reuse of foundations
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© Hasanul Islam, 2011
Author
Supervisor
of foundations
Hasanul Islam
Dr Richard Goodey
17 March 2011
City University London
School of Engineering and Mathematical Sciences
ii
Contents
Acknowledgements iii
Preface iv
1 Introduction 1
2 Principal Drivers for Foundation Reuse 4
3 Modes of Foundation Reuse 7
4 Ultimate Bearing Capacity of Pile Foundations 10
5 Assessment of Pile Integrity and Pile Capacity 17
6 Case Study 22
7 Conclusions 26
References 27
Bibliography 30
iii
Acknowledgments
After praising and thanking Allaah, I would like to thank my supervisor Dr Richard Goodey
for his continuous support and enduring patience from since the beginning of the project right till
the very end. I am truly grateful for the help Dr Goodey provided in seeking out the resources and
for his review of my drafts especially at times of urgency.
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Preface
Foundation Reuse has been reported as a highly relevant issue in modern-day
construction, and it concerns developers, building-owners and construction professionals. Eighty-
four respondents from the EU attested to its relevance via a questionnaire survey carried out by
the RuFUS Project in 2003 (Butcher, et al., 2006).
The aim of this project is to provide a technical yet simple outlook on the feasibility of
reuse of foundations in contemporary construction. It was not intended for the project to be
comprehensive, although care has been taken to specifically discuss this topic from a variety of
angles.
The initial chapters focus on introducing the topic at hand, reviewing the history and the
various modes of reusing foundations; the concluding chapters identify different methods of
verifying pile integrity, and discuss technical aspects such as the increase in Ultimate Pile Capacity
over time.
1
1. Introduction
Foundation reuse is not something new, and has been a key aspect in the development of
towns and cities, from controlling urban sprawl to preservation of historical heritage. This section
shall focus on defining what reuse of foundation is, stating the aims and objectives of the overall
project, and finally concluding by briefing the reader on the historical facet of this topic.
1-1 OBJECTIVES
This project seeks to verify the principal driving factors for the reuse of foundations,
identify the various modes of foundation reuse, analyse the phenomenon of increase in Ultimate
Bearing Capacity of pile foundations, and to critically appraise the methods for assessing the
capability of piles for reuse.
1-2 WHAT DOES REUSING FOUNDATIONS MEAN?
The reuse of foundations entails utilizing existing foundations at a site to provide support
for a new structure that is to be built upon it. Butcher, et al. (2006) state that reuse of
foundations does not always involve the usage of old foundations in or on the ground; rather
foundation reuse may even imply the re-usage of the facade of a building whilst the interior parts
of a building is rebuilt.
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1-3 HISTORICAL BACKGROUND
Reusing foundation for construction of new structures is not a new phenomenon and has
been implemented in practice throughout the ages. Chapman, et al. (2007) state the reusing of
foundations throughout history to be a norm rather than an
exception. In Great Britain, the castles and cathedrals of old
were constructed and then reconstructed upon old and existing
foundations. During the Elizabethan times, imposed laws
restricted people from building houses except on old
foundations in a bid to prevent urban congestion. Ackroyd (2001, as cited in Chapman, et al., 2007)
quotes that Kind Charles II was informed after the extinguishment of the Great Fire of London,
“Some persons are already about to erect houses again in the City of London upon their old
foundations.”
With the passage of time, the force loads of buildings increased due to their bulkiness,
and the level of performance expected from these buildings increased as well. The methods via
which foundation requirements are calculated had become far more reliable and advanced. As a
result of all of these, new buildings were often raised on completely new foundations, which
avoided aesthetic and structural damage.
Flack (2004) reports that up until the 1950s, most of the buildings around London were
built on shallow foundations and typically had one basement level. Post-1950s, due to the
increase in demand from developers to build taller buildings with longer floor spans, bored piles
have been in commonly applied thanks to the presence of London Clay in many construction sites.
Bored piling was chosen due to the quickness of installation, quietness and due to this being a
cost-effective method to support tall structures. This dramatic change in history was a cause for
concern.
Figure 1: The Great Fire of London, 1666
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As for the most recent times, Chapman, et al. (2006, as cited in Butcher, et al., 2006) state
that foundations have been re-engineered and successfully reused on projects such as railway
bridges and major building projects such as the Empress State Building and Thames Court.
