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Effective Bore Pile Design and Installation
Conference Paper · September 2012
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THE USE OF PILE LOAD TEST TO DETERMINE THE RESIDUAL SETTLEMENT
OF A PILE
by
BOLARINWA ADEMOLA
GEOTECHNICAL ENGINEER,
PORTSMAN FOUNDATIONS AND CIVIL WORKS,
12B, LUGARD AVENUE,
IKOYI, LAGOS, NIGERIA.
Abstract
This presentation reviews the accepted practice regarding the design, analysis and procedure of
pile load testing. A step-by-step guide on pile load test is enumerated and a case study of a pile
load test done in Lagos area of Nigeria is discussed. A comparison is made on the designed and
load test obtained settlement results. Also, included are general reflections and new trends in the
analysis of pile load test results.
Introduction
The behavior of single piles under axial loading has been examined in detail by many
investigators, and their findings were outlined in several publications (Davisson 1972; Meyerhof
1976; Vesic 1977; BS 8004: 1996; Das 1999; Nabil 2001). The load transfer settlement
relationship for single piles and pile groups is very complex. Most settlement analysis methods
are based on empirical methods and give only a rough approximation of the actual settlement,
Gilbert (1991). Calculations of settlement are rarely performed when designing ordinary group
pile and when they are done, the methods of calculation range from those using simple rules of
thumb to those incorporating detailed finite element analysis. However, the direct method for
calculating settlements from pressuremeter tests has been discussed by Frank (2012). A school of
thought from http://osp.mans.edu.eg/deepfoundation/ch9.htm explicitly stated the total
1
settlement equation for single bored pile of diameters less than 600mm in a semi-empirical
method below;
S = Ss + Spp + Sps … (1)
Where (Ss) is the elastic compression of pile shaft, (Spp) is the settlement caused by load
transferred at the pile tip, (Sps) is the settlement caused by load transferred along the pile shaft.
While the design of pile capacity is often verified by full-scale load testing, design for
settlement is almost totally without the benefit of full-scale verification. The bearing capacity of
a single pile may be estimated in one of three ways namely; (a) by load testing, (b) by static
formulae and (c) by dynamic formulae, Scottt (1980). The first, i.e pile load test is a major focus
in this paper with a case study. Generally the safe working capacity of a pile is dependent on
factors such as pile dimension, its founding depth, geotechnical properties of surrounding soils
strata and also method of installation. All these are best determined by a specialist piling
contractor. Pile testing is the main part of QA/QC of piling works.
Pile Load Test
Pile load test is a static load test of a pile or group of piles used to establish an allowable load.
The applied load is usually maximum of 150% to 200% of the design safe working load. The
primary objectives of pile load test are;
• To establish load-deflection relationships in the pile-soil system,
• To determine capacity of the pile-soil system, and
•
To determine load distribution in the pile-soil system.
These tests will confirm design assumptions or provide information to allow those assumptions
and the pile design to be modified, Geotechnical Engineering Bureau (2007). Pile load testing
provides an opportunity for continuous improvement in foundation design and construction
practices, while at the same time fulfilling its traditional role of design validation and routine
quality control of the piling works. In order to achieve this improvement, data from pile tests has
to be collected and analysed to enable the piling industry, both individually and collectively, to
make the best use of resources, Federation of Piling Specialist (2006).
2
Generally, many uncertainties are inherent in the design and construction of piles; it is therefore
difficult to predict with accuracy the performance of a pile. The best way to know the pile
behavior is to carry out a load test. The significance of a properly conducted load test is that, it
furnishes the actual soil resistance at site upon which design can be based reliably. In practice,
load tests will determine that the foundation is capable of sustaining working loads with
sufficient factor of safety (normally Factor of Safety = 2.5 – 3.0). Load test will also identify the
mechanism by which load is transferred to the soil.
Basically, there are two types of test conducted for axial types of loading namely; compression
tests and tension (pull out) tests.
Pile Details and Site Description
The tested pile is located within the site at 31/33, Waziri Ibrahim Crescent, Victoria Island,
Lagos. Table 1.0 below shows the pile details
Table 1.0: Pile Details
Auger Pile
Diameter
Type of test
Pile Safe Working
Length Load (kN)
(m)
(mm)
A-47
600
Compression
16.0
590
Date of Test
17th – 18th January,
2012.
The pile load test was carried out in accordance with BS 8004 “Code of Practice for Foundations”
Table 2 below shows the general site characterisation through executed subsoil investigations prior to the
design of piles. 4 no geotechnical boreholes were drilled to 60.0m each and 6 no. cone penetrometer test
were performed to refusal depth.
