The question asks to identify the specific diagram used in the liquid limit test, where water content is displayed on the y-axis and the number of strokes (blows) is shown on the x-axis. This plot is essential for determining a key engineering property of fine-grained soils.

Liquid Limit Test Fundamentals

The liquid limit test is a standard laboratory procedure in geotechnical engineering used to define the boundary between the plastic and liquid states of a fine-grained soil. This boundary is known as the liquid limit (\(LL\)), which represents the minimum water content at which soil behaves like a liquid and begins to flow under its own weight or a slight applied pressure.

• The test is performed using a Casagrande liquid limit device.
• A soil paste is prepared and placed in a brass cup, and a standard groove is cut through the center of the soil mass.
• The cup is then repeatedly dropped from a fixed height onto a rubber base by turning a crank.
• The number of blows (strokes) required for the two halves of the soil cake to come into contact over a distance of 12.7 mm (0.5 inches) at the bottom of the groove is recorded.
• At this point, a small sample of the soil is taken to determine its water content.

Flow Curve: The Key Diagram

To accurately determine the liquid limit, the test is typically repeated several times (usually 3-4 times) with the same soil, but at varying water contents. This results in different numbers of blows required to close the groove. The pairs of data (water content and corresponding number of blows) are then plotted on a specific type of graph called a Flow Curve.

• The water content (in percent) is plotted on the arithmetic (linear) y-axis.
• The number of blows (strokes) is plotted on the logarithmic x-axis.
• A best-fit straight line is drawn through these plotted points.

The liquid limit is then determined from this flow curve by identifying the water content corresponding to 25 blows. This is the standardized number of blows for defining the liquid limit.

Significance of the Flow Curve

The term "flow curve" aptly describes the diagram because it graphically illustrates the relationship between the soil's water content and its resistance to flow or deformation under a standard impact. It essentially shows how the soil's consistency changes with water content, moving towards a more fluid state. A steeper flow curve indicates that the soil loses its strength more rapidly with increasing water content.

Analysis of Other Options

• Saturation curve: This term is not associated with the liquid limit test plot. A saturation curve typically shows the relationship between water content and the degree of saturation of a soil, often used in compaction studies or for soil-water characteristic curves.
• Plasticity curve: While the liquid limit is related to soil plasticity, there is no standard plot specifically termed a "plasticity curve" in the context of the liquid limit test. Plasticity itself is a range of water contents, not a single curve from this test.
• liquid curve: This is not a recognized or standard technical term in soil mechanics for the specific plot described in the question. The correct and widely accepted term is "flow curve".

Therefore, the diagram where water content is plotted on the y-axis and the number of strokes on the x-axis for a liquid limit test is correctly known as the Flow Curve.