Yesterday, in sacrament meeting, one of the talks was about the Book of Joshua, and coincidentally, about the events that took place in his life. The speaker gave a rendition of the events described in chapter 6 of the Book of Joshua when the children of Israel besieged the city of Jericho and brought its walls tumbling down. Being the nerd that I am, I took hold of this thought and have been thinking about it ever since; seriously, I have no idea what the second speaker talked about. I've thought about it, and based on my knowledge of soil and wave mechanics, I believe that I can offer a scientific explanation to what happened in Jericho several thousand years ago.
The story goes that after the children of Israel had crossed the river Jordan (not to be confused with the Jordan river in Utah) and besieged the city of Jericho, the Lord counseled Joshua, who was the prophet to have his warriors march around the city once a day for six days, after which the priests would blow a trumpet made from a ram's horn. Then, on the seventh day, they would march around the city seven times, blow the rams' horns for an extended period of time, and all the warriors would shout at the top of their lungs. The Lord promised that if they did this, the city of Jericho would be theirs.
I'm not totally familiar with the construction methods or materials of the era, nor am I intimately familiar with the soil conditions of the region. Based on other passages in the Bible and the construction of other structures at the same time, it would be reasonable to believe that either bricks or stones would have been used with either some bituminous material as mortar, or with no mortar at all. Also of note, the city of Jericho was located somewhere around 825 feet below sea level in a deep river basin. This would generally lead to soft soils with fairly small particle size and low angularity, resulting in low shear strength.
Figure 1. Soil and resonance in the Walls of Jericho failure.
I've included a picture of my own drawing as Figure 1, above, that says at least a thousand words about what I think could have happened. The cross-section of the wall is shown to be about plumb (vertical) before the children of Israel arrive and besiege the city. As the priests carrying the ark of the covenant and the warriors march around the city once daily, they put a force on the soil (FI) that results in stresses, shown by stress bulbs underneath the stick figure. These stresses would result in a lateral force (FL) being applied to the bottom of the wall, causing it to rotate slightly off of plumb. The priests then blowing their trumpets made of rams' horns could insight a low-amplitude vibration in the wall that could weaken both the wall's structure and the soil beneath it.
The marching around the city once a day would cause a certain amount of rotation of the wall, but would be partially ameliorated by the dissipation of pore pressures (water pressure in the soil). With time, the pore pressures would dissipate and the lateral force applied to the wall would dissipate as well. However, on the seventh day, when the children of Israel marched around the wall seven times, it created a large lateral force that could not be reduced by the dissipation of pore pressures in time. This caused an even greater rotation in the wall than the previous six days worth of daily marches.
When the priests blew their horns and the people gave a loud shout, it's probable that the sound waves induced either a large-amplitude vibration, or outright resonance in the wall. With the wall in its out-of-plumb state, it's possible that this vibration could have caused an eccentricity in the structure that brought it down all by itself. It's also possible that the vibration in the wall could have caused a shear failure in the soil beneath that would cause a brick or stone wall to collapse just from the static load of its weight (W). Another important thing to note is that a continuous wall cannot rotate outward without cracks forming along the top. In reality, it probably would have been contributions from all three of these factors that brought down the walls of Jericho.
In my picture, it would appear that the wall would have collapsed on the children of Israel. I really have no idea how tall the wall was nor do I know how far Israel was standing from it. I've also exaggerated the rotations of the wall to make them easier to see in the picture. There's also a good chance the Israel stood at least a few feet away from the wall in anticipation of collapse. The distance of the wall collapse outward and the distance that the children of Israel would have had to have marched in order to create a significant lateral force on the bottom of the wall may have been similar, but it really depends on the height and subterranean depth of the wall, which I decidedly don't know.
Now, the best part about this, in my opinion, is that not even the brightest and most educated people in the world would have had the slightest clue about soil or wave mechanics in 1200 BC, when Israel overthrew Jericho. Galileo recognized that a wave could vibrate with more energy than was put into it (basically resonance) in the early AD 1600s. Even better, Karl Terzaghi didn't give birth to modern soil mechanics until the AD 1940s. Trust me, the children of Israel were not the most educated people in the world, either. They had spent generations being slaves in Egypt and 40 years wandering through a barren desert. I don't suppose that there would have been a wealth of scientific knowledge flowing through the minds of the people of Israel.
The reason that Israel was able to use these complex scientific principles to their advantage over the gentiles was that the Lord knew all of these principles and more that even I don't understand now. The Lord revealed to Israel what they should do in order to conquer. It would have been a seemingly useless and monotonous thing to march around the city once a day for six days and then seven times on the seventh. This is especially true when an army is used to taking over cities by force, as was the convention at the time. I can imagine soldiers whining because they just wanted to take over the city already and move on to the promised land. However, the Lord had a plan for them to take over the city. When they had followed that plan, they were able to begin attacking the city from every side and to utterly destroy it without much loss on their side. In the same way, the Lord will guide each of us to better things than we could ever imagine if we will exercise patience and follow the plan that he has set up for us.
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