Exposures to WBV are evident in the industrial world, in particular for earth-moving equipment including off-road vehicles. WBV comprises mechanical vibration or shock transmitted to the body as a whole (Griffin, 1990). When WBV is transmitted to the human body at the natural frequency of the body as a whole or of its individual parts a condition known as resonance will result. During resonance, the body or a part of the body will vibrate at a magnitude higher than the applied excitation force. In response, muscles will contract in a voluntary or involuntary way and cause fatigue or a lowering of motor performance capacity (Chaffin and Andersson, 1984).

In light of postural elements, WBV is a contributing factor in the development of musculoskeletal disorders of the spine among workers exposed to a vibration environment (Kittusamy and Buchholz, 2004; Kittusamy, 2003, 2002; Bovenzi and Zadini, 1992; Johanning, 1991; Bongers et al., 1988, 1990; Seidel and Heide, 1986). Low-back pain (LBP) is a prominent and unfavourable health effect of WBV. A review by the National Institute for Occupational Safety and Health (NIOSH) reported a significant positive association between WBV exposure and LBP in 15 of 19 WBV studies reviewed by assigning its highest ranking descriptor of ‘strong evidence’ to the WBV-LBP relationship (NIOSH, 1997). A variety of field investigations have reported on WBV exposure for mining and quarry machinery (Smets et al., 2010; Mayton et al., 2008, 2009a; Eger et al., 2006; Kumar, 2004; Miller et al., 2000, 2004). Smets et al. (2010) reported on a review of Canadian accident statistics for the Ontario Mining Industry, which showed that 16% of the traumatic injuries were associated with HT operation. Moreover, Kumar in his study of WBV on HTs concluded that HT operator exposure to WBV posed a significant health risk and noted that the exposure limit recommended in ISO 2631 was exceeded for a majority of the exposure time (Kumar, 2004; ISO, 1997).

The introduction to the ISO 2631-1 standard, among other things, states that the standard

“... does not cover the potential effects of intense vibration on human performance and task capability since such guidance depends critically on ergonomic details related to the operator, the situation and the task design. Vibration is often complex, contains many frequencies, occurs in several directions and changes over time. The effects of vibration may be manifold. Exposure to WBV causes a complex distribution of oscillatory motions and forces within the body. There can be large variations between subjects with respect to biological effects. WBV may cause sensations (e.g., discomfort or annoyance), influence human performance capability or present a health and safety risk (e.g., pathological damage or physiological change). The presence of oscillatory force with little motion may cause similar effects.” (ISO, 1997)

This paper discusses a continuation of the work conducted at two US eastern mid-Atlantic crushed stone operations with preliminary results reported previously (Mayton et al., 2008, 2009b). As a follow-up to this work, a more in-depth and systematic study was performed to further assess WBV exposure levels for drivers and operators of HTs and FELs relative to various factors that may influence them. The factors considered included vehicle, age, capacity, travelling with load and no-load, seat transmissibility, and vehicle speed. As part of a former NIOSH study that focused on implementing and evaluating ergonomic interventions in mining, a worker risk factor assessment was performed. Company management had received verbal and written responses from vehicle operators about back symptoms and vibration issues while performing their regular work cycles. Low-back discomfort was a frequently reported symptom at one quarry and vehicle vibration associated with bouncing and jarring was reported on a high percentage of cards. In some cases, employees indicated discomfort relative to the seating in vehicles. Quarry managers were thus interested in evaluating seating and operator exposure to WBV with the objective of learning how exposure levels compared to existing ISO/ANSI recommended standards.

NIOSH researchers conducted field studies and collected WBV exposure and global positioning system (GPS) data for a total of 10 vehicles and machines – three HTs and drivers and 2 FELs and operators operating at each of the two quarries (Table 1). The HTs were rear-dump, which differed by make/model, age, and capacity. The FELs were wheel-type and also differed by make/model, age, and capacity. Vibration measurements were recorded with an 8-channnel, digital data recorder (model PC208Ax, Sony Manufacturing Systems America, Lake Forest, CA). Other instrumentation (PCB Piezotronics, Inc. Depew, NY) included tri-axial accelerometers (models 356B18, 356B40), signal conditioning amplifiers (model 480E09), and in-line, 150-Hz low-pass filters (model 474M32). The floor- or frame-mounted accelerometer featured a frequency range of 0.3 Hz to 5 kHz and a charge sensitivity ranging from 949 mV/g to 1052 mV/g for the three orthogonal axes. The seat pad accelerometer featured a frequency range of 0.5–1 kHz and a charge sensitivity range of 97.4–105 mV/g for the three orthogonal axes. Vibration data were collected using accelerometers with pre-amplifiers and filters connected to a digital data recorder. The GPS unit, a Garmin Etrex C, was taped to the outside handrail of the HTs and FELs.

Installation was done at the maintenance shop and vehicle parking area. Two tri-axial accelerometers were installed, one on the frame of the haul truck or loader next to the cab window (frame or chassis measurement) and one (encased in a disk-shaped, rigid, black pad) on the seat at the operator/seat interface (seat measurement). Frame accelerometers were ordinarily mounted on the floor of the operator's compartment near the base of the seat, but space and setup constraints within the truck cab necessitated mounting the frame accelerometers on small ledges on the cab walls that were rigid and structurally connected to the floor. A 12-volt deep-cycle marine battery allowed researchers to avoid interruptions in data collection that occurred in earlier field work caused by random vehicle bouncing and premature disconnect from the terminals in the recorder and recorder shutdown. Measures for the cyclical nature of load-haul-dump activities were considered to be representative of exposures for the shift. Quarry management estimated that driver/operator exposure was 9 h for a 10-hr shift. Given cab constraints and the setup of data collecting instrumentation, researchers decided not to ride along in the vehicles to observe truck operation.

