4). A close relationship between these parameters is evident and can be described by the equation: eo= 3.352 exp C¢
(8)
where e'o is the void ratio at stress a o= 0.01 MPa and C¢ is the compression index. The correlation coefficient is 0.96. Such a relationship can also be deduced by combining (i) the correlation between porosity at the sediment surface and the liquid limit, and (ii) the equation established by Skempton (1944): Cc = 0.009(LL - 10%)
(9)
where C¢ is the compression index and LL the liquid limit. The relationship between void ratio and compression index implies that the particle properties in a given chemical environment determine the arrangement of the particles, and hence, the number and type of particle contacts, which control sediment properties such as porosity and compressibility. Discussion
The interdependencies observed between the various physical properties of fine-grained sediments have some implications. Among these, are (i) change in geotechnical properties due to bioturbation, (ii) estimation of the strength of interparticle bondings (expressed as stress value) at the depositional interface, and (iii) evaluation of erosion at the sea floor.
Bioturbation Bioturbation can change the physical properties of sediments considerably (e.g., Rhoads and Boyer, 1982; Richardson, 1983), and changes in geotechnical properties due to bioturbation of sediments are related to fabric changes (e.g., Chernow et al., 1986). However, a direct comparison between bioturbated and non-bioturbated sediments of similar composition is difficult to make because sediment physical properties are also affected by the conditions that prevent bioturbation, i.e., mainly rapid sedimentation and oxygen-free bottom waters. Because rapid sedimentation is
GEOTECHNICALPROPERTIESOF FINE-GRAINEDSEDIMENTS
void ratio
e
111
~at stress Ill / (MPa)
8.
ill
7.
I'/S% ., I y S ~
°°:°°°: o.ooos 0.001
iiii: .........j~S~y;iiiiii!i~"
compression index o
1
cc
Fig.5. State of consolidation of fine-grained marine surface sediments derived from the relationship between (1) initial porosity and liquid limit (Fig.3) and (2) compression index and void ratio at different stresses calculated fi-om eqn. (8) [e'o= 3.352 exp Co], and eqn. (4) [e=+~= e = - Cc lg (a=+ ~/ax) ]. T h e porosity and the liquid limit data for the surface sediments were transformed into compression index and void ratio values using eqns. (7) and (8). The stippled area refers to "average" surface sediments, and the ruled area refers to sediments with maximum porosity. In general, overconsolidation decreases with increasing initial void ratio.
not discussed in this paper, only anaerobic conditions are considered. Compared to deposits formed under oxic conditions more porous deposits normally form under euxinic conditions. This is because the particle arrangement is changed by the formation of organic-mineral complexes, resulting in an open fabric (e.g., Keller, 1982). However, the influence of bioturbation on sediment physical properties can be estimated indirectly, by analyzing the variations in physical properties between different cores as well as with depth in a single core, as demonstrated by Richardson (1983). Porosity is usually increased by bioturba-
tion, although it may also occasionally be reduced (e.g., Rhoads and Boyer, 1982). The range of porosity variations at the sediment surface due to bioturbation can be estimated in Fig.3 by quantifying the deviation from the trend line for all samples. The compression index can be lowered by as much as 20% when the deposits are completely homogenized, a process comparable to remolding in soil mechanical investigations (e.g., Mitchell, 1976). However, normal biogenic reworking results only in incomplete homogenization, (e.g., Chernow et al., 1986). Additionally, the effect of bioturbation on the compression index can be deduced from the data in Fig.5. Assuming that the fabric changes also result in a change in porosity, the difference between average porosity and actual porosity of sea floor sediments (Fig.3 and eqn. (7a)) implies changes in the compression index. Accordingly, assuming a maximum porosity increase in average muds due to bioturbation (Fig.3), the change in the compression index is about 12_ 2% for sediments with void ratios < 5 (= 83~/o porosity). Skempton's (1944) experimental work showed that the influence of increased porosity (e.g., by bioturbation) on the compressibility of muds decreases by as much as the deposits consolidate. At stress values exceeding 0 . 1 - 1 M P a (10-30m of overburden), most porosity-depth curves for similarly composed sediments do not show major deviations from each other, regardless of whether bioturbation is intense or not. Thus, the differences between sediments bioturbated to varying degrees are obliterated when they are compacted. This implies that the compaction properties deeper in the sediment depend on sediment composition and pore water chemistry, and to a lesser degree on fabric. In contrast, if bioturbation diminishes the porosity, its effect will be rapidly obscured by further compaction.
