Foundation Settlement Report

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FOUNDATION SETTLEMENT

By SEEMA S M

U07CE043

B.TECH IV

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INDEX

Sr.

No.

CONTENTS Pg no.

1. OBJECTIVE 32. INTRODUCTION 4

3. ANALYSIS OF FOUNDATION SETTLEMENT 14

4. CASE STUDIES 20

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1. OBJECTIVE:

The objective of the seminar is to understand the sources of foundation settlement, do

analysis of settlement of foundations and to find appropriate solutions.

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2. INTRODUCTION

2.1 Foundation Settlement

All foundations settle to some extent as the soil around and beneath them adjusts itself

to the loads of the building. Foundations on bedrock settle a negligible amount.

Foundations on certain types of clay may settle to an alarming degree, allowing

buildings to subside by amounts that are measured in feet or meters.

Foundation settlement in most buildings is measured in millimeters or fractions of an

inch. If settlement occurs at roughly the same rate from one side of the building to the

other, it is called uniform settlement, and no harm is likely to be done to the building.If large amounts of differential settlement occur, in which the various columns and

load bearing walls of building settle by substantially different amounts, the frame of

the building may become distorted, floors may slope, walls and glass may crack, and

doors and windows may refuse to work properly.

Accordingly a primary objective in foundation design is to minimize differential

settlement by loading the soil in such a way that equal settlement occurs under the

various parts of the building. This is not difficult when all parts of the building rest on

the same kind of soil, but can become a problem when a building occupies a site withtwo or more areas of different types of soil with very different load bearing capacities.

Most foundation failures are at tributable to excessive differential settlement. Gross

failure of a foundation, in which the soil fails completely to support the building, is

extremely rare.

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2.2 What Causes Foundation Settlement?

The causes of foundation settlement are rarely due to the design (or under-design) of

the structure itself. More commonly, damage is caused as changes occur within the

foundation soils that surround and support the structure. The following paragraphs are brief explanations for a few of the more common causes of foundation settlement.

Weak Bearing Soils

Some soils are simply not capable of supporting the weight or bearing pressure exerted

by a building's foundation. As a result, the footings press or sink into the soft soils,

similar in theory to how a person standing in mud sinks into soft, wet clay. In such

cases, footings may be designed to spread the load over the weak soils, thereby

reducing potential foundation settlement. However, the majority of settlement

problems caused by weak bearing soils occur in residential construction, where the

footings are designed based upon general guidelines and not site-specific soil

information.

Poor Compaction

Placement of fill soils is common practice in the development of both commercial and

residential subdivisions. In general, hilltops are cut down and valleys are filled in

order to create buildable lots. Properly placed and compacted fill soils can provide

adequate support for foundations. When fill soils are not adequately compacted, they

can compress under a foundation load resulting in settlement of the structure.

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Changes in Moisture Content

Extreme changes in moisture content within foundation soils can result in damaging

settlement. Excess moisture can saturate foundation soils, which often leads tosoftening or weakening of clays and silts. The reduced ability of the soil to support the

load results in foundation settlement. Increased moisture within foundation soils is

often a consequence of poor surface drainage around the structure, leaks in water lines

or plumbing, or a raised groundwater table.

Soils with high clay contents also have a tendency to shrink with loss of moisture. As

clay soils dry out, they shrink or contract, resulting in a general decrease in soil

volume. Therefore, settlement damage is often observed in a structure supported on

dried-out soil. Drying of foundation soils is commonly caused by extensive drought-like conditions, maturing trees and vegetation and leaking subfloor heating,

ventilation, and air conditioning systems.

Maturing Trees and Vegetation

Maturing trees, bushes and other vegetation in close proximity to a home or building

are a common cause of settlement. As trees and other vegetation mature, their demand

for water also grows. The root systems continually expand and can draw moisture

from the soil beneath the foundation. Again, clay-rich soils shrink as they lose

moisture, resulting in settlement of overlying structures. Many home and building

owners often state that they did not have a settlement problem until decades after the

structure was built. This time frame coincides with the maturation and growth of the

trees and vegetation.

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Soil Consolidation

Consolidation occurs when the weight of a structure or newly-placed fill soils

compress lower, weak clayey soils. The applied load forces water out of the clay soils

allowing the individual soil particles to become more densely spaced. Consolidationresults in downward movement or settlement of overlying structures. Settlement

caused by consolidation of foundation soils may take weeks, months, or years to be

considered "complete."

2.3 Other Causes of Slab Foundation Settlement

Poor soil preparation. Plasticity of clay soil.

Poor drainage.

