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42 CHapter FOUR FIELD METHODS FOR MATERIAL STIFFNESS MEASUREMENT Field Tests profiles under different impulse loads will be measured and analyzed with different theoretical models of distinct consti- Several in situ methods have been used to predict or interpret tutive behaviors to determine the modulus of various layers the resilient moduli or stiffness of unbound bases and sub- in the pavement system. The analysis uses backcalculation grades (pavement layers). These methods can be grouped routines that assume a different modulus for each layer of into two categories: nondestructive methods and intrusive the pavement and then use a specific algorithm to predict the methods. The following sections present a brief review on deflections of the pavement surface. If the predicted deflec- these methods. tion pattern and magnitudes match with the measured deflec- tions, then the assumed moduli are reported as the moduli of Nondestructive Methods the pavement layers. Nondestructive methods for determining the stiffness (E) GeoGauge are based on several principles, including geophysical prin- ciples. Some of the methods involve the measurement of GeoGauge is a portable instrument that can provide stiffness deflections of pavement sections subjected to impulse loads properties of subgrade and base layers. Stiffness properties and then employ backcalculation routines to estimate the are measured by inducing small displacements to the soil on stiffness properties of pavement layers such that the pre- a loaded region using a harmonic oscillator operating over a dicted deflections match with the measured deflections. The frequency of 100 to 196 Hz. Sensors of the GeoGauge will following sections briefly review some of the nondestructive measure both force and displacement, which in turn will be methods and then provide a synthesis of the available prac- used to measure soil stiffness properties. Stiffness property tices adopted by the states employing these devices. is determined by measuring and averaging stiffness values at 25 frequencies. Figure 44 presents a schematic of the cross- Dynaflect section of the GeoGauge. Detailed description and operation details can be found in Lenke et al. (2001). Dynaflect is a light-weight two-wheel trailer equipped with an automated data acquisition and control system. The pave- ment surface is loaded using two counter-rotating eccen- tric steel weights, which rotate at a constant frequency of eight cycles per second (8 Hz). This movement generates dynamic loads of approximately ±500 lb (227 kg) in mag- nitude (Choubane and McNamara 2000). The total load applied to a pavement system is a combination of the static weight of the trailer and the dynamic loads generated by the rotating weights. The deflections of the pavement system are measured by five geophones suspended from the trailer and placed at 1 ft intervals. Deflection data monitored during the loading is then analyzed using both theoretical and empiri- cal formulations to determine the modulus of subgrade and base layers. Falling Weight Deflectometer FWD applies an impulse load on the pavement surface by dropping a weight mass from a specified height and then measures the corresponding deflections from a series of geophones placed over the pavement surface. Deflection FIGURE 44 Schematic of GeoGauge (Lenke et al. 2001).
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43 Seismic Pavement Analyzer Light Falling Weight or Portable Deflectometers The seismic pavement analyzer (SPA) is an instrument Among nondestructive assessment of pavement layers, designed and constructed to monitor construction and the portable deflectometer–type devices have been receiving deterioration in the pavement layers (Nazarian et al. 1995, considerable interest by several DOT agencies. Similar to 2003, 2005). The SPA determines the Young’s modulus of the full-scale FWD-type tools, these devices utilize both elasticity and shear modulus of pavement layers. The por- dynamic force and velocity measurements by means of dif- table SPA (PSPA) is typically used for pavement material ferent modes such as transducers and accelerometers. These properties and the dirt SPA (DSPA) is used on constructed measurements are then converted to elastic stiffness of the subgrades and bases to determine the layer properties. The base or subgrade system, which is equivalent to homoge- SPA lowers transducers and sources to the pavement and dig- neous Young’s modulus of the granular base and subgrade itally records surface deformations induced by a large ham- layers, using equations that assume underlying layers as mer that generates low-frequency vibrations and by a small homogeneous elastic half-space. hammer that generates high-frequency vibrations (Nazarian et al. 