There are advantages and disadvantages for both mortared and mortarless construction. For a new stone arch bridge design, one can quite reasonably ask, which is better?
The Factors at Play: Surface Contact
The main advantage of mortar is that it results in a stronger structure, primarily through better distribution of weight. If you are very skilled at cutting stone, and therefore able to create large, perfectly shaped, precisely fitting blocks that meet up perfectly, mortar is almost certainly unnecessary. For most of us, we are probably not able to achieve such precision, though, of course, we certainly want to work towards that goal. In these cases, mortar will result in a stronger structure as it fills nooks and crannies between stones achieving good surface contact.
The Factors at Play: Economics
One major downside of using mortar is the economics of the situation. Mortar costs money. Actually, one bag of mortar is not all that expensive; in fact it is quite cheap. Even these days, you probably can find a good bag of premixed mortar for only a few dollars, a mere trifle. Unfortunately, as anybody who has built a mortared structure undoubtedly knows all too well, the most impressive thing about one bag of mortar is how woefully short it goes towards building even a modestly sized structure. You quickly find that you need many bags of mortar, perhaps even enough mortar to consist of one third of the volume of the structure. Though the individual bag is inexpensive, the small army of bags that goes into a single stone arch bridge, even a small one, starts to become rather pricey! On the other hand, the price of mortar can be reduced somewhat if you buy your own lime, portland cement and sand, to mix your own mortar mix. Then, too, tighter fitting stones means less mortar required.
Conclusions
There are other factors in favor of mortar and unfavorable to mortar, but for the DIY bridge builder the above two are the most significant. So, what do you do? As it turns out, the answer may be quite simple. For small stone arch culverts (spans of, say, 2 feet to 8 feet) the stress within the masonry is actually quite low, even under heavy loads.
Thus, the weight distribution of mortar is almost certainly not required if you are a good builder. For big bridges, the strain within the masonry is rather high, so mortar is a good choice for long-term stability. By the time the spans reach 30′, the stresses in the masonry becomes quite significant, making mortar the most prudent choice, though 30′ bridges are probably beyond the ambitions of most DIY builders! In the end, perhaps the best solution is to build small culverts mortarless and big bridges as mortared, as this seems to be the best balance between economics and durability. Be aware, of course, that mortarless masonry requires more care and time to build than mortared masonry to prevent stones being worked loose over time. If time is a concern more than money, using mortar may be the better option.
Stone Arch Bridges 