A few points.
1) Ground excavation in Texas is more difficult than in the rest of the country. Under the initial soil layer you have generally either limestone or clay. Limestone is hard to dig and the Texas clay reacts between the water table and the dry air, this leads to shifting and cracking. This is why basements are rare in Texas.
So you either have more effort up front (limestone) or a lot more maintenance (clay) when compared to other areas of the country.
Once you remove the topsoil you have to deal with this.
2) Generally large defensive fortifications not successful.
If you look through history unless you have a high troop density covering the fortification, weak spots will be found and at best you can hope that the defensive structures either channel them in some way or slow the enemy in retreat.
I would make small fortifications to allow for the people to hold up until that cavalry arrives (or day breaks) and use the rest of the earth moving resources to improve the farms.
3) There is a reason barbed wire was invented for and used so extensively in the American West. It is cheap, uses minimum resources, and can cover vast distances.
With very basic equipment it can also be used to relay phone signals.
http://www.geek.com/news/good-old-da...ences-1534417/