Cohesive soils compacted dry of optimum vs. wet.

Today we're going to delve into soil compaction. Can anyone tell me why compaction is important in civil engineering?

It helps to increase the strength of the soil, right?

Exactly! Compaction is crucial because it increases the soil's density and strength, reducing voids and making the ground more stable for structures. Now, let’s explore the effects of compaction on cohesive soils. Student_2, can you share what happens to the density when we compact soil?

The voids decrease, which increases the dry density of the soil mass.

Correct! And this process changes various soil properties. For example, lighter soil will undergo greater changes compared to heavier soils. Let’s remember this with the acronym DENSITY – 'Diminished voids Enhance the Natural Strength and Improve Together.'

What about shear strength? How does it change?

Great question! Shear strength is influenced by many factors including dry density and moisture content. In cohesive soils compacted dry of optimum, the structure tends to be flocculated, leading to higher shear strength compared to when compacted wet of optimum, which is more dispersed.

So it has to do with how closely the particles are arranged?

Exactly! When packed closely in a flocculated structure, particles interact more effectively. Let’s move to our next session where we cover permeability.

Continuing from our last discussion, let's focus on permeability. Who can remind us how compaction affects permeability?

Increased compaction reduces permeability because it decreases void space.

Spot on! Now, at the same density, soils compacted dry of optimum are generally more permeable than those compacted wet of optimum. Why do you think that might be?

Maybe because the particles are more flocculated when dry, which allows for more space between them?

That's right! That's a keen observation. The structure and arrangement of particles play a critical role in how water moves through soil.

What about when two soils have the same void ratio but different particle sizes?

Excellent point! Larger particle sizes generally lead to higher permeability, even at the same density. Remember this with PEARL – 'Permeability Increases with Enlarged And Rounded Limestone.' Let's summarize our learnings before we move ahead.

To wrap up, we discussed that compaction reduces voids, increases density and alters shear strength and permeability significantly. Those factors are crucial for construction! Let’s prepare for our next topic.

Now, let’s look at settlement—can someone explain how compaction impacts it?

Compaction increases density and decreases the void ratio, which reduces settlement.

Exactly! As settlement occurs, both elastic and consolidation settlements are reduced. Notably, cohesive soils compacted dry of optimum experience greater compression than those compacted wet. Can anyone explain why?

It could be that dry soils are more tightly packed, leading to less space for further settlement?

Yes! Dry compacted soils have less moisture between particles, which allows tighter packing and less potential for further compression. Let's use the mnemonic 'DENSER IS BETTER – Dry Ensures Nicer Settlement Efficiency Results.'

So why is knowing this vital for engineers?

Understanding these concepts helps in determining how much a structure might settle over time—critical for ensuring safety and stability! We will move to our next topic soon!