Foundations 1 Introduction

8/8/2019 Foundations 1 Introduction

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1 INTRODUCTION

1.1Terminology

Foundations: are the part of a construction that distribute on or into the ground the vertical and

horizontal loads acting on the structure.

Foundation Engineering: encompasses the topics related to shallow and deep foundations;

earth retaining structures and ground improvement techniques (i.e. the treatments of in situ soils

and rocks aimed at increasing their mechanical characteristics and, consequently, at using less

expensive foundations).

Geological Survey: is the investigation of the subsurface of a given site for the purpose of

creating a geological model of it, or a geological map. A detailed geological survey is

mandatory for the design of large constructions (e.g. high-rise buildings; bridges) and of

infrastructures (e.g. tunnels). The Italian building code (NTC) requires that a geological survey

be carried out for any kind of construction.

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Geotechnical Investigation: it aims at obtaining quantitative information on the physical

properties of soil and rock at a site which are necessary to design earthworks, foundations,

shallow and underground excavations, etc. It includes surface exploration (e.g. geophysical

methods, topographic survey, etc.) and subsurface exploration of the site. The later involves borings, to retrieve soil samples, in situ tests and laboratory tests.

Design stages: from the technical standpoint, the design necessary for relevant civil engineering

projects can be subdivided into three main stages. All of them involve parts of the geotechnical

investigation

Preliminary design: In general is prepared by the client for the engineering firms that will participate to the tender and that will present their offers. It defines the general characteristics of

the project and contains the main information necessary for the subsequent final design. It does

not necessarily provides details on the foundations. However, it contains the basic geological

and geotechnical information necessary for guiding the choice of the type of foundations and

their design.

Final design: it is based on the preliminary design and contains the details, dimensioning, etc. ofthe various parts of the construction and of the foundations. Various engineering firms could

present different proposals. The final design could contain an additional geotechnical

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investigation that provides, with adequate accuracy, the mechanical properties of soil/rock

necessary for dimensioning the foundations and the retaining structures.

“Construction

” design: this is necessary only for particular projects; it contains the details of

the procedure necessary for building up (for instance, long-span bridges) or excavating (for

instance, tunnels) the structure. It is usually prepared by the contactor. Additional geotechnical

investigation could be necessary for this last design stage.

A basic part of the geotechnical investigation is represented by borings, soil sampling and in situ

tests.

Borings: are necessary to identify the soil profile and the type of soil (clay, silt, sand, gravel)

constituting the various layers of the deposit.

Soil samples: are recovered from the borings and can be used for laboratory tests. To this

purpose the samples should be as undisturbed as possible. In practice, laboratory tests are

limited to cohesive soil specimens.

Sample Disturbance: depends on the reduction of the all-around stresses with respect to the in

situ conditions; on the shear interaction between the external part of the recovered soil and the

sampler; on the variation of the water content and density of samples.


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The borings should be extended to the volume of soil undergoing a “significant” stress/strain

change due to the foundation. This depend on both the depth of the layer and of its mechanical

characteristics. For instance, a moderate stress increase could lead to appreciable deformation in

a deep layer of particularly compressible soil and, hence, to appreciable settlements.

Roughly speaking, during the preliminary design stage one boring should be carried out within

areas of about 5000 m2. The area should decrease to 500 m2 during the final design.

The quality of the investigation should be constant throughout its various stages. In other words,

the preliminary investigation should have the same (good) quality of the subsequent ones so that

all of them provide reliable and consistent data.

The geotechnical investigation must be “flexible”, i.e. it should be possible to modify it if the

condition met during its first stages are different from the ones initially assumed.

When approaching a new construction site, a complete recovery of soil is required from the first

borings in order to obtain a detailed profile of all soil layers. If, subsequently, additional samples

are required for a specific layer, drillings can be carried out up to the sought depth (without soil

recovering) and samples can be retrieved only for that layer.

In general, it is preferable to recover as much soil as possible from the borings and to limit the

laboratory tests to the “significant” soil samples.