2.4 Structures settle into the ground for many different reasons.

The number one reason structures settle and foundation repair is later needed is

because of poor soil preparation when the structure was built. The soil is not prepared

before the concrete is poured. The soil, once it is graded, no longer remains at it's

NATURAL DENSITY. It is now disturbed, and is loose and porous. The property is

graded to level out the lot, sometimes cutting into a hill at one side and filling up the

low spots at the other side. The soil is rarely compacted to it's natural density.

What has happened is that the loose un-compacted soil compacted on its own, and the

slab settled into the ground. Un-compacted fill dirt will compact when it gets dry or

when it gets wet, bringing the structure down with it. Once a Bedrock foundationrepair specialist installs supports deeper into the ground to reach undisturbed soil, soil

that is naturally compacted at it's natural density, it is then strong enough to support

most structures. On loose soil that is un-compacted, even a light small shed will soon

settle.

There maybe water and moisture inside the trenches before the concrete is poured, and

they pour the concrete anyway. It looks good at the time, and they didn't have to spend

any time and money digging the wet muddy soil out of their trenches. No damage is

done there are cracks and damage. The soil again was not prepared properly before theconcrete was poured.

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The number two reason foundations settle is because of the plasticity of clay soils.

The clay will swell when it gets wet, and it will shrink when it dries. This swelling and

shrinking will work the weight of the structure down into the ground. Over time, the

foundation will remain more settled than it has risen.

Plasticity is the ability of the soil to shrink and swell. The higher the plasticity of the

soil, the more apt the soil is to rise up and then settle down. One side of the structure

may have more clays than the other side, and foundation may be sitting on more clay

than the neighbouring buildings, which accounts for some structures experiencing

more foundation problems than others.

The third main reason for foundation settlement is poor drainage. Water must

never pond around a foundation. The soil around the foundation must always shedwater away from the structure. To fix the foundation you must first fix the drainage.

Water is the enemy. It can wash out around foundation supports, but it can also cause

the clays to swell and shrink, damaging the foundation. Also, if there is too much

moisture in the soil, it can reach a saturated point where it will no longer support the

weight above it.

2.5 Factors Influencing Settlement

1. Size of footing

2. Depth of footing

3. Rigidity of footing

4. Tolerable settlement

5. Water table

6. Rate of settlement

2.6 Danger Signs

-Visible signs of cracking or deterioration in the foundation itself

- Garage door leaning

- Large cracks along brick facing

- Exterior window and wall separation

- Cracks in interior's wall

- Doors out of square that no longer shut tightly

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2.7 SOLUTIONS - How to prevent foundation problems

A foundation/structural engineer and an architect should be involved in the building process from the beginning of construction to protect the future owner.

The soil should be properly compacted before allowing any concrete to be poured.

The structure should high enough to insure proper water drainage.

The trenches should be properly inspected for standing moisture and wet spots,

and the wet soil should be removed and the area properly compacted before

concrete is installed.

2.8 Foundation Settlement: crack patterns, other evidence

The photograph shows a significant settlement crack in a poured concrete foundation

of a new (modular) home. This crack appeared first as a fine hairline crack. A

combination of poor site preparation of soils below the building footings

(uncompacted fill), portions of footings sitting on bedrock, and nearby blasting led to

differential settlement that produced this damage. Also some reinforcing steel mayhave been omitted from construction of the foundation wall.

A settlement crack is more likely to be wider at top than its bottom as the foundation

"bends" over a single point (or as one section of footing tips downward from its

neighbor), allowing differential settlement; it is possible for a settlement crack to

appear fairly uniform however if a foundation breaks vertically and then pursues

differential settlement. Settlement cracks need to be separated into initial settlement

due to construction or site factors and ongoing settlement due to site factors.

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http://www.inspectapedia.com/structure/FoundationCrackModular017DJFs.jpg



Settlement cracks are usually wider at the top of the crack, usually continuous, and

may occur multiple times in a wall

• Direction of downwards movement: If you draw an imaginary line orthogonal

(at a right angle to) the line of a diagonal foundation crack, usually the lineyou've drawn points to the direction of downwards movement in the wall.

However a diagonal crack may also indicate upwards wall-lift such as by frost

or expansive clay soils which are more active under one portion of a foundation

wall than another.

• Sink holes and foundation cracking: Settlement cracks may appear at the

opposite end of a wall when a reinforced masonry foundation is settling due to

presence of a sinkhole (for example in Florida). This condition can be

distinguished by observing or measuring the directions of floor or wall slope.

• Multiple settlement cracks in walls: Multiple cracks of either type (settlement

or lifting) may occur in a given area. Usually settlement cracks are visible both

outside and inside of the foundation wall if the wall material is exposed to view

at all.

2.9 Differential Settlement vs. Uniform Settlement in a Foundation:

(A）Building Before Settlement Occurs（B）Uniform Settlement

(C) Differential Settlement

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2.9.1 Differential settlement in a foundation: We use the term "differential

settlement" to describe a condition in which one portion of a building foundation is

moving down, (or up and down) at a different rate or in a different amount from other

portions of the foundation or wall.

