The "grove of death" - What does Marlow encounter here? What's the implication of the hole being as it is? Of there being smashed imported drainage pipes?

Marlow encounters natives who could no longer work. THey were left here to starve and die merely languishing in a grove of death,

{“They were dying slowly. They were not enemies or criminals. They weren’t even human anymore. They were shadows of disease and starvation lying in the gloomy green shade.}

I think the implication was that the Europeans were trying to build some sort of drainage or water system. The irony was that these people were dying of thirst right on top of it.