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The Earth Shrinkage Test Guide

 

An easy test to indicate earthwork shrinkage under controlled conditions.

 

1. Use a open form or box with smooth inner sides ( so that the test block can slip as it shrinks) measuring approximately 100mm wide, 100mm deep, but an accurate 1000mm long, as the length is where the measuring will be done. The testing box could be made from form-board, or alternatively, made from PVC storm pipe cut down its length and two caps glued to the ends, or plain timber sealed inside with a gloss coat.

Stormwater pipe cut to 1000 millimetres and capped

( click to enlarge) 

2. Apply form-release agent to the box (clean motor oil will suffice), and add earth mix and cement, and screed off excess. Leave to set overnight.

filling with testing samples The bottom test is loam only, the top has 50% loam and 50% clean sand.

( click to enlarge) 

3. The next day, you may observe that the block has already done some shrinking. You may take a measurement now if you wish. If the block has broken in two pieces, this may be due to insufficient slip in the testing box, and is of little concern. Numerous cracks however, in the case of the ‘loam only’ example (in figure right) would revealed a problem with the earth sample itself.

The next morning. note cracking in the 'loam only' test.

( click to enlarge) 

4. Keep the block damp for the first 7 days at least . Lean the testing boxes against a wall to keep the shrinkage gap localized to one point, and easy to measure. After 4 weeks measure blocks again, and compare the results. You should aim for a shrinkage of less than 4 mm over the 1000 mm. To reduce the amount of shrinkage, adjust the earth by adding friable sand. Note, measurements can fluctuate according to the levels of moisture in the block at any one time.

After 30 days sand and loam has contracted only 1 to 2 millimetres After 30 days the loam only test has contracted 4 to 5 millimetres

( click to enlarge) 

Seek expert advice if you remain uncertain about the suitability of your earth. There are also numerous publications about soil compositions and earth building available.


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