Differential settlement will damage the foundation or wall by producing (usually

vertical, possibly diagonal or stair stepped) cracks and other symptoms of wall

movement.

The large foundation crack in this poured concrete wall was caused by differential

settlement in a new foundation wall. All of this movement occurred during the first 13

months after the home was built.

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http://www.inspectapedia.com/structure/SinkingHouse107DJFs.jpg

http://www.inspectapedia.com/structure/FoundationCrackModular017DJFs.jpg



2.10 SHRINKING SOIL AND FOUNDATION SETTLEMENT

Concrete slab foundations are supported by the load bearing soils beneath them. When

this support isreduced, the foundation adjusts accordingly. In parts of the country where highly

expansive clay soils predominate, structural problems are common when the

foundations are not constructed to take into

consideration changes in the characteristics of the soil. One such situation involved a

single-family residence with extensive cracking to interior sheetrock and exterior brick

veneer. An engineering evaluation was requested to determine the cause of the

cracking.

A slab deflection survey was performed to determine the slope and curvature of thefoundation. The results showed localized settlement toward the front of the house such

that the slab sloped at a gradient of nearly two inches over 10 feet. The observed

cracking of the sheetrock and brick veneer was consistent with the measured slopes of

the foundation. A large oak tree was located fifteen feet from the foundation. It is

known that most trees will project root systems beyond their canopy (drip line) to seek

moisture. It was noted from the survey that the region of greatest settlement coincided

with the observed tree canopy and the probable extent of the root system.

The slab deflections were not indicative of any water leaks near or beneath thefoundation. Water leaks will cause excessive swelling of clay soils and ultimately

heave the slab upward in the affected area. In this case, however, the slab had not

heaved, but had suffered differential settlement at the side of the house where the oak

tree was located.

The observed differential settlement was caused by subsiding clay soils that shrank

over time and no longer provided adequate support to the slab. The culprit was the tree

which had, by way of its root systems, drawn water from the soil beneath the slab. Soil

contraction because of moisture extraction was expected to continue as long as the tree

roots remained viable beneath the slab.

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3. ANALYSIS OF FOUNDATION SETTLEMENT

Total Settlement (St) = Si + Sc + Ss Eq.(1.1)

Si – Immediate settlement

Sc – Consolidation Settlement

Ss – Settlement due to

secondary consolidation

Skempton and Bjerrum

Si = qoB Eq.(1.2)

q0 - Contact pressure at the base of footing

B - Least lateral dimension of footing

μ - Poisson’s ratio

Es - Soil Modulus

Is - Shape factor

Es - From Stress strain curve obtained from triaxial CU test

Janbu, Bjerrum, Kjearnsli

Eq.(1.3)

µ0,and

µ1 are coefficients to be obtained from chart for given depth and width of

foundation and depth of hard layer

Settlement of Rectangular foundations on soil layer

The settlement is given by

Eq.(1.4)

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21 s

s

I E

µ −

2

0 1 0

1Si q B

Es µ

µ µ −

=

0

i

q BS = I

E



Dimension less coefficient depending on ν , L/B, H/B

3. 1 SETTLEMENT OF SHALLOW FOUNDATIONS

3. 1.1 Types of foundation settlement

Foundation settlement under load can be classified according to two major types:

immediate or elastic, settlement, Se and consolidation settlement, Sc. Elastic settlement

of a foundation takes place during or immediately after the construction of the

structure .Consolidation settlement is time dependent and takes place as the result of

extrusion of the pore water from the void spaces of saturated clayey soils .

The total settlement of a foundation is the sum of the elastic settlement and the

consolidation settlement .

Consolidation settlement comprises two phases: primary consolidation settlement and

secondary consolidation settlement. Secondary consolidation settlement occurs after

the completion of the primary consolidation that is caused by slippage and

reorientation of solid particles under sustained load. Primary consolidation settlement

is more significant than secondary settlement in inorganic clays and silty clay soil.

However in organic soil, secondary consolidation settlement is more significiant.

3. 1.2 Elastic settlement based on the theory of elasticity

The elastic settlement of a shallow foundation can be estimated by using the theory of

elasticity. From Hooke’s law,

H H

S e = ∫ ε zdz = 1 ∫ (∆σ z – μ s ∆σ x - μ s ∆σ y )dz Eq.(1.5)

o E s o

Se =elastic settlement

Es =modulus of elasticity of soil

H=thickness of the soil layer

μs =Poisson’s ratio of the soil

∆σx , ∆σy, ∆σz = stress increase due to the net applied foundation load in the x,y and z

directions, respectively.

