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9-1. General Requirements
The geologic background of a site has an important bearing on the classification of soils and determining suitable engineering
characteristics. Soils are generally classified into two broad categories: (1) those that are derived in situ from bedrock, called
residual soils and (2) those that have been deposited at a site by wind, water, or ice, known astransported soils. Most soils are
formed from rock by weathering, which includes mechanical disintegration and chemical processes of separation. Soil layers
and strata are formed by freezing/thawing and leaching of top soils through the removal of soluble salts and colloids. Climate,
time, and vegetation also play a role in soil layer development.
For the most part, soil types are still defined using theUnified Soil Classification System developed by Arthur Casagrande in
1948. The major divisions include coarse-grained soils (gravels, sands) and fine-grained soils (silts, clays, organics), which are
also described in terms of the consistency and distribution of particle sizes and shapes (grading of coarse-grained soils) and
plasticity (stickiness of fine-grained soils). This classification system is also used to identify soil types for defining presumptive
load bearing values in IBC Table 1806.2. The following symbols are used by Casagrande:
1. Coarse-grained soils:
A. GW—Well-graded gravels or gravel-sand mixtures with little or no fines
B. GP—Poorly graded gravels or gravel-sand mixtures with little or no fines
C. GM—Silty gravels or gravel-sand-silt mixtures
D. GC—Clayey gravels or gravel-sand-clay mixtures
E. SW—Well-graded sands or gravelly sands with little or no fines
F. SP—Poorly graded sands or gravelly sands with little or no fines
G. SM—Silty sands or sand-silt mixtures
H. SC—Clayey sands or sand-clay mixtures
2. Fine-grained soils:
A. ML—Inorganic silts and very fine sands with slight plasticity (liquid limit [LL] < 50)
B. CL—Inorganic clays of low to medium plasticity, gravelly clays, and sandy clays with low to medium plasticity (LL < 50)
C. OL—Organic silts and organic silty clays of low plasticity (LL < 50)
D. MH—Inorganic silts, fine sandy or silty soils (LL > 50)
E. CH—Inorganic clays with high plasticity (LL > 50)
F. OH—Organic clays and silts with medium to high plasticity (LL > 50)
G. PT—Peat and other highly organic soils
Foundation types are selected based on the soil properties at a site, economic considerations, and the quality of labor available
to perform excavations and place materials. Shallow footings are usually the most economical and easiest to place, where the
depth is less than the width. Deep foundations often require special equipment and knowledge, and are classified with a depth
that is greater than about four times the diameter or square width. Micropiles form their own sort of classification as smalldiameter, high-strength elements placed as another type of deep foundation using tested procedures by highly qualified
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Shallow spread or continuous footings are used where bearing is adequate over a single stratum of soil close to the surface (or
that may be prepared for anticipated bearing loads with excavation and replacement with engineered fill), giving consideration
to the potential for immediate and differential settlements over time. Shallow mat-type foundations are often used when a
multitude of high column loads are present in such proximity as to provide little separation between individual spread footings.
Mats may be designed as rigid or flexible, and are also used for purposes of controlling settlement and building movement.
Deep foundations are usually needed when surface soils are poor and bearing loads need to be transferred to deeper soil
strata. They are also specified when construction occurs in close proximity to other structures to avoid adding surcharge to
basement walls or reducing the bearing effectiveness of existing foundations. Loads are transferred to the soil by end bearing
or skin friction (usually a combination of the two), and lateral forces are resisted by side bearing using methods associated with
the stiffness of the pile. Piles are placed in groups and tied together at the top with a pile cap. Drilled piers, or caissons, are
placed by creating a hole and placing fresh concrete or even a finished pier inside and are not always tied together with a cap.
Retaining walls can be built from each of the common materials but are most often masonry or concrete. The selection is
usually made as a matter of economics and quality of labor, and of the type of wall suitable for a particular task. The function of
a retaining wall is to hold back soil, water, grains, etc. and disallow them to relax back into their natural slope; this imposes
lateral forces predicted by equations that have been in use for decades. Different methods of design are used for a variety of
wall types commonly encountered:
1. Gravity walls depend on their weight for stability against imposed lateral forces. Stability is defined as a resistance to both
lateral translation (sliding) and overturning.
2. Cantilever walls retain the mass of soil by cantilever action above the top of a separate foundation, where stability is often
achieved by the weight of the soil over the heel (portion of the footing behind the retaining wall) in addition to the materials
of construction.
3. Restrained walls are used as basement walls, where the wall is pinned against translation at the top by a horizontal
diaphragm of a building or other structure and is pinned or fixed at the base to resist translation and sometimes rotation.
Having a fixed base increases the size of the foundation, but it results in a smaller horizontal force at the top, which can be
helpful in the design of the structure providing resistance.
4. Counterfort walls are similar to cantilevered walls, where the cantilevered portion is long or high pressures are involved, and
counterforts are provided behind the wall to tie the wall and base together.
5. Buttressed walls are similar to counterfort walls, except that the buttresses are placed at the front of the wall instead of the
back and resist imposed forces by compression.
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