UNDERWATER SOIL SAMPLING BASIC INFORMATION AND TUTORIALS


Common constructions that require some kind of underwater exploration program include bridge piers, port structures, pipelines, oil well platforms, land recovery (fills to extend the shore line or for an island), and the like. It is usually necessary to collect enough data to make a strength estimate.

Soil shear strength determines how much pile embedment is required or whether a fill will require special construction procedures. Estimates of settlement are also often required—both how much and how long it will take. This is very critical for land recovery operations, since the client will want to know when enough settlement has occurred so that construction of surface facilities can begin.

The in situ testing and recovery procedures for underwater samples, either in a freshwater or a saltwater environment, are not much different from those for dry land for water depths up to about 45 m. The principal differences are that the testing or drilling equipment is mounted on a barge that is towed to the test location and securely anchored and that casing is used, at least to the water bed and possibly 1 or 2 meters into the bed.

The casing strength is the principal cause for limiting the depth to about 45 m. For this situation the barge is securely anchored using four to six anchors so it does not shift or twist. Sometimes divers are used to observe visually if any construction difficulty will be encountered or if there are any existing underwater obstructions.

A barge-mounted drilling rig (drilling over the side) is a common method for drilling in rivers, in lake beds, and in the shallower water along the continental shelf for bridges, port structures, or land recovery. Penetration, vane, and pressuremeter tests described in the following sections can be made in the borings.

In deeper water (up to 1,000+ m) wave action requires alternative exploration equipment, such as a small ship converted to a drilling platform by installing a center well of 460 to 610 mm diameter from the deck through the hull and adding a drill rig. This configuration is sometimes called a drill ship.

Submarine-type vessels (sometimes called submersibles) are also used. In very deep water a platform might be constructed, off of which the exploration crew might work. Any of these equipment options will allow recovery of samples of reasonable quality.

Where wave action occurs, it is necessary to use casing with flexible joints, and a casing diameter large enough to allow passage of the sampling (or test device) tube. In deeper water the drill pipe may act as the casing (again using flexible joints). In this case the lower end of the pipe holds the auger bit, which produces an over-sized hole.

At the desired level a sampler is lowered through the drill pipe to the base of the hole and either driven or pushed into the soil below the bit.

There are also projectile-type devices that are lowered to the ocean floor from the drill ship to recover soil samples. Servomechanisms commanded from the surface may be used to force a sample tube into the soil using the weight of either the surface vessel or some kind of reaction device placed on the seafloor.

A projectile device may contain a gas or explosive charge to propel a sample tube into the soil, again using the weight of the total device as a reaction. Most of these types of devices are patented and/or proprietary. Deepwater divers are sometimes used to recover samples or to inspect the reaction device.

In situ tests are currently considered preferable to sample recovery, particularly for strength testing. It is difficult to recover good-quality samples from underwater because of the change in pore pressure when the sample is brought above water. As a minimum, air bubbles tend to come out of the pore water and occupy a greater volume, causing the sample to expand or even explode.

If the sample is still in the sample tube, the expansion may cause the sample to extend out of the tube end(s). Depending on the equipment, the sample recovery tube (about 50- to 75-mm ID and 610 to 100O+ mm in length) may be pushed or driven.

A pushed sample is generally of better quality than one obtained by driving the tube into the soil. Shorter tube lengths generally produce better-quality samples, since side friction is significant with all tube samples; if the sample is too long, it may become compressed from side friction between the sample and the inside walls of the sampler.

At a given site a few samples should be recovered for visual inspection and possibly some index tests (w#, W>L, Ip). A driven-tube recovered sample will often have excessive disturbance for strength testing, but the blow count to drive the tube gives some indication of soil strength, somewhat like the SPT test described in the next section.

A number of underwater exploration methods are described in ASTM (1971) and appear among the references cited by Focht and Kraft (1977), which the interested reader may wish to consult. Using the in situ vane test for underwater exploration is described in ASTM (1988). Olsen et al. (1986) described an elaborate marine sampling and testing program undertaken in 1979-1980.

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