Figure 2: Thames Court, London
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2. Principal Drivers for Foundation Reuse
In the past, the prime driver for reusing foundation was congestion in the ground,
however due to the contemporary changes in political and social attitude and the changes in
legislation, driving factors such as sustainability and environmental welfare have become amongst
the principal issues of concern. This section shall review the principal driving factors that may be
motivational, and perhaps act as inspiration for the developers to consider reusing existing
foundations.
As the urban sites become more and more congested, the need for reusing existing
foundations is becoming rampant. The working-life of the buildings in city centers has reduced to
20 years from an average of 50 years, possibly due to the quicker advancement of technology in a
shorter span of time and cultural need for aesthetic change. As a consequence, grounds at
Brownfield sites are congested with existing foundations. Other than that, existing infrastructure
in the ground and service tunnels pose as obstacles to the installation of new piles. In many cases
it is very expensive for developers to remove the old foundations (the cost is up to four times as
much as installing new piles), as shall be discussed, and also very costly to relocate service tunnels.
The developers would also have to reconsider their entire foundation design plan so that the
installation of the piles will not adversely affect the existing infrastructure; any adverse effect may
result in legal actions that the developers are likely to lose. To avoid all this, considerable ground
investigations and design models that will satisfy the owners of the infrastructure have to be
produced, and this can be costly. Reusing existing foundations in these cases seems ideal.
Furthermore, existing archaeology underneath the construction sites are protected by the
local byelaws, and oftentimes the type of foundation that is allowed to be installed in certain sites
are only those that will inflict the least amount of damage to the archaeology. Other than the
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archaeology underneath the ground, in case of driven piles, standing buildings (of historical
importance) also pose a threat to pile driving operations, especially if any damage is preconceived
to occur by the operations. Although in most cases the archaeology is preserved in-situ and is
hidden from public view; in certain cases the archaeology is
incorporated into the new building structures and displayed
for viewing, such as in the case of the Guildhall Art Gallery
(Ganairis and Bateman, 2004, cited in Butcher, et al., 2006).
Existing foundations may be utilized to support
higher loads than that experienced from previous structures
due to the advance in technology and understanding regarding the behavior of piles over time.
The Ultimate Bearing Capacity of the existing piles may increase, making them capable of carrying
higher loads than performed previously. If there was no time for the capacity of foundations to
increase, then one may consider if overcapacity had been incorporated into the previous design
for safety reasons. In that case the (additional) reserve capacity means that the piles will be able
to carry higher loads, albeit, at a lower safety rating. The relationship between Ultimate Bearing
Capacity of piles with time shall be discussed in proceeding chapters.
A key driver to the reuse of existing foundation is the economic factor, which is of interest
to the developers and buildings owners. As a result of reusing existing foundations, the need for
site investigation is reduced; the time taken to construct the new structure is reduced as well,
which would otherwise be lengthened due to installation of new piles. The reduction in the
project duration further reduces the costs to the developer by bringing closer the profits to be
made via sale or renting etc. of the properties, and as a result shortening the time for payback of
the (borrowed) capital (which most likely includes interest). Should the developer decide to
Figure 3: Guildhall Art Gallery Basement
with Preserved Roman Amphitheatre
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remove existing piles, this has proven to be an ardent and a very costly task. Again, reusing
existing foundations seems ideal for cost-effectiveness.
To add, the change in legislation regarding the disposal of waste and the classification of
waste products (such as hazardous waste) adds to the cost of installing new piles (especially in
case of bored piling) and removing the existing foundations.
The impact of construction-work on the environment also has to be taken into account.
The reuse of foundations can significantly reduce the use of natural resources, reduce the total
amount of energy used (since more piles will not be removed or installed), induce a reduction in
groundwater pollution that is caused by construction, reduce the amount of waste produced, as
well as reducing the amount of carbon emission caused by the utilization of plant and equipments
(for installing and removing piles).
Finally, reusing foundations will allow future buildings to be constructed without difficulty
or high expense, which would have otherwise resulted from unnecessary ground congestion. As a
result of heavy ground congestion, such that it is very difficult to build new structures atop the
site, there would raise cases where the development would proceed to more Greenfield sites, and
then under the same regime of installing new piles, over time ruining those sites as well.
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3. Modes of Foundation Reuse
As stated earlier, foundation reuse is not limited to the existing structures underneath the
ground, but may also refer to the re-usage of the existing façade of a building. In this section
however, the modes of foundation reuse related to pile foundations shall be identified and
compared, since this is the core concern of the overall project. Butcher, et al., 2006, have
identified four different types of foundation reuse, all of which shall be discussed as follows.