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Theoretically, if the foundation is perfectly flexible, the settlement may be expressed

as

S e = qo(αB’) 1- μ s² I s I f Eq.(1.6)

E s

qo = net applied pressure on the foundation

μs = Poisson’s ratio of the soil

E s = average modulus of elasticity of soil under the foundation, measured from

z=0 to about z=4B

B’= B/2 for center of foundation= B for corner of foundation

To calculate settlement at the center of foundation, we use

m’ = L Eq.(1.7)

B

The elastic settlement of a rigid foundation can be estimated as

S e (rigid) = 0.93 S e flexible,center) Eq.(1.8)

Due to the non-homogeneous nature of the soil deposits, the magnitude of E may vary

with depth.

E s = E Ʃ s(i) ∆ z Eq.(1.9) z

Es = soil modulus of elasticity within a depthZ = H or 5B, whichever is smaller

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3.1.3 Elastic settlement of foundations on saturated clay

Janbu et al proposed an equation for evaluating the average settlement of flexible

foundations on saturated clay soils.

S e = A1 A2 q o B Eq.(1.10)

E s

A1 is a function of H/B and L/B and A2 is function of Df /B.

3.1.4 Elastic settlement of sandy soil: Use of strain influence factor

The settlement of granular soils can also be evaluated by the use of a semi empiricalstrain influence proposed by Schmertmann et al (1978). According to this method, the

settlement is

Z2

S e = C 1 C 2 ( q-q ) Ʃ I z ∆ z Eq.(1.11)

0 Es

Iz = strain influence factor

C1= a correction factor for the depth of foundation embedment=1-0.5[q/ ( q-q )]

C 2 = a correction factor to account for creep in the soil

= 1+0.2 log (time in years/0.1)

Q= stress at the level of the foundation

Q= γ Df

For square or circular foundations

Iz =0.1 at z = 0

Iz =0.5 at z = z 1= 0.5B

and

Iz =0 at z = z2 =2B

Similarly, for foundations with L/B>10,

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Iz = 0.2 at z=0Iz =0.5 at z = z1 =B

Iz = 0 at z = z2 =4B

Where B = width of the foundation and L=length of the foundation .Values of L/B

between 1 and 10 can be interpolated.

The use of Eq.(1.11) first requires an evaluation of the approximate variation of the

elasticity with depth. This evaluation can be made by using the standard penetration

numbers or cone penetration resistances. The soil can be divided several layers to a

depth of z = z2 and the elastic settlement can be estimated. The sum of the settlement

of all layers equals Se.

3.1.5 Range of material parameters for computing elastic settlement

Several investigators have correlated the values of the modulus of elasticity E s, with

field standard penetration number, N 60 and the cone penetration resistance, qc.Mitchell

and Gardner (1975) compiled a list of these correlations.

Schmertmann(1970) indicated that the modulus of elasticity of sand may be given by

E s = 8 N 60 Eq.(1.12)

pa

N 60 = standard penetration resistance

pa = atmospheric pressure =100kN/m²

E s = 2qc Eq.(1.13)

Where qc = static cone penetration resistance

Schmertmann et al.further suggested that the following correlations may be used withthe strain influence factors

E s =2.5 qc (for square and circular foundations) E s =3.5 qc (for strip foundations)

3.1.6 PRIMARY SETTLEMENT

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Primary consolidation settlement relationships

As mentioned before, consolidation settlement occurs over time in saturated clayey

soils subjected to an increased load caused by construction of the foundation.

On the basis of the one-dimensional consolidation settlement,

S c = ∫ε z dz Eq.(1.14)

Where

Εz = vertical strain= ∆е 1+ еo

∆е = change of void ratio

= f (σ’ o , σ’ c and ∆ σ’)

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4. CASE STUDIES:

Geotechnical studies for foundation settlement in alluvial deposits in the City of

Rome (Italy)University of Roma “Roma TRE”, Dept. of Geological Science, L.go San Leonardo

Murialdo, 1 00146 Rome, Italy

Via degli Scaligeri, 29 00164 Rome, Italy

Received 14 March 2006;

Revised 31 July 2006;

Accepted 7 August 2006.

Abstract

This study identifies units characterized by specific geomechanical behaviours within

some Holocene alluvial deposits in the City of Rome. In particular, the highly

compressible units, which may be responsible for subsidence and settlement

phenomena below urban structures, have been identified. Investigations carried out

during this study have interpreted nearly 800 stratigraphic sections and physical–

mechanical features of the alluvial deposits of the Tiber and those of its right- and left-

bank tributaries. The analysis allowed to define, for each investigated deposit, a

representative series subdivided into lithotechnical units. When comparing the

stratigraphic and geologic features among the series, a remarkable difference in

geomechanical behaviour in the deposits from the Tiber's left- and right-bank tributaries and within the river's deposits themselves could be recognized. A yield

hazard zoning for urban structures related to recent alluvial deposits has also been

defined.

