Most of the material involved in constructing a massive earthen project like this involves a stone base with dirt and earth more or less used as filler. Massive wooden stakes are also used to support the structure. If a structure was made totally of dirt in the water, it would collapse upon the weight of men and siege towers. Waves would also easily erode it.
To accomplish the construction of this project, Alexander sent his soldiers into the woods of Lebanon to cut down trees to get the necessary supplies. The cedars in Phoenicia were very famous in ancient times so Alexander's men chopped down the trees and hauled them back to their camp. They sharpened one end of the tree and pounded it into the sand like a giant tent spike. This would protect the sides of their dam. While some soldiers pounded the giant trees into the ground, others were demolishing the ruins of the land based city of Tyre and bringing them to the shore to be dumped into the sea.
The people of Tyre did not really notice what was going on at first, as they may have simply thought Alexander was building beach fortifications. They never thought he would build an earthen bridge to them. However, soon it became apparent as to what Alexander and his army were doing and the people of Tyre became rightly alarmed. As the earthen structure began to grow in size day after day after day the people of Tyre began really mounting their defense. The people of Tyre began building anti-siege weaponry on their massive 150 foot walls that would fling massive stones and darts at the incoming Greeks. They also sent some of their agents ashore to make diplomatic alliances with several neighboring tribes. The people of Tyre had great wealth and if they survived, these tribes would be greatly rewarded.
So Tyre tasked these tribes with disrupting the gathering of resource efforts by Alexander's army. Tyre also landed a small contingent of its own men on the shore and tried to repel the stone gathering operation at the ancient ruins of Tyre. Having a considerable navy with their manned war galleys that protected their merchants from pirates, Tyre decided to respond even further. The war galleys rode right up to the construction side of the earthen dam walkway and bombarded the crew with all sorts of arrows and projectiles. While the first offense of Tyre slowly impeded Alexander's efforts, ultimately what they did was in vain. Alexander resumed his construction efforts right away and began building defensive structures around the camps. Alexander and his army built huge screens of wood and covered them with animal hides to protect them from any projectiles launched while they were working.
As the work advanced more defensive structures were built, especially at the sides and the front facing the city. These structures consisted partially of the great siege engines required for driving the massive wooden piles into the sea, along with catapults for throwing massive stones. Soldiers would also be able to throw stones, darts and shoot the galleys with bows and arrows from these siege towers which prevented them from getting too close to the workers.
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