SIMPLIFIED reinforcing the establishment, every one of them take

SIMPLIFIED METHOD FOR DESIGN OF UNDERPINNING PILES

Gaurav Patel, Sridevi Davuluri, Acthutaram
Kalanadhabatta

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Abstract: Existing buildings sometimes
experience excessive settlement under their design load or face the
prospect of excessive
settlement in the future if the foundation loads increases. Several methods of
foundation enhancement are available to arrest settlements or improve the future
performance of existing foundations. The method of underpinning by piles is
widely used, but despite the long-standing use of this method, approaches to
the design of piles for underpinning are not well developed. In this paper,
a design method based on the concepts of
pile-raft interaction is developed. The general concept of the simplified
design method is described, and then the method is formulated and explained in
detail. The application of the simplified method to a series of problems
analyzed by finite element methods is then described. Finally, comparisons are
made between the results of the simplified analysis and data from model footing
tests.

 

Introduction

Underpinning might be expert by expanding
the

foundation in depth or in breadth so it
either lays on a steadier soil stratum or appropriates its heap over a more
prominent region. Utilization of miniaturized scale heaps and fly grouting are
basic strategies in Underpinning. Underpinning might be essential for an assortment of reasons:

 

The first
establishment is basically not solid or sufficiently stable. The utilization of
the structure has changed. The properties of the dirt supporting the
establishment may have changed (conceivably through subsidence) or were
misrepresented amid outline.

 

The
development of close-by structures requires the unearthing of soil supporting
existing establishments. To expand the profundity or load limit of existing
establishments to help the expansion of another story to the working (above or
beneath grade). It is more sparing, because of land cost or something else, to
take a shot at the present structure’s establishment than to build another one.

 

Earthquake,
flood, drought or other natural causes have made the structure move,
accordingly requiring adjustment of establishment soils as well as footings.

 

Discussion

Following are the different underpinning methods used for
foundation strengthening:

·        
Mass concrete
underpinning method (pit method)

·        
Underpinning by
cantilever needle beam method

·        
Pier and beam
underpinning method

·        
Mini piled
underpinning

·        
Pile method of
underpinning

·        
Pre-test method of
underpinning

 

Whatever be the sorts of supporting strategy chose for
reinforcing the establishment, every one of them take after a comparable
thought of broadening the current establishment either longwise or breadthwise
and to be laid over a more grounded soil stratum. This empowers appropriation
of load over a more prominent zone.

 

One of the common remedial measures for foundations
that are settling excessively is to underpin with piles. This problem for
idealization and design purposes, can be considered as a “piled-strip
foundation” when the existing foundation is a strip footing, or as a
“piled-raft foundation” when the existing foundation is a circular,
square, or rectangular raft.

 

Piles often have been used in conjunction with raft or
strip foundations to diminish the settlement of structures or to ensure them
against exorbitant differential settlement. Method of designing piled-raft
foundations can be classified in two broad categories (Poulos 1991).

 

To start with, there are traditionalist strategies
that disregard the nearness of the strip or raft, and that assume that the
piles carry the total applied load. However, in any case a proportion of the
total load is inevitably transmitted directly from the strip, the raft, or the
pile cap to the ground. In some cases, about 45% of total load is carried by
the raft at the time of construction, and 25% at the time of occupation of the
building (Cooke et aI. 1981).

 

Second, there are more practical techniques in which
account is taken of the load sharing between the piles and the raft or strip.

 

Phung Duc Long (1993) has done a progression tests on
piled-footing tests and has concluded that when great contact among top and
ground surface starts, a huge extent of the applied load will be transferred to
the piles until their failure. After assembly of full limit of the piles, any
additional load will be transferred to the cap (raft). 

 

Phung Duc Long has proposed a streamlined strategy for
calculating the settlement of piled-raft foundations in which the conduct of a piled-raft foundation is
taken to be the same as that of a shallow footing under a reduced loading. In
other words, if the load carried by the raft (cap) P, is known, the settlement of a piled-raft foundation
establishment is around equivalent to the settlement of the shallow balance subjected
to a decreased connected stacking P.

 

Simplified Design Method – General Concept

The foreseen future settlement STF of an establishment
(either unpiled or with too few piles) is excessive, and it is wanted to
support the establishment so its last settlement will be lessened to a fair
esteem, STFP. The inquiries that a planner may have are either:

 

(1) For a given pile arrangement, what is the final
settlement STFP of the
underpinned foundation?

(2) for a specified value of STFP, what is the required piling arrangement?

 

The main steps involved in obtaining answers to these
questions are as follows:

1. Survey the expected future settlement STF of the
current establishment.

2. Survey the time ti; between the
beginning of stacking of the current establishment and initiation of
supporting.

3. Consider pile-soil-foundation interaction to assess
the increase in stiffness of the foundation due to the addition of piles.

