CONTRATO POR HONORARIOS

Littoral deposits form in the tidal zone and consist of

tidal lagoon, tidal flat, and beach sediments. Lagoon sediments include fine-grained sands and silts in the channels and organic-rich silt and clay in the quiet ar- eas. Organic matter and carbonates may be abundant. Tidal flat deposits consist of fine-grained dark muds, with lenses or stringers of sand and gravel, and are free of intermediate-size sediments. Beach deposits consist of clean fine- to coarse-grained sand with oc- casional stringers of gravel.

Estuarine Deposits

Estuaries are semienclosed coastal bodies of water that

have a free connection with the sea. The sediments consist of channel muds, silts, and sands deposited in response to seasonal river processes and tidal rhythms. Estuarine sediments typically grade seaward into fine- grained tidal deposits and landward into coarser- grained river (alluvial) deposits. Fine-grained tidal flats with salt marshes often fringe estuaries.

Deltaic Deposits

Deltas form at the mouth of rivers where they enter

the sea. They build up where there is no tidal or current

action capable of removing the sediment as fast as it is deposited. Deltas build forward from the coastline in a complex process that leads to the formation of a number of separate channels, isolated lagoons, levees, marshy ground, and small streams. As a result, deltas may consist of coarse and fine material, organic matter, and marl (a loose or friable deposit of sand, silt, or clay containing calcium carbonate). Coarse and fine materials alternate owing to the continual shifting of the stream. Suspended silt and clay in the main stream is flocculated by salts in the seawater to form marine mud in the seaward delta face, which is later covered by alluvial, lacustrine, and beach deposits as the delta grows.

The complex formations of the Mississippi River delta reflect the composite effects of the advancing delta and the encroaching sea. Pleistocene sediments consisting of dense clays, sands, and gravels underlie the delta. Sand and shell beaches, often 5 m high or more, are among the most suitable deltaic formations for foundation support. Conversely, difficult geotech- nical problems are associated with fine-grained and or- ganic delta sediments because of their low strength and high compressibility.

8.7 MARINE DEPOSITS

An averaged and idealized profile through the marine environment is shown in Fig. 8.10. The continental shelf extends from low tide to an average water depth of about 130 m (nearly 450 ft). The steeper continental

slope (average of 4 leads down to the more gently

sloping continental rise. The average water depth in the deep ocean is more than 3500 m (11,500 ft).

There are three main types of marine sediments:

lithogenous (derived from terrestrial, volcanic, or cos-

mic sources), biogenous (remains of marine organ- isms), and hydrogenous (precipitates from the seawater or interstitial water). An engineering classification sys- tem that incorporates compositional and depositional characteristics of these sediments was developed by Noorany (1989) as shown in Fig. 8.11. This system is patterned after the Unified Soil Classification System, the most widely used system for classification of ter- restrial soils for engineering purposes.

Biogenous sediments, formed from the remains of marine plants and animals, cover about half of the con- tinental shelves, more than half of the deep-sea abyssal plains, and parts of the continental slopes and rises (Noorany, 1989). They are abundant as coarse-grained

bioclastic sediments in shallow waters of the coastal

zones in tropical regions (between 30N and 30S lat- itude).

Figure 8.10 Idealized profile of the continental margin, with vertical exaggeration (after Heselton, 1969).

Neritic Deposits

The neritic, or continental shelf, environment extends to water depths of up to 200 m. In shallow water, dep- osition occurs when the intensity of wave-caused tur- bulence decreases. Generally there is a decrease in particle size and increased influences of biological and chemical factors in the seaward direction, although the sediment distributions may be irregular due to tidal currents and seasonal climatic variations. Neritic de- posits reflect sediment source areas and climatic con- ditions, with sandstone, shale, and limestone typically present in shelf areas. With the exception of the bi- ogenous sediments, the physical properties of conti- nental shelf deposits are essentially the same as those of comparable terrestrial soils.

Calcareous Sands Calcareous bioclastic sands are formed from the skeletal remains of corals, shells of mollusks, and algae. They are widely distributed in the

oceans in tropical and subtropical regions of the world. Most consist of porous or hollow particles. An electron photomicrograph of a calcareous sand is shown in Fig. 8.12. Special geotechnical features of the calcareous sediments are (Semple, 1988) that they are composed of weak, angular particles, particle sizes and size dis- tributions are variable, there is uneven cementation over short distances, and they have high void ratio rel- ative to silicate sediments. As a result, these materials may be the source of special geotechnical problems. For example, the side friction developed on driven piles in calcareous sands is often very much lower than anticipated based on the behavior of piles in quartz sand (Noorany, 1985; Murff, 1987; Jewell et al., 1988). Bathyal Deposits

The bathyal environment includes the continental slope and the continental rise. Bathyal sediments are typi-

211

Figure 8.11 Chart for classification of marine sediments (from Noorany, 1989). Reprinted with permission of ASCE.

Figure 8.12 Electron photomicrographs of calcareous sand

from Guam. Magnification is 45⫻(courtesy of I. Noorany).

cally fine sand, silt, and mud of high water content and low shear strength. The tectonic setting of the depo- sitional area and the characteristics of the continental source materials largely control the distribution, ge- ometry, and properties of these sediments.

Erosion, transport, and deposition of these sediments may be caused by the frictional effects of contour- following undercurrents that result in thick sequences of sediment ‘‘drift’’ consisting of alternating thin layers of very fine sands, silts, and muds (Leeder, 1982). Ap- preciable quantities of sediments can be transported from the continental slope and rise to the deep-ocean abyssal plains by slumps, debris flows, and turbidity flows.