3-1 COMPLETELY REUSING EXISTING PILES
Firstly, existing pile foundations may be completely reused without the addition of any
new piles. However the load-bearing capacity of the piles may be improved further before
subjecting them under the new loads through methods such as global ground treatments. Also,
the actual load exerted on the piles by the new structure could be reduced (during the design
process) to lower amounts than that exerted by the previous structure. This will ensure that the
pile head strain does not exceed beyond that which was previously experienced and that the pile
does not fail.
3-2 REUSE EXISTING PILES; SUPPLEMENT WITH NEW PILES
Secondly, the existing pile foundations may be supplemented with new piles. This method
is best applied when the new design load is higher than that experienced due to the load imposed
by the old structure. Furthermore, if the designers are unsure of the load-bearing capacities of the
piles, then this method is ideal so that the structure will not fail in case the piles have lower load-
bearing capacity than that which is desired. Also, if the layout of the existing foundations is not
supportive of the new column locations, then additional piles may be added to support those
piles.
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3-3 INSTALL NEW PILES; IGNORE EXISTING PILES
Thirdly, new pile foundations may be installed without removing the existing piles (and
just ignoring them). In these cases, new piles often need to be ‘squeezed’ in between the existing
piles due to the ground congestion caused by the existing piles. If it is not possible to squeeze the
new piles in between the existing piles, then the new piles may have to be installed away from
the designed column positions. As a result, large transfer structures occur between the columns
and the piles; higher capacity piles are thus needed, thereby increasing the cost and time of the
program. This is the oft-implemented method in contemporary construction due to the ease of
implementation, but unfortunately this leads to an increase in ground congestion and creates
problems for future development. Furthermore, if the site consists of existing historical
archaeology, then repeatedly installing new piles can cause severe damage, and this is highly
undesirable and may act adversely against the developer.
3-4 REMOVE OLD PILES; INSTALL NEW PILES
The fourth and final method of reusing foundation is to completely remove the old,
existing pile foundations, and then to replace them with new piles. In case of deep obstructions
which are on the column grid of the new structure, the piles may need to be removed, especially
if the existing piles do not have sufficient load-bearing capacity. Also, in cases where the old
foundations were installed through or around archaeological remains, it would be undesirable to
install more piles, so the old piles would have to be removed and the new piles installed in the
existing grid layout without disturbing the archaeological remains.
Reinforced piles may be removed from the ground by cutting out a circle of soil around
perimeter of the pile, thereby separating the pile from the surrounding soil, and then lifting the
pile from the ground. If the piles were not fully reinforced, then care has to be taken when
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removing them as this may lead to rupture of the pile shafts. A standard solution to this is to use
rock-boring equipment to break the piles into smaller pieces to be then lifted onto the ground.
There are several downsides to this type of foundation reuse. Other than the increase in
time and cost of the program, this method risks the lowering of the capacities of the new piles
due to the softening of the ground caused by removal of the old piles; voids created by the
disturbance may have to be backfilled by material that has to be suitable for both supporting the
surrounding soil structure as well as the new piles.
Figure 4: Four Types of Foundation Reuse
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4. Ultimate Bearing Capacity of Pile Foundations
The Ultimate Bearing Capacity of a pile foundation is the maximum load it can sustain
without failing. The Ultimate Capacity may equal to the sum of the (pile) base and shaft capacity.
In this section, the phenomenon of increase in the Ultimate Capacity of piles shall be analyzed
taking into account different types of piles and ground conditions, and case study of a proven test
where Ultimate Capacity of pile foundations increased over time shall be examined.
4-1 DEFINING PILE FOUNDATIONS
Pile foundations are used to transfer the load of a structure to a stable, bearing ground at
a depth, which can resist the compressive and tensile loads. Pile foundations increase the load
capacity of existing structures and provide support to stabilise unstable structures. Generally, pile
foundations fall into two different categories, ‘driven’ and ‘bored’.
A driven pile is a long and slender column, and its purpose is to provide support by
resisting the structural forces. Driven piles are manufactured from pre-constructed materials
(such as steel, concrete or timber) with predetermined shape and size. This form of piling is
installed by impact hammering, vibrating or pushing into the ground (Pile Drivers, 2011), which
displaces the soil in the ground as a result. Driven piles cannot penetrate obstructions.
