Mineralogy of Common Geologic Materials.

Clay minerals are complex aluminum silicates formed from the weathering of feldspars, micas, and ferromagnesian minerals. The weathering reaction is

The ﬁrst product, silica, is a colloidal gel. The second product, potassium bicarbonate, is in solution. The third product, hydrous aluminum silicate, is a simpliﬁed clay mineral. Their physical form is a wide, ﬂat sheet many times wider and longer than thick.

The three most common clay minerals, in order from largest to smallest in size, are
• Kaolinite
• Illite
• Montmorillonite or smectite—very expansive, weak clays Other somewhat less common clay minerals are
• Chlorite
• Attapulgite
• Vermiculite
• Sepiolite
• Halloysite, a special type of kaolin in which the structure is a tube rather than a ﬂat sheet
• Many others

Clays are arbitrarily deﬁned as particles less than 2 m in the long dimension. Clay particles are very small. If clay is observed under an ordinary optical microscope, the individual particles would not be seen. If a 1-in. cube of clay is sliced one million times in each direction, the resulting pieces are approximately the size of clay particles. If the surface areas of a thimbleful of clay particles are added up, the total would be 6 million square inches, the same surface area from about ﬁve truckloads of gravel.
At the microscopic level, the clay fabric may have either a dispersed (par-allel) or ﬂocculated (edge-to-side) structure. The clay fabric may have a signiﬁcant effect on engineering properties. Clay soils usually contain between 10% and 50% water by weight.

Silt is commonly found in ﬂat ﬂoodplains or around lakes. Silt is usually deposited by ﬂowing water or dust storms. Silt is composed of ground-up rock and is usually inorganic, that is, without plant material. A dry chunk of silt is easily broken by hand and is powdery.

Clays The particles in clay soils may have two general types of structures, depending on the concentration of polyvalent ions present in the groundwater.

If the particles are arranged in an edge-to-face arrangement, the clay has a ﬂocculated structure, in which the individual plate-shaped particles are clustered, as shown in Figure 3.1. Particles of this type are known to have a positive charge at the edges of the particle and a negative charge on the face of the particle, as illustrated in Figure 3.1a. If the specimen of clay is remolded, thereby destroying its natural structure, the plates may become parallel to each other, as shown in Figure 3.1b. Frequently, remolding results in a loss of shear strength compared to the undisturbed specimen. If the particles are arranged in parallel, the clay has a dispersed structure.

allel to each other, as shown in Figure 3.1b. Frequently, remolding results in a loss of shear strength compared to the undisturbed specimen. If the particles are arranged in parallel, the clay has a dispersed structure.

In addition to its mineralogical composition, the strength of clay is strongly dependent on the amount of water in the specimen. The past stress history of a stratum of clay is also of great interest to the geotechnical engineer. If the stratum was deposited in water and attained considerable thickness over a long period of time, a specimen at a particular depth is normally consolidated if equilibrium exists, with no ﬂow of water from the specimen due to the weight of the overburden. The specimen is overconsolidated if some of the overburden was removed by erosion or if the stratum was exposed to the atmosphere and later resubmerged. Consolidation may have occurred by desiccation if the stratum was uplifted and exposed to the air. Negative pore pressure develops due to capillarity and exposes the soil to positive com- pressive stress. The stratum will be overconsolidated; some formations of this sort have been resubmerged and exist in coastal areas in many parts of the world.
Silts Silt is a single-grained soil, in contrast to clay, where the grains are bonded due to chemical attraction. Silt grains are too small to be seen with the naked eye, ranging in size from 2 m to 0.074 mm. On drying, a lump of silt can be easily broken with the ﬁngers. Deep deposits of silt present problems to the engineer in the design of foundations. Chapter 2 notes problems with foundations in loess, existing principally above the water table.

Deposits of silt below the water table may be organic and compressible. Because of the small grains, water will ﬂow slowly through the soil, and drainage due to a foundation load could be lengthy. Deep foundations penetrating the deposit of silt might be recommended in lieu of shallow foundations.
Silt can hold water and is usually soft when wet. A wet silt will ﬂatten out when shaken, and the surface will appear wet. Silt is usually found in mixtures with sand or ﬁne sand. A mixture of sand with a smaller amount of silt is often called a dirty sand.

