Volume 4, Issue 2 June 2019

Special Contribution

Rapid Load Test of Piles
Tatsunori Matsumoto
Professor, Graduate School of Science and Technology
Kanazawa University, Kanazawa, Japan
Rapid load test of piles (RLT) is an alternative to the conventional static load test (SLT) and is widely used in Japan becauseof its time and cost effectivenss. In this article, history of the standardization of RLT in Japan, interpretation methods ofRLT signals, newly dveloped RLT device, and a case of comprison of th results of RLT and SLT on an open-ended steel pipe
pile are introduced.
Keywords: load-displacement relation, falling mass rapid load test method, interpretation method, standards
1. History of Rapid Load Test in JapanThe first rapid load test (RLT) in Japan was carried out by Takenaka Corporation in 1992 on a cast-in-place concrete pile.
The Statnamic was employed in the test. The well-known method of rapid load testing, Statnamic, was developed by thecollaboration of TNO in the Netherlands and Berminghammer Co. in Canada (Middendorp et al., 1993) [1].
A private research group for RLT was launched in 1993 led by Prof. Osamu Kusakabe (Prof. of Tokyo Institute of Technology
at that time, and the president of IPA at present). The research group was composed of general contractors, pilingcontractors, pile load test companies, pile manufactures and academics. The objectives of the research group were 1) tocompile the existing knowledge of RLT, 2) to examine basic characteristics and applicability of RLT to Japanese soils, and3) to establish interpretation methods of RLT. The outcomes of the activity of the research group were presented in the1st International Statnamic Seminar (1995) [2] held in Vancouver, Canada.
The research committee for RLT was formed in Japanese Geotechnical Society (JGS) in 1997, based on the activity of theresearch group, with research targets of 1) definition of rapid load test, 2) interpretation methods, 3) preparation oftesting manual and 4) standardization of testing method. The outcomes of the research committee were presented in
the 2nd International Statnamic Seminar (1998) [3] held in Tokyo, Japan. The research committee was upgraded to thestandardization committee of JGS in 1998. JGS 1815-2002: Method for Rapid Load Test of Single Piles [4] was standardizedin 2002. JGS 1815-2002 is the first standard for RLT in the world. After the standardization of RLT, use of RLT has been
widened in Japan.
Force

2. Definition of Rapid Load Test in JGS 1815-2002

ILT(DLT)JGS 1815-2002 clearly defines RLT. Fig. 1 shows anillustration of loading duration and characteristic curvesof a stress-wave propagating in a pile having a length, L.
Loading duration, tL, in impact load test (ILT) is 5 to 10 ms,
while tL in RLT ranges typically from 50 to 100 ms. Stresswave caused at the pile head propagates up and down inthe pile with a propagation speed, c. Relative loading
duration, RT, is defined asRLTTimePileTime(1)RT is the number of return traveling of the stress-wave inthe pile during the loading duration, tL. If RT is equal orgreater 5, the influence of the wave propagation in thepile could be negligible (Nishimura et al., 1998) [5] so thatthe pile body could be assumed as a rigid, although theinertial force of the pile body needs to be considered.
Dynamic load tests with the condition of RT ≥5 is definedas RLT.

Characteristic
curves ofa stress-wave

Length, L

RT = tL/(2L/c)

Loading duration of RLT, tL

Time
2L/c
2L/c: return travelling time of stress-wave in pile
c: longitudinal wave propagation velocity

Fig. 1. Comparison of loading duration [4]3Volume 4, Issue 2 June 2019
3. Rapid Load Test Devices
Loading mechanism of the well-known Statnamic device is launching areaction mass placed on the pile head by the gas explosion pressure.
At the same time, the gas explosion force pushes the pile gently
downward. The maximum acceleration of the reaction mass caused
during loading is about 5g. Hence, weight of the reaction mass is about
20% of the planned maximum load. An advantage of the Statnamic ishigh loading capacity up to 60 MN. However, repeated loading isdifficult in the Statnamic.
Another type of RLT device is a falling-mass type device. Fig. 2 is an
example of falling-mass type device, called Hybridnamic, developed by
Jibanshikenjo Co. Ltd. In the falling-mass type device, a hammer massis free-dropped from a certain height onto the pile head through a softcushion on it. Relatively wide ranges of the loading duration and themaximum force can be realised by adjusting combination of thestiffness of the cushion and the hammer mass. A great advantage ofthe Hybridnamic is that repeated loading is conducted very easily. It is
possible to conduct 10 blows (tests) on a pile in a day. It is a common
practice in the Hybridnamic to apply several blows on a pile withincreasing the drop height of the falling hammer.

Fig. 2. Falling-mass device with soft cushion(provided by Jibanshikenjo Co., Ltd.)

