Shafts 2 and 6
Shafts 2 and 6 were designed as structurally sound shafts, with no built in defects. However, during the construction of Shaft 2, the contractor reported that the hole was squeezing. After the design depth of 14.3 m (47 ft) was reached, the construction crew spent approximately 1 h pouring Shaft 4. Upon returning to Shaft 2, the measured depth of the hole was 1.2 m 4 ft shorter. At that point the cage was lowered into the hole, and concrete was tremied into the hole. Shaft 2 cage was the only cage that sank into the soil under its own weight and had to be supported during concrete placement. No anomalies were noted during the construction of Shaft 6.
Shafts 1, 3, 4, and 5
Several types of defects were integrated into the shafts including necking, voids, caving, and soft bottoms. The defects were made of a variety of materials including 1–10 gal. plastic pails, fiberglass, and wood insulation, 9–13 in. cardboard construction tubing, flexible PVC drain pipe, gasoline containers, and military water jugs. Some defects were filled with in situ soils to replicate inclusions on side walls, while others were left empty to simulate slurry pockets. Defects were rigidly attached to the rebar cage, using steel straps.
| Typically, defects were attached inside of the rebar cage. In some cases voids were extended outside the cage, to the outer perimeter of the shaft, using lightweight fiberglass insulation wrapped inside heavy PVC bags. When insulation was used, the dimensions of the defects shown in Figs. 6–11, were effective dimensions, which include the thickness of the insulation. Inside the cage, defects were positioned under each other along the same side of the cage, in order to leave enough room for the tremie pipe used in pouring concrete.We observed all defects while concrete was poured, and believe that all of them survived until they were covered with concrete. Shafts were not exhumed after testing and it could not be determined if one or more of the voids imploded due to the weight of the concrete. |  |
Voids
The voids included in this study ranged between 5 and 11% of the cross sectional area, and 0.3–1.5 m 1–5 ft in length. Typically smaller voids were made of 3.8–19 L 1–5 gal. sealed paint buckets, or 19 L 5 gal. water and gasoline jugs. Larger voids were made by combining several buckets inside cardboard construction tubes.
Soil Inclusions
Soil inclusions ranged between 5 and 17% of the cross sectional area, and 0.3–0.9 m 1–3 ft in length. Soil inclusions were made of 19–38 L 5–10 gal. containers filled with in situ soils to replicate inclusions on side walls. Several packages of wood– fiberglass insulation were also used to simulate soil inclusions.
Necking
Several necks were built into the shafts using 100 mm 4 in. corrugated flexible plastic tubing. The tubes were wrapped inside the reinforcing cage at specified locations, and cut section of the tubes were wrapped at the same location outside the cage. The resulting defect occupied approximately 45% of the cross sectional area. External necks occupied 10% of the cross sectional area.
Soft Bottoms
A cleaning bucket was not used to clean the bottom of any of the shafts. Technically, all shafts had a soft bottom. Nevertheless, shafts appeared to be relatively clean prior to pouring concrete.
