Table of Contents
liquid limit definition
There are 4 definitions available for liquid limit, we will discuss one by one.
Definition 1 (Casagrande practically definition )
The minimum moisture content in percentage (%) at which a cohesive soil changes from liquid state to plastic (flexible) state is called the liquid limit of soil.
Definition 2 (atterberg theoretical definition)
The minimum moisture content in percentage (%) at which a cohesive soil change from plastic to a liquid state is called the liquid limit of soil.
Definition 3
The minimum moisture content in percentage (%) at which a cohesive soil behaves like a viscous fluid is called the liquid limit of soil.
Definition 4 (lab definition)
In the lab, The liquid limit is defined as the minimum moisture content in percentage (%) at which a cohesive soil begins to behave as a liquid material and begins to flow under its own weight is known as the liquid limit.
Introduction to Liquid limit of soil
The liquid limit test is one of the most widely used tests in the soil engineering practice. Several properties, including mechanical properties (for example compressive index) have correlations with the liquid limit.
liquid limit apparatus
 | Porcelain evaporating dish |
 | No. 40 (0.425 mm) sieve |
 | Spatula |
 | Balance sensitive to 0.01 g |
 | Watering bottle, with distilled, dematerialized or tap water. |
 | Moisture drying containers with lid |
 | Liquid Limit Device (Cassagrande apparatus) |
 | Grooving Tool |
 | Oven a thermostatically controlled drying oven capable of maintaining temperatures of 230 ± 9 0F (110 ±5 oC). |
 | Desiccator |
Procedure
- Check and adjust the height of the drop of cup of the Cassagrande apparatus equal to 10 ± 0.2-mm.
- Take a sample of about 1000-g. from a thoroughly mixed portion of the material passing the No. 40 sieve, (0.425-mm).
- Further additions of water shall be made in increments of sufficient measured.
- When sufficient water has been thoroughly mixed with the soil to form a uniform paste of stiff consistency, a sufficient quantity of this paste shall be placed in the cup and spread with the spatula so that the depth of the paste in the middle of the cup (the point of maximum thickness) is equal to10-mm.
- The excess soil shall be returned to the mixing dish and covered to retain the moisture in the sample.
- The soil in the cup of the device shall be divided by a firm stroke of the grooving tool along the diameter so that a clean sharp groove is formed.
- The cup containing the sample prepared shall be lifted and dropped by turning the lever at a rate of approximately 2+0.1 revolutions per second until the two sides of the sample come in contact at the bottom of the groove along a distance of about ½ inches (13-mm).
- The number of shocks (blows) required to close the groove shall be recorded.
- Take a sample to determine the moisture content.
- The soil remaining in the cup shall be transferred to the mixing dish. The cup and the grooving tool shall then be washed and dried in preparation for the next trial.
- Repeat the procedure for at least four times and record the number of blows and the corresponding moisture content in each case. The object of this procedure is to obtain samples of such consistency that at least one determination will be made in each of the following range of drops: 25-35, 20-30, 15-25, so the range in the three determinations is at least 10 drops. The number of drops required to close the groove should be above and below 25.
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liquid limit graph or Flow Curve
The graph between the log of a number of blows on the X-axis and moisture content on the Y-axis is known as the liquid limit graph or flow curve.
Preparation of Flow Curve
A Flow Curve representing the relation between moisture content and the corresponding number of blows shall be plotted on a semi-logarithmic graph. The flow curve shall be a straight line drawn as nearly as possible through the plotted points.
Test Observations and Calculations:
After each trial when the groove is closed, the sample for determination of moisture content shall be taken in the containers for moisture content determination.
MATHEMATICAL FORMULAS
water content (%) = weight of water/weight of dry soil sample x 100
weight of water = (weight of container+wet sample) – weight of container+dry sample
weight of dry soil sample = (weight of container+dry sample) – (weight of the empty container)
The steps for the determination of moisture content are given in the following table. For the drying of the sample, it is placed in a thermostatically controlled oven with a temperature setting from 110 ± 5 0C.
liquid limit table
| Trial No | 1 | 2 | 3 |
|---|---|---|---|
| Dish No | T-10 | T-19 | T-9 |
| weight of Dish (gm) | 39.7 | 40.3 | 49 |
| Weight of Dish + wet sample (gm) | 78.6 | 78.9 | 90.1 |
| weight of Dish + dry sample (gm) | 69.7 | 69.4 | 79.4 |
| weight of water (gm) | 8.9 | 9.5 | 10.7 |
| weight of dry soil sample (gm) | 30 | 29.1 | 30.4 |
| Moisture content (%) | 29.67 | 32.65 | 35.20 |
| No of blows | 35 | 24 | 17 |
Liquid Limit Formula
The value of moisture content measured on the y-axis of the logarithm graph corresponding to the intersection of the flow curve with the 25 No of blows from the x-axis is the liquid limit of soil.
So from the above-plotted graph, the liquid limit is 32