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1.2 Geotechnical investigation

In addition, a number of in situ tests are used for determining the dynamic properties of soils.

Soil properties can also be determined through the back analysis of in situ measurements carried

out during construction/excavation works.

Means of investigation

Very frequent Less frequent

Soil profile - Borings - Penetrometer (static or dynamic)

- Trenches, wells, drifts

- Geophysical survey (strongly

suggested by NCT)

Mechanical

properties

Fine grained soils - Laboratory tests - Scissometer (or Vane test)

- Penetrometer

- Dilatometer

- Pressuremeter

- Plate load test

Coarse soils - Penetrometer (static or dyn.) - Plate load test

Water table Fine grained soils --- - Bishop type piezometers

Coarse soils - Casagrande type piezometers ---

Permeability Fined grained soils --- - Laboratory tests (e.g. oedometer)

Coarse soils - Pumping tests from wells ---


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1.2.1 Soil sampling


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Auger sampler

The auger sampler provides quite disturbed

soil and could be used only for a

qualitative identification of the soil profile.

The auger can be driven by hands or by

a mechanical equipment.


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Wash boring

This method, as the previous one, is soldom used in engineering practice. The hole is advanced

by an auger and then a casing pipe is pushed to prevent the sides from caving in. A stream ofwater under pressure is forced through the rod into the hole. The loosened soil in suspension in

water is collected in a tub.

This method does not provide soil samples.

It provides only a rough information

on the main soil layers.


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Shelby sampler (thin wall tube sampler)

Thin walled samplers are most conveniently used in moderately stiff

clays.

When used in soft soil or sand, a core catcher (trap valve or basket shoe)

is needed to keep the soil inside the sampler during withdrawal.

The borehole must be protected by a casing which advances after thesoil is recovered.


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Osterberg sampler (piston sampler)

The sampler advances through an hydraulic pressure. This reduces the disturbance and permits

sampling also stiff clays.


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Denison sampler (rotary drilling)

It is a large-diameter (15 cm), double-tube core barrel,

which is effective in obtaining samples of hard cohesivesoils, soft rock, cemented soils, and soils containing gravel

that cannot be obtained with push-type Osterberg samplers.

This sampler consists of a rotating outer barrel with cutting

teeth on the bottom and an inner barrel with a smooth cutting shoe.

The sample is captured in a very thin inner liner, which

facilitates retrieval and handling.

Core catchers should not be used unless absolutely necessaryto retain the soil sample. Care should be taken not to

overdrive the sample to avoid disturbance.


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1.2.2 In situ tests

- Scissometer test (or Vane test)

- Dynamic standard penetration Test, SPT- Dynamic cone penetration tests, DCPT

- Static cone penetration test, CPT

- Piezocone

- Pressumeter test (or Menard test)

- Dilatometer test

- Piezometers

- In situ pumping test- Plate load test

(Seismic tests are mainly used to assess the shear wave velocity of soil deposits. Among them

the following can be mentioned: Cross hole and down hole tests; Surface wave method; Seismic

refraction; Seismic cone; Field velocity probe FVP; etc.)

In geotechnical engineering the term “dynamic” denotes penetration tests in which the

penetrometer is driven into the soil by means of blows produced by a falling mass.

The term “static” denotes the tests in which the penetrometer is driven by an engine and

advances at a constant rate into the soil.


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Scissometer (or Vane test)

It is used to measure the undrained cohesion of clays.

Field Vane tester Pocket Vane tester Pocket

penetrometer


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Standard penetration test (SPT)

The SPT test is carried at a chosen depth within a boring. It uses a thick-walled sample tube,

with outside diameter of 50 mm, inside diameter of 35 mm and length of around 650 mm.The samples is driven into the ground at the bottom of the borehole by blows from a slide

hammer with a weight of 63.5 kg (140 lb) falling through a distance of 760 mm (30 in).

