Ask H&H: Fixing soft patches in an arena

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Q: At the livery yard where I keep my horse we have an arena with a five-year-old silica sand and rubber surface. The drains have been down for 15 years.

The arena used to be usable all year, but recently soft patches have started to appear, which become holes when a horse treads on them. Do we just need a top-up of sand? Or is the problem likely to be the drainage? What would you recommend to improve the surface?