Geotechnical Engineering Snapshot
Geotechnics is the branch of civil engineering concerned with the engineering behaviour of earth materials. Geotechnical engineering uses principles of soil mechanics and rock mechanics to determine the relevant properties of these materials.
To reduce risk of encountering poor soils and to provide cost efficient foundation, earthworks and any structure supported by the ground for that matter, accurate information on the properties of the underlying site soil properties are required.
The primary relevant properties are the deformation properties which determine the settlement and the shear strength which controls the bearing strength and the stability.
The general term "soils" include all material from soft earth to rocks and the science of soil mechanics relates to the study of the physical properties of soils.
Soil mechanics is a combination of knowledge gained from experience, site testing, laboratory testing and theoretical analysis.
The design of a foundation includes the geotechnic design relating to the ground and the design of the foundations and the interaction between the two.
Basic soil elements
Sand Cohesionless granules formed mainly from hard particles . In bulk it is very permeable owing to the voids. Primary characteristic, frictional resistance to shearing forces . Often found mixed with gravel. Gravel Similar to sand but comprised of larger particles of solid stone Clay Properties depend mainly on the consistency , the water content being the most important. Highly cohesive, has a definite mobility and yields under pressure when the moisture content exceeds 20%. Yielding takes place by expulsion of the water from the pores. Therefore the rate at which compression occurs is slow owing to the low permeability. Often mixed , or stratified with sand. Pure clays have a very a very small angle of internal friction which approach zero in soft clays and especially soft puddled clays. Silt Similar to course clay , fine grained micaceous silts can be mistaken for clay, although unsatisfactory for load-bearing because disturbance of ground water may completely alter the nature of the material. Organic silts containing organic matter are highly compressible and not suitable for load bearing.
Types of Soils
The classification of soils relevant to the construction of foundations and earthworks it different to the geological classification. In engineering a classification based on physical characteristics is adopted.
Sieve Size Description
over 200 Boulders
In reality soils are non uniform but are mixed and general descriptions as shown below apply
Slightly sandy GRAVEL <5% Sand
Very sandy GRAVEL 20%-50% Sand
Very gravelly SAND 20% to 50% gravel
Slightly Silty GRAVEL/SAND up to 5% silt
Very silty GRAVEL/SAND 15% to 35% silt
Clayey GRAVEL/SAND 5% to 15% silt
Sandy SILT/CLAY 35% to 65% sand
Very coarse over 50% cobbles/boulders
Sandy GRAVEL 5%-20% Sand
Gravel/SAND equal proportions
Slightly gravelly SAND up to 5% gravel
Silty SAND/GRAVEL 5% to 15% silt
Slightly clayey SAND/GRAVEL up to 5% clay
Very clayey SAND/GRAVEL 15% to 35%clay
Gravelly SILT/CLAY 35% to 65% gravel
In terms of effective stresses, the shear strength is often approximated by:
τ = σ' tan(φ') + c'
Where σ' =( σ - u), is defined as the effective stress. σ is the total stress applied normal to the shear plane, and u is the pore water pressure acting on the same plane. φ' = the effective stress friction angle, or the @angle of internal friction' after Coulomb friction. The coefficient of friction μ is equal to tan(φ'). Different values of friction angle can be defined, including the peak friction angle, φ'p, the critical state friction angle, φ'cv, or residual friction angle, φ'r. c' = is called cohesion, however, it usually arises as a consequence of forcing a straight line to fit through measured values of (τ,σ')even though the data actually falls on a curve. The intercept of the straight line on the shear stress axis is called the cohesion. It is well known that the resulting intercept depends on the range of stresses considered: it is not a fundamental soil property. The curvature (non linearity) of the failure envelope occurs because the dilatancy of closely packed soil particles depends on confining pressure.
Fill, and Ground improvement
Sometimes it is necessary to acheive adequate ground conditions by
-Placing naturual soil, crushed rock, stone fragments or suitable waste producs, - dewatering. - treating the ground -reinforcing the ground.