State of consolidation Combining the observed relationship between (i) the porosity and liquid limit of
112 surface sediments and (ii) the compression index and void ratio at a standardized stress using eqns. (7) and (9), it is possible to roughly estimate the forces between particles (expressed as stress values) at the depositional interface (Fig.5). A value of 0.0002MPa (corresponding to about the top 3-5 cm of overburden) was found for the average surface sediments represented by the trend line in Fig.3. Taking the sampled interval into account, the sediments are overconsolidated 1.5 to 3 times. A stress value of 0.00005MPa (corresponding to about 1 cm of overburden) was found for the maximum porosity at a given liquid limit.
Erosion Erosion of surface sediment can be estimated from the presented data using the method suggested by Skempton (1970). Assuming a constant sediment composition (referring to constant Atterberg limits) and determining the actual porosity and the liquid limit, the difference between the actual measurement and the average value (Fig.3) can be ascribed to erosional loss. This difference can be viewed in terms of overburden by constructing a compaction curve, using eqns. (9), (3) and (1). However, because there is a considerable change in the Atterberg limits deeper in the sediment (Skempton, 1970), this method is only applicable for near-surface sediments.
Conclusions (1) A close relationship between the initial porosity and the liquid limit of fine-grained surface sediments exists within the upper 2 cm of the sediment column. Because the liquid limit is related to sediment composition (including exchangeable cations) and the ionic composition of the pore water, it is deduced that initial porosity depends on these parameters. Consequently, initial porosity reflects equilibrium conditions between particle properties and the pore fluid at the depositional interface.
A.WETZEL (2) The Atterberg limits were found to be related to the specific surface area. Additionally, the latter reflects the particle properties of the argillaceous sediments. (3) A close relationship between the compression index and void ratio at a standardized stress supports the above-mentioned equilibrium and implies that the compression index is related to sediment composition. (4) Based on the relationships between geotechnical properties and porosity in the sediment, the state of consolidation of freshly deposited, fine-grained sediment can be quantified. Normally, argillaceous deposits at the sea floor are overconsolidated by 2 to 3 times due to the electrostatic forces between the particles. (5) The effect of bioturbation can be roughly estimated using the deviation from the trend for "average" muds; often, porosity increases by 5-10%, and overconsolidation decreases to a value of 1-2 (for low organic matter content). (6) Even if the initial porosity is changed by bioturbation, the compaction of the sediment will reach equilibrium conditions again under thicker overburden (if the chemical conditions remain reasonably constant), because the rearrangement of particles results only in changes in the compressional properties by about
5-10%. Acknowledgements H. Kassens (Kiel, F.R.G.) critically read an earlier version of the manuscript and L. Hobert (Albany, NY) improved the English. Parts of this study received financial support from the Deutsche Forschungsgemeinschaft. All these contributions are gratefully acknowledged.
References Atterberg, A., 1911. Die Plastizit~it der Tone. Int. Mitt. Bodenkd., 1:10 37. Bennett, R.H., Bryant, W.R., and Keller, G.H., 1981. Clay fabric of selected submarine sediments. J. Sediment. Petrol., 51: 217-232. Brunauer, S., Emmett, P.H. and Teller, E., 1938. Adsorption of gases in multimolecular layers. J. Am. Chem. Soc., 60:309 319.
