At the cutting edge of pile driving and pile testing

Middendorp, P. Allnamics Pile Testing Experts B.V., The Netherlands

Verbeek, G. E. H. Allnamics-USA, USA

Keywords: piling, vibratory driving, driveability studies, rapid load testing, Statnamic, StatRapid, PDP, VDP, PDA, VDA, DLT, RLT, SLT, STR, PDR, UPM, SUPM, pile testing equipment, wind mills, monopiles, EDC, SmartPile.

ABSTRACT: The worldwide challenges to achieve a greener planet, better communications, cost effective and fast means of transportation, and the protection against natural hazards (such as earthquakes, floods, tsunamis and hurricanes) continually force new developments in the area of pile driving and pile testing. In recent years the authors were involved in several projects responding to these challenges and some of them are briefly presented in this paper. The paper describes the vibratory driving of 22 m diameter steel tubular piles in China, the vibratory driving of windmill monopiles in Germany, Statnamic testing on a 2.3 m diameter instrumented bored pile with a length of 125 m in Malaysia, the application of an advanced rapid load testing device in Europe and recently developed pile testing equipment. 1 INTRODUCTION

The authors are privileged to be involved in many challenging projects the recent years related to pile driving and pile testing. From the numerous projects with interesting and even cutting edge aspects, a few have been described in this paper to illustrate recent progress in the areas of pile driving and pile testing.

2 HONG KONG-ZHUHAI-MACAU BRIDGE, CHINA

The construction of the Hong Kong-Zhuhai-Macau Bridge consists of a series of bridges and tunnels that will connect Hong Kong, Macau and Zhuhai, three major cities situated in the Pearl River Delta in southern China. Given the scope of this project, it is likely to become one of the landmarks in the area. The longest bridge section will be 22.8 km long and includes three cable-stayed spans between 280 m and 460 m. Construction formally began on December 15, 2009, and the bridge is due for completion in 2016. The 6.75 km tunnel section will allow large container vessels to pass to and from the South China Sea into and out of the Delta. The tunnel and bridges will transition on artificial islands, which are formed by two circular

earth-filled cofferdams; each consisting of 60 cells (which are 22 m diameter steel pipe piles), interconnected by 11 m long wing walls constructed from sheet piles. The wall thickness of the steel piles is 12 to 14 mm and on the inside of the piles additional steel reinforcing ribs are provided for shape stability. This technology is based on the “Japanese Construction Method of Steel Driven Caisson”.

In November 2010 the companies APE, APE-Holland, APE-China and Allnamics teamed up to convince the Chinese contractor First Harbor Marine Group China that a massive multi-vibro hammer could be used to drive 49 m long, 22 m diameter steel pipe piles weighing 600 tonnes each 25 m into the bed of the South China Sea, where the soil consists of silty clay, clay and sand with SPT N-values ranging from 8 to 40. Allnamics performed the vibratory driveability studies, which showed the feasibility of driving these gigantic piles into bottom of the sea to the required depth. The driveability study was done using the Allnamics-PDP program, which uses the Method of Characteristics to analyze the system as shown in Figure 1 (Middendorp 2004). For the soil fatigue modeling the Beta-Method proposed by Jonker (1987) and Jonker and Middendorp (1988) was applied

Figure 1:Vibro driveability model set up.

Figure 2. Vibratory driving of a 22 m diameter pipe pile.

On May 15, 2011 the world’s largest vibratory hammer drove the world’s largest pile in 7½ minutes (Figure 2), just as the driveability studies performed by Allnamics had predicted. At the highest production rate three piles were driven in a single day and seven piles in three days, and within 7 months, on December 8, 2011, the APE “Octakong” hammer drove the last of the 120 piles to the target penetration.

It is the authors’ opinion that in appropriate soils driving piles using vibratory hammers can not only reduce the installation time and the costs, but moreover minimizes the environmental impact during installation, especially for large structures, like artificial islands, flood barriers, windmill foundations, offshore platforms, jetties and harbor quays.

3 RIFFGAT WIND FARM PROJECT, GERMANY

The Riffgat Wind Farm project is located on the North Sea near the German island of Borkum and comprises the installation of 30 steel monopiles on which 30 wind mills will be placed. The monopiles weigh between the 480 and 720 tonnes, they are between 53 and 70 m long, with a diameter of 4.7 m on the pile top of the pile and between 5.7 and 6.5 m at the pile tip.

As a spin off from the successful vibratory driving at the Hong Kong-Zhuhai-Macau Bridge project the contractor Seaway Heavy Lifting choose the option of using a vibratory hammer for the installation of the monopiles. By choosing this innovative way of vibrating, they could adhere to the strict environmental rules which apply in Germany and keep the environmental impact due to noise and vibrations within acceptable limits. Using the traditional piling techniques with the conventional hydraulic hammers would result in noise levels that would cause major damage to marine life. Another advantage of vibratory driving is that the pile can be repositioned easily when the initial installation angle is too large, as was experienced during project execution.

