Geotechnical engineering

Geotechnical engineering is the branch of civil engineering concerned with the engineering of earth materials. Geotechnical engineering includes investigating existing subsurface conditions and materials; assessing risks posed by site conditions; designing earthworks and structure foundations; and monitoring site conditions, earthwork and foundation construction. Geotechnical engineers apply the principles of soil mechanics to analyse and design earthworks and buildings.

A typical geotechnical engineering project begins with a geotechnical investigation of soil and bedrock in an area of interest to determine their engineering properties including how they will interact with, on or in a proposed construction. Investigations can also include the assessment of the risk to humans, property and the environment from natural hazards such as earthquakes, landslides, sinkholes, soil liquefaction, debris flows and rock falls.

A geotechnical engineer then determines and designs the type of earthworks, foundations, retaining structures, and/or pavement subgrades required for the intended man-made structures to be built. Foundations are designed and constructed for structures of various sizes such as high-rise buildings, bridges, medium to large commercial buildings, and smaller structures where the soil conditions do not allow code-based design.

Foundations built for above-ground structures include shallow and deep foundations. Retaining structures include earth-filled dams, retaining walls, and excavation shoring. Earthworks include embankments, tunnels, levees, channels, reservoirs, and sanitary landfills.

Geotechnical engineers monitor earthwork and foundation construction to ensure that fills are properly placed and compacted, and that foundations are constructed as designed. Geotechnical monitoring also allows engineers to provide revised design criteria if the soil or rock conditions in the field do not match the conditions anticipated from the investgiation.

The fields of geotechnical engineering and engineering geology are closely related, and intersect in some areas. However, the field of geotechnical engineering is a specialty of engineering, where the field of engineering geology is a specialty of geology. Geotechnical engineering is also related to coastal and ocean engineering. Coastal engineering can involve the design and construction of wharves, marinas, and jetties. Ocean engineering can involve foundation and anchor systems for offshore structures such as oil platforms.

Soil mechanics

In geotechnical engineering, soils are considered a three-phase material composed of: rock or mineral particles, water and air. The voids of a soil, the spaces in between mineral particles, contain the water and air.

The engineering properties of soils are affected by four main factors: the predominant size of the mineral particles, the type of mineral particles, the amount of fine particles, and the relative quantities of mineral, water and air present in the soil matrix. Fine particles (fines) are defined as particles less than 0.075 mm in diameter.

The main topics of soil mechanics are permeability, consolidation, stresses within soils, and shear strength of soils.

Permeability (fluid) and seepage

Water flows through soils are governed by certain soil properties. The hydraulic conductivity of a soil is a function of the particle sizes within the soil, the mineral contents of the soil, and the porosity of the soil. Water flows through soils are a function of the hydraulic conductivity of the soil, the geometry of the soil conditions, and the hydraulic head across the soil region. Internal erosion of soil, called piping, can occur if the soil gradation and the water flow through a soil allow the smaller particles to move through the pore spaces created by the larger particles.

Consolidation

Consolidation is the reaction of soil to applied stress. A soil which has stress applied to it will compress due to expulsion of water from the soil and rearrangement of the soil particles. The rate at which consolidation occurs is dependent on the hydraulic conductivity of the soil. Compression of soils due to consolidation is only partially reversible by removal of the stress which caused the consolidation.

Stress

Stresses within soils can be subdivided into total (compressive) stress, shear stress, and effective stress, which is the total stress minus the pore water pressure, and is one of the most important quantities in soil mechanics. The pore water pressure, which is the hydrostatic pressure of free water at a point in the soil matrix, is also important in soil mechanics.

Shear strength

Most soil failure conditions occur in shear. The shear strength of most soils is dependent on the effective stress; for most purposes, this is modeled as a linear dependence, where the shear strength at zero effective stress is called the cohesion, and the slope of the line plotting maximum shear stress against effective stress is caled the friction angle. For materials with no cohesion, the friction angle is equal to the angle of repose, the steepest angle at which the material will remain stable. Sands and gravels typically have very low to zero cohesion. The cohesion and friction angle of clays can depend on the rate of shearing.

Geotechnical investigation

Geotechnical engineers perform geotechnical investigations to obtain information on the physical properties of soil and rock underlying (and sometimes adjacent to) a site to design earthworks and foundations for proposed structures and for repair of distress to earthworks and structures caused by subsurface conditions. A geotechnical investigation will include surface exploration and subsurface exploration of a site. Sometimes, geophysical methods are used to obtain data about sites. Subsurface exploration usually involves soil sampling and laboratory testing of the soil samples retrieved.

Surface exploration can include Geologic mapping, geophysical methods, and Photogrammetry, or it can be as simple as an engineer walking around on the site to observe the physical conditions at the site.

To obtain information about the soil conditions below the surface, some form of subsurface exploration is required. Methods of observing the soils below the surface, obtaining samples, and determining physical properties of the soils and rock include test pits, trenching (particularly for locating faults and slide planes), borings, and cone penetration tests.

Borings come in two main varieties, large-diameter and small-diameter. Large-diameter borings are rarely used due to safety concerns and expense, but are sometimes used to allow a geologist or engineer to visually and manually examine the soil and rock stratigraphy in-situ. Small-diameter borings are frequently used to allow a geologist or engineer examine soil or rock cuttings from the drilling operation, to retrieve soil samples at depth, and to perform in-place soil tests. A Cone Penetration Test (CPT) is typically performed using an instrumented probe with a conical tip, pushed into the soil hydraulically. A basic CPT instrument reports tip resistance and shear resistance along the cylindrical barrel. CPT data has been correlated to soil properties. Sometimes instruments other than the basic CPT probe are used.

