How to use "initial geometry" pop object field

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Hi everyone!!

 

I have done a ground of sand using grains of houdini shell. I want this ground to start with relaxing particles in frame 1, so when my sand balls falls from the sky, it interacts with the ground.

I have done this tutorial (https://vimeo.com/117734637) because this is what I need but instead with rigid bodies, with grains, but it doesn't work (it load the initial state but the solver doesn't read the sand balls)  

 

I have read that the pop object dop has a field called "initial geometry" that it might works, but I don't know how it works because I have to connect this field to a sop. And I don't understand how houdini knows what frame of the original simulation I want it to be the initial state. I also want that once I have the relaxed grains in frame 1, these interacts with the sand balls falling from the sky.

 

Hope you understand but I trying to say.

 

Thanks!