Question
QUESTION: I wish to increase the infiltration rate of a soil to around 1"-2" per hour to sustain a rain garden. Would coarse sand be an option ?

ANSWER: Hi Allan, Coarse sand should work fine.  The amount you mix with your soil will depend on the soil type.  Mix it as deep as possible.  Jim

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QUESTION: Thank you, is there a recommended amount by volume. We are looking at tilling in the sand to baout 18" and adding around 25% by volume. It appears to be loam right now.

ANSWER: 25% sand in loam should be enough, but in your case, there is no "too much" only "too little". I think 40% - 50% sand would be acceptable.  You could grow many groundcovers over the area or cover it with landscape fabric and any mulch.  Jim

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QUESTION: This really helping Jim, but one more question. If we went with say 30% sand in the top 12" and a mulch, do you think we would be able to infiltrate the water from the surface very quickly.

Answer
If there isn't a layer of clay somewhere below 12" AND it's not raining faster than the absorption rate...then yes, you should be fine.  I'd probably put a layer of landscape fabric between the mulch and the soil surface so that as it decomposes, it doesn't fill the air spaces and slow the water absorption down.  Jim