How to repair heavy duty nylon seam?
The way I see it, even the base material is not cut / prepared really properly. Even if you make a very good stitching, there is a chance that the material itself will fail during the next "shocking experiences".

I would do it in two steps:


stitch as good as possible, to make sure that the sand will stay inside. Please stitch each layer individually, for increased strength;
Wrap the area with strong ribbons (let's say, similar with the safety belts in cars; similar to wearing belts for pants). Be sure that the belts close with good quality metal, not with cheap plastic.