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.

This page includes some notes on soils and foundations, and their properties related to the support of engineering structures.    These pages have not yet been updated to include information in accordance with the Eurocodes.    The information is however still relevant for basic information reference.     This page does not include sufficient information to enable meaningful design work to be completed however I have tried to include lists of the relevant standards and I have provided links to reputable information sources with detailed content.

The safe and cost efficient design and construction of foundations, and earthworks requires accurate data on the properties of the ground.    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 and laboratory testing and theoretical analysis.

To illustrate the complexity involved in the geotechnical design required in construction an abbreviated list of the relevant factors is provided below (ref BS EN 1997-1).

Actions and their combined load cases.
Suitablility of ground to overall stability and ground movements.
Disposition and classification of various zones of soil, rock and elements of construction involved in calculation model.
dipping bedding planes.
Mine workings, caves, and other underground structures where relevant.
For structures on or near rocks....
      interbedded hard and soft strata.
      faults, joint, and fissures.
      possible instability of rocks.
      solution cavities or fissures filled with soft materials and containing solutions.
Relative to the local environment.
      effects of scour, erosion and local excavation.
     effects of chemical erosion.
      effects of weathering, freezing, long droughts.
      variations in ground water.
      presence of emerging gases
      miscellaneous effects resulting from time and environment eg. holes made by animals , tree roots etc.
earthquakes
subsidence
effect of new structures on existing structures services and environment

It is clear that the complexity and variability of the geotechnical design aspects of a construction is greater than for structural design.

The notes and information below relate generally to soils.  Outline notes on rock types and properties are found on webpage. Rocks.

Listed below are some of the definitions provided in the relevant code BS EN 1997-1.

Geotechnic action.... action transmitted to the structure by the ground, fill, standing water or ground water
Ground ... soil , rock and fill in place prior to execution of construction.
Structure... combination of connected parts, including fill during construction , designed to carry loads and provide rigidity.
Derived value...Value of geotechnical parameter obtained by theory, correlation or empiricism from test results.
Stiffness.. material resistance to deformation
Resistance...Capacity of a component, or cross-section of a component of a structure to withstand actions without mechanical failure .

BS EN 1997 introduces the use of geotechnical categories to identify the complexity of the design / preparation work required

Category 1 ..is for small projects with negligible-risk and where the fundamental requirements will be satisfied on the basis of experience and qualitative geotechnical investigations;

Category 2 is for conventional structures (e.g. foundations, retaining walls, embankments) with no exceptional risk or difficult soil or loading conditions;

Category 3 is for structures not covered by Categories 1 and 2 (e.g. very large structures, structures involving abnormal risks).

Geotechnical category 1 should only be applied if there is no excavation below the water table or if comparable local experience indicates that a proposed excavation below the water table will be straightforward.

Geotechnical category 2 should include conventional types of structure and foundation with no exceptional risk.
examples of structures / foundations complying with geotechnical category 2

Spread foundations;
raft foundations;
pile foundations;
walls and other structures retaining or supporting soild or water;
excavations
bridge piers and abutments;

Most routine geotechnical design work will fall into Geotechnical Category 2.

A very simple flowchart illustrating the steps undertaken in completing a Geotechnic design process are shown below:

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.    In theory all of the identified limit states should be satisfied and also the service limit state should also be satisfied.    In practice normally one limit state is controls the design and the other limits states need only be reviewed.

BS EN 1997 identifies that geotechnical design involves consideration of five ultimate limit states.

� Verification of static equilibrium (EQU)...Stability

� Verification of (structural) strength (STR)...Strength

� Verification of (ground) strength (GEO)...Strength

� Verification of resistance to uplift (UPL)...Stability

� Verification of resistance to heave failure due to seepage (HYD)...Stability

Serviceability Limit state(SLS) is the design state such that the structure remains functional for its intended use subject to routine loading.

Verification for serviceability limit states in the ground or structional section or interface shall be such that

E_d =< C_d

C_d = Nominal value or function of certain design properties of materials- (related to serviceability limit state

e.g.The ground strength , throughout the design life of the construction, is such that any resultant structural deformations are within the serviceability limits

The Ultimate limit design requires that the value of the product or the maximum expected forces or moments on a section and the associated partial margins should be less than the characteristic value of the strength of the sections divided by the relevant material partial safety margins.   Refer to notes below.

E _d <= R _d

The combination of partial factors relevant to the structural design is provided in combination 1 and combination 2 as tabled below .    The partial factors related to the geotechnic design is also shown. .   

Actions

Actions may be forces ( loads applied to the structure or to the ground ) and displacements ( or accelerations ) that are imposed by the ground on the structure, or by the structure on the ground.    Actions may be permanent e.g. self-weight of structures or ground , variable e.g. imposed loads on building floors, or accidental (e.g. impact loads).