3
Table 2: The Generalized Subsoil Profile of the Site
From
To
(m)
(m)
Generalised Strata Description
(Note * Refers to existing ground level.)
Cone
Resistance
Nspt/Cu
Values
(kg/cm2 )
0.0*
10.5/12.0
Very loose to loose, fine to medium grained SAND 2 - 135
being organic from 2.0m-6.50m in BH1/BH3, and
clayey from 6.4m-11.5m in BH4.
1-9
10.5/12.0
20.0/20.5
Medium dense to dense, fine to coarse grained 5 - 395
gravelly SAND becoming loose at the bottom
1 - 40
20.0/20.5
32.0/38.0
Soft to stiff organic CLAY being silty at the top. 5 - 95
BH-1 has medium dense sand between 25m and
32m.
Cu= 68-106 kPa
32.0/38.0
42.0/50.0
Medium dense to very dense, fine to coarse grained 95 - 495
SAND with gravel at places and with lateritic clay
and organic material at the top.
13 - 56
42.0/50.0
60.0
Dense to very dense, medium-coarse grained SAND with gravel at places and stiff sandy clay
running across the site at depths between 52.5m
and 58.5m (Cu=88 - 160kPa).
30 - 55
Preparation of Load Test Platform
The test pile was installed such that the top is exactly at the required height and the support
surface must be perfectly horizontal. Steel plates are then glued on top of the pile head while ensuring that
the lower surface of the protruding parts is clear of the ground surface. The distance between these parts
and the ground surface will be such that the test load is not carried by the ground. There should be enough
clearance around the pile cap to prevent resistance from the sides or in particular, the base of the cap. A
gap of 100 to 150mm is usually adequate below the cap.
Equipments and Instrumentation for Pile Load Test
Major equipment required for applying compressive load on a test pile generally include, but are
not limited to the following:
• test beams - primary and secondary
4
• bearing plates
• hydraulic jack of appropriate capacity (800tons); connected to hydraulic pump
• oil manometer of suitable capacity
• kentledge or Dead weights (normally in form of concrete cubes of 1m3 and 24kN or 2.4tons),
etc..
• 2 nos. steel reference beams
• 2 nos. dial gauges, capable of measuring movements within an accuracy of 0.01mm
Arrangement of Load Test Platform
The arrangement for an axial compression test is generally done using either;
(a) by means of a jack which obtains its reaction from kentledge heavier than the required test load
(see Fig. 1 & 2);
(b) by means of jack which obtains its reaction from tension piles or other suitable anchors (section
7.5.5.2 of BS 8004;1986).
In the case study presented in this paper, the former was applicable i.e (a).
Arrangement of the load test equipment for the kentledge method is as follows:
a) Two support lines are established equidistant from the test pile position.
b) A steel plate of adequate thickness is placed centrally on the pile head and the hydraulic jack
placed on the steel plate afterwards.
c) A second steel test plate is placed on the hydraulic jacks after which the primary beam is
placed over it in a position parallel to the established supports.
d) To ensure that there is a clearance between the primary beam and the hydraulic jack, the beam
is supported at each end and care is taken to ensure that there is a clearance between it and the
hydraulic jack. As a minimum, placement of the beam is such that its top level lies either at
the same level as that of the established supports or it is not more than 50mm below this level.
e) The secondary cross beams are then be placed over the established supports.
5
f) Afterwards, the kentledge is symmetrically placed to span across the cross beams which are
supported by the established supports.
g) The reference datum beams, each with one end set in concrete and the other free but
supported, are placed about 50cm from both sides of the pile head. This will allow the beam to
move as its length changes with temperature variation.
h) The dial gauges are placed on flat steel plates which are connected to the test pile head by
welding, after which glass plates, upon which the needle from dial gauges will rest, are then be
taped to the reference datum beams. The general arrangement of load is presented in Fig. 1 –
2.
Figure 1: The Typical Compression Pile Load Test Set-up by Kentledge (Dead Weight Method.)
6
Figure 2: A Schematic Section of Pile Load Test Arrangement
Pile Load Test Procedure
A detailed set of recorded results of load, time and movement measurement in tabular form and
graph of load against settlement with indicated stages of loading are found as attachments in the
appendix section.
The maintained load method is the usual method of carrying out a pile load test especially when the load
settlement relationship is required. It involves the application of the load in stages, with the load at each
stage being maintained constant until the resulting settlement of the pile virtually ceases before the
application of the next load increament.
