How did the Terracotta Army get burnt and destroyed?

The terracotta statues in the Pit 1, Pit 2, and Pit 3 of the Terracotta Army should be arranged in order at the beginning of construction. However, the scene of the excavation by the archaeological team was that some of the warriors in Pit 1 and Pit 2 fell down, some lost heads, and some had broken arms and legs... quite a mess. The wood shed and roof beam in the pits were also burned to ashes or coke, and the red burnt soil after the fire was still around the pit. The signs suggested that the terracotta warriors in Pit 1 and Pit 2 had been burned and destroyed. How did the Terracotta Army get burnt and destroyed? Who did set fire to the Terracotta Army? Why did Pit 3 survive without any trace of fire?
 

It was once assumed that the spontaneous combustion of the Terracotta Warriors was caused by the biogas in the pits. However, during the excavation, no large amount of humus was found. Although there was silt in the pits, the soil was relatively pure and did not have the conditions to produce biogas. Therefore, there is insufficient evidence for the self-combustion of Terracotta Army caused by biogas.
 

In some ancient funeral rites, there were the customs of setting fire to the burial objects and the buildings in front of the tombs. The ancient Chinese people believed that the dead could continue to enjoy the goods in the underworld only after the items were burned. Therefore, it was suggested that the Qin people set fire to the terracotta warriors themselves.
 

But there is also an obvious loophole in this statement, that is, Pit 3 had not been burned. Is it burning in turn, and pit 3 hadn’t been burned yet? Even if this is true, there are still doubts. Archaeologists found that there were 20 - 30 cm (7.8 - 11.8 inches) thick mud in the pits, which could not be accumulated in a short time. This indirectly showed that the terracotta figures have experienced a long period of time before getting burnt.
 

"Historical Record" or "Shi Ji" - the first of the official dynastic histories in China and many other historical books have records of Xiang Yu, who overthrew the Qin Dynasty burning the imperial palace of the Qin Dynasty (221 - 207 BC), excavating the Mausoleum of the First Qin Emperor and embezzling the plundered wealth. This may be the most direct historical evidence of Xiang Yu’s arson. However, there was no clear statement in the historical books that Xiang Yu burned the Terracotta Army in the Qin Shi Huang Mausoleum, and only the “Qin Imperial Palace” and “Underground Palace of Qin Shi Huang Mausoleum” were burned. In addition, Xiang Yu needed a lot of weapons to fight against Qin army. This is another possibility for him to destroy the Terracotta Warriors and grab weapons. But so far, there are not enough historical records to prove that Xiang Yu did burn and destroy the Terracotta Army.
 

Reasonable conjectures always need enough evidence to prove. So far, experts and scholars are still studying and exploring who burned the Terracotta Army and what was the purpose. We can only rely on more archaeological evidence in the future to give answers.