Test Pitting

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Trench excavation

"Trenching and test pitting are simple methods of shallow exploration of easily excavated rock or soil materials. Visual inspection of a wide section of strata is of great value in logging profiles and selecting samples. If bedrock is anticipated at a shallow depth, trenches, and test pits should be located on the centerline of the proposed structure and dug parallel with it. If bedrock is not at shallow depths, deep trenches, or test pits should be offset from the centerline to avoid damaging the foundation of the structure. Shallow trenches or test pits may be dug adjacent to the centerline for correlation purposes."

"Test pits are an effective means to explore and sample earth foundations and construction materials and to facilitate inspection, sampling, and testing. The depth of a test pit is determined by investigation requirements but is usually limited to the depth of the water table. Dragline, backhoe, clamshell, caisson drilling or auger equipment, and bulldozer pits are usually more economical than digging pits by hand for comparatively shallow materials explorations. Explosives may be required to break up large boulders. At the surface, the excavated material should be placed in an orderly manner around the pit to indicate depth of pit from which the material came to facilitate accurate logging and sampling. The moisture condition should be determined and recorded before drying occurs by exposure to air."

"Investigations in open, accessible explorations such as test pits, large diameter borings, trenches, and tunnels are inherently hazardous. Federal, State, and local regulations must be followed when planning and executing accessible investigations. Occupation Safety and Health Act (OSHA) regulations for excavation safety (29 CFR 1926.650-652) should be consulted prior to planning accessible explorations. Regulations require that competent personnel plan, design, and monitor excavations. Excavations greater than 5 feet in depth normally require sloping or shoring systems designed by professional engineers. Large diameter boring and deep trenches may be considered to be confined space and may require special ventilation, monitoring, and rescue safety equipment."

"Where pits or trenches penetrate or pass through foundation materials, trenches are backfilled and compacted to the density of the original in-place material. It is recognized that certain limitations exist in the use of trenching and test-pit excavating equipment for compacting fill material. However, every practical effort should be made to reestablish the in-place densities of foundation materials."

Definitions

• "Trenches - Trenches are long, narrow excavations. They are advantageous for studying earth materials on steep slopes and in exposed faces. Trenches made by power equipment, such as backhoes, power shovels, and bulldozers, may require hand trimming of the sides and bottom to reach relatively undisturbed material. The method is of particular value in delineating the rock surface beneath the principal spillway and in abutments and in exploring auxiliary spillway materials. Trenching may be the most feasible method of investigation in materials containing cobbles or boulders. Trenches may yield valuable information on potential rock excavation and core trench depth along the centerline of the structure, depending on its design."

• "Test pits - Test pits are large enough to accommodate a person with sampling equipment. They may be excavated by hand or by power equipment such as a clam-shell or orange-pool bucket. Power equipment should be used only for rough excavation and with extreme caution when approaching the depths at which undisturbed samples are to be taken. Cribbing is required in trenches and pits of depths of 5 feet or greater."

Best Practices Resources

National Engineering Handbook: Chapter 5 - Engineering Geology Logging, Sampling, and Testing, NRCS

Design of Small Dams, USBR

Citations:

Revision ID: 7062
Revision Date: 07/10/2023