GEOTECHNICALPROPERTIESOF FINE-GRAINEDSEDIMENTS Bryant, W.R., Deflache, A.R. and Trabant, P.K., 1974. Consolidation of marine clays and carbonates. In: A.L. Inderbitzen (Editor), Deep-Sea Sediments: Physical and Mechanical Properties. (Marine Science, 2.) Plenum, New York, pp. 209-244. Bryant, W.R., Bennett, R.H. and Katherman, C.F., 1981. Shear strength, consolidation, porosity, and permeability of oceanic sediments. In: C. Emiliani (Editor), The Oceanic Lithosphere. (The Sea, Vol. 7.) Wiley, New York, pp. 1555-1616. Chernow, R.M., Frey, R.W. and Ellwood, B.B., 1986. Biogenic effects on development of magnetic fabrics in coastal Georgia sediments. J. Sediment. Petrol., 56: 160-172. Dietrich, R., 1976. Vergleich von sedimentphysikalischen Eigenschaften und Echostiirken in einem Gebiet der westlichen Kieler Bucht. Thesis, Univ. Kiel. Gandais, V. and Viguier, C., 1986. Depot et compaction initiale de sediments sous-marins fins et carbonates de la region est-caraibes. Bull. Inst. G~ol. Bassin d'Aquitaine, 40: 5-16. Geotechnical Consortium, 1984. Geotechnical properties of sediments from Walvis Ridge, Deep Sea Drilling Project, Leg 75, Hole 532A. In: W.W. Hay, J.-C. Sibuet et al. Init. Rep. Deep-Sea Drill. Proj., 75 (2): 1109-1127. Keller, G.H., 1971. Engineering properties of Greenland and Norwegian basin sediments. Proc. Int. Conf. Port and Ocean Eng. Under Arctic Conditions, 1st, Vol. 2, pp. 1285-1311. Keller, G.H., 1982. Organic matter and the geotechnical properties of submarine sediments. Geo-Mar. Lett., 2: 191-198. Lambert, D.N., Bennett, R.H., Sawyer, W.B. and Keller, G.H., 1980. Geotechnical properties of continental upper rise sediments - - Veatch Canyon to Cape Hatteras. Mar. Geotechnol., 4: 281-306. Marine Geotechnical Consortium, 1985. Geotechnical properties of northwest Pacific pelagic clays: Deep Sea Drilling Project Leg 86, Site 576A. In: R.C. Heath, L.H. Burckle et al. Init. Rep. Deep-Sea Drill. Proj., 86: 723-758. McCave, I.N., 1984. Erosion, transport and deposition of fine-grained marine sediments. In: D.A.V. Stow and D.J.W. Piper (Editors), Fine-Grained Sediments: DeepWater Processes and Facies. Geol. Soc. London, Spec. Publ., 15: 35-69. Meade, R.H., 1964. Removal of water and rearrangement of particles during the compaction of clayey sediments - review. U.S. Geol. Surv. Prof. Pap., 497B. Mitchell, J.K., 1976. Fundamentals of Soil Behavior. Wiley, New York, 422 pp.
113 Nowell, A.R.M. and Hollister, C.D. (Editors), 1985. Deep Ocean Sediment Transport. Elsevier, Amsterdam, 418 pp. Parasnis, D.S., 1960. The compaction of sediments and its bearing on some geophysical problems. Geophys. J. R. Astron. Soc., 3: 1-28. Rabitti, S., Boldrin, A. and Vitturi, L.M., 1983. Relationships between surface area and grain size in bottom sediments. J. Sediment. Petrol., 53: 1032-1034. Rhoads, D.C. and Boyer, L.F., 1982. The effects of marine benthos on physical properties of sediments: a successional perspective. In: P.L. McCall and M.J.S. Tevesz (Editors), Animal-Sediment Relations. (Topics in Geobiology, Vol.2.) Plenum, New York, pp. 3-52. Richards, A.F., 1962. Investigations of deep-sea sediment cores. Part II (mass physical properties). U.S. Nav. Hydrogr. Off. Tech. Rep., 106:145 pp. Richardson, M.D., 1983. The effects of bioturbation on sediment elastic properties. Bull. Soc. Geol. Fr., 25: 505-513. Schultheiss, P.J. and Gunn, D.E., 1985. The permeability and consolidation of deep-sea sediments. Inst. Oceanogr. Sci. Rep., 201:94 pp. Seibold, E. and Berger, W.H., 1982. The Sea Floor. Springer, Berlin, 288 pp. Shephard, L.E. and Bryant, W.R., 1980. Consolidation characteristics of Japan Trench sediments. In: M. Langseth, H. Okada, R. von Huene, N. Nasu et al. Init. Rep. Deep-Sea Drill. Proj., 56 and 57 (2): 1201-1205. Shephard, L.,E., Bryant, W.R. and Chiou, W.A., 1982. Geotechnical properties of Middle America Trench sediments, Deep Sea Drilling Project Leg 66. In: J.S. Watkins, J.C. Moore et al. Init. Rep. Deep-Sea Drill. Proj., 66: 475-504. Skempton, A.W., 1944. Notes on the compressibility of clays. Q. J. Geol. Soc. London, 100: 119-135. Skempton, A.W., 1970. The consolidation of clays by gravitational compaction. Q. J. Geol. Soc. London., 125: 373-411. Taylor, E. and Bryant, W.R., 1985. Geotechnical properties of sediments from the Middle America Trench and slope. In: R. von Huene, J. Auboin et al. Init. Rep. Deep-Sea Drill. Proj., 84: 745-766. Terzaghi, K. and Peck, R.B., 1967. Soil Mechanics in Engineering Practice. Wiley, New York, 2nd Ed., 729 pp. Thiel, H., 1983. Meiobenthos and nannobenthos of the deep sea. In: G.T. Rowe (Editor), Deep-Sea Biology. (The Sea, Vol.8.) Wiley, New York, pp. 167-230. Trabant, P.K., Bryant, W.R. and Bouma, A.H., 1975. Consolidation characteristics of sediments from Leg 31 of the Deep Sea Drilling Project. In: D.E. Karig, J.C. Ingle et al. Init. Rep. Deep-Sea Drill. Proj., 31: 569-572.