Design values of actions ( F _d ) are calculated using the general equation:

F _d = γ _F. F _rep

where
F _rep is the representative  ( or characteristic F _k ) value of an action (or of the effect of an action);
F _rep = ψ. F _k
γ _F is the partial factor for an action (or γ _E , for the effect of an action).

The equation for the effect of actions should be

The general equation for the effect of actions should be

The part of the equation inside the brackets represent to combination of permanent and variable actions

In BS EN 1990 one of a number of equations for load combinations is equation 6,10

Note: The prestress term (γ_PP ) only applies to prestressed concrete applications

This is a quick, but conservative, method when compared to the alternative equations (6.10a and 6.10b)which are a little more complicated. 6.10b is generally the governing equation in the UK

Properties

Ground properties must be obtained from the results of tests or from other relevant data such as the back-analysis of settlement measurements or of failures of foundations or slopes..

Design values of ground properties shall calculated using

X _d = X _k / γ_M

where
X _k = the characteristic value of a material (ground) property
γ _M = the partial factor for the material property.

Design Resistance

The general equation for the resistance of the foundation / structure material

where
γ_Rd = the partial factor for the resistance model uncertainty + geometric deviations.
X_d,i = design value of material property
a_d = the design value of a geometric property e.g depth.

This can be simplified to

where
γ_M,i = γ_Rd .γ_m,i
.

Note: Limited to STR and GEO ULS

There are two sets of combinations to use for the STR and GEO ultimate limit states.     The values for the partial factors to be applied to the actions for these combinations and the partial factors for the geotechnical material properties are given in the tables below.    Combination 1 will generally govern the structural resistance, and combination 2 will generally govern the sizing of the foundations.    The partial factors for soil resistance to sliding and bearing should be taken as 1.0 for both combinations .

Structural design

Geotechnic design

The Eurocode standard generally requires completion of two calculations for a given foundation providing a check. e.g calculation to Design Approach 1 combination 2 and Design Approach 1 combination 1 are completed.   These are essentially the same calulations using different partial margins.    The first to satify the requiremt for (GEO -ULS) and the second to satisfy the requirement of (STR ULS). A simple example is shown below using the analylitical method and based on formula found in Appendix D of BS EN1997-1

Consider a 2m x 2m concrete pad supporting a column withstanding a permanent vertical load of 1000 kN and an imposed variable load of 180 kN.    The dimensions of the column are ignored for simplicity.

Two calculations sets are completed for this shallow foundation:

1) In accordance with BS EN 1997 Design Approach 1 , Combination 2

The permanent load resulting from the pad and foundation over the pad
G_pad,k= (A).(Depth).(γ_kt) + (A)..(γ_sat).(Cover)
G_pad,k= (16).(0,6).(25) + (16).(0,6).(18) = 412,8 kN
The permanent load through the column = P_k = 1000kN
The variable load through the column = Q_k = 180kN

The design value of the vertical loads =

V_d = γ_G ( G_pad,k + P_k ) + γ_G ( Q_k )
V_d = 1,0 . ( 412,8 + 1000) + 1,3 . (180) =1 412,8 + 234 = 1646,8 kN

The design value of the Resistance Rd is given by BS EN 1997 Annex D equation D.1

R_d / A� = (π + 2) c _u,d b _c s _c i _c + q _d

where
c_u,d = c_u,k /γ_cu
c_u,d = 30 / 1.4 = 21.4 kPa,
b_c = 1(horizontal surface),
i_c = 1 (loads purely vertical)
s_c = 1 ,2 for square pad
q_d = 18. (0.6 + 0.6) = 21,6 kPa = overburden pressure at base of pad.
A' = A = 16m² ( no loading eccentricity, no bending moments)

R_d = [(3,14 + 2) �.( 21,4 ). ( 1 ). ( 1,2 ) .( 1) + 21,6 ] . 16 = 2124,7 (kN)

V_D < R_D Therefore design is OK to ULS

2) In accordance with BS EN 1997 Design Approach 1 , Combination 1

The permanent load resulting from the pad and foundation over the pad G_pad,k= (16).(0,6).(25) + (16).(0,6).(18) = 412,8 kN
The permanent load through the column = P_k = 1000kN
The variable load through the column = Q_k = 180kN

V_d = γ_G ( G_pad,k + P_k ) + γ_G ( Q_k )

V_d = 1.35 (412,8 +1000) + 1.5 (180) = �1907,3 + 270 = 2177,3 (kN)

