A new take on sound-barrier walls.
By Jie Han, Robert Parsons, Matthew Pierson, and James Brennan
When residential areas are close to highways, sound barrier walls are often constructed to minimize noise from traffic on those roads. Under certain circumstances, mechanically stabilized earth (MSE) walls are used to support the sound barrier walls.
Figures 1 and 2 show an MSE wall supporting a sound barrier wall several feet away from an apartment building near the intersection of Interstate 435 and U.S. Highway 69 in Overland Park, Kan., a suburb of Kansas City. (Scroll down to view the images and the rest of the story.) Due to weather conditions in Kansas, the sound barrier wall can be subjected to substantial wind load.
The traditional design practice in Kansas has been to use reinforced concrete shafts with rock sockets isolated from the MSE wall to support the sound barrier wall as shown in Figure 3. The isolation of the shafts from the MSE wall simplifies the design of the shafts and the MSE wall.
In this design, the shafts and the MSE wall are designed independently without any interaction. Since it is assumed that there is no lateral support of the shafts from the MSE wall in this design, rock sockets are necessary to carry the lateral load from the sound barrier and are installed prior to the MSE wall construction.
Inner and outer casings are placed with a gap as the wall is constructed. The inner casing is filled with concrete to form the shaft once the wall construction is completed. This approach is convenient for simplifying the design, but construction of rock sockets is slow and costly.
Under this condition, the shafts are designed as cantilever beams, which require large-diameter shafts to resist the significant bending moment. The typical diameter of shafts used for this application ranges from 2.5–4.0ft. The requirement of rock sockets and large-diameter shafts makes this system very expensive.
An alternative design was proposed by the Kansas Department of Transportation (KDOT) and the University of Kansas as shown in Figure 4. In this design, the shafts are included in the MSE mass and seated on the bedrock instead of being keyed into the bedrock with rock sockets.
Different from the traditional approach, it is expected in the new design that the geosynthetic-reinforced soil mass provides lateral support to the shafts so that smaller diameter shafts without rock sockets may be used. As a result, this design provides a more economical foundation solution for the sound barrier wall, compared with the traditional design, with an estimated savings of more than $1,500 per shaft. Individual projects may have dozens or even hundreds of shafts.
Design and construction
To verify the proposed design, a research project was funded by KDOT through the K-TRAN Research Program to construct a full-scale MSE test wall in Kansas.
The test wall was 140ft long and 20ft tall and contained eight test shafts 3ft in diameter as shown in Figure 5. The wall system used in the experiment is an integrated system of components that included HDPE uniaxial geogrids, segmental units, and connectors. The test wall was designed according to AASTHO specifications without considering the existence of the shafts.
A typical design section is shown in Figure 6, which included five layers of stronger geogrids in the bottom half and five layers of weaker geogrids in the top half. The length of the geogrids was 14ft, which is equal to 0.7 times the height of the wall.
The spacing between geogrid layers was 2ft. The wall facing was formed by segmental blocks with nominal dimensions of 8in. high, 18in. wide, and 11in. deep. The individual geogrid layers were mechanically connected to the blocks by connectors ensuring a reliable structural connection of all system components. This wall had a 3-ft-deep embedment.
The test shafts were located at distances of 1, 2, 3, and 4 shaft diameters from the back of the wall facing. Figure 7 shows the construction of the MSE wall and the test and reaction shafts. In this photo, corrugated metal pipes (CMP) were preset in the backfill for the casting of shafts after the completion of the MSE wall.
The casings were installed by stakingshort sections of CMP for the shaft while the backfill and reinforcement were placed around the CMP. Additional sections of CMP were added as the fill progressed and steel cages were placed inside the CMP. Extruded, punched-drawn HDPE unaxial geogrid was used for this test wall and cut to fit around the shafts as shown in Figure 8.
There was no connection or anchorage of geogrid to the shafts. In Figure 9, the test shafts are located in the front row while the reaction shafts are located in the rear row, which was behind the reinforced fill. The test shafts were seated on the bedrock except one having the length equal to 75% of the wall height to determine the capacity of a “short” shaft.
High-quality free draining backfill material was used for the reinforced and retained fills. Details on the construction of this test wall can be found in the research report by Pierson et al. (2008).
Instrumentation and shaft lateral load testing
This test wall was instrumented with earth pressure cells behind the wall facing, strain gages in the geogrid layers, telltales on the geogrid layers and in the reinforced fill, inclinometer casings in the test and reaction shafts and in front of the test shafts, and targets on the wall facing for photogrammetry during the shaft lateral load testing.
Figure 9 shows the test shafts at different distances from the wall facing and the inclinometer casings. These casings were used during the shaft lateral load testing to measure the lateral movement of the shafts and the wall.
Figure 10 shows the targets placed on the wall facing, which were captured by a high-resolution camera located at a distance from the wall. The black zone on each target is 6in. long, which was used as a scale when the image was imported into the computer-aided design (CAD) software.
The red frames in Figure 10 show the original locations of the targets. The green lines within the red frames indicate the movement of the wall facing. Details on the instrumentation can be found in the paper by Pierson et al. (2009a).
Five single shafts and one group of three shafts were tested. Figure 11 shows the setup of the single shaft lateral load test while Figure 12 shows the setup of the group shaft lateral load test. Shafts were pushed toward the wall by one or two hydraulic jacks and the resulting displacements ranged from 4-9in. Measurements were taken during each test including the deflection of the shaft by LVDTs at the loading elevation and inclinometers, the movement of the wall facing by the targets, the internal movement and strains in the geogrid by telltales and strain gages, and the earth pressures behind the wall facing by the pressure cells.