Geophysical exploration is also sometimes used; geophysical techniques used for subsurface exploration include measurement of seismic waves (pressure, shear, and Rayleigh waves), using surface-wave methods and/or downhole methods, and electromagnetic surveys (magnetometer, resistivity, and ground-penetrating radar).

Engineering properties of soils

The following properties are commonly determined during a geotechnical investigation, and are often used in geotechnical engineering:

Bulk density

Total unit weight: Cumulative weight of the solid particles, water and air in the material per unit volume. Note that the air phase is often assumed to be weightless.

Dry unit weight: Weight of the solid particles of the soil per unit volume.

Saturated unit weight: Weight of the soil when all voids are filled with water such that no air is present per unit volume. This is typically assumed to occur below the water table.

Porosity

Ratio of the volume of voids (containing air and/or water) in a soil to the total volume of the soil expressed as a percentage. A porosity of 0% implies that there is neither air nor water in the soil.

Permeability

A measure of the ability of water to flow through the soil, expressed in units of velocity.

Consolidation

As a noun, the state of the soil with regards to prior loading conditions; soils can be underconsolidated, normally consolidated or over-consolidated.

As a verb, the process by which water is forced out of a soil matrix due to loading, causing the soil to deform, or decrease in volume, with time.

Soil Settlement

A decrease in total soil volume concurrent with a decrease in voids.

Shear strength

Amount of shear force which a soil can resist without failing.

Atterberg Limits

Liquid limit, plastic limit, and shrinkage limit used in defining other engineering properties of a soil and in soil classification.

Plasticity

A defining characteristic of soils, most notably clays and silts.

Foundations

A building's foundation transmits loads from buildings and other structures to the earth. Geotechnical engineers design foundations based on the load characteristics of the structure and the properties of the soils and/or bedrock at the site.

The primary considerations for foundation support are bearing capacity, settlement, and ground movement beneath the foundations. Bearing capacity is the ability of the site soils to support the loads imposed by buildings or structures. Settlement occurs under all foundations in all soil conditions, though lightly loaded structures or rock sites may experience negligible settlements. For heavier structures or softer sites, both overall settlement relative to unbuilt areas or neighboring buildings, and differential settlement under a single structure, can be concerns. Of particular concern is settlement which occurs over time, as immediate settlement can usually be compensated for during construction. Ground movement beneath a structure's foundations can occur due to shrinkage or swell of expansive soils due to climactic changes, frost expansion of soil, melting of permafrost, slope instability, or other causes. All these factors must be considered during design of foundations.

Many building codes specify basic foundation design parameters for simple conditions, frequently varying by jurisdiction, but such design techniques are normally limited to certain types of construction and certain types of sites, and are frequently very conservative.

In areas of shallow bedrock, most foundations may bear directly on bedrock; in other areas, the soil may provide sufficient strength for the support of structures. In areas of deeper bedrock with soft overlying soils, deep foundations are used to support structures directly on the bedrock; in areas where bedrock is not economically available, stiff "bearing layers" are used to support deep foundations instead.

Lateral earth support structures

A retaining wall is a structure that holds back earth at a slope steeper than the soil could stand on its own over the long term. Retaining walls stabilize soil and rock from downslope movement or erosion and provide support for vertical or near-vertical grade changes. Cofferdams and bulkheads, structures to hold back water, are sometimes also considered retaining walls.

The primary geotechnical concern in design and installation of retaining walls is that the retained material is attempting to move forward and downslope due to gravity. This creates soil pressure behind the wall, which can be analysed based on the angle of internal friction (φ) and the cohesive strength (c) of the material and the amount of allowable movement of the wall. This pressure is smallest at the top and increases toward the bottom in a manner similar to hydraulic pressure, and tends to push the wall forward and overturn it. Groundwater behind the wall that is not dissipated by a drainage system causes an additional horizontal hydraulic pressure on the wall.

Slope stability

Slope stability is the analysis of soil covered slopes and its potential to undergo movement. Stability is determined by the balance of shear stress and shear strength. A previously stable slope may be initially affected by preparatory factors, making the slope conditionally unstable. Triggering factors of a slope failure can be climatic events can then make a slope actively unstable, leading to mass movements. Mass movements can be caused by increases in shear stress, such as loading, lateral pressure, and transient forces. Alternatively, shear strength may be decreased by weathering, changes in pore water pressure, and organic material.

Notes and References

References

• Holtz, R. and Kovacs, W. (1981), An Introduction to Geotechnical Engineering, Prentice-Hall, Inc. ISBN 0-13-484394-0
• Bowles, J. (1988), Foundation Analysis and Design, McGraw-Hill Publishing Company. ISBN 0-07-006776-7
• Cedergren, Harry R. (1977), Seepage, Drainage, and Flow Nets, Wiley. ISBN 0-471-14179-8
• Kramer, Steven L. (1996), Geotechnical Earthquake Engineering, Prentice-Hall, Inc. ISBN 0-13-374943-6
• Freeze, R.A. & Cherry, J.A., (1979), Groundwater, Prentice-Hall. ISBN 0-13-365312-9
• Mitchell, James K. & Soga, K. (2005), Fundamentals of Soil Behavior 3rd ed., John Wiley & Sons, Inc.
• NAVFAC (Naval Facilities Engineering Command) (1986) Design Manual 7.01, Soil Mechanics, US Government Printing Office
• NAVFAC (Naval Facilities Engineering Command) (1986) Design Manual 7.02, Foundations and Earth Structures, US Government Printing Office
• NAVFAC (Naval Facilities Engineering Command) (1983) Design Manual 7.03, Soil Dynamics, Deep Stabilization and Special Geotechnical Construction, US Government Printing Office
• Terzaghi, K., Peck, R.B. and Mesri, G. (1996), Soil Mechanics in Engineering Practice 3rd Ed., John Wiley & Sons, Inc.