Beyond classification, engineers care deeply about several key soil properties.These include permeability, shear strength, compressibility, and density.Permeability measures how easily water can flow through soil pores.Shear strength measures how much shear stress soil can resist before it fails.Compressibility tells how much soil volume reduces under load.Density, or unit weight, affects stress at depth and slope stability.
Permeability is crucial when designing foundations exposed to groundwater.Coarse sands and gravels have high permeability and drain quickly.Clays have very low permeability and allow water to move only slowly.If drainage is poor, pore water pressures can rise and reduce effective stress.Reduced effective stress means reduced shear strength and larger settlements.Engineers often install drains or design special foundations to manage groundwater.
Shear strength is central to almost all soil and foundation problems.Think of the ground beneath a footing as a mass of particles resisting sliding and crushing.That resistance has two main components.One is cohesion, an apparent bonding between particles, especially in clays.The other is friction, related to how particles interlock and resist sliding.Shear strength is typically represented by the cohesion intercept and the friction angle.
The concept of effective stress connects shear strength, water, and external loads.Effective stress is the portion of total stress carried by the soil skeleton.Water in the pores carries the rest as pore water pressure.In most situations, shear strength depends primarily on effective stress, not total stress.If pore pressure rises, effective stress falls, and the soil becomes weaker even if total stress stays constant.This principle explains phenomena like quicksand and liquefaction.
Compressibility directly links soil behavior to settlement of structures.When a foundation applies load, the soil skeleton compresses and pore water pressures change.In coarse granular soils, drainage is rapid, and most settlement occurs quickly during or shortly after loading.In clay, drainage is slow, and settlement can continue for many years.Engineers distinguish between immediate settlement and consolidation settlement.Immediate settlement happens almost instantly due to elastic deformations.Consolidation settlement occurs gradually as water escapes from pores and effective stress rises.
Next, consider how engineers actually learn about soil at a project site.They begin with a site investigation, sometimes called a geotechnical investigation.The goal is to understand soil types, layering, groundwater conditions, and engineering properties.This investigation usually starts with desk studies of maps, previous reports, and aerial images.Then comes field exploration with borings, in situ tests, and sampling.
Boreholes are drilled at selected locations across the site.During drilling, engineers record soil changes with depth and groundwater levels.They collect samples for laboratory testing, paying attention to sample quality.In coarse soils, disturbed samples are often adequate for classification and density tests.In clay, undisturbed samples are essential for reliable strength and compressibility results.Special samplers like thin walled tubes help preserve the natural structure of clays.
In situ tests complement boring logs and provide continuous profiles of strength or stiffness.Two common tests are the Standard Penetration Test and the Cone Penetration Test.The Standard Penetration Test measures resistance to driving a split spoon sampler.The number of blows required gives an index of density and strength.The Cone Penetration Test pushes a cone with sensors into the ground at constant rate.It records cone resistance and sleeve friction, which correlate with soil properties.
Other in situ tests provide specialized information.Vane shear tests measure undrained shear strength of soft clays.Pressuremeter tests measure soil response to cylindrical expansion, giving direct stiffness and strength values.Permeability tests measure how quickly water flows through soil layers at depth.Together, these methods create a picture of subsurface conditions more reliable than any single test alone.
Laboratory testing refines this picture and provides design parameters.Basic tests include moisture content, specific gravity, and grain size distribution.Atterberg limits define plasticity characteristics.Compaction tests determine the relationship between moisture content and dry density for compacted fills.Shear strength tests such as direct shear and triaxial compression measure resistance under controlled conditions.Oedometer tests measure one dimensional consolidation behavior and compression index values.
With soil properties characterized, engineers can choose appropriate foundation types.Broadly, foundations fall into two categories, shallow and deep.Shallow foundations transfer loads to soil at relatively small depths compared to their width.Deep foundations extend much deeper to reach stronger layers or mobilize shaft resistance.The choice depends on soil strength near the surface, settlement tolerance, and economic factors.
Shallow foundations include spread footings, combined footings, mat foundations, and strip footings.A spread footing supports a single column and spreads the load over a wider soil area.A combined footing supports two or more columns when their footings would otherwise overlap.A mat or raft foundation supports many columns or entire buildings with a single large slab.Strip footings support load bearing walls or long rows of columns.
Shallow foundations are attractive when surface soils are reasonably strong and stiff.Their design focuses on two major issues.First is bearing capacity, meaning the ability of soil to support the load without shear failure.Second is settlement, meaning how much the foundation will move downward under load.Engineers must ensure both safety against failure and acceptable service performance.
Bearing capacity involves understanding how soil under a foundation will fail if overloaded.Classic solutions assume a failure surface forming beneath and around the footing.The ultimate bearing capacity is the load intensity that causes such a collapse mechanism.Engineers apply safety factors to obtain an allowable bearing capacity.For shallow foundations, bearing capacity depends on cohesion, friction angle, foundation width, depth, and shape.Groundwater depth and load inclination also influence it.
Settlement analysis is often more decisive than bearing capacity for serviceability.A footing might be safe from collapse but still settle enough to crack the building.Total settlement must remain within limits that the structure can tolerate.Differential settlement between supports must be even more tightly controlled.In granular soils, engineers estimate settlement using elastic theory and empirical correlations.In clays, they use consolidation theory, compression index, and time rate calculations.
Deep foundations become necessary when surface soils are too weak or compressible.Common deep foundations include driven piles, drilled shafts, and sometimes caissons.Piles are slender elements made of steel, concrete, or timber.They can be driven by hammering, pressed in, or formed by drilling and concreting.Piles transfer load through end bearing at their tip, shaft friction along their length, or a combination.
Driven piles are prefabricated and installed by impact or vibration.They displace soil laterally and can compact surrounding granular layers.Their installation is noisy and can cause vibrations, which may be restricted in urban areas.Drilled shafts, also called bored piles, are constructed by drilling holes and filling them with concrete and sometimes reinforcement.They cause less noise and vibration but require careful control to prevent collapse during drilling.
