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69 C H A P T E R 9 This chapter focuses on guidance on the use of ground zones within FHWA TNM. Appendix H (available on the NCHRP Project 25-34 web page at http://apps.trb.org/cms feed/TRBNetProjectDisplay.asp?ProjectID=2986) provides substantial detail on the methods and test cases that were used to develop the guidance. 9.1 Size and Location of Ground Zones 9.1.1 Background When ground surfaces other than the default ground intervene between roadway and receivers, ground zones are used to model those other ground surfaces. Several ques- tions arise concerning the location and size of these ground zones: â¢ General size. Are ground zones needed for very small patches of non-default groundâpatches such as suburban sidewalks and driveways? â¢ Precise coordinates. How precisely must ground zone coordinates be input to achieve reasonably precise sound levels? â¢ Location. Are ground zones needed more (1) toward the middle of the propagation path or (2) toward the ends near the roadway and receivers? The following section answers these questions with result- ing guidance for TNM input. 9.1.2 Resulting Guidance for TNM Input 18.104.22.168 General Size Ground zones are not needed for small patches of non- default ground such as suburban sidewalks and driveways. In general, a ground zone must cover about 20% of the source- receiver distance to have more than a 1-dB effect. 22.214.171.124 Coordinate Precision It is not necessary to be precise when entering X and Y coor- dinates for ground zones. Ground zone effects are very insensi- tive to the precise size and location of the zone. For example, it might take a change in width of 30 ft to cause a 1-dB change in the ground zoneâs effect, and even then the change might only occur under the most critical input geometry. 126.96.36.199 Location Ground zones are needed more toward the middle of the propagation path, generally in the area where the sound ray bounces off the ground toward the receivers. In general, ground zones are needed in this central area as long as they cover more than 10 to 20% of the source-receiver distance. If in doubt, it is best to include them to determine their effect. A sense of the effects of ground zones can be gained by examining the figures and graphs included in Appendix H. 9.2 Expanded List of Ground Types 9.2.1 Background During validation measurements for TNM 2.5, the U.S. DOT Volpe National Transportation Systems Center inves- tigated modifications to current TNM practice concerning ground typesâfor both ground zones and default groundâ with the aim of improving the match between computed and measured sound levels.36 The Volpe Center found the best match between computed and measured sound levels when an expanded set of effec- tive flow resistivity (EFR) values was used in place of TNMâs standard ground types (see next section). Ground Zones 36 Hastings, A. L. and J. L. Rochat, Ground and Pavement Effects Using FHWAâs Traf- fic Noise Model 2.5, DOT-VNTSC-FHWA-10-01 and FHWA-HEP-10-021, John A. Volpe National Transportation Systems Center, Environmental Measurement and Modeling Division, Acoustic Facility, Cambridge MA, April 2010.
70 9.2.2 Resulting Guidance for TNM Input 188.8.131.52 Expanded Set of EFR Values The Volpe Centerâs expanded set of EFR values for vari- ous ground types is collected mostly from the acoustics literature (see Table 13). Note that Volpe actually mea- sured the 5,800 EFR for hard-packed dirt. Also note that TNMâs built-in ground zone types are also included in Table 13. For best computation (especially for barrier design proj- ects), the use of EFR values from this table is suggested: â¢ Ground zones. Ground zones should be designated as âCustomâ with the appropriate EFR value from the table entered. â¢ Default ground. A built-in ground type can be selected from TNMâs pull-down list. Otherwise, the full default ground area can be overlaid with one or more new ground zones and with custom EFRs from Table 13. TNM does allow a ground zone to completely enclose another as long as their boundaries do not touch. 184.108.40.206 Distances beyond 500 ft In addition, Volpe validation showed that TNMâs built-in ground effects were too extreme for receiver distances beyond 500 ft or so. In particular â¢ Acoustically soft TNM ground types provide too much absorption, thereby under-predicting sound levels at large distances. â¢ Acoustically hard TNM ground types provide too much reflection, thereby over-predicting sound levels at large distances. Ground Type Description Additional Detail Average EFR Powder snow (built into TNM) -----a 10 Dry snow 4 in of newly fallen snow, on top of 16 in of older snow 20 ----- wons raguS 38 Granular snow (built into TNM) ----- 40 05 kcolmeh ro eniP roolf tseroF Lawn With 11.9% to 16.5% moisture content 58 Field (meadow) grass (built into TNM) ----- 150 Lawn root layer in loamy sand Volume porosity between 43.5% and 59.8% 188 212 sgnidliub cilbup dnuora ,erutsap hguoR nwaL Lawn (built into TNM) ----- 300 573 noitategev dna trid fo soitar suoiraV nwaL Soil Various types 278 374 sepyt suoiraV dnaS Loose soil (built into TNM) ----- 500 055 hsem ni-4 ot pu skcor llams suoiraV trid edisdaoR Dirt Roadside with rocks smaller than 4-in diameter 550 0561 selcihev yb dekcap draH tlis ydnaS Limestone chips 0.5-in to 1-in mesh 2750 0003 hsem delliF daor trid dlO Hard soil (built into TNM) ------ 5000 rg emos gnidulcnI trid dekcap-draH 0085 eploV yb derusaem RFEâleva Exposed dirt Rain packed 6000 000,01 ezis elcitrap suoirav ,weN tlahpsA Waterb Especially with wave roughness 10,000 005,21 dekcap draH tsud yrrauQ Pavement and water (built into TNM) ------ 20,000 005,72 tsud htiw delaes ,dlO tlahpsA Concrete Depends on finish 65,000 000,002 detniaP etercnoC a ----- = no additional detail. b This water entry derives from text in the Volpe report, rather from the actual Volpe table. Table 13. Expanded set of EFR values.
71 TNM users will want to be aware of this tendency when computing sound levels at distances beyond 500 ft. 9.3 Bodies of Water 9.3.1 Background Large bodies of water often require a TNM ground zone as input. This is especially important when (1) the body of water is toward the middle of the propagation path and (2) when it covers more than 10 to 20% of the source- receiver distance (both input criteria per Section 220.127.116.11 of this document). 9.3.2 Resulting Guidance for TNM Input When entering a ground zone for a body of water in TNM, analysts should recall that the ground zone includes no eleva- tion information, so they must enter a terrain line that com- pletely encloses the ground zone to define the water elevation. Because water surfaces are always horizontally flat, all points on that terrain line should have the same Z coordinate, that is, the waterâs elevation. Sometimes surrounding land does not slope gradually to the water. Instead, it sometimes drops abruptly down to the water from the waterâs so-called âbank.â Where this is the case, analysts must enter a second terrain line that lies close to the first terrain line.