Structural Engineering Blog: Minard Hall Collapse

2/4/2017

Case Study: Minard Hall Collapse - How Does Excavation Make a Building Collapse?

On December 27, 2009, the foundation and an exterior brick wall of Minard Hall, an university building situated on North Dakota State University, collapsed at the north side of the building. Before the collapse, excavation works, with an excavation depth of 25 feet deep, were in effect for the 108 year old building. Excavation works were for the building’s expansion and renovation project. Luckily, the collapse occurred during the early morning hours so there were no injuries.

The collapse was caused by excavation works close to the building. The excavation works created two problems that led to the collapse, the reduction of the load distribution of the soil pressure bulb and the vibrations to the soil caused by driving piles, which were constructed to support and protect a tunnel and a staircase adjacent to the excavation

The foundation that supported the Minard Hall was a spread footing. The footing transfers the loads from the superstructure above to the soil via a pressure bulb load distribution, also known as isobars. The pressure bulb distribution is also called a Boussinesq pressure bulb, as shown below. The vertical pressure is the same in all directions at points located at equal radial distance from the point of the load. The isobar is the line that connects these equal vertical pressures. The further an isobar is, the lower the pressure intensity is. The pressure bulbs are also zones of influence, in which they determine if the load distribution of the footing will have any effect on adjacent structures and/or volumes of soil. The excavated portions of soil beneath the footing of the Minard Hall removed some portions of the soil that helped to distribute the load from the superstructure via the pressure bulb.

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Excavation of the supporting soil also decreased the bearing pressure resistance, leading to settlement and cracks on the foundation walls. The allowable bearing pressure is the ability of soil below to resist the applied load of the building. The bearing capacity is the bearing pressure at which the soil will fail by producing shear failure AND have permissible settlement due to large compression loads. The soil beneath the footing got excavated, which reduced the area of soil that provides bearing pressure. The large compression loads from the superstructure also caused shear failure for the soil and soil settlement.

Another factor that caused the collapse of the Minard Hall was the vibrations due to driving piles adjacent to the excavation works. The soil beneath Minard was already composed of weak clays. The vibrations caused by the driving machines weakened the clays and their already weak strength to support the superstructure. Moreover, the vibrations can lead clays to easily move laterally to the excavated hole.

It was the combined effect of the reduction of the load distribution and bearing pressure and the vibration of soil that caused the collapse.

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Remediation efforts were immediately put into effect after the collapse.

The construction and excavation was ceased, in order to prevent additional collapse.
The already collapsed portion and its adjacent structures that were affected were demolished.
A shoring system was designed and constructed.
Additional testing, analysis, ground investigations of adequate bearing stratum, and stabilization of the structure and the foundation were put into effect for the next four to five months. Loose sand and fill were removed and replaced with compacted engineered fill.
Simultaneous work at the west side of the building and other site improvements were also taking place with the remediation works at the north side.
The north collapsed portion was redesigned.
The mechanical equipment room, which was supposed to be located at the basement, was removed and relocated to upper floors.

The north addition’s foundation and walls were finally finished in October 2011.

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Lessons learned

Before design, implement ground and geotechnical investigation, land survey, and soil testing and analysis.

Information about soil properties, such as soil strata, soil springs, and bearing pressures, should be gathered before foundation is designed.

The load distribution of the proposed works should be taken into consideration in the design stage.

The demolition works and the works added can change load paths. This can affect adjacent structures and/or site conditions.

The effects caused by the noise and vibration of driving equipment should be considered.

The equipment can cause disturbance to the public and effect to adjacent structures and ground conditions. This can be remediated by reducing the frequency and amplitude of vibratory driving and/or the oscillation of piling procedures. Noise and vibration free hydraulic equipment, predrilled holes, or low soil displacement piles can also be used.

Appropriate construction methods should be considered.

We should also think carefully about how our construction methods can affect the integrity of the structure itself and its adjacent site and ground conditions.