Primarily report on non-static pile load tests Literature review Quasi-static and Dynamic pile load tests --------N.Q.HUY-------- Primarily report on non-static pile load tests Table of contents: I. Introduction……………………………………………………………….3 II. Quasi – static pile load test methods……………………………………...4 2.1. Quasi – static pile load test procedures……………………………………4 2.2. Data collections…………………………………………………………...5 2.3. The interpretation methods and case histories…………………………….6 2.4. Correlation between the STATNAMIC and static results………………...16 III. Dynamic pile load test method…………………………………………....18 3.1. Dynamic pile load test procedures………………………………………...18 3.2. Data collections…………………………………………………………....19 3.3. The interpretation methods and case histories…………………………….20 IV. Some problems with non-static pile load test results……………………..27 4.1. Excess pore pressure………………………………………………………27 4.2. Loading rate effect………………………………………………………....31 V. Conclusion and recommendation………………………………………….33 References……………………………………………………………………...34 Appendix A ……………………………………………………………………..38 2 Primarily report on non-static pile load tests I. Introduction Pile testing, which plays an importance role in the field of deep foundation design, is performed by static and non-static methods to provide information about the following issues: (Poulos, 1998) - The ultimate capacity of a single pile. - The load-displacement behavior of a pile. - The performance of a pile during the test conditions. - The integrity of a pile (pile integrity test). For the purposes of verification the design axial capacity and the static load – settlement behavior of piles, the static pile load test has long been considered as the most reliable method but because of its high cost and time consuming, non – static pile load tests are looked as efficient substitutions. The two non – static testing methods, i.e. dynamic and quasi – static pile load test are objects of this report. The non – static pile load tests are performed by means of exerting an impact force on the pile head while measuring and recording the responses of the pile, from which the test results are determined. Duration of the impact force (T), longitudinal wave velocity of tested pile (c) and pile length (L) are T.c used as key factors to classify the testing methods. For instance, the relative wave length Λ = 2L T .c (Holeyman, 1992) or the wave number N = (Middendorp et al., 1995) or the relative duration w L T tr = (Karkee et al., 1997) have been used. The Research Committee on Rapid Load Test 2L c Methods in Japan (1998) proposed the classification of pile load test methods as shown in figure 1. In which, the practical boundary between static and non-static testing method is Λ= t r = 500 or N w = 1000; while the boundary between dynamic and quasi – static pile load test is Λ= t r = 5 or N w = 10. Figure 1 also shows the dynamic effects to be taken into account in interpretation of load testing results. Figure 1: Classification of pile load test methods. By review the published papers, firstly the quasi – static and dynamic pile load tests are presented with following aspects: · The testing procedures. · The interpretation methods and case histories. Primarily report on non-static pile load tests Then, some problems that affect the interpretation results are discussed such as the stress wave phenomena, the rate effect, and the excess of pore water pressure. Finally, some suggestions or further studies for improving the reliability of these non – static pile load tests are concluded. II. Quasi-static testing methods Quasi-static pile load test is a testing procedure with a relatively long duration of impact force, ranges from 100 to 200 milliseconds. In the loading method, if the impact force is developed by dropping a heavy mass, the testing method is correlative named Dynatest (Gonin et al, 1984) or Pseudo-static pile load test (Schellingerhout et al, 1996); if that by launching a reaction mass, the testing method is named as STATNAMIC pile load test (Birmingham & Janes, 1989). Although these tests are different in the way to create the impact force they are the same in the applied force vs. time manner. More details in each testing method will be discussed below. Generally the weight of falling mass or reaction mass is about 5%-10% of the intended maximum dynamic load on the test pile (Middendorp et al, 1992). 