When Things Go Wrong Structural Stress and Fatigue

When a structure is poorly designed or built, it may not be able to withstand all of the forces it has to face. When a structure has to face large

combinations of internal and external forces over a long period of time, the structure might weaken. This may result in structural stress. At first, signs of structural stress may disappear when the internal and external forces are reduced. For example, if you place an abnormally large book on the middle of a bookshelf, the shelf might bend. The

bend in the shelf is a sign of stress. When the book is removed,

the shelf may go back to its original shape. However, if the

shelf cannot withstand the stress, it might crack. Permanent

changes, like the bookshelf cracking, are signs of structural

How to Determine The Liquid Limit Of Soil

This test is done to determine the liquid limit of soil as per IS: 2720 (Part 5) – 1985. The liquid limit of fine-grained soil is the water content at which soil behaves practically like a liquid, but has small shear
strength. It’s flow closes the groove in just 25 blows in Casagrande’s liquid limit device. The apparatus used :-
i) Casagrande’s liquid limit device
ii) Grooving tools of both standard and ASTM types
iii) Oven
iv) Evaporating dish
v) Spatula
vi) IS Sieve of size 425µm
vii) Weighing balance, with 0.01g accuracy
viii) Wash bottle
ix) Air-tight and non-corrodible container for determination of moisture content
PREPARATION OF SAMPLE
i) Air-dry the soil sample and break the clods. Remove the organic matter like tree roots, pieces of bark, etc.
ii) About 100g of the specimen passing through 425µm IS Sieve is mixed thoroughly with distilled water in the evaporating dish and left for 24hrs. for soaking.



Procedure to Determine The Liquid Limit Of Soil
i) Place a portion of the paste in the cup of the liquid limit device.
ii) Level the mix so as to have a maximum depth of 1cm.
iii) Draw the grooving tool through the sample along the symmetrical axis of the cup, holding the tool perpendicular to the cup.
iv) For normal fine grained soil: The Casagrande’s tool is used to cut a groove 2mm wide at the bottom, 11mm wide at the top and 8mm deep.
v) For sandy soil: The ASTM tool is used to cut a groove 2mm wide at the bottom, 13.6mm wide at the top and 10mm deep.
vi) After the soil pat has been cut by a proper grooving tool, the handle is rotated at the rate of about 2 revolutions per second and the no. of blows counted, till the two parts of the soil sample come into contact for about 10mm length.
vii) Take about 10g of soil near the closed groove and determine its water content
viii) The soil of the cup is transferred to the dish containing the soil paste and mixed thoroughly after adding a little more water. Repeat the test.
ix) By altering the water content of the soil and repeating the foregoing operations, obtain at least 5 readings in the range of 15 to 35 blows. Don’t mix dry soil to change its consistency.
x) Liquid limit is determined by plotting a ‘flow curve’ on a semi-log graph, with no. of blows as abscissa (log scale) and the water content as ordinate and drawing the best straight line through the plotted points.

REPORTING OF RESULTS
Report the water content corresponding to 25 blows, read from the ‘flow curve’ as the liquid limit.
A sample ‘flow curve’ is given as