R_d / A� = (π + 2) c _u,d b _c s _c i _c + q _d

where
c_u,d = c_u,k /γ_cu
c_u,d = 30 / 1.0 = 30 kPa
b_c = 1(horizontal surface),
i_c = 1 (loads purely vertical)
s_c = 1 ,2 for square pad
q_d = 18. (0.6 + 0.6) = 21,6 kPa = overburden pressure at base of pad.
A�_f = A = 4m² ( no loading eccentricity, no bending moments)

R_d = [(3,14 + 2) �.(30,0 ). ( 1 ). ( 1,2 ) .( 1) + 21,6 ] . 16 = 3307 (kN)

V_D < R_D Therefore design is OK to ULS

Prior to inititation of detailed foundation design.    Site Investigations should be completed and a geotechnical report should be completed in accordance with BS EN 1997-2 Eurocode 7:Geotechnical design- Part 2 :Ground investigation and testing.

In order to allow the investigation to commence it is necessary to know the general form, size and loading related to the construction and the location of the structure on the site.   The aims of the geotechnical investigation are to estabish the soil rock and groundwater conditions and to determine the properties of the soil and rock, and to gather additional relevant information about the site.

The use of in-situ tests and off site laboratory tests are available to obtain the necessary rock and soil properties.    Various in-situ test procedures are available for obtaining the properties of the rock or soil. Details of the test equipment and the relevant methods of completing the tests are provided in the noted standard.   The main tests and short descriptive notes are provided below.

Tests Used for site investigations.

1) Cone Penetrations Test (CPT)
This test is used to determine the resistance of soil and soft rock to the penetration of a cone and the local friction on a sleeve.    The CPT consists of pushing a cone penetrometer vertically into the soil using a series of push rods.  The cone penetrator shall be pushed into the soil at a constant rate (controlled between 1,5 -2,5 cm/s ).     The penetrometer comprises the cone and, if appropriate, a cylinderical shaft or friction sleeve.    The cone tipe shall have a diameter of either 3,6 or 4,4 cm.  The penetration resistance of the cone ( q_c ) as well as, if appropriate the local friction on the friction sleeve shall be measured.  An advantage of CPT over the Standard Penetration Test (SPT) is a more continuous profile of soil parameters, with CPTU data recorded typically at 2 cm intervals.

2) Pressuremeter Test (PMT)
The pressuremeter test is an in-situ testing method which is commonly used to achieve a quick and easy measure of the in-situ stress-strain relationship of the soil which provides parameters such as the elastic modulus. This tests measures in-situ deformation of soil and soft rock caused by the expansion of a cylinderical flexible membrane under pressure.     The test involves inserting a probe containing a cylinderical flexible membrane into the ground either in a pre-formed borehole, or by self-boring or by full displacement pushing.  When at a predetermined depth the membrane is expanded under pressure and readings of pressure and expansion are recorded until a maximum expansion for the particular device is reached.

3) Standard Penetration Test (SPT)
This test is to provide an indication of the relative density of granular deposits, such as sands and gravels from which it is virtually impossible to obtain undisturbed samples.    The benefit of the test, and the main reason for its widespread use is that it is simple and inexpensive.    These tests determine the resistance of soil at the base of a borehole to dynamic penetration of a split barrel sampler (or solid cone) and obtaining of disturbed samples for identification purposes.   
The sampler is driven into the soil by dropping a hammer 63,5 kg mass onto an avil or drive head from a height of 760mm.    the number of blows (N) necessary to achieve a penetration of the sampler of 300mm ( after its penetration under gravity and below a seating drive ) is the penetration resistance.    The test uses a thick-walled sample tube, with an outside diameter of 50 mm and an inside diameter of 35 mm, and a length of around 650 mm.     This is driven into the ground at the bottom of a borehole by blows from a slide hammer.    The sample tube is driven 150 mm into the ground and then the number of blows needed for the tube to penetrate each 150 mm up to a depth of 450 mm is recorded.    The sum of the number of blows required for the second and third 150 mm. of penetration is termed the "standard penetration resistance" or the "N-value".     In cases where 50 blows are insufficient to advance it through a 150 mm interval the penetration after 50 blows is recorded.    The blow count provides an indication of the density of the ground, and it is used in many empirical geotechnical engineering formulae.

4) Flexible Dilatometer Test(FDT)..Using a cylinderical borehole dilatometer probe.
This test determines the deformability of rock (rock-dilatometer test, RDT) and soil (soil dilatometer test SDT) from measurements of the radial expansion of a boreholes section under a known radial pressure applied by means of a cylinderical dilatometer probe.    The test consists of inserting a cylinderical probe, having an outer flexible membrane into a borehole and measuring at selected intervals, or in a semi-continuous manner, the radial displacement of the borehole while inflating the probe under known radial pressure.

