Meaning of An Upturned Trough of Spoil

Meaning of An Upturned Trough of Spoil

The passage below is from Fathoms: The World in the Whale by Rebecca Giggs.

---------------------

I envisioned it jumbled in with household waste, amid defunct whitegoods and bags of rubbish; the skull, an upturned trough of spoil. After death, the whale’s putrefaction would generate yet more heat, scorching its bones and turning its organs black within the tight bind of its innards. If no one cut the body open, it might explode.

--------------------------

Here I am a volunteer who wants to help save a stranded whale. After its death, it is decided that the carcass should be brought to a landfill. And I am thinking about the meaning its dumping.

Here’s my questions on the underlined phrase.

The skull is the dead whale’s skull.

And the author likens it to ‘an upturned trough of spoil’.

Even if I’m right about this, I don’t know anything about ‘an upturned trough of spoil’.

Upturned is upside-down and a trough is a container in a barn but about spoil I have no idea of.

So I cannot visualize ‘an upturned trough of spoil’.

Help me.

Thanks.