These actions may be completed beneath foundations , as backfill to excavations and retaining structures, as general landfill or as embankments for infrastructure. The materials and methods should be selected such that the resulting ground system is suitable for design function over the design life of the construction. Fill and backfill beneath and around foundations should be compacted so that there is no possibility of subsidence.
Types of Foundations
There are two primary types of foundations shallow foundations as used for houses and smaller commercial properties and deep foundations as used for multi story buildings
Shallow Foundations-general notes and information Shallow foundations, often called footings, are usually embedded about a meter or so into soil. One common type is the spread footing which consists of strips or pads of concrete (or other materials) which extend below the frost line and transfer the weight from walls and columns to the soil or bedrock. The naming conventions for different types of footings vary between different engineers. Pad foundations are used to support individual or multiple columns, spreading the load to the ground below. They are generally square or rectangular in plan, with the plan area being determined by the permissible bearing pressure of the soil. The shape in plan will be dictated by the arrangement of the columns and the load to be transferred into the soil. The thickness of the pad must be sufficient to ensure distribution of the load. In general some reinforcement (either welded steel fabric or reinforcing bars, depending on the loads involved) may be required. Another common type of shallow foundation is the slab-on-grade foundation where the weight of the building is transferred to the soil through a concrete slab placed at the surface. Slab-on-grade foundations can be reinforced mat slabs, which range from 25 cm to several meters thick, depending on the size of the building, or post-tensioned slabs, which are typically at least 20 cm for houses, and thicker for heavier structures
Deep foundations -General Notes
A deep foundation is used to transfer a load from a structure through an upper weak layer of soil to a stronger deeper layer of soil. There are different types of deep footings including impact driven piles, drilled shafts, caissons, helical piles, and earth stabilized columns. Historically, piles were wood, later steel, reinforced concrete, and pre-tensioned concrete. The following notes are provided as background information.
Piling will normally be undertaken under the advice of specialist engineers. Piling may be the preferred foundation option under the following circumstances:
When the loads transmitted to the ground by the walls or columns of a building or other structure are of such a magnitude that the use of conventional foundations would result in uneconomically large bases.
When the loads cannot be taken on shallow foundations even if the bases occupy the whole of the building area.
When the upper strata are very weak or include unstable soils such as peat or unconfined sand the loads may be transferred through the faulty strata to a firm subsoil capable of supporting the loads.
When the upper strata are subject to large moisture movements piling may be desirable to ensure foundations at a stable level.
Two basic type of piling design are used: the first is , in effect , a column sunk into the ground whose base rests upon a suitable bearing such as rock.
Some of these types have belled bottoms to increase the bearing surface : the second type of pile is the friction pile which derives its support from the friction of the surface of the pile against the surrounding soil.
Obviously these piles also tend to gain support at the bottom bearing surface, but the principal design basis is frictional resistance to movement. The physical form of a particular pile depends upon the method of its sinking.
Piles can be preformed and driven into the ground or they can be cast in a formed hole .
Bored Piles- three types.
- A steel tube is dropped into the soils and then withddrawn and the soil retained in the bore is removed. An outer steel shell is driven down the bore as the work proceeds to prevent wall collapse. When concrete casting takes place the shell is removed in stages as the work proceeds.
- A rotating drill with shell casing cuts into the soil.
- A large diameter auger cuts broad and short piles
Two types of driven piles are used :steel piles which are either columns or formed sheets: concrete piles - usually precast with a steel tip to aid driving.
The sinking of driven piles or sheet piling can be by traditional pile-drivers or ,in some cases, by high speed vibration .
The latter is often used in built-up areas. Shells on bored piles are often sunk using the same methods.
When the load from a building requires more than one pile to provide adequate support , a number of piles are grouped in a "cluster" under a common pile cap which compensates for errors in positioning of the piles and also distributes the load evenly across the piles.