4. After the piling is installed calculate the
additional settlement, or of the required piling to obtain a final settlement
of STFP model footing tests in
which piles have been installed to reduce future settlements.

 

This approach requires the designer to estimate the
stiffness of the existing and the underpinned foundation, the interaction
factors among the various foundation elements, and the load distribution among
these elements. This can be accomplished most monetarily by means of a
straightforward inexact hand-computation technique, then the more complex
finite element method.

 

After obtaining the stiffness of the existing and
underpinned foundations, the load can be distributed among the foundation
Elements. After obtaining the stiffness of the existing and underpinned
foundations, the load can be distributed among the foundation Elements. The
settlement reduction can be predicted, about the time of pile installation, by
a simplified method that will now be described, about the time of pile
installation, by a simplified method.

 

Details of Simplified Design Method

Settlement After Underpinning – The final settlement
after underpinning of the existing foundation can be estimated by the following
equation:

STFP = Sti + X (STF –
Sti)

 

where STFP = total final settlement of foundation underpinned by piles; Sti = total settlement of existing
foundation at time t; of
underpinning; STF = total final settlement of existing foundation if
no underpinning is carried out; X = Krl
/ Kpr = ratio of stiffness of existing foundation to
underpinned foundation; Kr
= existing raft foundation stiffness, i.e. foundation stiffness before
underpinning; and Kpr =
piled-raft stiffness, i.e. foundation stiffness after underpinning.

Fig 1. Existing
Foundation

By dividing equation by STF, the following expression is obtained:

 

RSR = Us + X (1 – Us)

 

where RSR =
STFP, ISTF = ratio of
settlement reduction = (settlement with
pile) / (settlement without pile); and Us
= Sti / STF
= degree of total settlement (immediate plus consolidation) of foundation at time ti of underpinning =
(total settlement at time of pile installation) / (total final settlement of
existing foundation without piles).

Fig 2. Foundation Underpinned with Piles

 

To better comprehend the effectiveness of pile
installation for reduction of settlement, it is desirable to estimate how much
piles can reduce the remaining settlement of the foundation. Obviously, the settlement happening
before pile installation S, can’t
be adjusted for and ought not be considered in evaluating in assessing the effectiveness of
piles for settlement reduction.

Fig 3. Time-Settlement Behavior- Final
Settlement of Foundation STF
Is Excessive, with Underpinning Final Settlement Reduced to STFP

The overview of effectiveness of the pile installation
for underpinning the foundation will be obtained in relation to the ratio X of
stiffness of the existing foundation to the underpinned foundation. Add up to
Proportion of Load in Piles in the wake of Underpinning. The transfer of the
applied load on the existing foundation to the newly installed piles, is an
important issue in understanding the mechanics of underpinning. The greater the
transfer of load from the foundation to the piles, the smaller is the total
final settlement.

 

Calculation of Stiffness of Foundation Elements

In the past
area, it was demonstrated that the viability of piles on settlement diminished
and the measure of load exchanged to the piles are identified with the firmness
of an establishment prior and then afterward supporting. Hence, it is important
to depict how the firmness of the establishment with and without piles can be
computed.

 

By and large,
establishment uprooting or pile deformation might be identified with the
connected load on the establishment by the accompanying condition:

 

{P}
= K {S}

 

Where {P} =
Pile or Footing Load, K = Pile or Footing Stiffness, {S} = Foundation
Displacement.

 

Calculation of
Piled-Raft Stiffness

Connection
factors for the impact of heap assemble on raft estimations of heaped raft
firmness Kpr can be figured by the conditions proposed by Randolph
(1983). These have been displayed for an extensive variety of heap gather sizes
by Clancy and Randolph (1993). The load taken by the piles and the raft, Pp
furthermore, Pro separately, can likewise be figured (Randolph 1983; Clancy and
Randolph 1993).

 

Underpinning Tests

Makarchian (1995a) has
portrayed two arrangement of controlled show supporting tests that have been
completed in a regularly merged mud with two diverse stacking levels (257
what’s more, 200 kPa) and diverse heap lengths (50, 75, 100, and 125 mm), with
a heap distance across of 12.7 mm (0.5 in.).

 

The balance distance across
was 50.8 mm (2 in.), and the heap was introduced at its focus toward the start
of stacking (i.e., ti = 0). A mechanical assembly for performing
supporting tests was planned and fabricated, also, points of interest have been
portrayed by Makarchian (1995 a, b).