A bored pile extracts soil from the ground and then replaces the large bore with concrete
and reinforcement. This form of piling is ideal where there is need for small vibration and where
the piles need to be installed in grounds where the access condition is restricted (Van Elle, 2007).
4-2 DRIVING FACTORS FOR INCREASE IN ULTIMATE CAPACITY OF PILES
The increase in the Ultimate Capacity of pile foundations may occur due to multiple
driving factors. The nature and state of the soil in which the piles are driven, the physical
characteristics of the piles, including material type (i.e. concrete, steel, timber) and the
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dimensions, the methods of pile installation (i.e. driving, boring), the condition of the pile tip –
whether it is open-ended or close-ended –, the conditions of loading (such as piles being loaded
under tension or compression), the rate of load application, time elapsed post-installation; all of
these are driving factors to the change in ultimate capacity of pile foundations.
4-3 ULTIMATE CAPACITY INCREASE DUE TO CHANGE IN GROUND CONDITIONS
The Ultimate Capacity of a pile is usually estimated via static pile formulas and then this is
confirmed using pile load tests. As a driven pile is pushed into the ground, the soil around the pile
shaft and below the pile tip is heavily disturbed; excess pore pressure also builds up. The nature
of the soil in which the pile is driven remains a crucial factor for establishing the duration for the
complete dissipation of the excess pore pressure. Therefore, the pile load tests need to be
performed taking into consideration the duration of the pore pressure dissipation, otherwise the
pile capacity could be underestimated. Soil (such as sand and gravel) that has higher permeability
characteristic allows for the pore water pressure to dissipate quicker; vice versa for soil with low
permeability characteristic (such as silt and clay).
The Ultimate Capacity of piles in low-permeability soil (i.e. silt and clay) increases as a
result of the increase in the strength of the soil surrounding the pile shaft (due to reconsolidation).
In the case of soil with high permeability (i.e. sand and gravel), pile capacity may increase even
after there is a complete dissipation of pore water pressure. The complete dissipation of pore
pressure in case of high-permeability soils may take few hours to a few days.
4-4 INCREASE IN ULTIMATE CAPACITY OF BORED PILES
The Ultimate Capacity of bored piles may increase over time due to a number of factors.
Increase in the earth pressures against the surface of the piles over time may cause creep of soil
towards the pile, thereby increasing the radial stress on the pile shaft; hence increasing the pile
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capacity. Sustained loads on the bored piles can cause the soil structure to progressively become
more stable and increase in strength, and the loading and unloading cycles of the piles also have
similar effects.
4-5 INCREASE IN ULTIMATE CAPACITY OF DRIVEN PILES
Augustesen (2006) reports the increase in Ultimate Capacity for driven piles may be
caused by two different factors. The dissipation of excess or negative pore water pressures that
increase due to driving the piles lead to an increase in horizontal effective stresses that act against
the pile-shaft surface, thereby increasing the mobilized skin friction over time. Ageing is another
factor that leads to an increase in Ultimate Capacity for driven piles due the change in the
characteristics of the soil structure, change in the interaction between the pile-shaft and the soil,
as well as change of stress in the surrounding soil at a given pile. In the case of piles driven into
clay, thixotropy and the digenetic bonding of clay particles may play crucial roles.
Furthermore, steel-manufactured piles may chemically react (via cation exchange) with
the surrounding soil minerals and due to chemical bonding of the pile surface and the soil
structure, an increase the pile capacity may occur. Increase in pile capacity may also result from
the formation of digenetic bonding between the soil particles themselves following the complete
destruction of soil structure that results from the disturbance and displacements caused when
driving the piles into the ground. Evangelista and Picarelli (2000) state that digenetic bonding
refers to the inter-particle forces caused by matric suction. Matric Suction is defined (Chariot, n.d.)
as the suction induced by the soil matrix that causes the water to flow in unsaturated soil.
4-5-1 Increase in Capacity of Piles Driven into Clay
Chen, et al. (1999) discovered that when driving a pile into clay, a volume of clay equal to
the volume of the pile is displaced in the direction of least resistance. This seems to be an obvious
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empirical observation based on the fact mentioned earlier. Two zones formed around the pile,
the ‘Remoulded Zone’, where the displaced clay moved to, and the outer ‘Transition Zone’, where
the soil parameters changed in their characteristics slightly. The amount of change in the
transition zone depends on the properties of the soil, the driving methods, the dimensions of the
piles and the pile density. The soil beyond the transition zone retained its original properties.