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 (6 in) up to a depth of 450 mm (18 in) is recorded. The sum of

the number of blows required for the second and third 6 in of penetration is termed the "standard

penetration resistance" or the "N-value" or N SPT .

In cases where 50 blows are insufficient to advance it through a 150 mm (6 in) interval the penetration after 50 blows is recorded.

(split spoon barrel)

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Equipment

(hand operated; delivers (automatic; delivers

about 45% of free fall energy) about 90% energy)


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Recovered soil


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Split tube (or split barrel)


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Boring with SPT tests


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The blow count provides an estimation of the relative density of soil and it is used for the

empirical evaluation of mechanical parameters such as the friction angle and the modulus of

elasticity of granular deposits.

Advantages of SPT:

- Relatively simple to perform with respect to other in situ tests.

- Provides both quantitative data ( N SPT ) on the mechanical characteristics of soil and the soil

profile.

- The equipment is robust and relatively cheap.

- A number of empirical diagrams are available for interpreting the test results.

Drawbacks:

- It does not provide continuous results.

- Disturbed recovered samples.

- Limited applicability to cohesive soils and to soils containing boulders.

- Slower than other in situ tests due to the sample retrieval.

- Results influenced by overburden pressure, soil type, particle size, stress history of the

deposit.

- Significant variation of the measured penetration resistance due to the different

characteristics of the commercially available SPT devices.

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Factors affecting the number of blows N SPT


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Dynamic cone penetration test (DCPT)

This test is quite similar to SPT but for the tip of the device which is conical.

It provides the same number of blows ( N SPT ) of the SPT test and its results can be interpreted

through the same diagrams used for the SPT.

Advantages:

- The tools advances into the ground without need to bring it back to the surface every 45 cm.

- This test is particularly suitable for coarse grained soils (gravel).

- The presence of boulders usually do not stop the test.

Drawbacks;

-

It does not provide soil samples and it cannot properly identify the soil profile. Consequently,

the soil profile must be determined in advance by borings.


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The so called “light” dynamic cone penetrometer is sometime used for evaluating the density of

artificial fills and embankments, even though it does not provide reliable results.


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Static cone penetration test (CPT, Dutch cone)

The cone penetration test (CPT) is a method used to determine both the mechanical properties of

soils and their stratigraphy. It was developed in the 1950s at the Dutch Laboratory for Soil Mechanics in Delft University to

investigate soft soils. Based on this, it has also been called the "Dutch cone test".

The test method consists of pushing an instrumented cone into the ground at a controlled rate

(usually between 1.5 -2.5 cm/s).

The resolution of the CPT in delineating stratigraphic layers is related to the size of the cone tip,

which has a typical cross-sectional area of either 10 or 15 cm², corresponding to diameters of 3.6and 4.4 cm.

The tapered sleeve avoids the lateral friction on the cone, which measures only the base force.

The jacket measures the lateral frictional force.

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Sequence of steps of the CPT test:

-

The cone is first advanced by means of an inner rod and the tip resistance qc is measured.

-

The shaft is then advanced to the cone base and the skin resistance q s is measured.

-

Both cone and shaft are finally advanced simultaneously and the total resistance is measured,which should approximately be the sum of the previous base and skin resistance.


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- The so called “light” static cone penetrometer is sometime used for determining the

resistance of shallow soil layers but it does not provide reliable results.


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Piezocone

This is a CPT where a porous stone, connected to a manometer, is installed at the tip of the cone

to record the variation of pore pressure during penetration. This test is suitable for defining thestratigraphy of soft soil containing thin layers of sand and clay.


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Pressuremeter

This test can be performed in fine grained soils and leads to the estimation of the coefficient of

earth pressure at rest K 0 and of the shear modulus (in the horizontal direction) of soil.