The contract for the vibro-hammers was awarded to APE-Holland, who developed a modular hammer, the Super Quad Kong (SQK, Figure 3), specially developed for this offshore project. APE Holland cooperated on this project with Allnamics, who carried out the driveability studies for the monopiles.

Figure 3. Vibratory driving of monopile with SQK.

After driving the first monopiles it was clear that the capacity of this modular hammer was more than adequate: for the first piles in sandy soils as only 50% of the total power of these hammers was required, and it was possible to drive these monopiles to a penetration of 31 meters in one run. However, the Riffgat project also has locations with strong clay layers, and the excess vibratory power will be needed to drive the pile to a stable position with those soil conditions.

Since the owner of the field required a blow count as indication of the bearing capacity of the pile and since there was concern of soil strength degradation by vibratory driving, the last 10m of the piles still had to be driven with an impact hammer (using an IHC S-1800 hydraulic hammer). As part of the driveability studies Allnamics indicated that this was a rather conservative approach, and this opinion was supported during

actual driving by the fact that the pile at the start of driving with the impact hammer showed a blow count of 100 blows/25cm with an impact energy of 1200 kJ, which clearly demonstrated a strong recovery of the soil strength after vibratory driving. Moreover, before the hydrohammer could be applied, a Noise Mitigation System (NMS) had to be put around the pile, which clearly showed the difference in environmental impact between the two types of hammers.

Because the driveability predictions showed potential refusal before the target depth was reached it was decided to perform a small scale vibratory driving test at the clay locations, and to instrument the pile with strain and acceleration transducers near the pile to perform Vibratory Driving Analysis (VDA). The VDA results will then be used to tune the drivability studies for the monopiles. As these tests were completed shortly before this paper was written the results will be published elsewhere.

4 2ND PENANG BRIDGE, MALAYSIA

A third example of a cutting edge application of pile driving and pile testing is the construction the Second Penang Bridge which will act as the second crossing link from Penang Island to the western coast of the Malaysia peninsula. The length of the marine bridges is about 16.5 km and shall be supported by piled foundations, including steel pipe piles, cast-in-situ bored piles and spun piles. The Second Penang Bridge is being constructed by CHEC, (China Harbour) in cooperation with UEM Construction. The Malaysian Government signed a contract with UEM Group for the construction and management of Penang Second Crossing Bridge in November 2006. Jambatan Kedua supervises the construction, management and operations of the bridge.

To confirm the design assumptions a pile testing

program including static and rapid load testing was

performed. The rapid load testing was performed

by the company Geonamics using a range of

Statnamic devices, and the analysis and reporting

of the test data was performed by Allnamics during

the period 2010 and 2011. The test results were

interpreted in accordance with the Dutch Guideline

(2010) for the interpretation of rapid load testing

which offers two interpretation methods depending

on the soil type. 1. A method for piles in sand, gravel, silt

and piles on rock. This method is a simplified version of the unloading point method (UPM), originally developed in the Netherlands.

Figure 4. Offshore Statnamic test set up. Figure 5. Schematic Statnamic set up.

2. A method for piles in clay. This method

is a simplified version of the Sheffield method (SHM) originally developed in the U.K.

According to this guideline, the load on the pile can be considered a “rapid” load if the duration of the load fulfills the following

10 < Tf/(L/cp) ≤ 1000 (1)

where Tf is the load duration, L the pile length and cp the stress wave velocity of the pile material. If this condition is not met the pile load tests cannot be analyzed using the top signals only. In such cases additional analysis is required using the Segmental Unloading Point Method (SUPM) (Mullins et al. 2002) with strain measuring transducers placed at different pile levels inside the pile. For this project SUPM was applied whenever additional instrumentation (using Global Strain Extensometers) was provided.

The Global Strain Extensometer (Glostrext) (Hanifah et al. 2006) technology consist of a deformation monitoring system that uses advanced pneumatically anchored extensometers coupled with high-precision spring-loaded transducers, and a novel analytical technique to monitor loads and displacements down the shaft and the toe of foundation piles. The technology enables installation of instrumentation after pile-driving and thus virtually eliminates the risk of instrument damage during pile production and installation.

The piles tested were spun piles, steel tubular

piles and bored piles. The largest pile tested was a

bored pile with a diameter of 2.3 m and a length of

125 m (Figure 4Error! Reference source not found.).

test load of 65 MN was applied to the pile top and

Glostrext transducers were installed over the full

pile length. A schematic view of the offshore STN

set up is presented in Figure 5. The use of the Glostrext technology allowed for

the monitoring of the strain at several positions along the pile shaft. It should be noted, however, that to convert the measured strains to segment forces the modulus of the elasticity of the concrete and pile cross section at transducer locations have to be assumed, which introduces an interpreter dependent component in the SUPM analysis.

The results of the pile testing program proved that the design assumptions were correct and that the piles were able to transfer their top loads to the bearing layers and satisfied the requirements for settlement and ultimate capacity.

