Chapter 5, Problem 5.1P

(a)

To determine

Whether the statement, “In the same soil, the optimum water content would be greater for standard proctor compaction test than for modified proctor compaction test”, is true or false.

Answer to Problem 5.1P

The given statement is true.

Explanation of Solution

Sketch the compaction curve for standard proctor test and modified proctor test as shown in Figure 1.

Refer to Figure 1:

• The optimum moisture content of the standard proctor test is higher than the modified proctor test. The weight of the hammer used in the modified proctor compaction test is higher compared to that which is used in the standard proctor compaction test.
• Thus, the level of compaction achieved with the modified proctor compaction test is greater when compared to that of the standard compact test.
• Greater level of compaction is obtained with the use of hammer with higher weight.

Hence, the given statement is true.

(b)

To determine

Whether the statement, “Relative compaction cannot exceed 100%”, is true or false.

Answer to Problem 5.1P

The given statement is false.

Explanation of Solution

• The relative compaction is the function of maximum dry unit weight $\left({\gamma }_d\right)$ of soil obtained in the lab and field. The maximum dry unit weight $\left({\gamma }_{d,\max }\right)$ obtained in field and lab will vary, depending on the soil type and its moisture content.
• The expression of relative compaction (R.C) is given below:

$\text{R}\text{.C}=\frac{{\left({\gamma }_{d,\max }\right)}_{\text{Field}}}{{\left({\gamma }_{d,\max }\right)}_{\text{Lab}}}\times 100$

• The dry unit weight in the field might be more than the dry unit weight obtained in the lab, depending upon the soil type and moisture content.
• Therefore, the relative compaction can exceed 100%.

Hence, the given statement is false.

(c)

To determine

Whether the statement, “Sheepsfoot rollers are very effective for compacting clays”, is true or false.

Answer to Problem 5.1P

The given statement is true.

Explanation of Solution

• Different types of rollers that are used for compaction are smooth-wheeled rollers, pneumatic rubber–tired rollers, sheepsfoot rollers, vibratory rollers, and impact rollers.
• In sheepsfoot rollers, the pads or foots penetrate through the top layer of the soil and actually compact the soil starting from the bottom to the top and also aerate the soil.
• Therefore, the sheepsfoot rollers are most effective for compacting the clayey soil.

Hence, the given statement is true.

(d)

To determine

Whether the statement, “Strength of compacted clay is higher when compacted dry of optimum than wet of optimum”, is true or false.

Answer to Problem 5.1P

The given statement is true.

Explanation of Solution

• The strength of compacted clay increases with a decrease in the moisture content.
• The compacted clay has more shear strength when the clay is compacted with the moisture content to the drier side of optimum value.

Hence, the given statement is true

(e)

To determine

Whether the statement, “hydraulic conductivity of compacted clay is higher when compacted dry of optimum than wet of optimum”, is true or false.

Answer to Problem 5.1P

The given statement is true.

Explanation of Solution

• Hydraulic conductivity is the rate of flow of water through the porous medium measured with respect to time.
• Clay has more void spaces when it is compacted with the moisture content to the drier side of optimum value. Therefore, the compacted clay will have higher conductivity when it is compacted to the drier side of optimum than the wetter side of optimum.

Hence, the given statement is true.