Excavation of the new Thor Companies headquarters began on March 15.
Several backhoes and dump trucks were utilized to begin removing the soil from the site at the corners of Penn and Plymouth Avenues in North Minneapolis. In order to accommodate the below-grade parking, the site was excavated to a depth of 30 feet below grade. This is equivalent to the roof peak of a 2½ story house. Just think, your whole house could fit in this hole and never be seen again.
The piles driven into the perimeter of the site measured roughly 46 feet. Because they “stand up,” they are referred to as soldier piles. They extend eight feet above the original grade and eight feet below the final depth. The horizontal wooden boards are in fact, two feet by12 inches by eight feet timber lagging. Now here’s the counter-intuitive part. After the first board is installed at 12 inches below the original grade, all subsequent timbers are attached below the previous timber. The timber is attached as the soil is removed. This procedure is called the “earth retention system.”
Any excavation allows for two choices – sloped earth method or earth retention system. Sloped earth method, as titled, can create a 45 degree slope that is equal in both the horizontal and vertical dimensions. So a 30-foot-deep hole is also 30 feet wide in distance from the edge of construction to the nearest edge of flat land.
Because this horizontal flat land distance may not be available on urban sites, the earth retention system is generally preferred. Equipment and material storage and staging are best accomplished by this excavation methodology.
It was asked of me, by observation that a lot of dirt had been removed from the site and some dirt was also brought to the site. Why was this? A total of 41,000 cubic yards of material in 2,400 dump trucks was removed. A typical 2½ story house at 27’ x 36’ equals 810 cubic yards. This is enough dirt to fill the inside of 50 houses or roughly two and a half blocks of housing. A significant portion of the material removed was clay. Clay is unsuitable for the soil layer beneath a building because clay likes to move by expanding and contracting in all directions. Getting below the clay is imperative and was achieved at 27 feet below grade.
At the 30-foot depth the soil now had a 4,000 per square inch weight bearing capacity.
The soil that was brought onto the site is typically identified as engineered-fill. This is a mixture of soil types such as sand and clean black dirt, and other soils. For this site Class V (Roman numeral 5) material was utilized. Class V is crushed limestone, typically smaller than a pebble, but larger than a grain of sand. Approximately 1,100 cubic yards less than 3 percent of excavation was then installed at a depth of six inches and compacted as a suitable underlayment for the concrete footing and slab pours.
Look down at your feet. Without them you would not be able to stand up. Neither can a building. While there are some places on the planet that still use stone, modern day footings are calculated and designed with rebar (reinforcing bars) and concrete that can support all of the loads from the ground to the top of the building. Rebar is placed into the concrete formwork and wired together to create a cage or webbing and sit on metal or “plastic” chairs that allows the concrete to flow through and around. These two distinct materials bond to one another and together they become stronger than they can be separately.
The short rebar seen sticking up out of the perimeter footing is covered with fist-sized plastic balls called rebar caps. This allows the foundation walls to bond to the footings and create greater strength and stability to the structure. Those caps are for personal protection for the workers in case of a trip or a fall. They are not part of the structure and are removed prior to the concrete wall pours. Foundation walls can be stone, masonry or poured concrete.
None of these materials get built without the labor force that shows up with tools, knowledge and the ability to do the work. The men and women that undertake the construction represent great skills, knowledge and a desire to engage in a project that will bring pride to them and enjoyment to the community. This team has education, training, skill, knowledge, experience and even judgement to produce a first-class project for this and future generations.
We have now reached the bottom of “the hole,” and all excavation has ceased. This is like looking into an empty shoe box. The walls are straight, the corners are crisp and the “floor” is flat. The tower crane has been installed and we are ready for new construction.
Construction cranes are needed on the job site for the primary purpose of lifting and moving heavy loads. There are many different types of cranes but truck cranes, mobile or tracked cranes and tower cranes are utilized on building construction sites. The tower crane was placed in the northeast corner of the site, on its own footing so that it will reach all corners of the project. The crane is 132 feet high and the boom (arm) has a 230-foot radius. Fees were paid and permits were granted to allow the boom to swing over the public right-of-way, as well as, the adjacent private properties.
The crane functions with an operator and a rigger. The operator sits in a booth atop the crane while the rigger gives instructions from the ground with both hand signals and a walkie-talkie radio. This team has a bond of trust and dependency, so as not to drop any loads. This is one of the most important relationships on the job site. Safety is paramount for this team.
The crane was brought onsite by truck. It was assembled and shall remain onsite, and the building shall be built around it until approximately 95 percent completion. The crane will then be disassembled and trucked off the site leaving open the space it previously occupied. The construction will then be completed, floor by floor, until the roof encloses this final opening.
Thor has placed a sign on the boom that is back-lit with LED fixtures to indicate its transformation of this empty lot into a much needed economic development project for this community.