Presumed bearing values acceptable for category 1 structures and preliminary calcs for category 2

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 vegetable matter are highly compressible and not suitable for load bearing.

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.

In reality soils are non uniform but are mixed and general descriptions as shown below apply

Note:symbols not in line with BS EN 1997

The behaviour of a soil is dependent on the water content which is important in the choice of shear strength parameters including the internal angle of shear φ and the cohesion c.  If water is present the applied loads are supported by the pore water pressures.    For granular soils above the water table, pore water pressures dissipate almost immediately as the water drains away and loads are effectively carried by the soil structure.    For fine grained soils, which are not as free draining pore water pressures take much loinger to dissipate.     Water and pore water pressures affect the strength and settlement characteristics of soil.

In carrying out geotechnical design assessments it is necessary to distinguish between drained and undrained conditions ( short and long term conditions ).    Short term design relates to the pore water pressures and the design must be carried out on the basis of φ_u and c_u.    for the drained ( long term conditions ) when the pore water pressure has been dissipated the design should be based on φ' and c '

In terms of effective stresses, the drained 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.    The resulting intercept depends on the range of stresses considered: it is not a fundamental soil property.     The non linearity of the failure envelope occurs as a result of the close packing of the soil particles which depends on the confining pressure.

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.

 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.

Strip foundations are used to support walls or closely spaced rows of columns.     Strip foundations under walls , particularly domestic walls can be form 1:3:6 concrete of a thickness not less than the projection beyond the wall facing but not less than 150 mm.

Suggested strip foundation widths are provided in table below.

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.

Shallow Foundations / Spread Foundations- Notes relevant to design to Eurocodes

The geotechnical design of spread foundations (e.g. strip and pad foundations) is covered by section 6 of Eurocode 7, Part 1 and this gives three methods for design:

.. Direct method. - Calculations are carried out for each limit state.
.. Indirect method . - Experience and testing used to determine serviceability limit state parameters that also satisfy all relevant limit states.
...Prescriptive method. - Involves the use of presumed bearing resistance.

For most spread foundations, settlement will be the governing criterion; traditionally �allowable bearing pressures have been used to limit settlement.    The increasing the factor of safety on bearing resistances to control settlement may still be used with the prescriptive method except for soft clays where Eurocode 7 requires settlement calculations to be completed.

The prescriptive method may be used where calculation of the soil properties is not possible or necessary and can be used provided that conservative rules of design are used.    A member country may currently determine presumed (allowable) bearing pressures and these values may continue be used for category 1 structures and preliminary calculations for category 2 structures . (refer to table above showing presumed bearing values ) .    Alternatively, the presumed bearing resistance to allow for settlement can be determined by the specialist engineer and included in the geotechnical design report.

Approximate quick design method for shallow foundations
A general equation of allowable bearing capacities is based on work be Terzaghi is shown below

Table of factors

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 undertaked under the advice of specialist engineers.   

Piling may be the preferred foundation option under the following circumastances:

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 principlal 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 procedes.
- A rotating drill with shell casing cuts into the soil.
- A large diameter auger cuts broad and short piles

Driven 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.   

Multiple piles
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.

Basic calcs for estimates of pile capacities

Single bored piles in clay

Groups of piles below pile cap.
A goup of piles have a much lower overall capacity than the sum of the individual capacities.   An estimate of the efficiency of a group is provided by the following equation

Piles in granular soils

Pile caps
The construction load is transferred to the pile /piles using concrete platforms called pile caps.    Piles under the pile cap should be laid out symmetrically in both directions.     The column or wall on the pile cap should be centered at the geometric center of the pile cap in order to transfer the load evenly to each pile. Examples of pile layout pattern are shown below:

In general, piles should be spacing at 3 times of pile diameter in order to transfer load effectively to soil.    If the spacing is less than 3 times the diameter of the pile, pile group settlement and bearing capacity should be checked.

The pile cap thickness is normal determined by shear strength.    For smaller pile caps, the thickness is normally governed by deep beam shear. For large pile caps, the thickness is governed by direct shear. When necessary, shear reinforcement may be used to reduce the thickness of the pile cap

Table of suggested pile cap thickness to ensure that the critical design is based on sum of pile forces across centre and not on the punching shear.

Piles in accordance with Eurocodes

The design of piles is normally completed by specialists engineers and should be based on detailed experience of the site geology or preferably following extensive site tests.    BS EN-1997- specifically indicates that design of piles should be based on one of the following criteria

- the results of relevant pile load tests ;
- calculations that are based on valid load tests
- the results of dynamic tests ( based on valid load tests)
- observations of the performance of comparable piles.