Instead, spend your time writing articles and getting them into journals. You should be able to produce roughly one published article a year [0]. When you have 3-4 published or near published articles, take them and put them together into a dissertation.
Most universities will allow this and it's the approach to developing a dissertation that minimizes your risk (you don't bet everything on one experiment, you minimize the risk of last minute critiques and requests for changes, etc...).
[0] This might seem intimidating when you start out, but if you want a promising future in academia you will eventually need to be producing 5, 10 or more articles a year (with admittedly a lot of collaboration and assistance from other). Setting a strict goal of one article a year as a grad student should be obtainable in most fields and get you on the right track. It will also force you to be disciplined and to produce. When of the biggest risk for grad students is trying to be too perfect or too "big" in their work.