(The following is one in a series of cases I have investigated that illustrate the different types of structural failures and accidents that occur resulting in civil litigation, and the forensic engineering methods I used to investigate the cause)

The investigation is reported under the following main headings with several sub-headings:

• The case (a description of the failed structure – significant cracks in a building, the “lega”/technical issues, and my “client”
• “Forensic engineering” investigation of the failure and the methods used
• Cause (of the failure)
• Post mortem (an interesting side story and a lesson learned)

The case

I carried out my first “forensic engineering” investigation during my 5th year studying civil engineering at the University of New Brunswick (UNB).  This was at a time when I was an engineering student and had no understanding at all of forensic engineering, and wasn’t even qualified as a professional engineer.

Nevertheless, this was a significant and costly building failure but, fortunately, not a catastropic one.

We took our lectures in a room on the second floor of a two and a half story brick clad building with a full basement – the “engineering building” on the UNB campus.

During our 5th year the foundations of one wall of the building settled causing 1″ to 2″ wide, vertical cracks – as I remember the size, to appear in the front, left corner of a wall of the lecture room.  You could see daylight through the cracks.  This would be significant damage to an existing building

“Legal”/Technical issue

To me as a student with an interest in geotechnical and foundation engineering, the cause of the cracks was an issue of considerable interest.  I undertook to investigate and report on the cause to meet the requirements of one of my courses.

Client

My “client” in a sense was the professor who was giving the foundation engineering course.

“Forensic engineering” investigation

My “forensic engineering” investigation involved the following:

• Visually assess the exterior of the engineering building
• Determine how the building was constructed
• Research construction techniques

Visual assessment

A visual assessment of the exterior of the building found that an addition to the engineering building was being constructed immediately adjacent the existing building.  Consulting engineers for UNB had hired a contractor to build a new engineering building adjacent the old – only a few feet away.  Construction involved a deep excavation adjacent the shallow foundations of the existing building.

Building construction/Construction technique

I learned that the existing engineering building was supported on shallow spread footings founded in the natural soils.  Excavating near and well below natural foundation soils like these requires their support in some manner to prevent undermining the soils.

I saw during my visual examination that the contractor had installed a soldier pile shoring system to temporarily support the foundation soils beneath the existing building.

This type of foundation support system consists of steel piles driven vertically into the ground at regular intervals adjacent the existing building foundations.  The piles may also be installed in previously bored holes in the ground eliminating the ground vibration from pile driving.  As the excavation is taken deeper timber – lagging, is inserted horizontally between the piles to support or shore up the soil in the side of the excavation – in this case soil that is adjacent the existing building’s foundation soil.

A soldier pile shoring system is a good support system if constructed properly and its limitations kept in mind.

Research construction technique

I researched the shoring system and found that it “gives” or yields a little – deflects along it’s length in engineering terms, when mobilizing its strength to provide support to the soil it is retaining.  The retained, shored up soil behind the shoring system gives a little as well – moves sideways and away from the foundation soils to which it is providing lateral support.  This effectively undermines the foundation soils a little causing the soils to settle and the building foundations to settle as well.

This deflection is due to the piles bending along their length.  The piles will also deflect or tilt a little if they are not driven or embedded deep enough during installation.

This lateral movement of the shoring system and settlement of the soils and foundations is normal.  It can be negligible if the shoring system is properly designed and installed.  The movement can be significant causing damage to the foundations the shoring system is designed to protect if the support system is not well designed and installed.

Installing soldier piles by driving them in place causes the soils in the immediate area to vibrate.  Soil settles when it is vibrated.  Anything in the soil – like building foundations, settles as well.

Cause

I analysed the data that I had collected – the manner of construction of the shoring system and the results of my research, and concluded the cause of the failure and submitted my student engineering report.

In this case the soldier pile system deflected too much causing the foundation soils to yield or move sideways and settle in the process.  This caused the building walls to settle as well and the corners to crack and open up.  The deflection was probably due to a combination of the causes noted above:

• Vibration of the soils during installation of the piles
• Tilting of the soldier piles due to shallow embedment
• Deflection along the length of the piles

Post mortem

I passed my year so I must have got it right, not treading on any toes in the process – the engineers who approved the soldier pile system that failed were my professors who had formed a consulting engineering company to do this type of work.  Failures occur in spite of the best efforts of the best people.