Fig 4. Comparison of Underpinning Tests
with Finite Element Analysis and Simplified Method for Settlement Reduction by
Different Pile Lengths and Load Levels

 

The goal of the supporting
tests was to consider the impact of heaps utilized for supporting, and to
explore a few components impacting heap conduct and the heap exchange system.
The dirt utilized for the model supporting tests was a kaolin earth (meant as
CIC kaolin). This dirt was chosen because of its generally moderate rate of
solidification and high versatility record (LL = 74; PL = 32; and PI = 42). The
quality and misshaping of the CIC kaolin mud have been given by Makarchian
(1995a).

 

The streamlined technique
has been utilized to ascertain the settlement diminished acquired by two
distinctive heap lengths under two diverse connected load levels. Fig.  demonstrates an examination between the
aftereffects of supporting tests and both the limited component examination and
the disentangled strategy, and by and large great assertion is gotten. The heap
exchanged to the 100-mm long heap in test arrangement (II) was 200 N.

 

The pile stack as per the
simplified technique was ascertained to be 350 N. As already showed in this
paper, the bearing limit of the or heap gathering would be checked for the
extent of load exchanged to the heaps.

 

In that test, the pile came
to its full limit and no more load could be conveyed by the pile. The approach
portrayed by Phung Duc Long (1993) by utilizing was additionally used to assess
the settlement of the model supported balance. The esteem so figured was 4.2
mm, which is 19% higher than measured. This is thought to be reasonable
assertion for an exceptionally basic and estimated strategy.

 

Our Contribution in
the Term Paper

After research
on the various Reference Papers for the term Paper, we concluded that
Piled-raft foundation, in which piles are designed to reduce the settlement,
not to take the full load from superstructure, is an effective foundation
method for high-rise buildings. In any case, foreseeing the settlement for
heaped pontoons is a troublesome assignment for geotechnical builds because of
the mind-boggling heap top soil cooperation.

Most of the
accessible forecast strategies are not appropriate for pile-raft foundations as
they depend on the hypothesis of flexibility. The rearranged technique,
proposed in this paper, basing on the trial think about performed by the Author
can be utilized effortlessly for the reasonable outline of a pile-raft
foundations. This strategy can be utilized more viably if it is utilized as a
part of blend with FEM technique to foresee the settlement conduct of pile-raft
foundations.

 

The
examination for the contextual analysis completed by the Author for an
extensive skyscraper venture displayed in this paper is an unmistakable
delineation for the outline rehearse for pile-raft foundations. Applying heaped
pontoon establishment technique, many heaps can be spared in contrast with the
customary regular heap establishment. Sadly, the idea has not been acknowledged
in numerous nations.

 

Over the most
recent couple of decades, there has been impressive advancement of techniques
for computing settlement of unsupported heap gatherings and heaped footings. In
any case, most of the techniques depend on the hypothesis of flexibility. The
Author had made inferences not quite the same as those got from the
examinations basing on hypothesis of flexibility.

For example,
the comparisons of settlement of a piled footing with that of a free-standing
pile group show that due to the contribution of the cap, the increase in
stiffness of the piles footing, as compared with the corresponding
free-standing pile groups, is considerable.

 

Summary and
Conclusions

In this paper, a simplified
Method has been portrayed for the plan of supporting loads for foundations.
This technique includes estimation of the stiffness of the first foundation.
This method involves calculation of the stiffness of the original foundation
and the piles, together with a consideration of interaction between the
foundation elements. Simple expressions can be

derived for the remaining
settlement of the underpinned foundations and the amount of load carried by
each pile.

 

The application of the
simplified (hand-calculation) method to a series of problems previously solved
via finite element analysis by the writers is then described. It is found that
the simplified method can be used for calculation of settlement reduction by
piles and the proportion of load transferred to the piles with reasonable
accuracy. Reasonable agreement is also found with the results of model footing
underpinning tests.

 

 

References

Phung, D.L. (1993).
Footings with settlement -reducing piles in non-cohesive soil. Ph.D. Thesis,
Chalmers University of Technology, Gothenburg, Sweden.

 

Phung, D.L.
(1994). Piled footings with settlement reducing piles in non-cohesive soil.
Proc. Int. Conf. on Design and Construction of Deep Foundations, Orlando,
Florida. Phung, D.L. (2002).

 

Phung, D.L.
(2002). Foundation peer-review for Mega Tower, MTRC Kowloon Station Development
Package 7. WSP Report, July, Hong Kong.

 

Poulos, H.G.
(2011). Keynote lecture: The design of high-rise building foundations. Proc
1st.Int. Conf. Geotechnics for Sustainable Development – Geotechnics Hanoi
2011, pp 244-255, October.

 

Randolph, M.F.
(1983). Design of piled raft foundations. Cambridge Univ., Engineering Dept.,
Research Report, Soils TR143.

 

Sommer, H.,
Wittmann, P. & Ripper, P. (1985). Piled raft foundation of a tall building
in Frankfurt clay.