4.1. Introduction

The City of Rome is located in an area where there as been long-term human activity

and continuous transformations of the original terrain have been substantial. The

hydrographic networks of the Tiber's right and left banks have been modified morethan once during historical times; sometimes, it has even been obliterated by urban

growth. These changes mean that nowadays, it is very difficult, at least in the

historical centre, to recognize the original terrain. Many of Rome's streets run along

the ancient courses of the Tiber's tributaries and many buildings are situated on

alluvial deposits now buried by anthropogenic debris. Many of these structures

overlying alluvium and debris have been damaged by subsidence and seismic wave

amplification. The effects are visible in the buildings' uniform or differential

settlement.

In the past, the alluvial deposits were considered as continuous bodies consisting of clayey–silty and sometimes sandy sediments. Instead, the formations were found to be

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mainly heterogeneous. The geotechnical characterization of these units is mandatory

for evaluating the intrinsic geological environment's hazard in urban areas, where the

risk can reach very high values. This study has been carried out through the analysis of

borehole and geotechnical data from three left-bank tributaries and three right-bank

tributaries of the Tiber River. Data for the Tiber's alluvial deposit from various partsof the city were also included. Based on this geotechnical analysis, a subdivision into

lithotechnical units has been created for each deposit to make correlations and

comparisons among the different deposits. The same level of detail was not possible

for all of the stream valleys, since it was not always possible to obtain geotechnical

data. Nonetheless, it has always been possible to define a stratigraphic series that

would represent the examined deposit by subdividing it into units after stratigraphical

and sedimentological observations.

4.2 Geological, geomorphological, and hydrogeological setting in the City of

Rome

The dominant feature on the Tiber River's right bank is the “Monte Mario” ridge. It

lies parallel to the Tiber valley and reaches nearly 140 m a.s.l.; to the south, it is 60 m

a.s.l. Monte Mario is made up of marine deposits .The “Monte Vaticano” Unit, (blue

clays) affects the whole structural setting of the City of Rome and is remarkably thick,

consolidated, and is the aquatard for hydrogeological units of Rome. These marinedeposits are exposed at the base of the “Monte Mario” ridge. To the east and west of

the ridge “Monte Vaticano” lies at various depths in paleogeomorphic depression in

tectonically deformed “bedrock.” Above those marine sequences we can follow the

change from marine to coastal, then to continental sedimentation. The continental and

coastal facies trace the flow of the Paleotiber, which is parallel to the coast running SE

along a continuously subsiding belt, producing conglomerate deposits tens of metres

thick. From the Middle Pleistocene ash and other airborne products from an intense

volcanic activity of the “Sabatini” and “Albani” volcanic districts reached the area of

Rome. The geomorphologic and hydrogeologic setting was deeply modified by this

volcanic activity. The Paleotiber's course was deviated arriving to the present-day

position within the boundary between the Albani and Sabatini products. Later, the last

glacial lowstand was 120 m below today's level.

This produced a deepening of the Tiber's course and its tributaries' as they eroded the

volcanic deposits first, followed by the continental Paleotiber's deposits then, and later

carving deeply into the underlying and well-consolidated “Monte Vaticano” Unit.

When the sea level rose, the deep gorges were in-filled with alluvial deposits, which

are the object of this paper.

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The succession of these events created the present-day terrain of the City of Rome: the

area consists of a central plain, which is the Tiber's alluvial plain; the relief on the

river's right bank is the “Monte Mario”–“Monte Vaticano” structural high; at the

Tiber's left bank lies an articulated area that, in the historical city centre, corresponds

to the “Seven Hills”

Fig. 2. Geological map of Rome

4.3 Methodology

This study is predominantly concerning areas where growth of the modern City of

Rome has occurred over the last 50 years and where it will presumably developed in

the future. Several hundreds of recent drillings carried out from geological–technical

investigations were analyzed and interpreted using previous data from published

stratigraphies. Initially, each alluvial deposit was subdivided into units based on

physical data from stratigraphic descriptions (colour, sedimentological features,

organic matter content, consistency, etc.). Then, for each of these subdivisions, the physical–mechanical parameters from laboratory or on-site tests were determined; this

gave a representative series for each alluvial deposit by subdividing them into

“geotechnical units”.

