a pile of意思为“一堆……”根据惯用法表示单数含义。
piles of意思是“成堆的……”,后面谓语要用复数相应的形式。
1. a pile of后接名词复数或不可数名词（a pile of既可以修饰名词，也可以修饰不可数名词），根据惯用法一般视为单数含义，请看：
There in front of me was a great pile of old cans.
我的面前是一大堆旧罐头。
There was a pile of clean laundry on her bed. 她床上有一堆刚洗好的衣物。
A pile of ironing is hardly inspiring. 一堆需要熨烫的衣服无法让人提起兴致。
Heaped up in one corner was a pile of junk. 堆在一角的是一堆废物。
There was a great big pile of books on the table.
桌上有很大一摞书。
There was a (neat) pile of books on the desk. 桌子上有一堆（整齐的）书本。
On the table beside an empty plate was a pile of books.
桌上空盘子的旁边是一堆书。
On the table was a pile of books and a scatter of papers.
桌子上放着一摞书和零落的几份报纸。
Under a wide shelf that holds coffee jars stands a pile of magazines
放咖啡罐的一个宽架子下方有一摞杂志。
I was looking for my handbag, which was buried under a pile of old newspapers.
我正在找我那被埋在一堆旧报纸下面的手提包。
If we thought Dad's pile of chicken manure was bad, this was 10 times worse.
如果我们认为爸爸那堆鸡粪很糟糕的话，这就是那个的十倍。
高考真题- 2015 四川
特例用复数的例子：
A pile of presents were heaped under the Christmas tree.
圣诞树下有一大堆礼物。(摘自Easier English Student Dictionary)
2. piles of（后面可加可数名词，也可以不可数名词；表复数概念，作主语时谓语一律用复数形式。）
一堆堆的，成堆的；一大团；一大批
I have got piles of work to do.
我有大量工作要做。
There were neat piles of kindling wood against the wall.
墙边整齐地放着几堆引火柴。
There are piles of marvellous stones in the garden.
园中奇石磊磊。
There were piles of litter in the streets.
街道上垃圾成堆。
The bookshelfves were full and there were piles of books all over the floor.
书架上装满了书，地上也一堆一堆的书。