I am in CA and have always maintained an inground pool.
It all depends on whether there is ground water on the other side of the gunnite shell and how deep it is. Assuming the pool is 15 x 30 feet with a 4 ft shallow end and an 8 ft deep end and is rectangular, not rounded: the weight of the gunite would be approx. 87,000 lbs (6 inchXXX@XXXXXX.XXX/cc). An equivalent weight of water would fill the deep end pool plus another foot. If the ground water exceeded that level on the other side of the gunite the pool would pop out of the ground and float on whatever level water you have. If you drill a hole through the bottom or side of the pool 1 or 2 feet from the bottom the lift pressure will be relieved as water comes out and you will have no worries. Gunite is very strong and the pool can remain empty quite a while if there is no: water, land movement , earthquakes, or other mechanically damaging prospects.
That is true as well, however, if you have an empty pool and high ground water table in your area and should your region experience heavy rains, the ground water can create pressure on the walls of the unfilled pool and create damage. It is important to know what the ground water status is in your region when draining your pool.
That is the biggest worry. My parents live in NJ, drained their pool to have work done on it. Heavy rains came, ground water level increased... pool "popped" a nice size crack.
It is bet to refill the pool as soon as possible, there are no minimum or maximum guidelines; however, the longer it remains empty, the more room for damage.
I would stronly recomend filling the pool at least half way for now. But YES it definently needs to be filled or the Gunite will crack excessively. I would recomend doing this as soon as possible. For further reference. Never allow your pool to be completly drained for more then 2-3 days. If longer then that time frame you are at risk for serious damages.
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Yes, sun and other evironmental elements can cause drying, flaking and cracks when a pool is left drained.
It is however, most important not to underestimate the potential for wall damage and cracks due to pressure applied on the walls related to ground water. This factor is extremely important even in existing pools. Any reputable pool construction firm or pool maintinance company will not guarantee you a crack-free pool if your groundwater source isn't completely controlled prior to undertaking any pool draining efforts..
My son-in-law's family owns a large pool construction firm and upon consulting with him, he has mentioned that a pool should only be drained in urgent situations. It is never recommended to completely drain the pool. He suggests 10-14 days is the max and only recommends this to his clients in situations that are beyond their control. However, there are no guarantees. The age of the pool is also a mitigating factor. The older the pool, the more chance for cracking.