Scheme of Menard pressuremeter Self-boring pressuremeter


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Details of a pressuremeter Results of a test ( p0 is considered as the

geostatic horizontal stress)


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Flat dilatometer (DTM, Marchetti dilatometer)

With respect to the pressuremeter, the flat dilatometer has the advantage of reducing the

disturbance due to its penetration into the soil. It is also used to evaluate the undrained cohesionof clays and other mechanical parameters.


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Results of tests


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Piezometers

These are devices used to measure the in situ pore pressure. They can be subdivided into two

categories: those that measure the height to which the column of water rises against gravity;those that measure the pore pressure thorough a manometer.

Open standpipe piezometer or Casagrande piezometer (for granular soils)


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Pneumatic, or Bishop, piezometer or pore pressure cell (for cohesive soils)


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Hydraulic conductivity tests

The coefficient of hydraulic conductivity k can be obtained from a steady state pumping

test

Main difficulties:

- Reaching the steady state regime during

the test.

- Determining the distance A from the well

at which the water table reaches its

undisturbed elevation.Hhr 0

A


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Piezometers or observational wells can be used for determining the change of the free surfaceelevation


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Plate load test

It is sometime used to evaluate

the in situ elastic modulus of theWinkler constant of in situ soils.


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Drawback: the stress increase induced by the plate reaches a depth smaller than that involved by

the actual foundation.

Results of plate load tests


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Various relationships have been proposed to extrapolate the settlement measured during the load

test to that of the actual foundation,

B0 size of the plate

B size of the foundation

S 0 settlement of the plate

S settlement of the foundation


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1.2.3 The Calibration Chamber

The interpretation of the data from SPT, DCPT, CPT, etc. is based on diagrams that have been

obtained through the Calibration Chamber.

This is a large experimental device in which the penetration test is performed under controlled

conditions.


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The testing procedure of the calibration chamber is divided into two stages.

First stage:

- The calibration chamber is first filled with the chosen type of soil. The pluvial deposition is

used for granular soils. The soil falls within the chamber from a hopper. The sought relative

density of soil is obtained by changing the aperture of the hopper and the height of fall.

- An upper plate with a hole at its centre is fixed to the chamber.

- Vertical and horizontal pressures are applied to the soil through flat jacks in order to account

for the influence of depth.

- The penetrometer is mounted on the top plate and the penetration tests is performed.

This first stage provides empirical relationships between the type and density of soil and the data

recorded during penetration (e.g. number of blows of the SPT).

Second stage:

- Triaxial test samples are prepared having the same density of the soil in the calibration

chamber. The moist tamping technique is used for granular soils.

- The triaxial tests lead to the mechanical parameters (elastic modulus, friction angle, etc.).

The second stage relates the mechanical parameters to the results of the in situ test (e.g. N SPT ).


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1.2.4 Interpretation of SPT and DCPT tests

Terzaghi’s classification of sands Classification of clays

( N 60 is the N SPT value obtained

when 60% of the energy of

blows is transferred to the rod)

Empirical values of the friction angle ’ [] for granular soilsDescription N SPT D r Silty sand Fine sand Coarse sand Gravel

loose 4-10 0.15 20 30 33 36

medium 10-30 0.150.35 2224 3234 3537 3739dense 30-50 0.350.65 2528 3536 3840 4042

very dense >70 >0.85 30 37 41 45

Density of sand N 60Very loose 0-4

Lose 4-10

Medium 10-30

Dense 30-50

Very dense >50

Consistency of

Fine-grained soil

N SPT q u [kPa](unconf. compress. strength)

Very soft 0-2 30 >400


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N 70 is the N SPT value obtained when 70% of the energy of blows is transferred to the rod.

Relative density

; void ratio


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Relationships between and friction angle of granular soils

(Peck, Hanson and Thorburn, 1974)

(if no soil type is mentioned, usually the diagrams refer to medium sand)


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(De Mello, 1971)


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Relationship between and compressibility modulus (D’Appolonia et al., 1970)


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1.2.5 Interpretation of CPT tests

Schmertmann classification of soils based on the results of Begemann cone


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Schmertmann interpretation of the results of Fugro cone (1976)


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Diagram proposed by Trofimenkov for sand (1974)


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1.2.6 Laboratory tests The laboratory tests have been discussed in the Geotechnical Engineering course. Only the main

of them are recalled here.