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4.4 Lithostratigraphic characterization of alluvial deposits at the Tiber River's

left bank

4.4.1 Alluvial deposit of the “Fosso di Grotta Perfetta” stream

4.4.1.1 Geographic setting

The “Fosso di Grotta Perfetta” stream flows in a secondary valley and is a left-bank

Tiber tributary. For its low-energy, marsh-environment characteristics this stream

was considered to be a swamp in the past, the area is long and narrow. It is 8.5 km

long and 2 km wide. The lower basin of the stream is now almost completely

urbanized, the final branch was channeled during historical times. As of today, the

area is known for its instabilities for constructions, principally related to settlement

and rigid rotation of structures, caused by the soil's bad geomechanical characteristicsand inadequate foundations. The examined area follows the Grotta Perfetta stream

course for about 3 km, and a tangential band of about 1 km into the valley of the

Tiber.

4.4.1.2. Subdivision into depositional units and geotechnical parameters

Based on the physical characteristics of the soil, within the alluvial deposits and

beneath the anthropic cover, six main units in the central portion of the valley can bedistinguished and linked to geotechnical data from nearly 218 undisturbed samples.

About 100 new in situ geotechnical tests were performed. The following are

descriptions of the analyzed units and their physical–mechanical parameters.

Beneath the anthropic cover, from the most recent to the most ancient unit, deposits

from the last historical flood can be observed, which are 3–4 m thick, clayey–silty

sands with floated anthropic inclusions. The soils are of pyroclastic origin with some

tuff element. The geomechanical behaviour is definitely frictional, the cohesion is

almost zero; the gravel–sand fraction percentage is higher than in the overlying

anthropic cover. Underlying the historical flood deposits are the brown clays.Theyare silty clays and clayey silts; consistency is medium at the top and gradually

decreasing with depth. Water content and Atterberg's values are rather high; the

consistency index is medium to low, decreasing with depth, typical of plastic to

soft/plastic physical states. Deeper, the first brown–blackish, high organic matter

content unit can be observed. Locally, greyish, brown, and/or greenish lenses are

present. Geotechnical analysis highlighted particular parameters that are commonly

identified with geotechnically poor soils. The dominant feature is the high organic

matter content, which reached 15–25% of the total weight. Densities are particularly

low, void ratio being sometimes higher than 4.00. Often the moisture content is

higher than the liquid limit; the liquidity index is > 1 indicating liquid–plastic states.

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Such characteristics are typical of metastable or sensitive soils, for which stable

states are only possible onsite (with confining pressure); whenever they were

disturbed by both static and dynamic tension, they lose their strength. In particular,

when subjected to dynamic loading, they can produce local collapse phenomena.

Shear strength parameters are definitely low and has high compressibility; withvalues of compression index C c often more than 2.00.

Around − 5 m a.s.l there is a substantial decrease in organic matter which identifies

the top of the greenish clays The water content and Liquid Limit, despite still being

high, reveal a certain degree of geotechnical improvement compared with the

overlying organic clays. This is particularly due to the absence, or minor presence, of

organic matter (less than 10% found only in particular horizons). Nevertheless, the

shear strength resistance is still low similar to that of the above-mentioned organic

clays. Compressibility characteristics reveal compression index C c values of 0.6–1.5,which, although typical of highly compressible clays, indicate a slight increase in

stability compared with the overlying organic clays.

Finally, an organic matter-rich unit characterized by physical–mechanical parameters

similar to those of the above-mentioned blackish and organic ones is also present.

This layer changes from black to grey in colour at depths exceeding 35–40 m

indicating a decrease in organic matter and an increase of cohesion where coarse

horizons of floated pozzolan fragments are observed. This geotechnical and

stratigraphic setting allowed the layer to be subdivided into two sub-units: the upper

AO2′ unit and the lower AO2 unit. The AO2′ unit is characterized by parameters

similar to that of the AO1 unit.

High water content and Atterberg's values were observed, with very low, sensitive

and metastable soil characteristics again observable. The AO2 unit instead shows

better mechanical parameters. It is also characterized by the presence of diffused

floated pozzolan horizons, lens-shaped and decimetre-sized; only locally, these

horizons can be up to 1–3 m thick. Their presence improves the geotechnical

properties of the clayey unit; the higher permeability allows some local consolidation

within the organic clays. The bed of the AO2 unit has geotechnical conditions

compared to the AO2′ which is also evident when analyzing the water content,density, and strain values. These characters define the geotechnical parameters

representative of the various lithotechnical units in the Holocenic alluvial deposits in

the “Fosso di Grotta Perfetta” stream.

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Table 1.

Geotechnical parameters summary table of the lithotechnical units identified in the

“Grotta Perfetta” deposit

Average

thickness

(m)

γn

(kN/m3)

WC

(%)

LL

(%)

IP

(%)

φ′ C ′

(kPa)

Cu (kPa)

Historical

floods

3 16–19 32–65 //–78 NP-

23

30–

47

0–38 –

Brown clays 7 15–19 30–93 43–

123

17–

58

23–

30

5–35 21–74

Organicclays

6 12–15 91– 186

102– 171

33– 93

13– 22

9–25 16–55

Green clays 8 12–17 49–

154

84–

151

24–

79

17–

29

3–15 7–19

Organic

clays

5 14–16 42–

184

63–

164

18–

76

13–

26

5–25 15–33

Organic

clays

15 16–19 28–84 46–

104

17–

65

25–

30

0–0.3 15–30

// indicates no data.