As for the increase in Ultimate Capacity, the driven piles induce high pore pressures
within the remoulded zone. As the water dissipates from around the pile surface, reconsolidation
occurs in the remoulded zone; as a consequence the undrained shear strength increases resulting
in higher pile capacity. In certain cases, the remoulded zone may even become adhesive with the
pile surface, and after the process of reconsolidation, the remoulded zone may move
simultaneously with the pile, acting as one body, thereby increasing the perimeter of the pile. This
results in further increase of pile capacity.
4-5-2 Increase in Capacity of Piles Driven into Sand
When driving a pile into a sandy deposit, Chen, et al. (1999) stated that generally the
increase in Ultimate Capacity is expected to occur shortly after installing the pile. As stated earlier
the duration for the complete dissipation of excess pore pressure may take a few hours to a few
days after pile installation. It has also been reported by various researchers (Tavenas and Audy,
1972, cited in Chen, et al., 1999) that over a long period of time the pile driven in sand gains a
substantial increase in pile capacity. Chow, et al. (1998 cited in Chen, et al., 1999) reported pile
capacity increase by 85% during time intervals of six months and five years after installing open-
ended piles into dense marine sand.
Looking at these discoveries, one may conclude that the colossal increase in capacity of
piles driven into sand (in the long-term) cannot be explained by the dissipation of excess pore
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water pressure, since the dissipation occurs very shortly after the pile installation. Therefore, the
increase in Ultimate Capacity may be attributed to the aforementioned chemical bonding
between the pile surface and soil minerals, digenetic bonding between the soil particles, increase
in stiffness and strength of soil due to ageing, or due to the creep of soil towards pile shaft
increasing the earth pressures against it.
4-6 ULTIMATE CAPACITY INCREASE OVER TIME
Lied (2010) states that during the year 2006, tests conducted by the NGI showed that soil
generally demonstrates time-dependant behavior, and the soil grains gain additional strength and
stiffness with time due to the time-dependant processes such as ageing. The capacity of piles also
increases with time after installation due to the time-dependant processes in the soil. It was also
found that the increase in pile capacity with time depends on soil type, albeit, due to insufficient
data available which links pile capacity increase with soil parameters, the relationship cannot be
confirmed to be absolute.
4-6-1 Case Study
Powell and Skinner (2006) report that a test was carried out at Lodge Hill Camp,
Chattenden, Kent; the test site was based upon London Clay. This site was originally a ‘Greenfield’
site designated for testing the performance of foundations piles by the BRE.
The piles (numbering 20) used for testing were designed to be economical but large
enough to resemble a typical bored pile generally used in construction sites. To ensure reliability
of the test results, care was taken to make sure all the piles produced were similar in
characteristic. The piles were also designed with low capacities so as to minimize the size of the
reaction system, and this was deemed suitable for a testing environment. Furthermore, to ensure
that the piles were not affected by the seasonal variations, they were dug 4m beneath the ground
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surface; and initially (the piles) settled 9.7m deep, since the length of each individual pile was
5.7m (and the diameter of each pile was 300mm).
The piles were installed in the ground using a four-stage process:
1. A 400mm bore was drilled using a rotary technique to a depth of 4m. An outer
casing (of diameter 380mm) made of light corrugated steel was dropped into the
open bore. An inner casing (of diameter 320mm) was then dropped within the
outer casing to a depth of 5m. At the top of the bore, the gap between the inner
and outer casing was lined with polystyrene filler to prevent any soil or concrete
from pouring in between.
2. The main bore was drilled 10m deep into the ground using a 300mm diameter
auger. The base of the bore was left unclean so that the pile capacity would be
derived primarily from the shaft resistance.
3. A single piece of high grade steel bar (of diameter 32mm) was inserted through the
centre of the bore, all the way down to 10m.
4. Concrete (of design strength 35 Nmm-2
) was poured into the inner casing right up
to the top of the inner casing to fill up the bore and form the bored pile.
During the test, the loading on the piles was applied in incremental steps of 25kN for a
minimum duration of 1 hour up until the appliance of that loading induced a settlement of 0.1mm
per hour. The tests were terminated after the settlement reached the required value of 20-30mm.
The piles were tested and then retested.
At the end of the test it was found that the ‘virgin’ piles (which were not previously tested
before) had an increase of 25% in Ultimate Capacity, three and a half years after their installation.