105
Interrelationships Between Porosity and other Geotechnical Properties of Slowly Deposited, FineGrained Marine Surface Sediments ANDREAS WETZEL Geologisch-Pal~iontologisches
Institut der Universitdt, Bernoullistrasse 32, CH 4056 Basel (Switzerland) (Received by publisher November 1, 1989)
Abstract Wetzel, A., 1990. Interrelationships between porosity and other geotechnical properties of slowly deposited, finegrained marine surface sediments. Mar. Geol., 92: 105-113. The pore volume, expressed as porosity or void ratio, of fine-grained marine deposits at the sediment surface (0-2 cm) is closely related to other geotechnical properties such as liquid limit, plastic limit, compression index, and specific surface area. Equations have been formulated for the interdependencies between (1) average porosity at the sediment surface (n.) and liquid limit (LL) In. = 100 (0.0378LL + 0.43)/(0.0378LL + 1.43)], (2) liquid limit (LL) and specific surface area (Sg) [LL = 1.01 88 + 46.5 if LL > 50], and (3) compression index (C¢) and void ratio at a stress of 0.01 MPa (eo) [e'o= 3.352 exp Col. From the relationship between geotechnical properties and initial porosity, the state of consolidation of surface sediments and the influence of bioturbation often lead to an increase in porosity (5-10%) and compressibility (up to 10%), and a decrease in the overconsolidation ratio (from 2-3 to about 1) at the sediment surface. The interrelationship between initial porosity and physical properties can be explained by the dependence of the geotechnical properties on the electrostatic forces between the particles and the adsorbed water around them.
Introduction
The depositional interface has been a subject of investigation for a long time, because it is there that sediment, water and organisms interact (e.g., McCave, 1984; Nowell and Hollister, 1985). The sediments are subjected to permanent changes under the influence of organisms and water movement. This leads to the question of whether under such circumstances defined relationships can exist between sediment composition and geotechnical properties such as porosity, compressibility and sonic velocity. Many investigators have suggested that the 0025-3227/90/$03.50
porosity of fine-grained surface sediments correlates with other geotechnical parameters, a correlation which may reflect the composition and depositional environment of the sediments (e.g., Skempton, 1944; Parasnis, 1960; Meade, 1964; Bryant et al., 1981). However, most of these relationships are restricted in their validity because they are often subject to a large degree of error or were deduced from only a small number of observations. Generally applicable equations have therefore not yet been formulated. With the use of a large data base it is the purpose of this paper to quantify some of the relationships between the geotechnical properties of surface sediments.
© 1990 Elsevier Science Publishers B.V.
106
A.WETZEL
Materials and m e t h o d s
Sediment composition Fine-grained, slowly deposited marine sediments of various compositions were investigated (Table 1). The selection of samples with respect to these criteria has some implications regarding their physical properties: (i) "Fine-grained" means that the samples contain at least 35% clay minerals, so that theoretically all coarser grains can be surrounded by argillaceous particles and the cohesive components therefore dominantly affect the behavior of the deposits (Mitchell, 1976). (ii) Only marine sediments were considered, so effects resulting from varying ionic composition of the pore water and its influence on the exchangeable cations can be excluded. (iii) Slow deposition of the samples excludes the effects of sedimentation rate on the initial porosity, although this parameter has not yet been quantified.
Sampling interval The sediment surface is not always sharply developed, and the transition from water through an increasingly concentrated suspension to cohesive sediment may be gradual (e.g., McCave, 1984; Nowell and Hollister, 1985). Normally, only the cohesive sediment is usually at rest, as indicated by its colonization by TABLE 1 Composition of the investigated sediments (in percent) Clay Sand Silt Diatoms Radiolaria Foraminifera Calcareous nannofossils Corg
4 100 0 40 ~~ <602 } 0-60 0 10 : 0 10 < 403 0 20 0-40 0-5
> 401 < 602
~Minimum value for clay-sized material; 2Maximum value for coarse-grained components; ~Maximum value for all biogenic components.
more-or-less immobile organisms (e.g., foraminifera, bacteria, etc. Thiel, 1983). Although the boundary between sediment and water is known in theory (when the biologic definition is accepted), the sediment is sometimes too soft for undisturbed samples to be taken from the surface sediment layer. In order to obtain reproducible geotechnical data the interval between the cohesive surface and the 2 cm depth was sampled as one unit. It is probable that sampling this interval would provide representative samples, because bioturbation homogenizes a layer at least as thick as this (e.g., Seibold and Berger, 1982).