1995, 2003, 2005). The test at a site is relatively quick Factors that influence the stiffness estimation of field one, taking 1 minute. A schematic of the test setup can be devices also influence these methods, and hence some varia- seen in Figure 45. tions in moduli values are expected with the same group of devices that operate on different principles. A few of these A spectral analysis of surface waves is performed in the Light Falling Weight or Portable Deflectometers, which are field to determine the shear wave velocities and the related abbreviated as LFWD, LFD, PFWD, or LWD in the litera- moduli of layers. The moduli determined from SPA tests are ture, are described in the following sections. For simplicity’s at small-strain levels and they are different from the resilient sake, these devices are abbreviated as LWDs in the remain- modulus values that are representative of medium- to high- der of the synthesis report. strain levels (Nazarian et al. 1995). In general, the moduli or stiffness measured from nondestructive tests are repre- PRIMA 100 Equipment. The PRIMA 100 equipment is a sentative of stiffness properties at small- to medium-strain portable LWD, which can be used to measure in situ material levels, whereas the MR is representative of stiffness at small- modulus. Figure 46 shows the equipment. to high-strain levels depending on the strength of the soil specimen tested (Nazarian et al. 1995). These variations in The device consists of a handheld computer, mass, guide strains contribute to the differences in the field moduli and rod, load cell, velocity transducer, and a 200-mm diameter laboratory-measured moduli. plate. A mass freely falls from a known height along the FIGURE 45 Schematic of SPA (Nazarian et al. 1995).
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44 place the LWD on the testing location, then turn it slightly to smooth out the level surface; (3) set the trigger mecha- nism to the desired falling height of 25, 50, or 75 cm; (4) lift the weight until it connects with the trigger mechanism; (5) press the go button on the handheld computer; (6) activate the trigger mechanism while holding the top of the guide rod to keep the instrument steady; and (7) record the load and displacement readings and repeat the same to perform the test and record readings for five times. Analysis of the col- lected data will provide the repeatable stiffness results. Loadman PFWD. The Loadman PFWD was originally developed in Finland and used to test granular base courses (see Figure 47). The device utilizes a single 10 kg weight that is dropped from a fixed height of 800 mm (2.6 ft) (Steinert et al. 2005). The Loadman has loading plate sizes of 132, 200, and 300 mm (5.2, 7.9, and 11.8 in.). The device is capable of measuring deflections ranging from 0 to 5 mm (0 to 0.2 in.), with a time of loading between 25 and 30 ms and a maxi- mum dynamic load of roughly 23 kN (5,171 lbf). The Loadman PFWD (Loadman 2) uses two types of sen- sors: a load cell and an accelerometer (Steinert et al. 2005). The revised Boussinesq’s stress expression is used to deter- mine the modulus from the Loadman results. For each mea- surement, the Loadman displays the maximum deflection and the calculated bearing capacity modulus, among others. A few other LWD devices are also used in the United States, and these details are documented in the following synthesis sections. FIGURE 46 Prima 100 (LWD) (Petersen and Peterson 2006). The methods described previously frequently and recently used nondestructive techniques for interpreting stiffness properties of subgrades and unbound bases. The following guide rod shown in Figure 46, and it impacts a load cell at the sections summarize various field practices attempted by sev- lower end of the rod. A velocity transducer, which protrudes eral research studies supported by the state DOTs, test-related through the center of the plate, measures velocity. Velocity field experiences, moduli interpretations and comparisons is integrated with respect to time to determine displace- between interpreted moduli and MR results, and assessments ment, and a time history of the impact load and displace- of software used for backcalculations of moduli. ment are then displayed (Petersen and Peterson 2006; Petersen et al. 2007). This LWD weighs about 40 lb with approximately half of its weight being in the falling mass (22 lb). The PRIMA 100 can be adjusted such that the height of fall can be varied resulting in the possibility of measuring modulus at different stress states (Petersen and Peterson 2006). The following steps described by Petersen and Peterson (2006) explain the test procedure: (1) locate a smooth and level spot for the test; (2) assemble LWD and FIGURE 47 Loadman PFWD (left) and display portion of unit (right) (Steinert et al. 2005).