Force required to drive a fence post into soil

AI Thread Summary
The discussion focuses on determining the force required to drive a 6ft post into soil, emphasizing the impact of soil type and moisture content on driving ability. Key insights include the importance of using a tapered post end to facilitate penetration and reduce resistance, as blunt ends require significantly more force and can cause instability. The use of hydraulic systems, particularly drop hammers, is highlighted as an effective method to enhance driving efficiency by reducing the hydraulic power needed. Real-world applications often involve vibrating pilings to aid in insertion, especially in soils with high clay content and moisture. Overall, understanding soil mechanics and post design is crucial for effective fence post installation.
rafha
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Homework Statement:: what force is required to drive a 6ft(1.8 m) 4 inch (100 mm) post 2 ft into the ground? I'm wondering how different soils affect driving ability and where I could find information on soil resistive force. A 20 ton hydraulic tractor mounted cylinder is used to press the posts into the soil. imagine the tractor has infinite mass and the output force of the cylinder will be fully transferred into the post.

thanks for any help!
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[Mentor Note -- moved from the schoolwork forums -- appears to be a general technical question]
 
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The soil type and water content are very important parameters. An auger is used to prepare the hole in dense soils without big rocks, or where noise is a problem.

You must study the way that particles or soil flow around the end of a blunt or a sharp penetrator. No theory can beat driving one hundred fence posts, while thinking about the forces involved. Pile drivers follow a similar theory.

The blunt end of a post is not simply pushed onto the soil. The end is cut to make a symmetrical wedge or a point which then opens, separates, and compacts the soil sideways, allowing the post to penetrate and to set solid.

A post is not simply pressed linearly into soil. The post is hammered into the soil using a drop hammer that is repetitively lifted by hydraulics and dropped onto the top of the post. That reduces the power required by the hydraulics, by a factor of about ten.

A blunt or square-cut end requires too much force and punches an over-sized hole in the soil. The post would wobble in the hole until the soil was able to flow back to hold the post.

A blunt end cannot easily be hammered into soil because too much energy is reflected back up the post from the square cut flat end. A tapered end impedance matches the compression wave traveling down the post from the hammer, into moving the soil aside. With a tapered point, the top of the post is less damaged.
 
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Baluncore said:
The soil type and water content are very important parameters. An auger is used to prepare the hole in dense soils without big rocks, or where noise is a problem.

You must study the way that particles or soil flow around the end of a blunt or a sharp penetrator. No theory can beat driving one hundred fence posts, while thinking about the forces involved. Pile drivers follow a similar theory.

The blunt end of a post is not simply pushed onto the soil. The end is cut to make a symmetrical wedge or a point which then opens, separates, and compacts the soil sideways, allowing the post to penetrate and to set solid.

A post is not simply pressed linearly into soil. The post is hammered into the soil using a drop hammer that is repetitively lifted by hydraulics and dropped onto the top of the post. That reduces the power required by the hydraulics, by a factor of about ten.

A blunt or square-cut end requires too much force and punches an over-sized hole in the soil. The post would wobble in the hole until the soil was able to flow back to hold the post.

A blunt end cannot easily be hammered into soil because too much energy is reflected back up the post from the square cut flat end. A tapered end impedance matches the compression wave traveling down the post from the hammer, into moving the soil aside. With a tapered point, the top of the post is less damaged.
thanks for the reply! I should have mentioned I would be driving pointed posts.
My question still remains how much force would be required to literally 'press' the post into the soil, assuming the soil is free from stones, has high clay content, and is about 50% water and air mix.
also assume a perfect transfer of force between the cylinder and the post and no movement of the cylinder above the post.
 
If we are talking about real-world applications and not just theoretical calculations, it's pretty common around here to "vibrate" pilings as they are pressed into the ground (and not hammered). The soil characteristics are close to those described in post #6, maybe with a higher water content.
 
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