Maximum load to be applied on a single pile for this method will not exceed 2.0 x S.W.L. The load is
applied in increments of 25% of the design load. Each load increment is maintained until the rate of
settlement is not greater than 0.05mm/30minutes or until a maximum of about 2 hours have elapsed,
whichever occurs first.
7
The maximum load is maintained on the pile for 6hours, except in the event that the average rate of
settlement is not greater than 0.05mm/30minutes. Unloading of pile is done in decrements of 25% of the
maximum load or as specified by the client. A loading scheduled was prepared by the Geotechnical
Engineer using the relationship between pressure, force and area. That is, pressure applied by the jack is
directly proportional to its applied force (load on pile) and inversely proportional to the cross sectional
area of the jack. Table 3.0 below shows the configuration of some available hydraulic jacks we use in
practice.
Table 3.0: Available Jacks for Pile Load Test
Hydraulic Jack Details
S/No.
Make
Model No.
Capacity (tons)
Effective Area (cm2)
Calibration
Date.
1.
ENERPAC
2501
250
176
Nov. 2010
2.
ENERPAC
CLASG50012
500
730
Nov. 2010
3.
ENERPAC
800
961
Nov. 2010
8
Typical loading schedule prepared for the load test in the case study is shown in Table 4.0 below
Table 4.0: Pile Load Test Sequence for Ø600mm Pile
Safe Working Load (SWL)
:
55.90tons
Pile Depth
: 16.0m
Maximum Load (2.0 x SWL)
:
88.50tons
Pile No.
: A47
Capacity of Hydraulic Jack
:
500tons
Date
:
Percentage
Applied Loading
Pressure
Minimum load holding time
(tons)
(kg/cm2)
LOADING
UNLOADING
SECOND CYCLE
LOADING
UNLOADING
FIRST CYCLE
Loading (%)
0
0
0
10 minutes.
25
14.750
19
30 minutes or 0.25mm/hr.
50
29.500
38
30 minutes or 0.25mm/hr.
75
44.250
57
30 minutes or 0.25mm/hr.
100
59.000
76
1hour or 0.25mm/hr.
75
44.250
57
10 minutes or 0.25mm/hr.
50
29.500
38
10 minutes or 0.25mm/hr.
25
14.750
19
10 minutes or 0.25mm/hr.
0
0
0
30 minutes or 0.25mm/hr.
100
59.000
76
30 minutes or 0.25mm/hr.
125
73.750
95
30 minutes or 0.25mm/hr.
150
88.500
114
12 Hours or 0.25mm/hr.
125
73.750
95
10 minutes or 0.25mm/hr.
100
55.900
76
10 minutes or 0.25mm/hr.
75
44.250
57
10 minutes or 0.25mm/hr.
50
29.500
38
10 minutes or 0.25mm/hr.
25
14.750
19
10 minutes or 0.25mm/hr.
0
0
0
30 minutes or 0.25mm/hr.
9
Before and after the application of each stage of loading and unloading, reading is made at two opposite
sides of pile head by means of dial gauges. During loading, additional readings are taken at interval of 10
minutes. However, where the rate of settlement does not show any appreciable difference in two
consecutive readings, the reading is taken at a reasonable interval or next stage of loading or unloading is
performed. The Survey instrument is also used to monitor the settlement as a form of check for the
fleximeter readings.
Pile Load Test Results and Discussion
The compression pile (A-47) with cross sectional diameter of 600mm is tested to a maximum 150% of the
proposed safe working load, i.e. test load of 885kN. The design compressive SWL for the pile is 590kN.
The test for pile (A-47) was carried out in two cycles, first to design test load of 590kN, then to 1.5 x
design test load (1.5 x 590 = 885kN) during the 2nd cycle.
Table 5: Pile Load Test Result Summary
Pile
No.
100%
Movement
Movement
(mm)
125% of
SWL
(kN)
SWL
Movement
(mm)
150% of
SWL
(kN)
(mm)
Residual
Movement
(mm)
1.01
737.50
1.65
885
3.03
1.41
(kN)
A-47
590
In the first cycle, progressive and increasing loads till 100% SWL was applied on the pile and on
attainment of the 100% maximum load it was maintained for a period of about 1-hour, during which pile
movements as loading progressed were recorded, see summary in Table 5.0 above. The percentage
increase or decrease in successive loading or unloading was 25%. During the second cycle loading, the
test was taken to the maximum test load 1.5xSWL i.e. 885kN which was maintained continuously for
12hours and the corresponding movement was recorded. The full records of load test are found in the
appendix section. The load – settlement curve is shown in Fig. 3; load – time curve in Fig. 4, and the
settlement/deflection – time curve in Fig. 5
Pile (A-47), under the maximum applied load of 885kN, pile movements recorded for this pile is 3.03mm
as shown in the Table 5 above. The test load was then gradually removed over this second and final cycle
until the entire test load was fully removed.
