Page images


38. Purpose of Test Installations. - The properties of all earth materials can be accurately determined in the laboratory for various working conditions. The designer of modern earth structures uses the results of these laboratory tests to design and analyze the structure. In addition to making tests on the fill to control embankment construction, elaborate test apparatus are installed in most dams now built by the Bureau to determine the behavior of the earth embankment and appurtenant structures during construction and later reservoir operation.

Piezometer installations are designed to contact, transmit to terminal wells, and record pore water pressure in the embankments and foundations of earth dams. Embankment settlement installations provide a means of measuring the consolidation within the embankment, settlement of foundation, lateral displacement of the embankment, and seepage through the embankment. Surface settlement installations are made to record cumulative settlement of the surface of the embankment and horizontal deflection normal to the axis of the dam.

In O'Sullivan Dam, 36 piezometer tips, 2 embankment settlement installations, and 66 surface settlement points were installed.

A. Installation of Test Apparatus

39. General. - According to a provision in specifications No. 1400 it was contemplated that the Government excavate the embankment material and place the settlement-measuring apparatus in position after the completion of each 5-foot lift of the embankment. The contractor was then to backfill and compact all excavated material around the apparatus. However, upon curtailment of the Government force-account activities it became necessary to make arrangements with the contractor for the excavation and backfill work in connection with the piezometer and embankment settlement apparatus installations. The work involved in establishing the surface settlement points was performed by Government forces.

40. Piezometer Apparatus. - The piezometer installation at O'Sullivan Dam consists of 36 tips. Thirty-three tips are located at station 170+00 at various elevations and axis offsets and one tip each is located at an elevation of approximately 970 at stations 168+00, 172+00, and 174+00 (fig. 29). These piezometer tips are connected by l/4-inch outside-diameter plastic tubing to gages in a terminal well 270 feet downstream from the axis station 170+03. When practicable the tubing was cut to the prescribed length extending from the tip to the well. Where lengths of tubing had to be joined, brass flared-type couplings were used. Trenches for the tubing and tips were dug a minimum of 2 feet deep (fig. 30) to insure adequate protection during the backfilling operations. It was found that the trench could be dug to adequate depth and to a reasonably true grade with a road patrol grader, followed by widening and trimming of the bottom and ends of the main trenches and digging of the side trenches for the tips by hand. Loose backfill material was hand placed directly in contact with the tubing and tips, after which it was rolled with the wheels of the grader. Thereafter, the grader was used to place and also compact the backfill material. To limit seepage at lower levels a small amount of bentonite was placed around the tubing at several points along the main trench between side trenches in zone 1 material. Although not considered to be necessary in this material, the bentonite was used as a precautionary measure.

Tubing from tips; located in the cutoff trench, was brought to the original ground elevation in sloping trenches dug in the downstream slope of the cutoff trench. These sloping trenches were filled with compacted zone 1 material. Four to six inches of selected fine material was placed around the tubing where the trench for tubing passed through gravelly material. The tubing passes through the entrance pipe into the terminal well. The entrance pipe was filled with bentonite and sealed on each end with oakum and calking compound to prevent any leakage of moisture around the tubing into the well.

The terminal well, constructed of reinforced concrete, is 4 feet 5 inches by 5 feet 8 inches in plan and 27 feet deep. In the terminal well manifold system, l/4-inch

[ocr errors][ocr errors][ocr errors][graphic]
[ocr errors][graphic][merged small][merged small][graphic]

inside-pipe-size brass fittings and pipe were used. One-fourth-inch outside-diameter plastic tubing was used in the terminal well, and all connections were made with flared-type brass fittings.

The terminal well including ventilating blowers, air ducts, and ladders were installed by the contractor. The personnel from the residency staff installed the gates, manifold piping, and all plastic tubing in the terminal well, placed the system in operation, and took initial readings. No difficulty was encountered in securing balanced readings after all air had been removed from the tubing.

41. Embankment Settlement Apparatus, - The embankment settlement apparatus (fig. 32) consists of a series of telescoping l-l/2-inch and 2-inch pipes having lengths of 3 feet and 4 feet, respectively. Three-foot sections of channel iron are bolted and welded at right angles to the middle of the 3-foot lengths of l-l/2-inch pipe.

Each anchored pipe section and crossarm moves with the embankment surrounding it and is independent of the rest of the installation, thereby permitting measurement of differential movement between successive crossarms. The crossarms in each installation are installed at 5-foot nominal intervals and their alinement is such that any two successive crossarms lie in vertical planes at right angles to each other. A special engaging device, or "torpedo," is lowered into the pipes by a steel measuring tape to obtain elevations of the crossarm at their respective measuring points, beginning at the upper crossarm and continuing to the bottom of the installation. In this operation the lower end of the crossarm is engaged by pawls in the torpedo and the tape is tightened against the pawls and the exact location of the crossarm measured by means of a measuring scale. When the measurements are completed the torpedo is permitted to strike the latching plate at the bottom of the installation, which causes the pawls to latch closed allowing the torpedo to be withdrawn. A comparison of the measured elevations with the original elevations gives the vertical movement of the embankment.

Two embankment settlement installations, comprising 63 crossarms, were made in O'Sulllvan Dam. Installation A, consisting of 37 crossarms, is located 45 feet upstream from axis station 170+10 with the base crossarm established on top of lean concrete in the base of the cutoff trench. Installation B, consisting of 26 crossarms, is located 50 feet downstream from axis station 170+10 with the base crossarm set on undisturbed alluvium.

After the base crossarm was set and a pipe cover was placed on the 1-1/2inch pipe, a minimum depth of 6-1/2 feet of embankment was placed over the pipe cover. Then a trench about 2 feet wide, 4 feet long, and 3 feet deep was dug over the base installation and a hand-auger hole dug to expose the pipe cover. After sufficient material was removed around the pipe to permit the removal of the pipe cover without loose material falling into the pipe, the pipe cover was removed. Then the 2-inch pipe was slipped over the l-l/2-inch pipe of the preceding installation, and the next crossarm with oakum and pipe cover in place was inserted into the 2-inch pipe. The excavated material was then backfilled and hand tamped while the 2-inch pipe was held vertical (fig. 31). The placement of embankment was resumed and after placing another 6-1/2-foot lift over the pipe cover, the next crossarm was installed. Identical procedure was used in setting each succeeding crossarm. However, the long axis of the trench for any adjacent installation had to be rotated 90° about the centerline of the crossarm pipe from the axis of the trench of the preceding installation to permit their crossarms to lie in vertical planes approximately perpendicular to each other.

Surveyors of the resident engineer's staff established the base crossarm of each installation and determined the location of all subsequent crossarms. Elevation readings were taken after each installation. Prior to December 1, 1949, it was assumed that a base crossarm established on bedrock remained at a constant elevation which was used to determine elevations of other crossarms. On that date this practice was discontinued and elevations of all crossarms have since been determined from benchmarks outside the embankment area.

42. Surface Settlement Points. - Sixty-six surface settlement points were established on the surface of the completed embankment in five lines parallel to the axis of the dam (fig. 33). The number of points at each location are as follows: (a) 24 points located 25 feet upstream, (b) 24 points located 50 feet downstream, (c) 8 points

[graphic][ocr errors][ocr errors][ocr errors][merged small][ocr errors]
« PreviousContinue »