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For example, a pile was tested two and a half months after its instalment in the ground to show
that 300kN of loading was required to get a settlement of 3mm.
Retested piles however did not show any increase in capacity. The initial capacity of the
piles at the time of retesting was lower than that of the piles when they were at the ‘virgin’ state.
Piling Blanket
Outer Casing
Inner Casing
Concrete Pile
Figure 5: Layout of Tested Piles
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5. Assessment of Pile Integrity and Pile Capacity
Before placing old, existing piles into use, checks must be performed to test the pile
integrity and the load-bearing capacity to ensure neither failure nor any undesired settlement
occurs. Pile testing methods are best identified by the duration for which the force is applied and
strain induced on the pile. To test the pile load capacity, large loads are applied to the piles for a
long duration, and to assess the pile integrity, small energy low strain tests are performed. Four
pile testing methods shall be discussed here. In this section various methods of testing load-
bearing capacities of piles, as well as checking their integrity, and even measuring their length and
mass shall be critically appraised.
5-1 HOW TO ESTIMATE AXIAL CAPACITY OF PILE FOUNDATIONS?
Estimating the capacity of piles is highly desired; this is required for building safe
structures over the piles, but this also may provide economic benefits by allowing the designer to
calculate how many piles may be required for the structure in question, thereby reducing the
error of underestimating the capacity of piles which may lead to larger and unnecessary amount
of piles being used. Static Design Equations, Pile Driving Formulae, Static Loading Tests or Stress
Wave Analysis may be used to estimate axial capacity of single piles.
The calculation of pile capacity is based on the theoretical concepts derived from the
sciences of soil mechanics, and one mainly relies upon empirical experience to determine the
capacity (Tomlinson, 1995 cited in Augustesen, 2006). Clausen and Aas (2001a), as cited in
Augustesen (2006), verified that due to the fact that the mechanical properties of the soil in the
remoulded zone and the stresses acting against the piles are unknown, the calculation of pile
capacity is primarily based on evidence from empirical experience. Therefore, one must be careful
in calculating the capacity of driven piles and take into consideration the fact that the physical
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properties of the soil around the pile may change, and hence make changes based on correlations
derived from other tests regarding the change in soil parameters during pile installation.
5-2 FORMULAE
Skov and Denver (1998), as cited in Augustesen (2006), provide a semi-logarithmic
equation that describes a linear relation between time, t, and Ultimate Capacity, Q, of piles in clay:
Where, Q is the vertical ultimate capacity at time, t, following the end of pile installation;
Q0 is the reference ultimate capacity determined at reference time, t0; Δ10 refers to the capacity
increase corresponding to a ten-fold increase in time – this may be a function of a relevant soil
parameter or used as a constant.
Meyerhof (1976), as cited in Chen, et al. (1999), stated the Static Pile Capacity in sand
may be calculated using the following empirical equation:
Where, Qf is the Shaft Friction; Qb is the End Bearing Capacity; N is the Standard
Penetration Test values; As is the area of the pile shaft; and Ap is the cross-sectional area of the
pile.
The resulting estimated value of pile capacity from the above formulae may not
necessarily reflect the actual capacity due to the effect of time. The value should be recalculated
after conducting tests following significant amount of resting period.
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5-3 MAINTAINED LOAD TEST (MLT)
Incremental loads are applied at a given rate on the pile in this method until the rate of
induced settlement goes below the specified criteria, and the resulting pile settlement is
monitored. The total duration of the test last between 24 to 48
hours including time taken to set up the equipment. This method
is ideal for testing piles that work directly under load points. MLT
is a method that suits all soil conditions and pile types, and can
test for very high loads, i.e. 30MN. However the long duration of
testing and high expense may be off-putting and health and safety risks have to be carried out
since the equipment set-up involves working at height.
5-4 CONSTANT RATE OF PENETRATION TEST (CRP)
The purpose of this test is to determine the Ultimate Bearing Capacity of a pile, which is
derived mainly in terms of the shaft friction. The pile is loaded at a constant rate until failure or a
maximum specified test load is achieved. The duration of this test is less than 24 hours excluding
the equipment set up. The measured penetration of the pile is then plotted against the load
applied. CRP is ideal for all pile types. However, it only suits cohesive soils, and due to the high
rate of loading, the Ultimate Capacity may be over-predicted.
