«A Design Guide for Earth Retaining Structures Hugh Brooks Civil & Structural Engineer John P. Nielsen Civil & Geotechnical Engineer Basics of ...»
A Soil Primer The mantle of our earth is composed of water, rock and soil. It is the soil or rock that supports our structures. We need to understand what soil is, how it behaves, and the properties we need for design. Soil is a collective term for any mixture of sand, silt, or clay. Soil is not “dirt”, which we sweep off the floor and wash from our clothes. Dirt is a colloquial term contractors often use, such as “We underestimated the fill quantity and need to import 200 more yards of dirt (a “yard” in that terminology means one cubic yard).
Soil is the result of the decomposition of rock. Rocks decompose by weathering, freezing and thawing, by crushing and grinding along earthquake faults, along planes of failure in landslides, by the overland movement of glaciers, the tumbling action of rivers and streams, and from the corrosive inorganic acids present in the atmosphere and derived from plants. Additionally, we must add heat, temperature changes and pressures within the earth.
Before the mid-1920s, determining how large a footing was needed to support a structure was rudimentary. It consisted primarily of driving rods into the soil and observing the resistance, auger borings, test pits, and usually load testing a small area and observing tolerable deformations from which a footing could be safely sized. Recommended bearing capacities were published in the handbooks of the day. For instance, the 1916 New York Building Code listed capacity of various soils. An example: “Sand and clay mixed or in layers” allowed “2 tons per square foot”.
A pioneer to advance soil behavior to a science was Karl Terzaghi (1883-1963) who in 1925 published Erdbaumechanik, which loosely translates to mechanics of soil in construction, followed in 1926 by Principles of Soil Mechanics. Later, in 1948, he and Peck published the classic Soil Mechanics in Engineering Practice. His studies were based upon application of the theory of elasticity to mass materials. From his work, and that of others, the term soil mechanics evolved into geotechnical engineering.
Moving ahead to today, types of soil – sand, silt, or clay, primarily – are classified by particle size and the composition of the mixture. The distribution of grain size in a soil sample is determined by a grain size analysis. For example, in a sieve test a sample is passed through successively smaller sieves, and the amount by weight retained on each sieve is noted as a percent of the total.
With this information the geotechnical engineer can classify the soil per the most-used Uniform System for Classification of Soil (USCS) that is reproduced in Appendix B. Sieve sizes use a numbering system where the number indicates the number of openings per inch. For example, a #4 sieve has four openings per inch, or ¼” each, and a #200 sieve has 200 openings per inch, and so forth.
Some common designations of soil are:
There are other classifications systems, such as the AASHTO system (American Association of State and Highway Transportation Officials), but the USCS classification is most often referred to in the foundation investigation reports you will read.
Soil is further classified as being cohesive, non-cohesive, or somewhere in between.
Cohesive soil derives its strength primarily from the cohesive bond between particles. Examples include fine-grained silts and clays.
Non-cohesive, or granular soil, derives its strength from inter-particle friction between grains.
Sand and gravel are examples of non-cohesive soil. Non-cohesive soil is the type usually assumed for analysis of pressures against a retaining wall.
Expansive soils usually consist of clay, but some silt is also expansive. Expansive soil can lift footings if water is present or shrink upon drying. Some clays are highly expansive and change in volume with changes in water content. Such swelling can cause considerable pressure on retaining structures. It is for this reason that clay backfill should be avoided, and if the site contains expansive soil, the geotechnical engineer will recommend measures to minimize its effect, mainly by removal and replacement with suitable material. It is important that water not be allowed to penetrate expansive soil.
Frost line is a term used in colder climates in the northern US, whereby upper portions of the ground may freeze seasonally or permanently, with depths ranging from a few inches to eight feet or more. To prevent the added pressure of swelling because of freezing, foundations should be placed below the frost line. The geotechnical engineer and applicable building codes will address this local concern. In areas where the ground is permanently frozen to a great depth, such as Alaska, local expertise and experience will apply.
Bearing capacity of a soil is an estimate of its capability to support a vertical load in compression.
The shearing strength of the soil is the controlling factor for determining its bearing capacity. The shear between particles can be either frictional resistance (sliding friction between particles) or in the case of a clayey soil, cohesion and perhaps interparticle friction. Sandy soil requires confinement to develop shear strength, as for example a lack of confinement is illustrated when you step on sand at the beach you will notice the sand displaces sideways under your feet. This illustrates the lack of frictional forces at work.
When soil samples (cores retrieved from drilling) are taken to the laboratory for testing, the geotechnical engineer will calculate the bearing capacity of the particular soil by determining its angle of internal friction, , its unit cohesion, c, and its unit weight.
Most soil mechanics texts will thoroughly cover the several types of shear tests available to the geotechnical engineer.
The basic equation for shear resistance developed along a plane of rupture is:
s = shear strength; p = effective normal stress and c = cohesion, both usually expressed in psf;
and = effective angle of internal friction.