Strategy The geotechnical parameters were determined on the same sample eleven times. Obviously, this number is too low to evaluate whether important relationships exist. Consequently, additional data were taken from the literature to enlarge the data base. Data from the literature were utilized on sediments that are (i) of similar composition (Table 1), and (ii) sampled from the same interval (0-2 cm). The locations of these samples are given in the figure captions.
Methods To quantify the interdependencies between various geotechnical properties of finegrained, clayey sediments at the depositional interface, parameters are required that reflect (i) the sediment composition, which determines the electrostatic properties of the particles, and (ii) the ionic composition of the water, which influences the thickness of the adsorbed water and the exchangeable cations (when stress at the sediment surface is ignored). With a given sea water composition, and hence, constant chemical conditions, sediment composition is closely related to porosity, liquid limit and compressibility (expressed as the compression index). Additionally, the specific surface
GEOTECHNICALPROPERTIESOF FINE-GRAINEDSEDIMENTS
107
area w a s also determined. These parameters were used in this study. Porosity The porosity (n) was calculated from the grain density and water content (corrected for salt content) of a sample with a known volume, with the formula: n = (w'ps/w'ps + 1) 100
(%)
(1)
where w is water content and ps is grain density. In some geotechnical studies the void ratio is used in place of porosity. Porosity (n) and void ratio (e) are related to each other by the equations: e
=
(n/lO0)/(1
-
n/100)
n = (e/1 + e) 100
(%)
(2) (3)
Atterberg limits Atterberg (1911) introduced the liquid and plastic limit to provide an empirical but quantitative measure for describing the plasticity of clays. The tests became internationally standardized. Briefly, the liquid limit of fine-grained sediment is the water content at which, in the remolded state (i.e., artificially completely homogenized), it passes from a plastic to an almost liquid condition. The plastic limit is the water content at which the remolded argillaceous material passes from the plastic to a friable or brittle condition (e.g., Terzaghi and Peck, 1967).
sediment compressibility is quantified by the compression index. This is the slope of the straight line part of the void ratio versus the log stress curve (Fig.l), calculated by the Terzaghi equation (Terzaghi and Peck, 1967): e.+l = e x - Cc lg (ax+l/a~)
where the indices refer to the void ratio (e) at a defined stress (a), and C¢ is the compression index. Specific surface area The specific surface area was determined using a StrShlein Areameter II instrument. The test is based on the BET (Brunauer, Emmett and Teller, 1938) method which measures N 2 adsorption on the internal surfaces of a sample at the temperature of fluid nitrogen. Reliability of data The geotechnical data used in this study were taken from various sources. However, their reliability should be affected only to a 0 0.01. I
,
,
1 I
,
2 I
,
,
actual void ratio T e3
1
,
,
3 |
/7 ///
/
e2
,
[
1 //,'/
//
/
e
_
void ratio
I
initial
void ratio
O. 1 ~/ --
~
preconsolidation stress
~
6"2
. / Cc= 1 . 0 4 ~
Compressibility The compressibility of a sediment is determined by means of a standardized compression test (e.g., Terzaghi and Peck, 1967) in which a sediment volume is compressed by applying different known stresses under confined conditions. The test results provide information on (i) previous consolidation (expressed as stress value - - the so-called pre-consolidation stress, a°) which the sediment has experienced and (ii) compressibility. For stresses exceeding the preconsolidation stress ("first loading"), the
(4)
/
1 (MPa) G'
stress
Fig.1. Theoretical curve of void ratio (e) versus the log of effective stress (a) showing typical compression behavior of a pre-consolidated sediment. When stress increases, the void ratio does not change significantly until the preconsolidation stress is reached. Then, during "first loading", there is a linear relationship between void ratio (e) and log effective stress (a); the inclination of the graph is defined as the compression index (Co) by eqn. (4): e3=e2-Cc lg
108
A. WETZEL
w
water
~
content
Fig.2. Relationship between liquid limit, plastic limit and specific surface area for fine-grained marine sediments (31 samples; off northwest Africa, Sulu Sea, central Pacific and Gulf of Mexico). This pattern suggests that the water adsorption capacity of a sediment depends mainly on surface properties of particles when the chemical conditions are similar. Stars indicate surface sediment samples for which initial porosity and compressibility were also determined. All data were determined by the author.
~
(4) 100-
• lk liquid limit
lk
0 ple$tl¢ Ilml|
o
~
e-
•
50.