Figure 6: Maintained Load Test
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5-5 DYNAMIC LOAD TEST
A pile head is subjected to hammer blows from which the
measured response parameters are analyzed to predict the
resistance of the soil that would be mobilized by the pile under
static load conditions. The settlement performance of the piles can
also be predicted. The duration for the test can range between 15 to
30 minutes only depending on the type of pile. Furthermore, this
method is also relatively cheap and suitable for bored and driven
piles. Due to the high rate of loading, this test cannot take into
account matters such as consolidation or creep.
5-6 TESTING PILE INTEGRITY
Testing the integrity of existing piles is a crucial factor to be considered before putting
them to use. Different integrity tests deal with pile materials or even test both the pile and the
soil together. Pile Integrity Tests are better than the Pile Loading Tests for a number of reasons.
Pile Loading Tests are not cost or time-effective, only a small range of piles are tested hence faulty
piles may be ignored, and the actual dimensions of the piles cannot be determined using the load
tests either.
Pile Coring and Excavation are traditional methods employed to test the integrity of piles.
Cores are drilled in piles using percussion equipment and then the inner structure is examined
using a camera which relays the video on a screen. The size of the coring is around 50-100mm in
diameter. The pile integrity is determined by the drill resistance, as well as the composition and
colour of the material. This method is relatively quick. Excavating piles results in pile-exposure up
to ground level after which the piles may be observed for their material constituent and their
dimensions (Simons and Menzies, 2000).
Figure 7: Dynamic Load Test
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Gardner and Moses, (1973) as cited in Simons and Menzies (2000), mention Sonic Testing
as a method to test pile integrity. Sonic pressure waves are passed horizontally between a
transmitter and receiver that are vertically cast into the piles. An oscilloscope records the time
taken for sound waves to travel from the transmitter to the receiver. Zones of weak concrete
appear as fainting marks of signals on the oscilloscope and lengthen the travel time. Sonic Pile
tests may be used to determine the length of the pile as well.
Also, stated is the method of Vibration Testing in which a motor vibrates vertically and
exerts a force on the pile head. This method does not require much preparation of the pile or pile
head, and is primarily used to detect major defects. The results may be affected by extraneous
vibrations caused by moving plant at site, so the test either has to be carried out at night or after
placing the entire site at standstill. This test method boasts the capability to determine pile length,
mass, stiffness, as well as the recognising the damping effect of the soil that surrounds the pile.
5-6-1 Case Study
In terms of practical application, Hungspruke, et al. (2003) state that Ultrasonic tests were
performed at a building in Buenos Aires, on the existing concrete columns to determine their
actual strength by comparing the results with the concrete samples which were earlier tested for
strength. Gamma-ray test was also carried out to determine the location of the steel
reinforcement as par the depictions on the drawings of the original structure; and based upon
these results the column capacity was evaluated and judged against the super-imposed loads
from the new structure.
22
6. Case Study
Examining practical applications of foundation reuse can provide better understanding of
the procedures involved. This section shall provide an outlook on two cases where foundations
were reused, the reason for the reuse, and the benefits gained thereof.
6-1 BELGRAVE HOUSE
Belgrave House was originally designed by Ove Arup and it was raised to a seven-storey
structure above a single-storey basement. It is located on Buckingham Palace Road. The building
site is restricted by Victorian-style buildings on one side. The area that covered the old
development was roughly 110m in length and 43m in width. The existing piles relied upon the
45m+ thick layer of London Clay which rested 7.5m below the ground level. The new structure, for
which the construction work commenced in November 2001, was built on the site of the old
structure, which was demolished.
During the stage of desk study, it was found that adjacent to the building site, two multi-
stacked tunnels were planned for construction 36m below ground for the New Crossrail. As a
result of this, the designer had to take into consideration the ground movement that would be
Figure 8: The New Belgrave House
23
caused by the future tunneling. Furthermore, the new building extended beyond the footprint of
the old structure; hence new foundations needed to be installed which were located next to the
Crossrail tunnels. Also, on the other side of the building site, the London Underground’s District
and City line passed at a shallow depth.
The site plan showed the presence of a congested ground filled with existing under-
reamed piles which imposed heavy restriction on the design on the new structure. Clearly these
existing piles were an obstruction to the construction of new piles; ignoring these existing piles
altogether and installing new piles upon which the building structure would completely rely
would mean that the ground would be further congested for future developments at the site.
