e septic tank system installers place vertical standpipes deep into the gravel at one or both ends of a trench. Although there is no evidence that the trench functions any better because of these pipes, they do serve to mark the location of the trench, to allow monitoring of the performance of the trench, and to provide access for pumping out the trench and, should it be necessary, adding restorative chem- icals (see Chapter 8). Materials 1. Three-fourths to 2.5 in. gravel. Total depth of gravel times width of trench times length of trench in feet, all divided by 27, gives the number of cubic yards needed. 2. Four-inch Schedule 40 PVC (ASTM D2665-68), ABS (ASTM D2661-73), clay pipe (ASTM C700-75), or other drain pipe. Solid pipe for connections and risers and perforated pipe for the trenches. Perforations are 3/8 in. to 3/4 in. holes. Common spacing for the holes is 3 in. apart, but greater spacing (on the order of 12 in. or more) should provide more even distribution. 3. Elbows and tees as needed. Cement for connections. Forty- to 60- lb. untreated building paper, geotechnical fabric, or other decom- posable liner. 4. Shovel, rake, level, and string. Construction Before you lift a shovel, be sure that your absorption system will be located to meet setback requirements, will not collect water from rainfall, and will be able to be gravity fed from the septic tank. Also, consider the availability of space to locate a replacement field, should one become necessary. Attention to environmental and physical details now can save money and frustration later. Trench location can be marked by sprinkling flour, lime, or some other powder in an outline. Remember the advice to the amateur carpenter: "Measure twice, cut once." And remember also the advice to anyone hoping to move water from a hole into the sur- rounding soil: "Don't dig when the soil is damp." The first construction problem is how to get the trench bottom level. The answer varies with the installer. On flat land an experi- enced backhoe operator can come prett