Detailed exploration of the ocean margins indicates that debris flows are probably a much more important depositional process on the seafloor than has been pre- viously suspected. For example, debris flow deposits of enormous extent have been identified that were gen- erated by large sediment slides on the northwestern African continental margin. The flow traveled on a slope as flat as 0.1 for a distance of several hundred kilometers. The deposits cover an area of about 30,000 km2_{and originated from a massive slump of about 600}

km3 _{on the upper continental rise where a prominent}

slide scar now exists. The exact triggering mechanisms for such events are unknown in most cases; however, earthquakes are believed to be the cause of some of them.

Abyssal Deposits

Deep-ocean (abyssal) deposits consist primarily of brown clays and calcareous and silicious oozes, with thicknesses of 300 to 600 m. Terrigenous deposits are derived from land, whereas pelagic sediments settle from the water alone and contain the shells and skeletal remains of tiny marine organisms and plants. Accu- mulation rates range from less than a millimeter per thousand years in the deep sea to a few tens of centi- meters per year in near-shore areas close to the mouths of large rivers (Griffin et al., 1968). Oozes contain more than 50 percent biotic material.

Calcareous ooze, composed of empty shells or tests, covers about 35 percent of the seafloor for water depths up to about 5 km. It is usually nonplastic, cream to white in color, and composed of easily crushed sand- to silt-size particles. Brown clay is found beneath most of the deeper ocean areas. Its origin is believed to be atmospheric dust and fine material circulated by ocean currents. About 60 percent of this material is finer than 60 m, and the clay fraction contains chlo- rite, smectite, illite, and kaolinite, with illite often the most abundant. Brown clays have high water contents, moderate-to-high plasticity, and low strength. Siliceous ooze, composed of plant remains, is found mainly in the Antarctic, northeast of Japan, and in some areas of the equatorial Pacific.

Except near their surface, deep-sea deposits are nor- mally consolidated and highly compressible. There is an apparent overconsolidation of the near-surface ma- terial at many locations. This evidently reflects bond- ing developed as a result of the extremely slow rate of deposition and physicochemical effects (Noorany and Gizienski, 1970). Much of the available data on the mechanical properties of deep-seafloor soils pertains to material from the upper 6 m.

8.8 CHEMICAL AND BIOLOGICAL DEPOSITS

Evaporite deposits formed by precipitation of salts

from salt lakes and seas as a result of the evaporation of water are sometimes found in layers that are up to several meters thick. The major constituents of sea- water, their relative proportions, and some of the more important evaporite deposits are listed in Table 8.5. In some areas alternating layers of evaporite and clay or

FABRIC, STRUCTURE, AND PROPERTY RELATIONSHIPS: GENERAL CONSIDERATIONS 213

Table 8.5 Major Constituents of Seawater and Evaporite Deposits

Ion Grams per Liter

Percent by Weight

of Total Solids Important Evaporite Deposits Sodium, Na⫹ Magnesium, Mg2⫹ Calcium, Ca2⫹ Potassium, K⫹ Strontium, Sr2⫹ Chloride, Cl⫺ Sulfate, So42⫺ Bicarbonate, HCO3⫺ Bromide, Br⫺ Fluoride, F⫺ Boric Acid, H3BO3 10.56 1.27 0.40 0.38 0.013 18.98 2.65 0.14 0.065 0.001 0.026 34.485 30.61 3.69 1.16 1.10 0.04 55.04 7.68 0.41 0.19 — 0.08 100.00 Anhydrite CaSO4 Barite BaSO4 Celesite SrSO4 Kieserite MgSO4H O2 Gypsum CaSO42H O2 Polyhalite Ca K Mg(SO )2 2 4 2H O2 Bloedite Ma Mg(SO )2 4 24H O2 Hexahydrite MgSO46H O2 Epsomite MgSO47H O2 Kainite K Mg (Cl / SO )4 4 4 11H O2 Halite NaCl Sylvite KCl Flourite CaF2 Bischofite MgCl26H O2 Carnallite KMgCl3 6H O2 Adapted from data by Degens (1965).

other fine-grained clastic sediments are formed during cyclic wet and dry periods.

Many limestones have been formed by precipitation or from the remains of various organisms. Because of the much greater solubility of limestones than of most other rock types, they may be the source of special problems caused by solution channels and cavities un- der foundations.

More than 12 percent of Canada is covered by a peaty material, termed muskeg, composed almost en- tirely of decaying vegetation. Peat and muskeg may have water contents of 1000 percent or more, they are very compressible, and they have low strength. The special properties of these materials and methods for analysis of geotechnical problems associated with them are given by MacFarlane (1969), Dhowian and Edil (1980), and Edil and Mochtar (1984).

Chemical sediments and rocks in freshwater lakes, ponds, swamps, and bays are occasionally encountered in civil engineering projects. Biochemical processes form marls ranging from relatively pure calcium car- bonate to mixtures with mud and organic matter. Iron oxide is formed in some lakes. Diatomite or diatoma-

ceous earth is essentially pure silica formed from the

skeletal remains of small (up to a few tenths of a mil- limeter) freshwater and saltwater organisms. Com- pacted fills of diatomaceous earth can have very low dry unit weights (1.0 to 1.2 Mg / m3_{) and high moisture}

contents (40 percent or more). The material may be- have as a dense granular material at stresses below

about 50 kPa, owing to the roughness and interlocking of the diatoms, but becomes more compressible under higher stresses owing to crushing of the diatoms (Day, 1995).

8.9 FABRIC, STRUCTURE, AND PROPERTY

RELATIONSHIPS: GENERAL CONSIDERATIONS