4. Interpretation method of RLT signals
Generally, force applied to the pile head, Frapid, is measured
via a load cell, and acceleration at the pile head, , ismeasured via accelerometers. The velocity of the pile, v, isobtained from time integration of the measured . An
optical displacement meter is employed to measure thepile head displacement, w.

FrapidInertia of pile ，Ra = M 

If Tr = tL/(2L/c) ≥5, the pile body could be treated as a rigid
body having a mass of M. Fig. 3 shows a modelling of pileand soil during RLT. Frapid is the sum of inertia of the pile,
Ra, and the soil resistance, Rsoil (Eq. 1). It is assumed thatsoil resistance, Rsoil, is the sum of the static soil resistance,
Rw, and the dynamic soil resistance, Rv (Eq. 2). Rd is
assumed to be proportional to the pile velocity, v, with aconstant value of damping factor, C. Hence, Rsoil is readily
obtained from the measured Frapid and  (Eq. 2).
(1)

Rsoil = Rw + Rv = Rw + Cv = Frapid − M(2)

C = ( Rsoil max − Rw ULP ) / v *(3)Rw = Rsoil − Cv(4)Dynamicsoil resistance,
Rd = C v（Linear dashpot）

Fig. 3. Modelling of pile and soil during RLT [1, 6]

Force (MN)
Rw ULP Rsoil max

0123450

Pile head disp., w (mm)Frapid = Ra + Rsoil = M + Rsoil

Static soil
resistance, Rw（Non-linearspring）

Fig. 4 is an example of thus obtained Rsoil vs w. The point at
the maximum displacement, wmax, is call Unloading Point(ULP). The velocity of the pile, v, is zero at ULP. Hence, Rsoilat ULP is regarded as the maximum of the static soil
resistance, Rw ULT. The damping factor, C, is estimated fromthe difference of Rsoil max and Rw ULT (Eq. 3). Finally, Rw vs w
is obtained using C and v (Eq. 4). Rw vs w is called "derived
static load-displacement curve".

Rsoil

1020

Rw30
wmax

40
50

Cv*
Unloading point, ULP(v = 0)Fig. 4. Unloading Point Method (ULPM) for obtaining
static load-displacement curve4Volume 4, Issue 2 June 2019
If multiple blows with increasing the drop height of hammer are carried, interpretation of RLT signals becomes easier, as
shown in the next section.
5. Comparative SLT and RLT on an Open-ended Steel Pipe Pile
Results of comparative SLT and RLT on an open-ended steel pipe pile (Hoshino et al., 2012) [7] are briefly introduced inthis section.
The pile having a length of 15.5 m, an outer diameter of 1.0 m and a wall thickness of 14 mm was installed in the sandy
ground by a water jet vibratory installation method (Fig. 5). The bottom 4.5 m section of the pile was embedded into thesand rock layer having SPT N-values greater than 50.
The conventional SLT was carried out for the pile with a loading sequence shown in Fig. 6. It took 6 hours to complete fiveloading cycles of the SLT. Each load step was maintained for one hour, and finally the maximum load of 9000 kN wasapplied.

Testpile
0 10 20 30 40 50 60
SPT N-value

10000

FillPile head load (kN)02

Depth, z (m)46Sandy silt
Sandy silt

81012

Sand rock

Silty sand8000
6000
4000
2000

1400

16
18

1234

5678

Elapsed time (hr)Sand rockFig. 6. Loading sequence in SLT20Fig. 5. Soil profile at the test site

RLT of the pile was carried out subsequently to the SLT. A
total of 8 blows were applied to the pile using a hammer
mass of 22 tonnes. Fig. 7 shows Frapid vs time in the lastblow. The loading duration, tL, was 60 ms that resulted inthe relative loading duration Tr = tL/(2L/c) = 9.7 (c = 5000m/s). As mentioned above, Tr =9.7 ≥ 5 satisfies thecriterion for RLT.

Pile head force, Frapid (kN)

10000

Fig. 8 shows Fsoil vs w in all the 8 blows. The red line is theconnection of ULPs which can be regarded as the staticload-displacement curve.

8000
6000
4000
2000
060

Static load-displacement curves from the SLT and the RLTs
are compared in Fig. 9. Note here again that the RLTs wereconducted after the 5th loading cycle in the SLT.