/
oO
oO °
: *
o£ O o
° 0
0
o
O0
Sp®cific surface a r e a (rn=g -~) ! 0
|
I
50
100
s t a n d a r d i z e d a n d h e n c e a c e r t a i n a c c u r a c y of t h e m e a s u r e m e n t s h a s b e e n achieved. T h e e r r o r d u e to l a b o r a t o r y p r o c e d u r e s for p o r o s i t y is _+2%, for l i q u i d l i m i t + 3 % , a n d for c o m p r e s s i o n i n d e x _ 5 % . T h e e r r o r i n specific s u r f a c e a r e a d e t e r m i n a t i o n is _ 2.5%. H o w e v e r , d i s t u r b a n c e s d u e to s a m p l i n g a n d s u b s a m p l i n g p r o c e d u r e s a r e difficult to e s t i m a t e a n d t h e y m a y i n f l u e n c e t h e r e s u l t s of t h i s study.
Sg
Results m i n o r degree by the fact t h a t t h e y were o b t a i n e d i n m a n y d i f f e r e n t l a b o r a t o r i e s , because the test procedures are internationally
Three relationships between the geotechnic a l p r o p e r t i e s of s u r f a c e s e d i m e n t s a r e con-
(%) porosity at floor at the the sea sea floor
100-
__
-
.---
- e- _ ~• _ - - . . . . . .e-.
,
50
0
. . . .
0
!
50
,
,
,
,
I
100
. . . .
!
150
•
'
•
'
I
200
water content .atthe !i~ui.d limit '
250
•
(%)
Fig.3. Relationship between the porosity and liquid limit of fine-grained marine surface sediments (sample interval 0 2 cm). This graph indicates equilibrium conditions between the pore volume formed during deposition and the electrostatic particles properties, in combination with the ionic composition of the pore water (the latter two are reflected by the liquid limit). The dashed line is the regression line for all 91 data points, while the continuous line marks the observed, upper limit. Stars indicate samples for which compressibility and specific surface area were also determined by the author (off northwest Africa, Sulu Sea, central Pacific and Gulf of Mexico). Other data were taken from Richards (1962, Atlantic), Keller (1971; north Atlantic), Dietrich (1976; Baltic Sea), Lambert et al. (1980, 1986; east Atlantic), and National Geophysical Data Center (Boulder, CO) reports MGG 03005010 (off New Jersey), MGG 03005011 (Caribbean), MGG 03005012 (off New Jersey), MGG 03195001 (Caribbean) and MGG 09005001 (Gulf of Mexico).
109
GEOTECHNICALPROPERTIESOF FINE-GRAINEDSEDIMENTS
e'~ void ratio at stress ~o = 0 . 0 1 MPa
/ • •
,
/: 6 - - ..
5
el
•
....~. •
•
4
•
4 oe
8 •
I
I
"~'
compression Index
0 0
1
2
3
Cc
Fig.4. Relationship between compression index (C¢) and void ratio (e'0) at a standardized effective stress (a'o = 0.01 MPa); derived from 384 compression test data. The continuous line is the regression line for all data. Stars mark surface sediment samples for which initial porosity, liquid limit and specific surface area were also determined. In addition to 112 determinations by the a u t h o r (off northwest Africa, Sulu Sea, central Pacific and Gulf of Mexico), data were t a k e n from B r y a n t et al. (1974; Gulf of Mexico), T r a b a n t et al. (1975; Aleutan Trench), Shepard and B r y a n t (1980; J a p a n Trench), Shepard et al. (1982; Middle America Trench), Geotechnical Consortium (1984; southeastern Atlantic), Schultheiss and Gunn (1985; n o r t h Atlantic), Taylor and B r y a n t (1985; Middle America Trench), Marine Geotechnical Consortium (1985; northwest Pacific) and Gandais and Viguier (1986; Caribbean).
sidered: (i) Atterberg limits and specific surface area, (ii) porosity and liquid limit, and (iii) void ratio and compression index.
Atterberg limits and specific surface area are related to each other (Fig.2) by the equations: L L = 1.01 Sg ÷ 46.5,
L L > 50
(5)
P L = 0.43 Sg + 13.5,
P L > 20
(6)
S p e c i f i c s u r f a c e area a n d A t t e r b e r g l i m i t s
The Atterberg limits depend mainly on (i) the type and amount of the clay fraction, (ii) exchangeable cations and (iii) pore water chemistry (e.g., Mitchell, 1976). Under given chemical conditions, factors (ii) and (iii) are assumed to be constant, and parameter (i) can be approximated by the specific surface area (Rabitti et al., 1983).
where L L is the liquid limit, P L the plastic limit and Sg the specific surface area. The water adsorption capacity of marine argillaceous material reflected by the Atterberg limits and by the specific surface area is important because it may govern the porosity formed during deposition.
