Soil Compaction Tests

1) The Sand Cone Method

The sand-cone method is used to determine in the field the density of compacted soils in earth embankments, road fill, and structure backfill, as well as the density of natural soil deposits, aggregates, soil mixtures, or other similar materials. It is not suitable, however, for soils that are saturated, soft, or friable (crumble easily).

Characteristics of the soil are computed from

Volume of soil, ft³ (m³)=[weight of sand filling hole, lb (kg)] /[ Density of sand, lb/ft³ (kg/m³)]

% Moisture = 100(weight of moist soil – weight of dry soil)/weight of dry soil

Field density, lb/ft³ (kg /m³)=weight of soil, lb (kg)/volume of soil, ft³ (m³)

Dry density=field density/(1 + % moisture / 100)

% Compaction=100 (dry density)/max dry density

Maximum density is found by plotting a density–moisture curve.

2) Load-Bearing Test

One of the earliest methods for evaluating the in situ deformability of coarse-grained soils is the small-scale load-bearing test. Data developed from these tests have been used to provide a scaling factor to express the settlement r of a full-size footing from the settlement r1 of a 1-ft²(0.0929-m²) plate. This factor r /r1 is given as a function of the width B of the full-size bearing plate as

r/r1 = ( 2B / 1 + B )²

From an elastic half-space solution, E’s can be expressed from results of a plate load test in terms of the ratio of bearing pressure to plate settlement k_v as

K_v ( 1 – m² ­ ) p / 4

E’s = ___________________

4B / ( 1 + B )²

where m represents Poisson’s ratio, usually considered to range between 0.30 and 0.40. The E’s equation assumes that r1 is derived from a rigid, 1-ft(0.3048-m)-diameter circular plate and that B is the equivalent diameter of the bearing area of a full-scale footing. Empirical formulations, such as the r /r1 equation, may be significantly in error because of the limited footing-size range used and the large scatter of the database. Furthermore, consideration is not given to variations in the characteristics and stress history of the bearing soils.

3) California Bearing Ratio

The California bearing ratio (CBR) is often used as a measure of the quality of strength of a soil that underlies a pavement, for determining the thickness of the pavement, its base, and other layers.
CBR = F / F_o

where

F = force per unit area required to penetrate a soil mass with a 3-in² (1935.6-mm² ) circular piston (about 2 in (50.8 mm) in diameter) at the rate of 0.05 in/min (1.27 mm/min);

F_v = force per unit area required for corresponding penetration of a standard material.

Typically, the ratio is determined at 0.10-in (2.54-mm) penetration, although other penetrations sometimes are used. An excellent base course has a CBR of 100 percent. A compacted soil may have a CBR of 50 percent, whereas a weaker soil may have a CBR of 10.

4) Soil Permeability

The coefficient of permeability k is a measure of the rate of flow of water through saturated soil under a given hydraulic gradient i, cm/cm, and is defined in accordance with Darcy’s law as
V = kiA

where V = rate of flow, cm³ /s, and A = cross-sectional area of soil conveying flow, cm² .

Coefficient k is dependent on the grain-size distribution, void ratio, and soil fabric and typically may vary from as much as 10 cm /s for gravel to less than 10–7 for clays. For typical soil deposits, k for horizontal flow is greater than k for vertical flow, often by an order of magnitude.