10
Figure 3: Load – Settlement Curve for Pile No. A-47
11
Figure 4: Load – Time Graph for Pile No. A – 47
Figure 5: Deflection – Time graph for Pile No. A – 47
12
Table 6: Comparison between Designed and Load Test Settlements Results
Design Results
Load Test Results
Load (tons)
Settlement (mm)
Load (tons)
Settlement (mm)
59.00
1.16
59.00
1.01
73.75
1.74
73.75
1.65
88.50
3.26
88.50
3.03
Note that, 10kN = 1ton.
Conclusion
This paper has attempted to simplify a direct method of obtaining the residual settlement of a
single pile. Comparison of the designed and as built settlement values were also done in order to
avoid mispredictions, see Table 6 above for details. The comparison between the designed settlement
and load test obtained settlement for Pile No. A-47 under a designed safe working load of 59.0tons shows
little or no deviation. Italian software called “KK” was used in computing safe working load for the test
pile and corresponding settlements anticipated.
The designed settlement at safe working load of 59.0tons for the pile was 1.16mm while the load test
obtained settlement was 1.01mm, hence a deviation of 0.15mm. The time required by the pile under the
load test at safe working load of 59.0tons to reach a settlement of 1.01mm is 1.66hrs which is considered
insignificant considering a long term settlement results. See Table 6 above for a comprehensive analysis
of the designed settlement and load test obtained settlement. Figure 3 – 5 also shows a three dimensional
representation of a single pile behaviour. The three dimensions being Time, Load and Settlement. The
maximum pile movement for Pile No.A-47 under these loading is far less than 10% of the pile diameters
i.e. 60mm, the boundary that would have theoretically taken the piles to their plastic limits. The tested
pile (A-47) rebounded after the test loads had been fully removed with residual movements/settlement of
about 1.41mm recorded after the load was fully removed. This is an indication of a good quality piles
with good elastic properties.
13
References
British Standard Code of Practice for Foundations BS 8004: (1996).
Das Braja M. (1999). “Principles of Foundation Engineering”. 4th Edition, Brooks/Cole
Publishing Co., Pacific Groove, Calif.
Davisson, M.T. (1972). “High Capacity Piles, Proceedings, Soil Mechanics Lecture Series on
Innovations in Foundation Construction” American Society of Engineers, ASCE, Illinois
Section, Chicago. Pp 81-112.
Federation of Piling Specialists, F.P.S. (2006). “Handbook on Pile Load testing”
www.fps.org.uk/fps/piletesting/loadtestinghandbook.pdf (Accessed on 20th July, 2012.)
Fleming, W.G.K. (1992). “A New Method for Single Pile Settlement Prediction and Analysis”
Geotechnique, Vol. 42, No. 3. Pp 411 – 425.
Frank, R. (2012). “Some Aspects of Research and Practice for Foundations Design in France”.
Université Paris-Est, Ecole des Ponts ParisTech, UR Navier, Geotechnical teamCERMES.Pp 25 – 42. http://www.sloged.si/LinkClick (Accessed on 2nd June, 2012.)
Geotechnical Engineering Bureau. (2007). “Geotechnical Control Procedure” New York State
Department of Transportation.
Gilbert, G. (1991). “Design of Pile Foundations”. US Army Corps of Engineers. Engineers
Manual No. 1110-2-2906.
http://osp.mans.edu.eg/deepfoundation/ch9.htm. (Accessed on 29th June, 2012.)
Meyerhof, G.G. (1976). “Bearing Capacity and Settlement of Pile Foundations”. J. Geotech.
Engrg.Div., ASCE, 102(3), 195-228.
14
Nabil, F.I. (2001). “Axial Load Tests on Bored Piles and Pile Groups in Cemented Sands”
Journal of Geotechnical & Geoenvironmental Engineering. Vol.127, No. 9, Pp 766 – 773.
New Trends and Developments in Geotechnical Engineering. International Society of Soil
Mechanics and Geotechnical Engineering (ISSMGE) International Conference.
Vesic, A.S. (1977). “Design of Pile Foundations”. Nat. Cooperative Hwy.Res. Program
Synthesis of Pract.No.42, Transportation Research Board, Washington D.C.
15
APPENDIX
LOAD TEST RECORDING SHEETS
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