Measuring the soil’s drainage capacity (K value)


Measuring the soil’s drainage capacity (K value)

The K value in the soil is the speed at which water is able to seep into the actual soil where the drywell is to be placed. This value is easy to measure.

Simple way to find out the soil type:

1.   Use a post hole digger to dig a 1 meter deep hole where you want to place the drywell.
2.   Fill the hole with water and saturate the soil – after 5 minutes, refill the hole.
3.   After 15 minutes, measure how much the water level has fallen:
  ♦  If the water has disappeared completely, it is coarse sand 
(K value = 1 x 10-3 m/sec.).
  ♦  If the water level has fallen by at least 10 cm, it is fine sand 
(K value = 1 x 10-4 m/sec.).
  ♦  If the water level has fallen by at least 1 cm, it is silt 
(K value = 1 x 10-5 m/sec.).
4.   If the water level has not fallen after 15 minutes you must measure at longer intervals:
  ♦  If the water level has fallen by only 1 cm after 1 hour, it is
sandy clay
(K value = 1 x 10-6 m/sec.).
  ♦  If it takes 5 hours or more for the water to fall 1 cm, then it is
silty clay
(K value = 1 x 10-7 m/sec.).
5.   Based on the measured seepage speed you can see how big the actual drywell must be using the table dimensions below:

The following number of blocks must be used:

 Drainage area: 100 m2
 Soil type K value Length of drywell with…
– whole BIO-BLOK® elements – half BIO-BLOK® elements
 Coarse sand 1 x 10-3 1.08m  =  2 pcs. 2.16m  =  2 pcs.
 Fine sand 1 x 10-4 3.24m  =  6 pcs. 4.32m  =  4 pcs.
 Fine soil (silt) 1 x 10-5 6.48m  =  12 pcs. 10.80m  =  10 pcs..
 Sandy clay 1 x 10-6 10.48m  =  20 pcs. 18.36m  =  17 pcs.
 Silty clay 1 x 10-7 16.20m  =  30 pcs. 30.24m  =  28 pcs.
All values are approximate.

Some more complicated methods for calculating the seepage capacity:

♦    Dig a hole where you want to place the drywell to the depth required, e.g. 0.5 x 0.5m. Total area = 0.5 x 0.5 = 0,25m2
♦    Before doing the test you must saturate the soil. Then fill the hole with water, e.g. 0.5m deep. Water volume = 0.5 x 0.5 x 0.5 = 0,125m3
♦    You can then measure the water that disappeared per time unit. For example, the water may have gone within one hour, equivalent to 3,600 sec.
♦    You can then calculate the K value: K = m3 / (m2 x sec.) = m/sec.
          K = 0.125 / (0.25 x 3,600) = 1.4 x 10-4 m/sec.

This value can then be used for sizing the actual drywell.

Alternatively the K value can be measured with an infiltrometer. With this tool you can test the soil's capacity to receive the seeped water and then calculate how many litres of water will seep down per m2 per day.

The result from measuring with an infiltrometer can, for example, be 5 l/m2 x day. This result must be converted to a K value which is measured in m/sec. This is obtained by multiplying the result with a factor of 500 and the measurement result is given in m3.

K = 0.005 x 500 / 1 x 24 x 60 x 60 = 2.8 x 10-5 m/sec.

A factor of 1000 is only used in optimal seepage conditions, i.e. the drywell's location is far above the groundwater level.

For information or if you are interested in buying an infiltrometer, please contact Tønder Beton by phone: +45 74 72 17 33.

With numerous or large drywells the soil's seepage speed (K value) must always be measured as it gives a more precise measurement of the actual drywell.

Testing the soil’s seepage capacity

Studies of the soil’s seepage capacity can be carried out in two ways:

♦    Analysis of grading curve
♦    Infiltration test

Analysis of grading curve

Based on a grading curve for an actual soil it is possible to define the hydraulic conductivity empirically. The most used formula is Hazens formula:

     K = 116 x 10-4 x d102

d10 is the grain size in mm defined as the aperture of a screen which lets 10% of the weight of the sample through (materials with a diameter larger than 20mm are removed beforehand). The disadvantage is that a soil sample cannot be representative of a larger area and that such empirical formulae are burdened with great uncertainty.

Infiltration test

The method is based on there being a connection between the sinking speed for clean water (rainwater) in a pilot hole and the soil's infiltration capacity. Infiltration tests are used for determining a soil type's hydraulic conductivity.

Pilot hole
At least two test holes are excavated by the expected base for the rainwater drywell. The holes must be at least 0.25m x 0.25m and at least 0.3m deep. The holes must be situated at least 5m apart.

Forming a pilot hole

Water saturation of the soil
♦    Before doing the infiltration test you must saturate the soil. Put approximately 0.05m gravel in the bottom of the hole. Then fill the hole with water so that it reaches at least 0.2m above the gravel layer. Keep the hole filled for approx. 30 minutes.
♦    Stop the water input and measure the water's sinking speed.
♦    If the water level sinks by more than 0.2m in 15 minutes, stop the water saturation and perform the infiltration test.
♦    If the water level sinks by more than 0.2m in 15 minutes the water saturation must carry on until the sinking speed is almost constant. In practice, this can be done by measuring the sinking speed at approx. 30 minute intervals. When the difference of the sinking speed in two subsequent measuring periods is less than 0.005m, the water saturation can stop.

Measuring the infiltration capacity

♦    Adjust the water level in the hole so that there is 0.15 m above the gravel in the bottom.
♦    Put a straight edge over the hole and measure with a prober to the water surface.
♦    Now you can determine how far the water sinks in, for example, 10 minutes (you can use other times depending on the sinking speed).
♦    Then convert the sinking speed to m/sec.
♦    The sinking speed in m/sec is the soil's infiltration capacity for clean water.
♦    When defining the soil's infiltration capacity the worst (lowest) of the measured infiltration capacities should be used.


The infiltration test of two different samples shows that the water sinks 50mm in 10 minutes in test 1 and 60mm in 10 minutes in test 2.

The sinking speed for rainwater in test 1 in mm per second will then be:

50mm / 10 min. x 60 sec- = 0.0833mm/sec. = 8.3 x 10-5 m/sec.

and the sinking speed for rainwater in test 2 in mm per second will then be:

60mm / 10 min. x 60 sec. = 0,100mm/sec. = 10-4 m/sec.

The hydraulic conductivity to be used for sizing seepage systems for rain water is the lower of the two values of the whole system, i.e. 8.3 x 10-5 m/sec.

Source of test of ground's drainage capacity:
Drainage of rain water in drywells
Directions from Rørcentret (Danish Pipe Centre) 009
June 2005