Technology Edge Streamlines Giant Concrete Pour

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The highly anticipated beachfront Seacoast Inn located near Imperial Beach, San Diego will incorporate luxury suites, a spa, restaurant and other specialized services within a traditional four-story, wood-framed structure, underground parking garage–and an impressive foundation. The 200-ft x 238-ft foundation is unique in that it is built on a 27-inch thick mat slab, instead of the more typical 3-6 inches, pushing Lusardi, the general contractor, to apply innovative tools and techniques to meet the project’s tight timelines and engineering requirements.

The thicker, reinforced slab will hold back the high water table created by the hotel’s close proximity to the Pacific Ocean. In addition, the slab required a number of drains which created numerous ridges and valleys to insure effective drainage.

Stephen Mayes, field engineer with Lusardi, says, "The crux of the problem was in the required ridges and valleys. We needed to place the concrete and screed to achieve the elevation changes called for in the design, all while completing the job as quickly as possible. The ridges and valleys sloped to drains with percentages of 1.5% to 2%. In fact, some of the elevation changes came to as much as four inches."

Conventional methods for layout, preparation and pour would have required that carpenters construct a number of forms and cold joints to account for the ridges and valleys. Unfortunately, this technique would have taken 3-4 days to set up, while creating additional time-consuming problems. For instance, the team would have had to figure out how to get the screed hooks out of the ground. Screed hooks or conventional methods for concrete screeding usually require some kind of anchoring into subgrade. Anchoring was not possible on the hotel foundation to avoid costly damage to waterproofing systems.

Instead, Lusardi put together a plan to place and finish 4,150 cu yds of concrete in one pour without cold joints and at the correct elevations–in just 13-hours. The contractor put together a highly skilled team to pour, screed and smooth the concrete, equipment that includes four large pumps with booms able to span over 250 ft, 400 concrete trucks and a Trimble robotic total station.

Prior to the pour, Mayes used the architectural 2D model of the foundation to locate about 200 control points. He then downloaded the points from the model to the Trimble® LM80 handheld controller, which was connected to a Trimble® SPS710 robotic total station. In the field, he shot the elevations and the high points. Ridges are the high points in the pour with valleys sloping to the lowest points in the pour (drains).

On September 14, the first of 400 cement truckloads began arriving at the site from 4:30 a.m. to 6 p.m. As the wet concrete pumped from the trucks, Mayes and his robotic total station moved in position to see whole project. Throughout the pour, he moved from one end of the foundation to the other checking elevations with the total station until the entire foundation surface was smoothed and leveled per the engineering requirements.

Mayes says, "Our challenge was to stay within one-quarter inch tolerance across the entire surface–which we were able to maintain because of the robotic total station’s mobility and digital data management. As I went back to perform layout of columns and masonry walls, I spot checked elevations and they were all within tolerance. On a project like this, with numerous ridges and valleys, the model-to-field connect is the only way to ensure speedy, accurate results."

Vicki Speed is a freelance writer based in Denver, Colorado.

A 1.426Mb PDF of this article as it appeared in the magazine—complete with images—is available by clicking HERE