Soil Compaction

Compaction is the process of densifying soil by reducing air voids using mechanical energy (rolling, ramming or vibration), at constant water content.

Effects on engineering behaviour

1. Shear strength increases: particles interlock more closely and dry density rises.
2. Compressibility and settlement decrease: a dense, stiffer soil settles less under the load.
3. Permeability decreases: voids are reduced, so seepage through dams and embankments is lower.
4. Bearing capacity increases: safe load for foundations and pavements is higher.
5. Swelling and shrinkage decrease (when compacted wet of optimum), and volume change from water absorption is lessened.
6. Liquefaction potential is reduced for sands due to the denser state.
7. Slope stability improves: because of higher strength and lower pore pressure.
8. Water absorption and frost damage decrease.

Compacting wet of optimum gives lower permeability and a more flexible soil (good for dam cores); compacting dry of optimum gives higher strength and stiffness (good for road subgrade) but more swelling.

The dry density achieved by compaction depends on:

1. Water content: at low water content the soil is stiff and resists compaction. Water lubricates the particles, so density increases up to the optimum moisture content (OMC) where $\gamma_d$ is maximum. Beyond OMC, water occupies space and density falls.
2. Compaction effort (energy): more effort (more blows, heavier rammer, more passes) gives a higher maximum dry density and a lower OMC. Increasing the effort beyond a limit gives little gain.
3. Type of soil: well-graded coarse soils give a high $\gamma_{d,max}$ and low OMC with a steep curve; clays have a low $\gamma_{d,max}$, high OMC and a flat curve. Poorly graded sands give a low density.
4. Method of compaction: impact (hammer), kneading, vibration, and static pressure give different densities. Vibration is best for sands; kneading for clays.
5. Lift thickness and number of passes in the field.
6. Type of roller and speed: sheep-foot for clay, vibratory for sand.
7. Admixtures: lime, cement or salts change OMC and density.
8. Temperature, and initial moisture condition.

Relative compaction (RC) is the ratio of the dry unit weight (density) achieved in the field to the maximum dry unit weight obtained in the laboratory compaction test (Proctor test), expressed as a percentage:

It is used as the field control of compaction. Specifications normally require RC of 95% (standard Proctor) for embankments and 98 - 100% for pavement layers or the top of fills. Not to be confused with relative density $D_r$, which is for cohesionless soils.

Given

$\gamma_{d} = 17$ kN/m³, $w = 16\%$ (OWC), $G = 2.65$, $\gamma_w = 9.81$ kN/m³.

Void ratio

Degree of saturation

Zero air void unit weight at $w = 16\%$

Answer: $e = 0.529$, $S = 80.1\%$, $\gamma_{d,zav} = 18.26$ kN/m³.

Given

$\gamma_{d} = 18$ kN/m³, $w = 15\%$, $G = 2.7$, $\gamma_w = 9.81$ kN/m³.

Void ratio and porosity

Degree of saturation

Zero air void unit weight

Answer: $n = 32.0\%$, $S = 85.9\%$, $\gamma_{d,zav} = 18.85$ kN/m³.

(i) Compaction curve

Mould volume $= 1000$ cm³ = 1 litre, so bulk density $\rho = m/1000$ g/cm³ (Mg/m³), and

ZAV: $\rho_{d,zav} = \dfrac{G\rho_w}{1+wG}$ with $G = 2.7$.

Plot $\rho_d$ (or $\gamma_d$) against $w$ and draw a smooth curve through the points, which rises to a peak between tests 3 and 4 and then falls (the curve stays below the ZAV values).

 rho_d
 1.90 |         .-*-.
 1.85 |       *      *
 1.80 |    *            *
 1.75 | *
      +---+---+---+---+---+-- w%
       11  12  13  14  15  16

From the smoothed curve:

• Maximum dry density $\approx 1.90$ Mg/m³, i.e. maximum dry unit weight $\approx 18.6$ kN/m³
• OMC $\approx 13.2\%$

(ii) Relative compaction

Field dry density:

Answer: MDD $\approx 1.90$ Mg/m³ (18.6 kN/m³), OMC $\approx 13.2\%$, relative compaction $\approx 90\%$ (below the usual 95% requirement).

Dry unit weight of each test is found from the bulk density of the 1000 cm³ mould, $\gamma_d = \dfrac{\gamma_{bulk}}{1+w}$.