Always check with the Building Department having jurisdiction over the project to determine the code(s) adopted by the jurisdiction and if any local amendments apply. The following codes are most often adopted or cited.
Building Codes International Building Code (IBC) This standard building code has been adopted by most jurisdictions, some with local modifications (California Building Code, for example). The IBC was a culmination of efforts to merge into one national building code the Uniform Building Code, Southern Building Code, and Standard Building Code. The IBC is compiled and published by the International Code Council (ICC), County Club Hills, Illinois. The series of International Building Codes (e.g. plumbing, electrical, etc.) are collectively referred to as the “I-Codes”. The IBC Website is www.iccsafe.org. The current edition is 2012. IBC 2012 references or modifies other standard codes, principally ASCE 7-10 Minimum Design Loads for Buildings and Other Structures.
Uniform Building Code (UBC), '97 This now defunct code, the last in a series first published in 1927 by the International Conference of Building Officials, was the dominant code in the Western states until replaced by the International Building Code and California Building Code.
California Building Code (CBC) This California code was first published in 2001 to replace the ’97 Uniform Building Code. It is an adaptation of the IBC with minor modifications and is essentially the same as the IBC. The current edition is 2013. See www.bsc.ca.gov.
NFPA 5000: Building Construction and Safety Code (National Fire Prevention Association) NFPA 5000 has been promoted in some States. It addresses construction protection and occupancy features necessary to minimize danger to life and property. The current edition is NFPA 5000: Building Construction and Safety Code, 2012 Edition. The NFPA web address is www.nfpa.org. This code references ACI 318, ASCE 7 and ACI 530 for structural design issues.
Referenced Publications IBC 2012, CBC ’10, and other regional codes, often refer to the following standards for structural
Lateral Earth Pressures The purpose of a retaining wall is to retain soil and to resist the lateral pressure of the soil against the wall. Most lateral pressure theories are based upon the sliding soil wedge theory. This, in simple terms, is based upon the assumption that if the wall is suddenly removed, a triangular wedge of soil will slide down along a rupture plane, and it is this wedge of soil that the wall must retain. The development of the soil wedge theory was discussed in Chapter 3. There are two basic equations for computing lateral earth pressures: the Coulomb equation and the Rankine equation.
The Coulomb Equation The Coulomb Equation where Ka is the coefficient of active pressure, which takes into account
backfill slope, friction angle at wall face, angle of rupture plane and angle of internal friction:
= Angle of backfill slope = Angle of internal friction of the soil = Wall slope angle from horizontal (90° for vertical face, ° if the back of wall is battered outward or 90° if wall battered inward) = Angle of friction between soil and wall (usually assumed to be 2/3 to 1/2/)
The Coulomb equation should only be used for gravity, segmental, gabion, and cantilevered walls having a short heel dimension. The reason is that the Coulomb equation includes a soil-to-wall friction angle, designated δ, which assumes the moving soil mass contacts the wall face and activates a shear resistance as the wall deflects. This friction angle δ is generally assumed to be between 0.5 and 0.7 times the phi () angle. For the case of a cantilevered wall with a larger heel dimension the soil between the stem and the failure plane can be considered a rigid mass, then δ in the Coulomb equation or Mononobe-Okabe equation (discussed in the seismic chapter) can be taken a equal to because with a cantilevered wall the soil above the heel will move in mass with the wall so that wall friction cannot develop, the failure plane being through the heel of the footing.
If the backfill is level, the inside wall face is vertical, and if zero friction is assumed between the
soil and wall, then the Coulomb equation reduces to the familiar Rankine equation:
The Rankine Equation The Rankine equation is a simplified version of the Coulomb equation that does not take into account wall batter or friction at the wall-soil interface. As such, it is a conservative approach to the design of retaining walls. An example of its use will be described later for both the Coulomb and Rankine equations. For the case for vertical walls with a level backfill and zero wall friction, the lateral pressure factor Ka will be the same by either approach Rankine’s approach was to evaluate the stress at a point in the backfill by using Mohr’s circle concepts to obtain the minimum lateral stress at a point in the backfill. The minimum lateral stress corresponds to the “active” case. Integration of that stress with respect to depth leads to a second-order equation (the well-known triangular distribution) for the total lateral force against the wall.
The use of the Rankine approach is recommended for most cantilevered retaining wall designs. It is conservative because it predicts a larger active force than that of Coulomb. It’s also simpler to calculate for most walls, and easily handles sloping backfills and surcharge loads.
The Rankine Equation for active pressure:
Earthquakes – An Overview Although our planet Earth is an incomprehensible 4.5 billion years old it is still cooling and adjusting. The tectonic plates (tectonic from the Latin: “building”) that wrap our earth continue to float, move, and rotate, as in past eons, to shape our topography and build mountains. And cause earthquakes as they lurch along their boundaries.