Silt is usually a poor foundation material unless it has been compressed and hardened like siltstone. Many types of silt are compressible under low foundation loads, causing settlement. As a construction material, silt is difﬁcult to work with. It is difﬁcult to mix with water, is ﬂuffy when dry, and pumps under compaction equipment if too wet. Silt particles can be ﬂat, like clay particles, or blocky, like sand particles.
Sands and Gravels Sand also is a single-grained soil that exists in a wide range of sizes, from ﬁne (0.074 to 0.4 mm), to medium (0.4 to 2 mm), to coarse (2 to 4.76 mm). The grains of sand can have a variety of shapes, from subrounded to angular, and deposits of sand can have a variety of densities.
Laboratory tests will reveal the effect of such variations on strength and stiffness. If the sand is loose, vibration from nearby motorized equipment can cause densiﬁcation and settlement.
Sand is a granular material in which the individual particles can be seen by the naked eye. It is often classiﬁed by grain shape—for example, angular, subangular, or rounded. Sand is considered to be a favorable foundation material, but deposits are subject to erosion and scour, and need protection if
they occur near a waterfront or a river.
Deposits of sand allow easy ﬂow of water and do not hold water permanently. Water rising vertically through sand can cause quicksand. Any deposit of sand that holds water is a mixture containing ﬁner material, either silt or clay. If a deposit of sand contains water and the deposit is surrounded by relatively impermeable deposits of clay or silt, a perched water table exists with a water table higher than exists outside the perched zone. Perched water sometimes results in construction difﬁculties if the perched water table is not
recognized.
One problem with sand is that excavations often cave in. A slope in sand steeper than 1.5 horizontal to 1 vertical rarely occurs. The angle of the steepest natural slope is called the angle of repose.
The mineral composition of the grains of sand should receive careful attention. The predominant minerals in many sands are quartz and feldspar.
These minerals are hard and do not crush under the range of stresses com- monly encountered. However, other types of sand, such as calcareous sands, can be soft enough to crush. Calcareous sands are usually found between the Tropics of Cancer and Capricorn and present special problems for designers.
One project for which the calcareous nature of a sand was of concern was an offshore oil production platform founded on piles driven into a deposit of lightly cemented calcareous soil off the coast of Western Australia. Openended steel pipe piles were driven to support the structure. Construction followed an intensive study to develop design parameters for axial loading. King and Lodge (1988) wrote that ‘‘it was noted with some alarm that the 1.83 m diameter piles not only drove to ﬁnal penetration more easily than any of the pre-installation predictions but also that the piles free-fell under their own weight with little evidence of the expected frictional resistance.’’ The calcareous sand was composed largely of the calcium carbonate skeletal remains of marine organisms (Apthorpe et al., 1988). During pile installation, the tips of the piles apparently crushed the grains of calcareous soil and destroyed the natural cementation. The cementation present in the sand was likely a natural consequence of the deposition and prevented development of the normal axial resistance along the length of the piles. The offshore project illustrates how the strength of sand grains and natural cementation can require special attention for designs of some foundations.
Gravel, as sand, also exists in many sizes from ﬁne (4.76 to 19 mm) to coarse (19 to 75 mm).
Driving piles into deposits of sand and gravel may present difﬁculties. If the deposit is loose, vibration will cause a loss of volume and allow the pile to penetrate; otherwise, driving of a pile is impossible. The problem is especially severe for offshore platforms where a speciﬁc penetration is necessary to provide tensile resistance. Various techniques may be employed, but delays in construction may be very expensive.
Cobbles range in size from 75 to 1000 mm, and the size of boulders is above 1000 mm. A glacier can deposit a layer of cobbles and boulders with weaker soil above and below. Often, piles cannot be driven into such materials and drilling or coring for a drilled shaft foundation is the only practical alternative (see Chapters 5 and 11).

Rock or Bedrock Rock or bedrock refers to deposits of hard, strong material that serves as an end-bearing foundation for piles or drilled shafts and, in some instances, as support for a shallow foundation. Characteristics of importance for rock are compressive strength and stress-strain characteristics of intact specimens. In some instances, the secondary structure of rock, re- ferring to openings, joints, and cracks, is of primary importance. The secondary structure can be deﬁned quantitatively by the rock quality designation
(RQD), a number obtained by dividing the sum of the length of core fragments longer than 100 mm that were recovered by the overall depth that was cored (Chapter 4).