Grain size distribution


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Sieves for grain size analysis (mesh size n means that the number of squares in one inch is n

both horizontally and vertically).


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The particle size distribution of fine soil (silt, clay) is determined through sedimentation using a

hydrometer on the basis of Stokes law.


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Atterberg limits

The Atterberg limits define the states (solid, semi-solid, plastic and liquid) of silts and clays.

They are expressed in terms of water content w=V w /V tot .

The Shrinkage Limit (SL) is the water content below which further loss of moisture does not

lead to volume reduction. It represents the boundary between solid and semi-solid states. It is

much less commonly used than the liquid and plastic limits.

The Plastic Limit ( PL) is the water content below which a thread of soil rolled on flat, non-

porous surface breaks into fragments when reaching a diameter of 3 mm. It represent the

boundary between semi-solid and plastic states.

The Liquid Limit ( LL) is the water content at which

a soil changes from plastic to liquid states.

The test is performed through Casagrande cup.


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The Plasticity Index ( PI ) defines the water content range within which the soil is in a plastic

state ( PI = LL-PL). Clays have high PI , silts have low PI . Soils with PI close to 0 are non-

plastic and have vanishing silt or clay component.

Depending on their PI , soils can be classified as follows: 0-1 non plastic; 1-5 slightly plastic;

5-10 low plasticity; 10-20 medium plasticity; 20-40 high plasticity; >40 very high plasticity.

Index properties

The Liquidity Index ( LI ) defines the relative position of the natural water content w between

liquid and plastic limits: LI=(w-PL)/(LL-PL)

The undrained cohesion of the remolded soil can be roughly estimated on the basis of LI .

The Consistency Index (CI ) is defined as 1- LI .

The Activity ( A) is the ratio between PI and the percent in weight of clay particles (less than 2

μm). High activity, exceeding 1.25, implies large volume change when wetted and large

shrinkage when dried. Soils with high activity are very reactive chemically. The clay is

considered inactive if A is less than 0.75.

The Sensitivity is the ratio between the undrained compressive strength of intact soil and that of

the remoulded soil having the same water content.


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The soil Unit Weight is usually determined in the laboratory by measuring the weight and

volume of a relatively undisturbed soil sample trimmed by means of a brass ring of know radius

and height (i.e. of known volume). The unit weight of granular soil can be determined by in situ

tests (in situ sand cone test, nuclear densometer).

Uniaxial Compression Test: provides the undrained cohesion of clay samples.

Triaxial Test: provides stiffness and shear strength characteristics of soils in drained or in

undrained conditions. In engineering practice

it is used for cohesive soils.

It can also be used for particular studies on

granular soils. In this case the sample should

be reconstituted at the sought relative density

thorough, e.g., the moist tamping technique.

The test consists of three main stages:

- sample saturation (back pressure technique);

- sample re-consolidation (isotropic or anisotropic);

- loading stage up to failure (drained or undrained).

The vertical load is increased in compression tests,

keeping constant the cell pressure, while the cell


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pressure is decreased in extension tests, keeping constant the vertical load.

The main testing procedures are: UU (unconsolidated undrained); CU (consolidated undrained);

CiD (consolidated isotropically drained); CaD (consolidated anisotropically drained).

The quantities to record during the loading stage are: vertical deformation; vertical load; volume

change for drained tests or pore pressure change for undrained tests.

Example of results of CD tests


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Example of results of CU tests


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Oedometer Test provides data on the one dimensional consolidation process of clays and

silts.


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Direct Shear Test: provides the drained shear resistance of the soil sample along a pre-

defined plane.

Annular or torsional or ring direct shear test

(Hvorlsev, 1936)


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Qualitative results of direct shear tests

Foundations 1 Introduction

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