5 RAPID LOAD TESTING INSTEAD OF HIGH STRAIN DYNAMIC TESTING

There is a growing awareness that pile capacities

determined by signal matching techniques such as

CAPWAP and DLTWAVE heavily depend on the

assumptions made by the person analyzing the test

results and yield a wide range of results, especially

for cast in situ piles (Holeyman et. al. 2001, Viana

da Fonseca 2008). Consequently it is virtually

impossible to calibrate these signal matching based

methods against static load tests, since calibration

requires consistent results for each method. This is

one of the reasons that in 2010 the Dutch CUR

commission adopted the rapid load testing

technique (over the high strain dynamic testing) as

the results are consistent and virtually independent

of the person analyzing the test data. In response to market demand the company

Cape-Holland has manufactured rapid load testing devices (the so-called Statrapid, Figure 6) based on dropping a mass on a soft cushioning system. The Statrapid (STR) devices are a further development of methods described by Gonin et. al. (1984), Schellingerhout and Revoort (1996), and Miyasaka et. al. (2008). The Statrapid system consists of a lifting and guidance frame, a modular drop mass (allowing for a mass up to 40 tonnes), a catch mechanism and a modular soft spring system (Figure 7). The duration of the load and the maximum load can be adjusted by varying the drop mass weight, the spring stiffness and the lifting height. The system is hydraulically operated and has sensors for a proper vertical and stable position. It should be noted that the dead weight of the drop mass can also be used for the first (static) loading cycle.

The catch mechanism catches the drop mass

after bouncing up from the springs. This allows for

successive cyclic testing (Figure 8) with increasing

loads similar to static load cycling testing (since

just dropping the mass on the springs would result

in successive load cycles with decreasing loads). In

accordance with the CUR and ASTM standards the

load, displacement and acceleration are measured

near the pile top and then recorded. For easier

calibration 3 load cells are applied instead of one

big one. As mentioned above the required load duration

for rapid load testing depends on the pile length. The modular set up allows for the tuning of the StatRapid system for piles with different lengths before going to site. With the Allnamics-RLT simulation program the set-up can be checked to satisfy the RLT condition.

In addition a pile testing monitoring system developed by Allnamics allows the monitoring of the required signals. This same system, the PDR (Figure 9) can also be used for pile driving analysis (PDA), high strain dynamic testing or dynamic load testing (DLT) in addition to rapid load testing (RLT). A separate system called Reyca has been developed to measure the pile top displacement during a rapid load test. The PDR can be

connected to a PC notebook/computer by Ethernet cable or Wi-Fi. A typical StatRapid pile test result and its analysis results according to the Dutch guideline is presented in Figure 10.

Figure6. Statrapid device for rapid load testing of piles up to

8MN.

Figure 7. Statrapid modular spring cushioning system.

Figure 8. StatRapid succesive cyclic loading test results.

Figure 9. PDR data acquisition system with optional wireless

data transmission, suited for PDA, DLT, RLT and VDA

measurements.

Figure 10. Statrapid test result according to the Dutch

Guideline.

6 EDC SYSTEM AND VALIDATED SIGNAL MATCH

In the last decade an alternative to pile driving analysis was developed in Florida, USA by

embedding data collectors (strain gauges and accelerometers) into the concrete at both ends of the pre-tensioned pre-stressed pile (a so-called EDC system). This approach, which has been studied extensively by the University of Florida and the Florida Department of Transportation (FDOT) and applied in numerous FDOT projects in recent years, allows for direct monitoring of the pile toe condition, instead of assessing the situation at the buried pile toe by interpreting the recorded stress waves at the top of the pile.

Recently SmartStructures Inc. in the USA teamed up with Allnamics to expand their software program suite (SmartPile Acquisition and Smartpile Review ) with 2 additional modules: SmartPile Simulate (a tailored version of Allnamics-PDP) and SmartPile Signal Match (a tailored version of Allnamics-DLT). With this expanded software suite the user can simulate the pile driving before it happens, monitor the pile driving as it happens (by reviewing not only the acceleration and strain measurements at the pile top, but also at the pile tip), compare the predicted results (such as transferred energy and blow count) with the actual results, and perform a signal match on the strain and acceleration data measured at the pile top. And whereas signal matching programs cannot generate a unique solution (which means that the outcome depends on the person performing the match), the outcome of SmartPile Signal Match can be validated by comparing not only the calculated and measured signal at the pile top, but also those at the pile toe. This provides the user with a validated signal match, ensuring an enhanced reliability of the results.

At the time this paper is written this enhanced program suite is being finalized and it expected to be used commercially in the second half of 2012.

Figure 11. Screenshot SmartPile Review.

Figure 412. Smartpile comparison of predicted and actual

results.

7 CONCLUSIONS

New pile driving techniques and pile testing methods have become available that can meet the challenges presented by projects to address the needs to achieve a greener planet, better communications, cost effective and fast means of transportation, and the protection against natural hazards requirements

In suitable soil conditions the pile installation by vibratory hammers seems to go faster and with minimal environmental impact compared to standard impact pile driving. Still impact pile driving will be needed, if only in areas with strong cohesive soil, and tools are available to reduce the environmental impact when using impact hammers.

Rapid load testing devices, analysis methods and monitoring equipment are available to produce consistent and reliable pile testing results. Rapid load testing devices are available and under development for pile test up to 100 MN.

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