The first step in the soils evaluation process is to dig test holes. For information on establishing the location of the test holes see the Septic System Design page. Per State and Municipal regulations, the new septic system will have to be placed at least 6 feet above the bottom of the test hole. In other words, if the test hole was 10 feet deep, the drain field could be no deeper than 4 feet, severely restricting the type of system that could be installed. For this reason, it is usually desirable to dig the test hole as deep as practical, or until bedrock or groundwater are encountered. Most test holes are excavated with a backhoe that is capable of digging to 16 or 18 feet deep.
Once the test holes are dug, the field technician will visually classify the type of soils observed. The technician will also look for the presence of bedrock, impermeable soils, or groundwater. In addition, he or she will identify the soil profile in/on which the septic system should be placed. A perforated pipe is then placed vertically in the excavation and the hole is back filled. This is referred to as the test hole monitoring tube and it is used to monitor for the presence of groundwater. The Municipality of Anchorage requires that groundwater levels be monitored for a period of at least 7 days.
It is important to note that groundwater levels can vary significantly, depending upon the locale and time of year. If the lot has high groundwater during winter conditions, to the extent that the installation of a septic system is marginal, it is likely the conditions will be unsuitable for an onsite septic system after spring run-off recharges and raises the groundwater table. In short, if the groundwater conditions are marginal for the installation of an onsite septic system, it is essential to know whether the data was obtained during winter or spring conditions. The importance of this cannot be overemphasized.
After the test hole is backfilled, the backhoe operator will then dig an additional hole off to the side. This depth is referred to as the "perk bench". The bench is excavated to the soil profile which was specified by the field technician. The percolation test is then performed in order to determine the application rate used for the design of the septic system. The percolation test involves digging a 6 inch diameter hole that is 12 inches deep into the bench that was excavated by the backhoe operator. This is usually done with a post hole digger. The hole is then filled with a standing head of water for a period of at least 4 hours. this is called the presoak period. The MOA will waive the requirement of a presoak for the soil if when filled with water to a depth of 12 inches, two times, it will drain away completely in less than 10 minutes each time. For most soils, a presoak is required.
After the presoak period the percolation test is performed. The "perk" test is performed by filling the perk hole with 8 inches of water and measuring how much the water level drops in 30 minutes. If all of the water drains away in less than 30 minutes, the time interval is changed to 10 minute readings. At the end of the 30 minute period, the hole is again filled to the 8 inch level and the process repeated. A minimum of three 30 minute readings are taken. From this, the percolation rate is calculated by dividing 30 minutes by the drop in inches during the 30 minute period. The following is a general classification of the percolation rates:
- Less than 1 minute per inch: Coarse sands/gravels that may require the installation of a sand filter to slow down the flow of waste water through the soil
- 1-5 minutes per inch: Excellent
- 6-15 minutes per inch: Good to Fair
- 15-30 minutes per inch: Fair
- 30-60 minute per inch: Fair to Poor
- 60 - 120 minutes per inch: poor
- Greater than 120 minutes per inch: Unsuitable for an onsite septic system
With the soil profile identified, the groundwater levels established, and the percolation rate determined, the engineer can then proceed with the septic system design
Purchasing Undeveloped Land?
If possible, negotiate the transaction so that the sellers pay for the soils testing (however, you should retain the right to select the engineer), with the agreement that you will reimburse them for their costs upon closing of the sale, assuming the test results are acceptable. Bear in mind that most sellers will not be receptive to this idea. Nonetheless, by heeding this advice, you won't risk investing money into a piece of land that turns out to be undeveloped.
In addition, you should retain the right to accept, or reject, the suitability of the percolation results. The reason for this is that if you make your only stipulation that "the lot be suitable for installing an onsite well and septic system", you may be bound to purchase a lot that, although suitable, will require an investment of $30,000 - $50,000 worth of engineering, drainage systems, and "high-tech" septic system technology in order to make it developable. If this turns out to be the case, then you want to right to re-negotiate the price of the lot.