▁may ▁not ▁offer ▁ade qu ate ▁wheel ch air ▁access . ▁The ▁American ▁Academy ▁of ▁Orth op edic ▁Sur ge ons ▁states , ▁“ So il , ▁pack ed ▁d irt , ▁grass , ▁and ▁tur f ▁are ▁not ▁recommended ▁for ▁sur f acing .” ▁ ▁A ▁thick ▁layer ▁of ▁l ush ▁green ▁grass , ▁mo ist ▁from ▁recent ▁ra ins , ▁may ▁seem ▁soft ▁enough . ▁But ▁in ▁practice , ▁the ▁grass ▁we ars ▁away ▁and ▁either ▁turns ▁to ▁mud ▁in ▁wet ▁weather ▁or ▁d ries ▁to ▁a ▁hard ▁cr ust ▁in ▁dry ▁weather . ▁ ▁Common ▁surfaces ▁ ▁Engine ered ▁wood ▁fi ber ▁( E WF ): ▁This ▁material ▁consists ▁of ▁sh red ded ▁wood ▁into ▁sizes ▁and ▁aspect ▁r ati os ▁determined ▁by ▁a ▁series ▁of ▁specified ▁sie ves . ▁The ▁wood ▁ch ips ▁are ▁spring y ▁enough ▁to ▁meet ▁A ST M ▁F 1 2 9 2 ▁for ▁impact ▁at ten u ation ▁and ▁firm ▁enough ▁to ▁meet ▁A ST M ▁F 1 9 5 1 ▁for ▁wheel ch air ▁access . ▁Engine ered ▁wood ▁fi ber ▁is ▁made ▁from ▁bi ode grad able ▁vir gin ▁wood . ▁A ▁wood ▁fi ber ▁surface ▁tends ▁to ▁compact ▁over ▁time ▁and ▁needs ▁to ▁be ▁to pped ▁off ▁every ▁two ▁to ▁three ▁years . ▁P oured ▁rub ber ▁sur f acing : ▁This ▁is ▁a ▁se am less ▁rub ber ▁surface ▁composed ▁of ▁two ▁layers ▁that ▁is ▁pou red ▁in ▁place ▁( PI P ). ▁The ▁first ▁layer , ▁or ▁" we