Test results and discussion
Significant amounts of test data were obtained from this field testing, most of which are available in the publications by Pierson (2008) and Pierson et al. (2009b). Several key results and observations from the testing are presented here.
Table 1 summarizes the lateral load capacities of shafts in the MSE wall obtained from the lateral load testing. Except for Shaft BS (15ft long), all shafts were 20ft long. Shaft BG is one of three group shafts. All other shafts were tested in a single shaft test.
The peak load capacities are reported at the top displacement of the shaft at 0.5, 0.75, 1.0, 2.0, 4.0in., and an ultimate state. It is shown that the peak load of each shaft increased nonlinearly with the top displacement of the shaft. Lateral capacity increased substantially with the distance of the shaft from the wall facing.
Using Shaft D (located close to the end of the reinforced zone) as a reference, Shafts A (i.e., the shaft closest to the wall facing), B, and C had approximately 27%, 91%, and 95% peak load compared with the reference shaft, respectively, at the top displacement of 1in. Therefore, the shaft had a significant load capacity once the shaft was located at a distance of two times the diameter of the shaft.
Table 1 shows that the short shaft (Shaft BS) had more than 60% load capacity as the regular single shaft (Shaft B) at the same distance to the wall facing. Table 1 also shows that the center shaft (Shaft BG) in the group had 68-94% load capacity as the regular single shaft (Shaft B), which indicates a group effect for the shafts spaced at 15ft apart.
After the group load test was performed, a section was excavated to examine the geogrid between two shafts. The aperture size of the geogrid was measured to determine its elongation. The maximum strain in the geogrid was 3% and occurred at the shaft and the strain level decreased to 0 at a distance of 57in. from the near edge of the shaft. This result indicates that the surrounding geogrid was involved in resisting the lateral load even though the geogrid was cut to fit around the shafts.
Due to the pattern of the facing blocks and the rough masonry appearance of their surfaces, the aesthetics of the wall system were only affected slightly by the wall movement, resulting from the lateral shaft testing. The deflection of the wall facing was only seen from the top of the wall looking down, or from the side looking at the wall facing parallel. The movements of individual blocks were visible only upon close inspection, even for wall displacements in excess of 6in.
Despite the significant loadings and displacements imposed during the experiment, the wall system remained fully intact. The mechanical connections of geogrid to block likely contributed to robustness of the wall system and the system’s ability to maintain integrity after such large displacements were imposed. Additionally, the textured surfacing and finish of the segmental blocks hide the local deformation of the wall facing well, as shown in Figure 13.
Shafts in MSE walls are used to support sound barrier walls near highways and major roads when a residential area is nearby. The traditional design, which isolates the shafts from the MSE mass to simplify the design, requires rock sockets and large-diameter shafts, and thus, is very costly.
An alternative design was proposed and verified through a full-scale MSE test wall in this research. In this design, the shafts are seated on the bedrock and supported by the MSE mass. The field single and group shaft lateral load testing demonstrated that the shaft could carry significant loads when the shaft was located at two times the shaft diameter (36 in.).
There was a group effect when the shafts were spaced at 15ft apart and located at a distance of two times diameter of the shafts. Even though the geogrid layers were cut around the shaft, they were involved in resisting the lateral load from the shaft.
The segmental blocks were tolerable to the differential movement induced by the shaft and effective in hiding the local deformation even at the wall facing deflection more than 5in. As a result, the alternative design approach investigated appears to be technically viable for the specific wall system used in the testing.
This research has demonstrated an economic alternative to the standard KDOT method, allowing future noise wall construction to occur more economically.
Jie Han is an associate professor in the Department of Civil, Environmental, and Architectural Engineering at the University of Kansas and is the coprincipal investigator on this research project.
Robert Parsons is an associate professor in the Department of Civil, Environmental, and Architectural Engineering at the University of Kansas and is the principal investigator on this research project.
Matthew Pierson is a Ph.D. candidate in the Department of Civil, Environmental, and Architectural Engineering at the University of Kansas and is the graduate research assistant working on this project.
James Brennan is an assistant geotechnical engineer with the Kansas Department of Transportation and the monitor of this research project.
This research project was financially sponsored by the Kansas Department of Transportation.
The KDOT maintenance and geotechnical group provided its great help in constructing and testing the wall.
The contributions of Tensar International Corp., Midwest Block and Brick, Applied Foundation Testing, Great Plains Drilling, and Dan Brown & Associates were essential to the successful completion of this project. Their sponsorships and contributions are greatly appreciated.
Pierson, M. C., Parsons, R. L., Han, J., Brennan, J. J., and Vulova, C. (2009a). “Instrumentation of MSE wall containing laterally loaded drilled shafts.” Proceedings of IFCEE 09, ASCE Geotechnical Special Publication No. 187, 353-360.
Pierson, M. C., Parsons, R. L., Han, J., and Brennan, J. J. (2009b). “Capacities and deflections of laterally loaded shafts behind an MSE wall.” Journal of the Transportation Research Board, 2116, 62-69.
Pierson, M. C., Parsons, R. L., Han, J., Brown, D. A., and Thompson, R. W. (2008). Capacity of Laterally Loaded Shafts Constructed behind the Face of a Mechanically stabilized earth Block Wall. Final Report, Kansas Department of Transportation, 237 pages.