Percolation Testing
Percolation Testing
Once the site has been surveyed, is to perform percolation tests. While performing the tests, observe the soil characteristics and watch for groundwater within the test area. The site may be considered unsuitable if the following occurs: the soil appears to have too much sand or clay; groundwater is encountered; and/or the percolation rates are too slow. If the site is determined to be unsuitable, the septic system will need to be relocated. If another location cannot be found, then an alternative treatment system will need to be designed.
Percolation testing may be carried out with a shovel, posthole digger, solid auger or other
appropriate digging instruments. Percolation tests shall be accomplished uniformly throughout
the area where the absorption field is to be located. Percolation tests determine the acceptability
of the site and serve as the basis of design for the liquid absorption.
(1) Three or more tests will be made in separate test holes uniformly spaced over the proposed absorption field site. The average of the six tests shall be determined and will be used as the final result.
(2) Dig or bore a hole to the required depth of the proposed trenches or bed, with dimensions
necessary to enable visual inspection during percolation testing.
(3) Carefully scratch the bottom and sides of the excavation with a knife blade or sharp-pointed instrument to remove any smeared soil surfaces and to provide a natural soil interface into which water may percolate. Add 50 mm of gravel (of the same size that is to be used in the absorption field) to the bottom of the hole. In some types of soils the sidewalls of the test holes tend to cave in or slough off and settle to the bottom of the hole. It is most likely to occur when the soil is dry or when overnight soaking is required. The caving can be prevented and more accurate results obtained by placing in the test hole a wire cylinder surrounded by a minimum 25 mm layer of gravel (of the same size that is to be used in the absorption field.)
(4) Carefully fill the hole with clear water to a minimum depth of 300 mm above the gravel or sand. Keep water in the hole at least 4 hours and preferably overnight. In most soils it will be necessary to augment the water as time progresses. Determine the percolation rate 12 to 24 hours after water was first added to the hole. In sandy soils containing little clay, this pre-filling procedure is not essential and the test may be made after water from one filling of the hole has completely seeped away.
(5) The percolation-rate measurement is determined by one of the following methods:
(a) If water remains in the test hole overnight, adjust the water depth to approximately 150 mm above the gravel. Measure the drop in water level over a 30-minute period. This drop is used to calculate the percolation rate.
(b) If no water remains in the hole the next day, add clean water to bring the depth to approximately 150 mm over the gravel. From the batter board, measure the drop in water level at 30-minute intervals for 4 hours, refilling to 150 mm over the gravel as necessary. The drop in water level that occurs during the final 30-minute period is used to calculate the percolation rate.
(c) In sandy soils (or other soils in which the first 150 mm of water seeps away in less than 30 minutes after the overnight period), the time interval between measurements will be taken as 10 minutes and the test run for 1 hour. The drop in water level that occurs during the final 10 minutes is used to calculate the percolation rate.
The percolation rate is the number of minutes it takes to drop 25 mm. Table lists percolation rates and the corresponding absorption field sizing factor (liters/m2/day). The sizing factors are used, in conjunction with average daily demand (ADD), to determine the size of the absorption field. The following is an example of how to calculate the percolation rate:
Example 1: Calculating Percolation Rates - In 30 minutes, the measured drop in the water
level is 15 mm.
Minutes/25 mm = Time/(drop/25 mm) = 30 minutes/(15 mm/25 mm) = 50 Minutes/25 mm
where,
Minutes/25 mm = Minutes for water to drop 25 mm.
Once the site has been surveyed, is to perform percolation tests. While performing the tests, observe the soil characteristics and watch for groundwater within the test area. The site may be considered unsuitable if the following occurs: the soil appears to have too much sand or clay; groundwater is encountered; and/or the percolation rates are too slow. If the site is determined to be unsuitable, the septic system will need to be relocated. If another location cannot be found, then an alternative treatment system will need to be designed.
Percolation testing may be carried out with a shovel, posthole digger, solid auger or other
appropriate digging instruments. Percolation tests shall be accomplished uniformly throughout
the area where the absorption field is to be located. Percolation tests determine the acceptability
of the site and serve as the basis of design for the liquid absorption.
(1) Three or more tests will be made in separate test holes uniformly spaced over the proposed absorption field site. The average of the six tests shall be determined and will be used as the final result.
(2) Dig or bore a hole to the required depth of the proposed trenches or bed, with dimensions
necessary to enable visual inspection during percolation testing.
(3) Carefully scratch the bottom and sides of the excavation with a knife blade or sharp-pointed instrument to remove any smeared soil surfaces and to provide a natural soil interface into which water may percolate. Add 50 mm of gravel (of the same size that is to be used in the absorption field) to the bottom of the hole. In some types of soils the sidewalls of the test holes tend to cave in or slough off and settle to the bottom of the hole. It is most likely to occur when the soil is dry or when overnight soaking is required. The caving can be prevented and more accurate results obtained by placing in the test hole a wire cylinder surrounded by a minimum 25 mm layer of gravel (of the same size that is to be used in the absorption field.)
(4) Carefully fill the hole with clear water to a minimum depth of 300 mm above the gravel or sand. Keep water in the hole at least 4 hours and preferably overnight. In most soils it will be necessary to augment the water as time progresses. Determine the percolation rate 12 to 24 hours after water was first added to the hole. In sandy soils containing little clay, this pre-filling procedure is not essential and the test may be made after water from one filling of the hole has completely seeped away.
(5) The percolation-rate measurement is determined by one of the following methods:
(a) If water remains in the test hole overnight, adjust the water depth to approximately 150 mm above the gravel. Measure the drop in water level over a 30-minute period. This drop is used to calculate the percolation rate.
(b) If no water remains in the hole the next day, add clean water to bring the depth to approximately 150 mm over the gravel. From the batter board, measure the drop in water level at 30-minute intervals for 4 hours, refilling to 150 mm over the gravel as necessary. The drop in water level that occurs during the final 30-minute period is used to calculate the percolation rate.
(c) In sandy soils (or other soils in which the first 150 mm of water seeps away in less than 30 minutes after the overnight period), the time interval between measurements will be taken as 10 minutes and the test run for 1 hour. The drop in water level that occurs during the final 10 minutes is used to calculate the percolation rate.
The percolation rate is the number of minutes it takes to drop 25 mm. Table lists percolation rates and the corresponding absorption field sizing factor (liters/m2/day). The sizing factors are used, in conjunction with average daily demand (ADD), to determine the size of the absorption field. The following is an example of how to calculate the percolation rate:
Example 1: Calculating Percolation Rates - In 30 minutes, the measured drop in the water
level is 15 mm.
Minutes/25 mm = Time/(drop/25 mm) = 30 minutes/(15 mm/25 mm) = 50 Minutes/25 mm
where,
Minutes/25 mm = Minutes for water to drop 25 mm.
Table: Soil Treatment Areas in Square Meters
Percolation Rate, Water Absorption of Soil
Minutes for Water to (m2/liters/day)
Drop 25 mm
Faster than 0.1 Soil too coarse for sewage treatment
0.1 to 5 0.020
6 to 15 0.031
16 to 30 0.041
31 to 45 0.049
46 to 60 0.054
Slower than 60 Soil too fine for sewage treatment
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