80

100

120

140

160

Time, t (ms)Fig. 7. An example of RLT load

Load-displacement curve in the 5th loading cycle of the SLT and Fsoil vs w from the RLTs are shown in Fig. 10, in which thepile head displacement at the start of loading was zeroed for comparison purpose. It is seen that the curve derived fromthe RLT is almost equal to that obtained from the SLT.
51200 01000 0
8000
6000
4000
2000
0010203040

Pile head displacement (mm)Static pile head force (kN)

Fig. 8. Fsoil vs w from RLTs with different falling height of hammer, and ULPs
10000

from SLTfrom RLT

8000
6000
4000
20000020406080100

Pile head displacement (mm)Fig. 9. Load-displacement curve in SLT and Fsoil vs w from RLTs

Static pile head force (kN)

Soil resistance, Fsoil (kN)

Volume 4, Issue 2 June 2019

10000
8000
6000
4000

from SLT(5th loading cycle)

2000

from RLT
00

1020

30

405060Pile head displacement (mm)Fig. 10. Comparison of static load-displacement curves from SLT and RLTs
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Volume 4, Issue 2 June 2019
Fig. 11 shows the axial forces in the pile measured in the SLT and the RLTs at the maximum pile head load of 9000 kN, andthe distributions of the shaft resistance estimated from the axial forces in the pile. The results from the SLT and the RLT
are comparable, showing that RLT is an alternative to the conventional SLT.
024Depth, z (m)from SLT
from RLT

from SLT
from RLT

6810121402000 4000 6000 8000 10000

Pile axial force (kN)

0200

400

600

800

Shaft resistance (kPa)(a) Axial forces in pile(b) Shaft resistanceFig. 11. Axial forces in pile and shaft resistance
The above case study was carried out for the sandy ground. Brown (2009) [8] pointed out that the static loaddisplacement curve derived using UPM (Unloading Point Method) is still influenced by the strain rate effects of thesurround soils, especially in clay ground. The initial parts of the static curves (working load range) from SLT and RLT arecomparable, while the ultimate load derived from ULM is overestimated by 50% in maximum. This information is usefulwhen RLT is used for design purpose of piles in clayey grounds.
6. Concluding Remark
The author's belief is that the performance of a pile installed using Press-in Method is higher than that of the same pile
installed using the other installation methods. This belief, however, needs to be verified through load testing onconstructed piles. A handy load test may be possible using the Press-in machines, although the maximum load is limited
to 3 MN. The rapid pile load testing is one of useful tools to conduct load tests on many piles, because of its time and costeffectiveness as well as a reasonable reliability.
References[1] Middendorp, P., Bermingham, P. and Kuiper B.: Statnamic testing of foundation pile, Proc. 4th Int. Conf. on Applicationof Stress-Wave Theory to Piles, The Hague, pp.585-588, 1993.
[2] Proc. 1st Int. Statnamic Seminar, Vancouver, Canada, 1995.
[3] STATNAMIC Loading Test (Kusakabe, Kuwabara & Matsumoto Ed.), Proc. 2nd Int. Statnamic Seminar, Tokyo, Japan,
1998.
[4] Standards of Japanese Geotechnical Society for Vertical Load Tests of Piles, JGS 1815-2002: Method for Rapid Load
Test of Single Piles, 2002.
[5] Nishimura, S., Shibata, A., Yamashita, K., Kita, N. Ogita, N. and Ishida, M.: One dimensional stress wave simulation
analysis of Statnamic load test, evaluation of boundary between Statnamic and dynamic loading tests, Proceedings
the 2nd Statnamic Seminar, Tokyo, pp.337-344, 1998.
7Volume 4, Issue 2 June 2019[6] Kusakabe, O. and Matsumoto, T.: Statnamic tests of Shonan test program with review of signal interpretation, Proc.
1st Int. Statnamic Seminar, Vancouver, Canada, pp.113-122, 1995.
[7] Hoshino, M., Sato, K., Takano, K., Kamei, S. and Nishimura, S.: A comparison between static load test and rapid load
test for a steel pipe pile installed by water jet vibratory technique, Proceedings of the 9th International Conference on
Testing and Design Methods for Deep Foundations (IS-Kanazawa 2012), Kanazawa, pp.581-585, 2012.
[8] Brown, M.: Chapter 2 Recommendations for Statnamic use and interpretation of piles installed in clay, Rapid LoadTesting on Piles, pp.26 - 36, Taylor & Francis Group, London, UK, 2009.

◆ A brief CV of Prof. Tatsunori Matsumoto

Professor Tatsunori Matsumoto obtained his Bachelor of Engineering and Master of Sciencefrom Kanazawa University, Japan. He joined the Department of Civil Engineering of Kanazawa
University in 1981 as research associate. He became an Associate Professor in 1991 andpromoted to a Professor from August in 1999.
He retains an active involvement in research into pile dynamics and deformation of pilefoundations including piled rafts subjected to load combinations. He has published more than
200 technical papers including more than 45 Journal papers.
He was the Chairman of IS-Kanazawa 2012: The 9th International Conference on Testing andDesign Methods for Deep Foundations held in Kanazawa, Japan, from 18 to 20 September,
2012.

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