Computation of dry density

$\gamma_{bulk} = \dfrac{\text{mass (g)}}{1000\ \text{cm}^3}$ (in Mg/m³ = g/cm³)

i) Compaction curve, $\gamma_{d,max}$ and OMC

Plot $\gamma_d$ (y-axis) against $w$ (x-axis) and draw a smooth curve through the six points. The dry density rises to a peak between 12.8% and 13.6% and then falls. A parabola through the three points around the peak gives the crest at about $w = 13.1\%$ with $\gamma_d \approx 1.90$ Mg/m³.

 γd
 1.90 |         . * .
 1.85 |      *         * 
 1.80 |    *             
 1.75 |  *                 *
 1.70 | *
      +----------------------- w %
        11  12  13  14  15  16

• Maximum dry unit weight, $\gamma_{d,max} \approx 1.90$ Mg/m³ ($\approx 18.6$ kN/m³)
• Optimum moisture content, OMC $\approx 13\%$

ii) Relative compaction

Field dry unit weight:

iii) Degree of saturation at $\gamma_{d,max}$

Answer: $\gamma_{d,max} \approx 1.90$ Mg/m³, OMC $\approx 13\%$; RC $\approx 90.4\%$; $S \approx 83\%$ at maximum dry density.

The field soil has 35% water content, far wetter than OMC, so its RC is low and the sample is not properly compacted.

Placement water content is the water content at which the soil is placed and compacted in the field. It is usually specified relative to the OMC (for example OMC $\pm 2\%$). Dry-of-optimum placement gives higher strength and stiffness but higher permeability and swelling potential; wet-of-optimum placement gives lower permeability, lower strength and better flexibility, as needed in the core of an earth dam.

The theoretical maximum dry density is the dry density that a soil would have if all air were removed from the voids, i.e. at $S = 100\%$ for the given water content. It is represented by the zero air void (ZAV) line:

It is an upper limit of dry density: the actual compaction curve always lies below it, since in practice 2 - 5% air remains in the soil.

The zero air void (ZAV) line (or saturation line) is the curve showing the relation between dry unit weight and water content when the degree of saturation is 100% ($V_a = 0$):

It is the theoretical maximum dry unit weight for each water content. The compaction curve lies to its left and never crosses it, because air cannot be fully expelled by compaction.

Zero air void

The zero air void (ZAV) line is the plot of dry unit weight against water content for a fully saturated soil ($S=100\%$, no air):

It is the upper limit of the compaction curve.

Precautions during field compaction

General: use the correct roller, lift thickness (150 - 300 mm), number of passes; control the water content near OMC; check density with field tests (sand replacement, core cutter); remove stones and organic matter; maintain a uniform moisture by mixing; avoid over-compaction.

Homogeneous earth dam:

• Compact on the wet side of OMC (about OMC + 1 to 2%) to get a flexible, low-permeability fill that can adjust to settlement without cracking.
• Use sheep-foot or pneumatic rollers; bond each layer by scarifying the surface; keep a uniform water content; compact in thin layers; provide a filter drain to control seepage.
• Avoid over-wetting, which would cause pore pressure build-up and slope instability during construction.

Highway subgrade:

• Compact on the dry side or at OMC to get high strength and stiffness (CBR); the top 300 mm to be compacted to 95 - 100% of the standard/modified Proctor MDD.
• Protect from rain and keep drainage working, because wetting later can cause swelling and strength loss; moisture can be adjusted to OMC by sprinkling or aeration.
• Use vibratory rollers for granular soils.

Dry side or wet side?

For earth dams and canal linings (impermeability and flexibility), prefer the wet side of OMC. For highway subgrade and where strength is important, prefer the dry side up to the OMC, as the soil has higher strength, but there is risk of swelling and water absorption if the soil gets wet later.

It is not fully possible to maintain the exact optimum moisture content in the field, but it can be kept within about $\pm 2\%$ of the OMC.

Reasons:

• Water content changes after sprinkling, because of evaporation by sun and wind or addition by rain during hauling, spreading and rolling.
• The borrow soil is non-uniform, so moisture is variable and cannot be mixed perfectly.
• The field OMC differs from the laboratory OMC, since the field compaction energy and method are different.
• Water content is checked only in selected spots, and the test results take time.

Compaction

Compaction is the process of increasing the density of soil by reducing the air voids using mechanical energy such as rolling, ramming or vibration, with no change in water content.