4.5 Lithostratigraphic characterization of the Tiber River alluvial deposit

4.5.1 Introduction

When compared with tributary alluvial deposits, there are more studies of the

deposits of the River Tiber. These studies are indispensable for individualizing the

stratigraphical and evolutionary relationships between the Tiber and its tributaries.

The investigated area is located in the Historical Centre and has been characterized

by a number of boreholes that often reached down to Pliocenic bedrock.

4.5.2 The stratigraphic series representative of the Tiber River's alluvial deposit

and its geomechanical parameters

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Almost all of the studies about the alluvial deposits of the Tiber within the City of

Rome agree on both their subdivision and geomechanical characterization. There is

also a general agreement on the reconstruction of the depositional evolution of the

Tiber during the Holocene. For the Historical Centre, the authors agree on

characterization of the following stratigraphical units.Sandy silts and dark-greenish clayey silts; subdivided into two different ones, a silty–

clayey (LAV) unit and a sandier one (SLV).

S: coarse sands constituting the filling after an erosional phase of the Tiber River.

AG: silty clays and greyish clayey silts with thin organic levels, relatively more

frequent at an elevation of − 15 a.s.l.

SLG: silty sands, grey sandy silts, and coarse sands.

G: base gravel: mainly made up of carbonate clasts from the erosional phase.

Our analysis corresponds to the above studies, even though the data come from

different areas.

Summary table of the principal geomechanical parameters representative of the units

observed in the Tiber's alluvial deposits

Units Averagethickness

(m)

γn(kN/m3) Wn(%) LL(%) IP(%

)

φ C′(kPa)

LAV Greenish

clayey silts

7 18–20 20–0 43–71 12–

41

18–

25

15–30

SLV Greenish

silty sands

7 17–20 20–

40

26–65 8–

26

27–

35

0–20

S Medium

sands

8 17–20 20–

30

ND–

27

// 30–

40

0–10

AG Grayish silty

clays

23 15–20 20–

60

34–93 11–

59

15–

26

5–25

SLG Grayish silty

sands

15 18.5–20 20–

30

24–46 //–

22

25–

35

0–15

// indicates no data.

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The left-bank deposits show physical–mechanical parameters remarkably poorer

than those from the right bank and from the River Tiber itself.

The differences in shear resistance between the left-bank and right-bank deposits

are in the graphs compare three CPTs on the three deposits we examined.

The plasticity conditions can be compared by examining the Casagrande's Plasticity

Chart. It highlights the high plasticity of the organic silty deposits from the “Grotta

Perfetta” stream.

The deposits of the Tiber and of its right-bank streams show normal activity indices

( A = 0.4–0.8), whereas the left-bank deposits show higher values ( A = 0.8–2),

typical of “high-activity” soils.

Prominent for the objective of this study is the remarkable deformability of the left-

bank deposits when compared with the right bank and the deposits of the Tiber. The

deformational features are here compared with three representative oedometric

graphs. The organic pelitic–silty deposits of the “Fosso di Grotta Perfetta” stream

clearly show a very high compressibility whose natural void index reaches very

high values (e0 = 1.5–2.5).

Thus, these normally consolidated alluvial deposits show a relatively higher deformability when associated with organic sediments, which are distributed

throughout the left-bank deposits. The pelitic–silty beds in particular are potentially

prone to subsidence and settlement if loaded by landfill. The Tiber's deposits and

the right-bank deposits instead show fairly similar and comparable deformability

characteristics.

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4.6 Conclusions

The analysis of nearly 800 stratigraphic logs and a number of laboratory tests gave

the opportunity to subdivide the alluvial deposits of the Tiber and its tributaries into

depositional units and to define their geomechanical characteristics. This division

into units is very important for evaluating the hazard in different areas of the

territory. Such hazards are derived from the fact that alluvial deposits are subject toseismic amplification and the structure of organic-rich units are more ‘sensitive’

and can be affected by phenomena similar to the liquefaction of silty sands below

the water table. Such sediments are mostly underconsolidated, hence, more

subjected to settlement.

Through data derived some alluvial deposits in the City of Rome (including those of

the Tiber), it is possible to identify areas potentially at risk from instabilities caused

by the presence of organic matter. Units with high organic matter content, widely

dispersed and several metres thick, are mainly within a portion of the stream withshallow gradients. The Tiber's deposits only show thin layers of organic matter and

generally have better geotechnical characteristics than deposits underlying the left-

bank tributaries. All the deposits that have been subjected to loading by backfills

and buildings in the historical centre over thousands of years have completed their

consolidation process and are no longer affected by subsidence.