110
A. WETZEL
Porosity and liquid limit The porosity of fine-grained surface sediments were found to be closely related to the liquid limit (LL) (Fig.3). The relationship follows the equation n a = 100 (0.0378LL + 0.43)/(0.0378LL + 1.43) (7a) n u = 100 (0.0438LL + 0.5)/(0.0438 + 1.5)
(7b)
where na is the trend line for the porosity for all data points, n u is the upper limit of the porosity data and LL is the liquid limit. The regression coefficient for equ. (7a) is 0.87. Equations (7a) and (7b) are valid only for a liquid limit higher than 20; below this value there are no data available. Regression line n~ also fits the porosity - - liquid limit relationship which was found by Skempton (1970) in samples taken from nine drill holes. Therefore, the observed regression may reflect "average" conditions for marine deposits, conditions which also average over any changes caused by bioturbation. The scattering of the data points around the regression line is ascribed to (i) variations in sediment composition, especially to the different intratest porosities of microfossils, and (ii) to a varying degree of bioturbation (see below). The close relationship between liquid limit and the porosity of surface sediments implies that in the marine environment the particles reach a state of equilibrium during accumulation (e.g., Bennett et al., 1981). Consequently, the porosity of the surface deposits reflects the sediment composition (including exchangeable cations) as well as the ionic composition of the pore water. In this context it is interesting to note that a relatively sharp upper limit of porosity exists at a given liquid limit.
Compression index and void ratio The compressibility, expressed as a compression index, is related to the void ratio (e'o) at a standardized stress a:=0.01 MPa (Fig.4). A close relationship between these parameters is evident and can be described by the equation: eo= 3.352 exp C¢
(8)
where e'o is the void ratio at stress a o= 0.01 MPa and C¢ is the compression index. The correlation coefficient is 0.96. Such a relationship can also be deduced by combining (i) the correlation between porosity at the sediment surface and the liquid limit, and (ii) the equation established by Skempton (1944): Cc = 0.009(LL - 10%)
(9)
where C¢ is the compression index and LL the liquid limit. The relationship between void ratio and compression index implies that the particle properties in a given chemical environment determine the arrangement of the particles, and hence, the number and type of particle contacts, which control sediment properties such as porosity and compressibility. Discussion
The interdependencies observed between the various physical properties of fine-grained sediments have some implications. Among these, are (i) change in geotechnical properties due to bioturbation, (ii) estimation of the strength of interparticle bondings (expressed as stress value) at the depositional interface, and (iii) evaluation of erosion at the sea floor.
Bioturbation Bioturbation can change the physical properties of sediments considerably (e.g., Rhoads and Boyer, 1982; Richardson, 1983), and changes in geotechnical properties due to bioturbation of sediments are related to fabric changes (e.g., Chernow et al., 1986). However, a direct comparison between bioturbated and non-bioturbated sediments of similar composition is difficult to make because sediment physical properties are also affected by the conditions that prevent bioturbation, i.e., mainly rapid sedimentation and oxygen-free bottom waters. Because rapid sedimentation is
GEOTECHNICALPROPERTIESOF FINE-GRAINEDSEDIMENTS
void ratio
e
111
~at stress Ill / (MPa)
8.
ill
7.
I'/S% ., I y S ~
°°:°°°: o.ooos 0.001
iiii: .........j~S~y;iiiiii!i~"
compression index o
1
cc
Fig.5. State of consolidation of fine-grained marine surface sediments derived from the relationship between (1) initial porosity and liquid limit (Fig.3) and (2) compression index and void ratio at different stresses calculated fi-om eqn. (8) [e'o= 3.352 exp Co], and eqn. (4) [e=+~= e = - Cc lg (a=+ ~/ax) ]. T h e porosity and the liquid limit data for the surface sediments were transformed into compression index and void ratio values using eqns. (7) and (8). The stippled area refers to "average" surface sediments, and the ruled area refers to sediments with maximum porosity. In general, overconsolidation decreases with increasing initial void ratio.