Measurement periods for HTs ranged from 22.1 to 98.9 min with a mean of 68.3 and standard deviation of 18.9 min. Similarly, measurement periods for FELs ranged from 43.8 min to 99.2 min with a mean of 72.0 min and standard deviation of 23.5 min.

Truck routes began and ended in the pit and plant storage pile areas or the shop area (as was the done in earlier field work). Instrumentation were switched on just prior to the truck departing this area and returned to the same area at the end of the measurement period for uninstalling the instrumentation. Weather conditions during both studies were dry, warm and sunny to partly cloudy. The roadways were dusty and required constant watering for dust abatement. All of the trucks and their respective seats were considered to be in good working order.

ISO 2631 and ANSI S3.18 (ISO, 1997; ANSI, 2002) were used to evaluate the WBV exposures for haulage truck drivers. For the x, y, and z directions (Figure 1), wRMS and VDV with overall totals of wRMS and VDV were used to evaluate driver/operator exposure. Considering an eight-hour exposure period, the European Union Good Practice Guide for WBV (EUGPG) recommends, for the worst-case axis, wRMS accelerations of 0.5 m/s² as the action level and 1.15 m/s² as the exposure limit. In using VDV to assess vibration, the EUGPG recommends 9.1 m/s^1.75 as the action level and 21 m/s^1.75 as the dose limit for an eight-hour exposure. The ISO/ANSI standards are slightly more conservative with recommended wRMS accelerations of 0.45 m/s² as the action level and 0.90 m/s² as the exposure limit and, for VDV, 8.2 m/s^1.75 as the action level and 16 m/s^1.75 Moreover, the EUGPG recommends measurement periods totalling a minimum of 20 min or longer, and if shorter periods are unavoidable, measurement periods should be at least 3 min long and repeated if possible, for a total time of more than 20 min.

Vibration transmitted through the seat was determined by the ratio – transmissibility (T) – of vibration level at the vehicle frame or chassis to the vibration level at the seat. A value greater than 1.0 (times 100%) would indicate a higher vibration level at the seat and that the seat is amplifying rather than attenuating the vehicle ride vibration. Griffin points out that comparing the accelerations on the seat with that at the seat base is the most direct method of obtaining accelerations. Impedance methods offer another means for measuring or predicting transmissibility. The seat effective amplitude transmissibility (SEAT) is given in two different ways by the following equations (Griffin, 1990): (1)TwRMS=SEAT%=[∫Gss(f)Wi2(f)df]∕[Gff(f)Wi2(f)df](2)TVDV=SEAT%=VDV(seat)∕VDV(floor)×100. In equation (1) G_SS(f) and G_ff(f) are the seat and floor acceleration power spectra and W_i (f) is the frequency weighting of the human response to vibration. In equation (2), VDV are the seat and floor or frame vibration dose values (VDVs). In this study, the authors used both wRMS, and VDV for the seat and frame of the truck cab to approximate and compare T values.

This study compared exposure levels on HTs and FELs with existing ISO/ANSI and EUGPG guidelines and evaluated the influence of factors such as load/no-load conditions, speed, load capacity, vehicle age, and seat transmissibility on vibration exposures. Predictably, recorded vibration at the chassis and seat increased with increasing HT speed. Increases in transmissibility were not evident with increasing vehicle load or vehicle speed. Conversely, decreases in transmissibility were evident with increases in HT size and age. The wRMS exposure levels for HTs, compared to the ISO/ANSI standards and the EUGPG, were, in all but one instance, within the HGCZ for the dominant axis of exposure and, in 43% of the cases for VDV, above the ELV for the ISO/ANSI guidelines. The y-axis (lateral or side-to-side direction) was most often the dominant axis. Roughly half of the dominant axis exposures were either in z-axis (vertical or up-and-down direction) or the y-axis. Of the 275 HT separate incidents (not normalised to an equivalent 8-hr shift), 129 were within the HGCZ and 146 of 275 were below the HGCZ. Of those within the HGCZ, 56% of the 129 incidents were the vertical or z-axis, whereas 44% were the lateral y-axis. For FELs, the wRMS levels dominant x-axis (fore-aft or front-to-back direction) and were predominantly within the ISO/ANSI and EUGPG EAVs. In contrast, VDV levels all in the dominant x-axis, were within and above the HGCZ for the EUGPG and above the HGCZ for ISO/ANSI guidelines. The higher VDV levels in the fore-aft direction are not surprising and are indicative of the quick and sudden start/stop bucket filling-/-emptying activities associated with FELs during loading operations. Future work should continue ongoing data collection efforts to monitor vibration exposure data on HTs and FELs and solicit feedback from the respective drivers/operators over a longer period of time.

Operating conditions and the mining processes shortened sampling time in some instances. There was also an obvious change in the working environment (pit and bench location changes for loading operations), and truck driving routes over the 12-month period were a limiting factor in drawing conclusions and comparing to earlier field work. Moreover, the accuracy of the GPS information was limited somewhat by fewer visible satellites at a relatively high vertical angle, which degraded to some extent the accuracy of the horizontal coordinates. The nature of the current data made examination of interactions rather problematic, such as interactions between two continuous independent variables (i.e., speed and vehicle age). While such analyses can be done, such interactions can be confounding and are extremely difficult to interpret.