DEPTH OF PAVING WOODEN BLOCKS The proper depth of wooden paving blocks is a matter yet to be determined. Shallow blocks are likely to split because the pressure upon them under heavy traffic is so great that the fibers are pulled apart, or, as technically stated, the wood fails in longitudinal shear. Deeper blocks will not fail so easily; that is, a block three inches deep may soon give way under heavy traffic, while one four inches deep may stand up well.
Since longleaf pine has so far been regarded as the standard paving block wood, the Director of the Government Forest Products Laboratory recently made an interesting comparison of its longitudinal shearing strength with that of a number of other woods, and also indicated the depth it would be necessary to have blocks of these woods to give the same shearing strength as a longleaf pine block inches deep. The results of the comparison are given in Table 13.
Of course, as the Director states, the depth of a block is not the only thing to be considered in wood pavement. Other conditions—such as cost of material, and ability to take creosote— eliminate some of the woods listed in Table 13 from practical consideration for paving blocks.
Table 13 Longitudinal Shearing Strength of Wood Blocks Dean Necessary Shearing Strength to Equal Parallel to Strength of Long Grain leaf PineSpecies of Wood (Lbs. per eq. in.) (Inches) Pignut hickory 2,710 2.18 Sugar maple 2,385 2.48 Rock elm 2,154 2.74 Beech 1,908 3.1 Red maple 1,789 3.3 Longleaf pine 1,688 3.5 Tupelo 1,577 3.75 Sycamore 1,554 3.8 Yellow birch 1,428 4.14 Tamarack 1,372 4.31 Western yellow pine 1,300 4.54 Norway pine 1,262 4.68 Douglas fir 1,180 5.01 Eastern hemlock 1,148 5.15 Shortleaf pine 1,135 5.2 White spruce 1,134 5.21 Lodgepole pine 974 6.07 Redwood . 674 8.78