9 - Compaction / Forest Road Drainage Installation Practices in the Pacific Northwest

Forest Road Drainage Installation Practices
in the Pacific Northwest

9 - Compaction	Base Preparation, Fill Compaction, Fill Depth

Base Preparation for Pipe Culverts

Benefits of Compaction

achieves higher fill density;
increases soil bearing strength;
provides lateral culvert support;
reduces pumping up of water;
prevents culvert piping failures.

Prepare the base for a pipe culvert by placing a layer of well-graded material at a slope appropriate for the culvert. Although rarely available, the ideal material is one inch minus unwashed crush rock or equivalent, but any pitrun gravel, till, ripped or shot rock will be suitable as long as it contains a high percentage of coarse, angular fragments. Soils with a high proportion of silt or clay are often impossible to compact. Remove round boulders and sharp angular rocks to avoid puncturing the pipe. Compact material with a compactor or at least the excavator bucket. Secure pipe with some fill on both sides to stop it from rolling, and check culvert slope before placing fill.

Fill Compaction for Pipe Culverts

Figure 1: Fill Compaction

Compacting fills around culverts provides a number of benefits (above) that minimize structure failures and reduce long-term maintenance costs. The bigger the structure the more critical is compaction. However, do not over-compact. After completing the base preparation, compact fill around the bottom of the pipe, i.e., the haunches. This is the most critical and difficult area to compact, and hand tamping may be needed. Uncompacted fill in this area will result in piping failures. Compact the fill in 30 cm (12 inch) lifts on both sides of the structure simultaneously, or with not more than one lift difference between sides; this avoids uneven forces on the culvert wall. As with the base preparation, ensure that large stones and sharp rocks do not touch the culvert, especially not on top of the pipe, to avoid puncturing the wall. Fill contaminated with moisture (water, snow, ice) or organics can not be properly compacted and will result in failures.

Wooden Culverts and Bridges

Compaction is important in behind the fill retaining walls such as wooden sill logs, log cribs, lock blocks or concrete ballast walls. Uncompacted fills in these areas settle and create safety hazards (potholes, washboards) which can also initiate damage to the bridge deck. Unevenly compacted fills can further weaken the integrity of the structure as bridge designs rely on equal support from both fills.

Fill Depth

Minimum fill depth on top of culvert pipes varies with vehicle load, materials used and compaction methods. Even on small culverts, always have at least 30 cm (12 inches) of well-graded compacted fill depth, more with lower quality fill or off-highway loads. As a rule of thumb, try to achieve a depth of 60cm, keeping in mind that wear and tear (traffic, grading) will reduce the fill depth over time. Always follow the specifications of manufacturers and professional designs.

9 - Compaction