2.1. Quasi-static pile load test procedures 2.1.1: Dynatest and Pseudo-static pile load test The Dynatest (Gonin et al, 1984) or Pseudo-static pile load test (Schellingerhout et al, 1996) is carried out by drop a heavy ram with a coiled spring to the head of the test pile. This creates a slow-rising and long-lasting impact force to pile head as theoretical calculated in figure 2. The coiled spring is attached to the pile head (in Dynatest) or to the bottom of the falling mass (in Pseudo-static test). The reduction of the coiled spring stiffness and increasing of drop mass is a feasible way to lengthen the duration of the impact force (Holeyman, 1992). Figure 2: The force as a function of time for different drop heights. (Schellingerhout et al, 1996) The loading equipment is mounted on a small tracked vehicle, with a ram weight of 15-25 tons. The required measurements for the test are pile head force and displacement. The measurement devices for the test consist of a load cell and an optical displacement measuring device. The load cell that is placed on the pile top and optical displacement measuring device is placed at a distance of about 10m or more from the test pile to eliminate the influence of ground vibrations. When the pile head has been prepared, the rig is positioned, the measured devices are installed the test can start. First, the ram weight is lifted to a predetermined height vary from 0,1-1,4m by two jacks. Then a number of blows are executed to the pile by freely drop the ram from increasing heights (Schellingerhout et al, 1998). After impact to the pile head, the ram bounces and is picked up at its highest position by automatic catching system. 2 Primarily report on non-static pile load tests In good condition, 10 piles can be tested a day. 2.1.2: STATNAMIC pile load test In 1989, Birmingham Corporation Limited (Canada) and TNO Building & Construction Research (The Netherlands) had jointly developed a so called STATNAMIC test, a new pile load testing method, with a specific testing device. This STATNAMIC test device consists of a pressure chamber, reaction mass and a catching system (Janes et al, 1989). The pressure chamber comprises of a piston and a cylinder is used to produce high pressure gases by burning of solid fuel for launching the reaction mass. As consequence, a reaction force pushes the pile downward. The reaction mass is a series of concrete or steel rings, whose weight is about 5-10% of total desired load to be applied to the test pile (Middendorp et al., 1992), so it is easily transported and installed. The catching system is gravel catching system or hydraulic catching mechanism, which is used to catch the reaction mass before it falls down on the pile head again. Procedure to perform the STATNAMIC test is little different between two kinds of catching system but generally following these steps: prepare the pile head, place the testing device on the pile top, secure the reaction mass to the cylinder and start the test. The test is started by burning a volume of solid fuel in the pressure chamber, creating an increasing high pressure gases that push the reaction mass upward with the acceleration of about 20g while the equally opposite force pushes the pile downward. The downward velocity and acceleration of the pile are normally 0.5 - 1.0m/s and 1.0g – 3.0g, respectively. A typical load-time diagram is show in figure 3. The peak and duration of the applied force can be controlled by adjusting the weight of reaction mass, the volume of solid fuel or the characteristics of pressure chamber. During the test, the applied force is directly measured by a pressure transducer housed in the piston base, the pile head displacement is measured by use of laser sensor. Nowadays, the gravel catching system device gets the testing capacity up to 40 MN and can perform one test a day whereas that of hydraulic catching mechanism is 8 MN and 3 or 4 tests a day respectively (Birmingham, 2000). Figure 3: Typical STATNAMIC load-time diagram. 2.2. Data collections 2.2.1. Measurements During the quasi-static pile load test event the indispensable collected data are applied load and displacement at pile head as a function of time. In addition, some more items such as acceleration at pile head and pile toe, velocity at pile head, axial strain of the pile are collected upon the testing aims, the interpretation methods, and the availability of measurement devices. The measurement data can be divided into two groups (Holeyman –1992): dynamic measurements, which are force, pressure, stress, or strain; and kinematic measurements, which are displacement, velocity, and acceleration. The applied force at the pile head is measured by a load cell, which is mounted directly between the loading device and the pile head. This device should be checked before the test for the accuracy (accurate to 0,1% - Janes et al, 1991) and rated capacity. Normally, the applied force at the pile head is derived from the compression of the coiled spring for Dynatest or measured by the pressure gauge fixed in the pressure chamber for STATNAMIC test. 3 Primarily report on non-static pile load tests The displacement at the pile head is measured by a special displacement transducer, which is capable of measuring displacements directly and continuously.