Furthermore, the piles could not be removed and replaced by new piles since most piles were
under-reamed and not fully reinforced.
Hence, Whitbybird retained all the existing piles, and installed reinforced concrete rafts
which would then allow the uniform distribution of the structural loads into the existing piles.
These rafts catered for better control of ground movements, thereby reducing the effects on the
future Crossrail tunnels.
The design consultants appointed for the project, Whitbybird, planned rigorous testing
procedures to determine the integrity of the existing piles as well as estimating the impact to
occur if the piles were to be demolished.
Cichy (2006) states the reuse of existing piles had shortened the construction program,
resulted in lower foundation costs and minimized the risk that would otherwise occur from the
existing obstructions in the ground. All of this was possible due to the reuse of the existing
foundations; the need for a lesser number of new piles resulted in lower cost, as well as the time
required to install these piles.
24
As for the design strategy in reusing the existing piles, Whitbybird looked into the design
drawings, contacted the piling contractor, and this resulted in the attainment of excellent
information which proved useful for assessing the feasibility of reusing the existing foundations.
Assessments of realistic loads (SLS) and design loads (ULS) was made on each pile as well as
examination of case studies on the load-settlement behavior of similar piles in London clay
proved very resourceful.
When the old building was demolished, the impact on the existing piles had to be checked
to test their integrity. The response of the load-settlement was taken to be similar to an unload-
reload scenario of a pile load test, i.e. the piles were reloaded after reconstruction. Due to the
demolition, there was a risk that the existing piles below the reinforcement cage would crack. To
minimize the risk, pile integrity tests were carried out for all exposed piles. Care was also taken to
arrange the structural elements of the new building such that eccentric loads were not applied on
the existing piles; hence the design of the foundation layout did not require much manipulation,
and new piles were installed only where necessary.
The durability of the piles was determined by coring samples and performing
petrographic examination, which determines the material components which compose the pile
shaft. This examination also verifies whether the blend of concrete has been mixed in accurate
proportions or with desired materials as required for sustainable durability, otherwise corrosion
and deterioration of the piles can occur causing structural instability.
To take extra precaution, the presumed load carrying capacity was reduced for the
existing piles and diverted to the new piles. A larger sample of existing piles were also tested out
in the lab, however the results showed that there was no need for such precautionary measures
25
since the piles had high integrity. The construction work for Belgrave House was completed in the
winter of 2003.
6-2 WORLD TRADE CENTRE
Currently at the Ground Zero site, the development of the new World Trade Centre will
see almost half of the old foundations of the collapsed building being reused. Preliminary desk
studies did not provide much design information about the old foundations. So some of the piles
were exhumed for testing and most of them proved “Okay”, albeit some of the concrete strengths
were a bit low, that is in the encasement and not in the core.
Due to the heavily congested ground condition, difficulty was encountered whilst
installing new raking piles to resist the lateral loads which the old piles were unable to cope. The
new structure has had to rely fully on vertical piles which are fixed to a thick concrete foundation
mat.
Mueser Rutledge geotechnical partner George Tamaro said about the congested ground
condition, “It’s like trying to put additional foundations in the back of a porcupine – trying to
thread needles through needles.”
Whilst reusing the old foundations, the
engineers had to be careful since surplus
information was not available about the existing
piles. So whilst the new piles were fixed to the
foundation mat, the existing piles were pinned,
which meant that the lateral load imposed on the
original piles would be reduced by a remarkable
amount of 60%.
Figure 9: Construction work at the World Trade Centre
26
7. Conclusions
Reuse of foundations is becoming an increasingly essential requirement at construction
sites, particularly in urban centres like London. Ground congestion, preservation of historical
heritage, sustainability issues and economical factors all drive for the need to reuse existing
foundations at the Brownfield sites.
With the advancement of technology, the integrity of pile foundations and their capacity
can now be accurately determined, allowing for a swift design plan, considering the need to install
new piles only in cases of dire necessity.
Reusing foundations benefits the design of the structure which in turn brings about
economic benefits as well. With the progress of time, the Ultimate Capacity of pile foundations
increases, thereby allowing the piles to carry higher loads than before; the number of piles
required is thus reduced.
Just as it is important to preserve the historical heritage of a site, catering for future
developments on that same site is equally significant. Reusing foundations will prevent the
ground from being over-congested, thereby allowing the Brownfield sites to be successfully used
over and over again, and all the while using up less Greenfield sites.
27
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