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Create a new solid and add the Grid effect, setting the Size to ‘Width Slider’, the Width to about 100 (or whatever seems right to you) and the color as needed.
Once you have that grid the way you want, make the layer 3D (click the 3D option in the layer’s Switches), type ‘r’ to see the rotation property and set the X Rotation to -90. Type ‘p’ to see the position properties and lower the y-position so the bottom edge of the grid goes off the bottom of the screen.
It should look like a 3D grid on the floor at this point, but cropped to the layer size.
To expand the layer, add the Repetile effect, set the Expand Up to 20000 (max amount) and set the Left and Right to something high (like 5000) until it fills the width of the screen.