Objective:
The Proctor Compaction Test
determines the Optimum Moisture Content (OMC) and Maximum Dry Density
(MDD) of soil. These values are crucial for earthworks, road
construction, and foundation stability, ensuring soil compaction meets
design requirements.
ASTM Standard Code:
- ASTM
D698 – Standard Proctor Test
(Used for light compaction, 600 kN/m² energy).
- ASTM
D1557 – Modified Proctor Test
(Used for heavy compaction, 2,700 kN/m² energy).
Apparatus
Required:
- Proctor
Mold – Cylindrical mold of 944
cm³ (4-inch diameter) or 2124 cm³ (6-inch diameter).
- Rammer
(Hammer) –
- Standard
Proctor: 2.5 kg, 305 mm drop.
- Modified
Proctor: 4.5 kg, 457 mm drop.
- Weighing
Balance – Accuracy of 0.01g.
- Oven
– Maintains temperature at 110 ± 5°C.
- Mixing
Tools – Trowel, scoop, and spatula.
- Water
Sprayer – To adjust soil moisture.
- Straight
Edge – For leveling the compacted
surface.
Test
Procedure:
1. Soil Preparation:
- Collect
about 3 kg of soil sample and dry it in an oven at 110 ± 5°C.
- Sieve
the soil using a 4.75 mm sieve for fine-grained soils and 19
mm sieve for coarse-grained soils.
2. Mixing Water with Soil:
- Add
water incrementally (3-5%) to the soil and mix
thoroughly.
- Prepare
five different moisture content samples to determine OMC.
3. Compaction in the Mold:
- Fill
the mold in three layers, each compacted with 25
blows using the rammer.
- For
Modified Proctor, use five layers with 25 blows per layer.
- Trim
excess soil using a straight edge.
- Weigh
the compacted soil with the mold.
4. Dry Density Calculation:
- Determine
the bulk density (γb): γb = Compacted soil weight/volume
- Find
dry density (γd): γd=γb/1+w
where w = moisture content
(%).
5. Plot Moisture vs. Dry Density
Curve:
- Plot
a graph of dry density vs. moisture content.
- The
peak point represents the Maximum Dry Density (MDD) and Optimum
Moisture Content (OMC).
Results Interpretation:
|
Soil Type |
OMC (%) |
MDD (g/cm³) (Standard Proctor) |
MDD (g/cm³) (Modified Proctor) |
|
Sand |
8
– 12 |
1.6
– 1.8 |
1.8
– 2.0 |
|
Silt |
12
– 18 |
1.4
– 1.6 |
1.6
– 1.8 |
|
Clay |
18
– 25 |
1.3
– 1.5 |
1.5
– 1.7 |
A
higher compaction energy (Modified Proctor)
results in higher dry density and lower optimum moisture content.
Limitations
of the Test:
- Not
Suitable for Coarse Aggregates – The test is not effective
for gravel larger than 19 mm.
- Does
Not Consider Field Conditions – Laboratory conditions do
not replicate field variability.
- Effect
of Soil Type – Highly organic soils
and saturated clays do not compact well.
- Energy
Application Variation – Improper hammer dropping
height or number of blows can affect accuracy.
Conclusion:
The Proctor Test (ASTM D698 &
D1557) ensures proper soil compaction, which improves load-bearing
capacity and reduces settlement. It is essential for roadways,
embankments, and foundations to achieve safe and stable soil structures.
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