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Deploying Sonar Systems to Create 3D Maps for Underwater Excavation

By Roberto Folchi
Managing Director
Hans Wallin
Technical Director
Nitrex srl
Lonato del Garda, Italy

Charges deployed from the barge.
The construction of the third water intake of the Southern Nevada Water Authority at the Lake Mead reservoir required an underwater excavation at 100 meters depth, and a large basalt surface had to be deepened by 20 meters. No new technology was available to fit the project, so the old imprecise yet effective method of using shaped demolition charges was chosen. However, no shaped charge was found in the market that fit the underwater specifications of the project, so a new one had to be conceived, along with a deployment system and technique, monitoring and survey procedures.

Basalt is strong and resilient, with a high resistance to metallic jet penetration, close to that of ceramic, which is why it is used in the defense industry to protect tanks against grenade impact. The new shaped charge needed to create a high-energy jet slug to break through the basalt, which required adequate water-free standoff distance between the explosive charge and the rock to be fractured. To keep the charge standoff water-free at 100 meters depth, a specially fitted shell had to be coupled to the charge.

To monitor excavation and deploy charges, Nitrex used a combination of an ROV, a single-beam sonar and a multibeam echosounder.

Excavation and Underwater Survey
The rock mass to be excavated was a basalt mainly composed of plagioclase. There was an upper layer of stones, sand and silt. Thickness was quantified by sub-bottom profiling and mechanical coring. The upper layer had been weathered by atmospheric exposure before being covered by water when the Hoover Dam was completed.

The progressive density increase from the top to the bottom of the loose sand-silt formation and from the upper layer of basalt affected accuracy, leading to overestimation of the thickness of the loose materials overlaying the solid rock.

An open-air survey of an outcrop in the nearby village of Callville offered a general view of the consistency of the rock to be blasted. Rock mass appeared to be fractured within three main families of thermal shrinkage joints, and some minor faults were noted. Some joints showed earth filling but were generally well-closed into the rock mass. Non'interconnected cracks were seen in the rock matrix.

The upper standing layer of stones, sand and silt was removed by air lifting and clam shell. Material was mucked aside by swinging the drag line boom, with the bucket/clam- shell kept below the water table. The hole in the rock, which was needed to host an intake shaft/riser, was excavated by means of a custom-made shaped demolition charge.

Because of the water pressure at that depth, sealing for water tightness was an issue. No divers were needed. Instead, charges were attached to a steel frame and deployed. They were monitored with Saab Seaeye Ltd.'s (Fareham, England) Falcon 12108 ROV, equipped with an MS 1000 sonar and a camera, both from Kongsberg Maritime AS (Kongsberg, Norway).

Georeferenced positioning of the charges was performed by Leica Geosystems Inc.'s (Norcross, Georgia) mojoRTK system, which was linked to an underwater tracking system, the Scout ultrashort baseline from Sonardyne International Ltd. (Yateley, England), with a precision of less than 1 meter. Excavation progress was monitored by Kongsberg's EM 3002 multibeam echosounder to generate lake-bottom profile maps for use in assessing mucking and charge performances. The high density of profile lines and low systematic errors allowed for a 3D map, so volume calculation could be assessed even if moved rock height represented less than 1 percent of lake depth at that point. To continue this article please click here.

Roberto Folchi is a mining engineer with 30 years of experience in explosives engineering. He is the managing director and shareholder of Nitrex srl, working in the explosives sector for demolition, controlled blasting, survey and monitoring, risk assessment and research.

Hans Wallin has more than 40 years of industrial experience, with 27 years in the explosives industry. He has held leading positions in development, production, health, environment, safety and security related to the production of explosives for the defense sector.

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