Compaction versus consolidation

Methods of compaction

Field compaction methods

1. Tamping (rammers)
2. Smooth-wheel (static) rolling
3. Sheep-foot (tamping foot) rolling
4. Pneumatic-tyred rolling
5. Vibratory rolling
6. Dynamic compaction (heavy tamping), vibroflotation (deep)

Compaction curves: Standard and Modified Proctor

The Modified Proctor test uses more energy (heavier hammer and greater drop, 5 layers), giving a higher MDD and lower OMC than the Standard Proctor test (3 layers).

 gd
  |        .-"-.   Modified
  |       / MDD2 \
  |      /  .-"-. \ Standard
  |     /  / MDD1\ \
  |    /  /       \ \  ZAV line
  |   /  /         \ \ (above)
  +-----|---|------------ w
       OMC2 OMC1

Notes: The peak of the Modified Proctor curve is higher and to the left (OMC$_2$ < OMC$_1$).

A compaction curve plots dry unit weight ($\gamma_d$) against water content ($w$) for a fixed compaction effort. The dry unit weight rises with $w$ up to a peak, then falls.

 gd
  |                    ZAV line
  |         MDD  .-"-.  .
  |  . - - - - -/     \   .
  |            /       \    .
  |          /          \
  |  dry   /      wet     \
  | side  /       side
  +------+----------------------- w
       OMC

Features:

• Maximum dry density (MDD): the peak value of $\gamma_d$ at the top of the curve.
• Optimum water content (OWC or OMC): the water content corresponding to MDD.
• Dry side of optimum: the left of the peak, where $w <$ OMC. The soil is stiff, and has flocculated structure.
• Wet side of optimum: the right of the peak, where $w >$ OMC. The soil is dispersed with lower strength; the curve approaches the ZAV line.
• Zero air void (ZAV) line: $\gamma_d = G\gamma_w/(1+wG)$ for $S = 100\%$; the compaction curve lies always below and to the left of it.

The ZAV line is drawn using the specific gravity of the solids; it lies above the compaction curve as air always remains.

 gd
  |  Sand ___.-~~~~~-.___
  |  (flat, dips)
  |
  |        Clay .-"-.
  |            /     \
  +-------------------- w

In sand, the highest density is obtained when it is either completely dry or fully saturated; the minimum occurs at about 4 - 8% water content because of bulking (capillary tension holds particles apart). In clay the density follows the typical bell-shaped curve with a clear optimum.

Given

$\gamma_{d} = 18$ kN/m³, $w = 15\%$, $G = 2.65$, $\gamma_w = 9.81$ kN/m³.

Degree of saturation

Further compaction

The maximum value the dry unit weight can reach at the same water content is when all air is expelled ($S = 100\%$), on the ZAV line:

Answer: $S = 89.5\%$; the soil can at most be compacted to 18.60 kN/m³ (in practice slightly less, since some air always remains).

Given

$D = 10$ cm, $H = 20$ cm, $G = 2.65$, $w = 15\%$, $\gamma_w = 9.81$ kN/m³.

Assumption: the weight "30 kN" is a misprint for 30 N, since 30 kN in this small mould would give an impossible unit weight of about 19,000 kN/m³.

Volume and unit weights

Void ratio

Degree of saturation

(ii) Field requirement

Taking the specimen's dry unit weight as the maximum (laboratory) dry unit weight:

Answer: (i) $\gamma_d = 16.61$ kN/m³, $e = 0.565$, $S = 70.3\%$; (ii) field $\gamma_d \ge 15.78$ kN/m³.

Degree of compaction (relative compaction) is $RC = \gamma_{d,field}/\gamma_{d,max}$. First find $\gamma_{d,max}$ from the lab test, then compare with the field value.

Lab dry density (mould volume = 950 cm³)

Plotting $\gamma_d$ against $w$ gives a peak at

• $\gamma_{d,max} \approx 1.865$ g/cc, OMC $\approx 14.6\%$

Check of the embankment

Required dry density $= 0.95 \times 1.865 = 1.77$ g/cc.

Answer: RC = 95.4% (> 95%), field dry density 1.78 g/cc > required 1.77 g/cc. The embankment construction is satisfactory.

The margin is small, so the result is acceptable but only just; the moisture content of the field fill should also be checked to be near the OMC.

Sand cone test: volume of the hole = mass of sand filling it / density of sand. Then the field density and water content follow. The specification requires both

• $\gamma_d \ge 0.95\,\gamma_{d,max} = 0.95 \times 1.95 = 1.8525$ Mg/m³
• $w$ within $13.5 \pm 2\%$, that is 11.5% to 15.5%

Location 1

Location 2

Comparison

Answer: The specification is not satisfied. Location 1 fails on density (RC 92.8% < 95%) though its water content is acceptable. Location 2 is just below 95% (94.9%) and its water content (16.05%) is above the 15.5% limit. Both areas need re-rolling (and drying at location 2).

The masses are taken as grams, as given; the result depends only on their ratios.