not discussed in this paper, only anaerobic conditions are considered. Compared to deposits formed under oxic conditions more porous deposits normally form under euxinic conditions. This is because the particle arrangement is changed by the formation of organic-mineral complexes, resulting in an open fabric (e.g., Keller, 1982). However, the influence of bioturbation on sediment physical properties can be estimated indirectly, by analyzing the variations in physical properties between different cores as well as with depth in a single core, as demonstrated by Richardson (1983). Porosity is usually increased by bioturba-
tion, although it may also occasionally be reduced (e.g., Rhoads and Boyer, 1982). The range of porosity variations at the sediment surface due to bioturbation can be estimated in Fig.3 by quantifying the deviation from the trend line for all samples. The compression index can be lowered by as much as 20% when the deposits are completely homogenized, a process comparable to remolding in soil mechanical investigations (e.g., Mitchell, 1976). However, normal biogenic reworking results only in incomplete homogenization, (e.g., Chernow et al., 1986). Additionally, the effect of bioturbation on the compression index can be deduced from the data in Fig.5. Assuming that the fabric changes also result in a change in porosity, the difference between average porosity and actual porosity of sea floor sediments (Fig.3 and eqn. (7a)) implies changes in the compression index. Accordingly, assuming a maximum porosity increase in average muds due to bioturbation (Fig.3), the change in the compression index is about 12_ 2% for sediments with void ratios < 5 (= 83~/o porosity). Skempton's (1944) experimental work showed that the influence of increased porosity (e.g., by bioturbation) on the compressibility of muds decreases by as much as the deposits consolidate. At stress values exceeding 0 . 1 - 1 M P a (10-30m of overburden), most porosity-depth curves for similarly composed sediments do not show major deviations from each other, regardless of whether bioturbation is intense or not. Thus, the differences between sediments bioturbated to varying degrees are obliterated when they are compacted. This implies that the compaction properties deeper in the sediment depend on sediment composition and pore water chemistry, and to a lesser degree on fabric. In contrast, if bioturbation diminishes the porosity, its effect will be rapidly obscured by further compaction.
State of consolidation Combining the observed relationship between (i) the porosity and liquid limit of
112 surface sediments and (ii) the compression index and void ratio at a standardized stress using eqns. (7) and (9), it is possible to roughly estimate the forces between particles (expressed as stress values) at the depositional interface (Fig.5). A value of 0.0002MPa (corresponding to about the top 3-5 cm of overburden) was found for the average surface sediments represented by the trend line in Fig.3. Taking the sampled interval into account, the sediments are overconsolidated 1.5 to 3 times. A stress value of 0.00005MPa (corresponding to about 1 cm of overburden) was found for the maximum porosity at a given liquid limit.
Erosion Erosion of surface sediment can be estimated from the presented data using the method suggested by Skempton (1970). Assuming a constant sediment composition (referring to constant Atterberg limits) and determining the actual porosity and the liquid limit, the difference between the actual measurement and the average value (Fig.3) can be ascribed to erosional loss. This difference can be viewed in terms of overburden by constructing a compaction curve, using eqns. (9), (3) and (1). However, because there is a considerable change in the Atterberg limits deeper in the sediment (Skempton, 1970), this method is only applicable for near-surface sediments.
Conclusions (1) A close relationship between the initial porosity and the liquid limit of fine-grained surface sediments exists within the upper 2 cm of the sediment column. Because the liquid limit is related to sediment composition (including exchangeable cations) and the ionic composition of the pore water, it is deduced that initial porosity depends on these parameters. Consequently, initial porosity reflects equilibrium conditions between particle properties and the pore fluid at the depositional interface.
A.WETZEL (2) The Atterberg limits were found to be related to the specific surface area. Additionally, the latter reflects the particle properties of the argillaceous sediments. (3) A close relationship between the compression index and void ratio at a standardized stress supports the above-mentioned equilibrium and implies that the compression index is related to sediment composition. (4) Based on the relationships between geotechnical properties and porosity in the sediment, the state of consolidation of freshly deposited, fine-grained sediment can be quantified. Normally, argillaceous deposits at the sea floor are overconsolidated by 2 to 3 times due to the electrostatic forces between the particles. (5) The effect of bioturbation can be roughly estimated using the deviation from the trend for "average" muds; often, porosity increases by 5-10%, and overconsolidation decreases to a value of 1-2 (for low organic matter content). (6) Even if the initial porosity is changed by bioturbation, the compaction of the sediment will reach equilibrium conditions again under thicker overburden (if the chemical conditions remain reasonably constant), because the rearrangement of particles results only in changes in the compressional properties by about
5-10%. Acknowledgements H. Kassens (Kiel, F.R.G.) critically read an earlier version of the manuscript and L. Hobert (Albany, NY) improved the English. Parts of this study received financial support from the Deutsche Forschungsgemeinschaft. All these contributions are gratefully acknowledged.
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