Areas with significant organic matter content on the left bank are subsiding now,

whereas the right-bank alluvial deposits are no longer subsiding.

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Case study: Reinforced Earth retaining wall approaches

Thursday, June 10, 2010, 11:03 Hrs [IST]

Dhananjoy Das and Saikat Chatterjee present an interesting case study of how the

highly-flexible Reinforced Earth technology was successfully deployed on a bridgeover a canal in Kolkata. Marshy land had made construction difficult.

A bridge was to be built over Bagjola Canal to meet the increasing traffic and

developmental demands. The banks of the canal were made of filled up soil on a low

and marshy lands. For PWD, the construction of main bridge structure was not so

critical due to its pile foundations. Whereas, it was hard to construct its approaches in

such a marshy land with 'N' value ranging from 2-3 only, up to a depth of 15m from

ground level in most of the places. Conventional RCC retaining wall was not a feasible

solution in such ground condition. Therefore, the owner of the project opted for highly

flexible Reinforced Earth Retaining Wall and the work was awarded to Reinforced

Earth India.

The solution: It was a challenging engineering task to build Reinforced Earth Walls

under such extreme weak ground conditions with very low bearing capacity to support

the load of the structure. Therefore, it was imperative to improve the foundation soil to

withstand the load to be imposed from the wall. PVD (prefabricated vertical drain) in

combination with stage construction preloading was selected as a best possible

solution at that location followed by two layers of PET uniaxial geogrids to improve

the foundation soil and ensure the safety of the wall under global stability. Proper instrumentation and stage construction were done to monitor settlement and

development of additional pore water pressure with each stage of construction and

subsequent dissipation during pause period.

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Owner: West Bengal PWD

Main contractor: Mackintosh Burn Ltd

Construction stage: April 2005 to November 2007

RE wall area: 5,062 sq. m (as built)

Engineering Data

Facing: 1.5mx1.5m Cruiciform Terraclass panels

Connection type: Galvanised tie strips

Reinforcement type: High Adherence (HA) galvanised steel strips

Fill soil properties: Phi=32 deg., ?min=15KN/m3, ?max=17KN/m3

Foundation type: N' value = 2-3 up to depth 15m

Foundation system: Prefabricated vertical drain

PVD work: PVD had been inserted up to a depth of 15m (average), in 1.5mx1.5m

square grids, below a depth of 2.0m from existing ground level. The job was done at a

very fast pace and the overall PVD work at an approximate plan area of 7,000 sq.

metres, had been completed within two weeks.

Instrumentation: Four settlement plates and ten Piezometric tubes had been installed at

both the approaches to measure the periodical settlement and pore water pressure

respectively.

Stage construction: Stage construction

technique had been adopted to stabilise

the foundation soil with every stage of

construction by allowing dissipation of

pore water through PVD along with

very sophisticated sub-surface drainage

system. As the wall height was

approximately 9.0m, three stages of rest

periods had been considered to achieve

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the maximum settlement in stages. At 4.0m, 7.0m and 9.0m, average rest periods of 45

days each had been provided.

The gain in shear strength of foundation soil at each stage of construction allowedconstruction of next stage of RE wall. During this rest periods instruments were

monitored at regular basis, and the next stage of construction started after reaching the

desired settlement and dissipation of pore water pressure. Below is one example of

settlement and piezometer monitoring report at one of the locations.

Notable observations: A maximum 319mm settlement had been observed; differential

settlements had been observed varying from 99mm to 319mm; and post-construction

settlements had also been observed in some places.

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REFERENCES

1.) Braja M. Das, Principles of Foundation Engineering, Thomson, India Edition.2.) Website: http://www.sciencedirect.com/ 3.) Geotechnical studies for foundation settlement in Holocenic alluvial deposits in

the City of Rome (Italy ) Maria Paola Campolunghi Giuseppe Capelli, Renato

Funiciello and Maurizio Lanzini University of Roma “Roma TRE”, Dept. of

Geological Science, L.go San Leonardo Murialdo.4.) Soil mechanics and foundation engineering, vol. 43, no. 1, 2006

Settlement analysis based on recommendation of building code 50-101-2004

V. A. Barvashov.

5.) Settlement analysis of foundations.Willmore, M J

Proc lnt Conference on Foundations and Tunnels, London,arch 1987 VI, P25-29. Publ Edinburgh: Engineering Technics Press. 1987

http://www.sciencedirect.com/

http://www.sciencedirect.com/

Foundation Settlement Report

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Transcript of Foundation Settlement Report