Leaha Martynuska
Construction Management Option
Wilsdorf Hall – Materials Science Engineering & Nanotechnology Building
University of Virginia
7. Breadth Analysis: Alternate Underpinning System
7.1 MSENT Underpinning Background
MSENT is being constructed on a very small site situated between three
existing buildings. Construction at this location required the installation of a
detailed underpinning system to support the existing foundations. The
underpinning system installed on the existing chemistry building failed, cracking
the existing foundation, and therefore causing a schedule delay. A structural
engineer did not design the underpinning system. The specification section for
shoring and underpinning was a ‘performance specification’ wherein an AE
indicated what is required, but does not indicate a specific manufacturer or
method. Therefore the specification said to underpin the building, don’t let it fail,
and the allowable tolerances are….
The chemistry building was underpinned using a mine shaft method in the
following steps:
•
A pit was excavated next to the building footing
•
A 5’ x 5’ x 5’ hole was dug against the footing
•
Excavation was completed to 4’ underneath the footing
•
Wood shoring was installed to shore the inside of the pit
•
Once completely underneath the building, a shaft was dug down to
approximately 33’ with wood boards at the exterior of all four
sides
•
This shaft was then poured solid with concrete up to 2” +/- below
the existing footer
•
Once the concrete was cured the 2” gap was dry packed with grout.
75
The Pennsylvania State University
AE Faculty Advisor: Dr. David Riley
Leaha Martynuska
Construction Management Option
Wilsdorf Hall – Materials Science Engineering & Nanotechnology Building
University of Virginia
7.2
MSENT Underpinning Constructability Review
It is unknown why the building foundation actually cracked. An
investigation was conducted by an outside forensic engineering firm who
concluded that the design was inadequate and did not comply with project
specifications. Yet the contractors who installed and designed the system believe
the design was adequate and the building cracked as a result of an unforeseen site
condition. There are several different soil/material planes within the height of the
excavation. At the intersection of two of the planes there is a slick surface which
caused the soil not to be as self supporting as normal and put an unexpected
amount of force onto the underpinning pits. This exceeded the values normally
used in underpinning design and caused the building to shift and crack.
One side claims the failure could have been prevented in either case if the
design had been upgraded to comply with the contract documents. Yet the
opposing side claims that standard design practice would not have prevented the
movement. Currently, it is being accepted that any failure was not the result of
faulty installation, but instead of an inadequate design (to support loads under the
unforeseen conditions).
All required tests
were performed on the
underpinning system,
including the concrete
which was tested to
ensure it reached the
proper compressive
strength. The tiebacks
(used to resist lateral
forces) which were the
point of failure were also tested. There were two tests completed, a short test
which loaded each tie to alignment, to 0.25 design load, to 0.5 design load, to 0.75
design load, to 1.0 design load, and to 1.2 design load. Each step was tightened,
76
The Pennsylvania State University
AE Faculty Advisor: Dr. David Riley
Leaha Martynuska
Construction Management Option
Wilsdorf Hall – Materials Science Engineering & Nanotechnology Building
University of Virginia
held for a specific duration then its movement was recorded (about 10 minutes
total). If movement exceeded an allowable standard then a second test was
performed with a more complicated stepping process, longer duration for each
step and final step of 1.33 design load (total time about one hour). This test was
to be done for any failure (there was none on the project) and 5% of the total
number of tiebacks on the job. The 5% selected were the ones with the highest
strength requirements, the hardest to install and the rest random.
Additional tiebacks were installed to make the system stronger, and each
was tested to the more stringent one hour test. Then two were randomly selected
to be tested at 1.33 design load and this took a total of 8 hours.
77
The Pennsylvania State University
AE Faculty Advisor: Dr. David Riley
Leaha Martynuska
Construction Management Option
Wilsdorf Hall – Materials Science Engineering & Nanotechnology Building
University of Virginia
7.3 Factors Affecting Underpinning Selection
Foundations need to be underpinned either when a structure is being
disturbed by foundation movements or when proposed new works are likely to
cause structural failure if the foundations are not improved. There are many
factors to consider when deciding how exactly to underpin an existing structure:
•
Size and depth of excavation
•
Soil conditions
•
Ground water
•
Surface drainage conditions
•
Weather and moisture conditions
•
Routes and depths of existing underground services
Size and Depth of Excavation
The size and depth of excavation needed to support an existing structure
needs to be known to avoid any potential failures. These two factors help to
decide which approach to take to underpin a building.
Soil Conditions
A variation of soil conditions can result in the failure of an installed
underpinning system. This is a very important factor to investigate prior to
selection of the underpinning system to be used. Any pockets of sand, fissures, or
results of previous soil disturbances, etc. prove that the soil is unstable and may
be unfit for underpinning.
Ground Water
Ground water is also very important in the determination of how to go
about underpinning an existing structure. The location of the water table needs to
be known before excavation can begin. The water table indicates the level to
which the ground freezes and thaws. This needs to be known because soil tends
78
The Pennsylvania State University
AE Faculty Advisor: Dr. David Riley
Leaha Martynuska
Construction Management Option
Wilsdorf Hall – Materials Science Engineering & Nanotechnology Building
University of Virginia
to break down and not be as strong and supportive under extreme weather
conditions.
Surface Drainage Conditions
On site surface drainage is very important when installing an underpinning
system. It eliminates any problems with additional moisture or flooding during
installation. It also improves soil conditions, making soil stronger and more
stable.
Weather and Moisture Conditions
Weather and moisture conditions in areas where underpinning is to be
installed are also important factors when choosing which system to install. Soil
strength can break down through heavy rain or frost affecting the system. Also,
clay has the possibly of shrinking and drying out during extreme conditions.
Routes and Depths of Existing Underground Services
It is important to know what is underneath the surface, especially where
underground utilities are located. Gas, electricity, and telephone lines, are just a
few of the many utilities usually located underground. The location of these
utilities needs to be known prior to excavation to eliminate any emergency
situations.
79
The Pennsylvania State University
AE Faculty Advisor: Dr. David Riley
Leaha Martynuska
Construction Management Option
Wilsdorf Hall – Materials Science Engineering & Nanotechnology Building
University of Virginia
7.4
Construction Management Role in Underpinning
A construction management team, including supervisor play a very
important role in the installation of any system. An underpinning system may be
very detailed or not depending on the location of the construction site and the
structure needing support. Regardless of the size or shape of the underpinning
system, the project management team has several tasks:
•
Safety is the number one concern when on a jobsite, therefore the
number one task is to ensure safety during all times.
•
During installation regular checking is required to ensure the
system work is being done correctly. This is also necessary to
ensure safety of workers and other people (especially when on a
tight site).
•
Consequences of accidental impact need to be thought about to
ensure a safety plan goes into action immediately. No possibilities
of accidents can be overlooked.
•
The sequence of installation needs to be closely watched to ensure
the system is being installed correctly. For example the concrete
needs to be poured immediately after excavation is complete.
80
The Pennsylvania State University
AE Faculty Advisor: Dr. David Riley
Leaha Martynuska
Construction Management Option
Wilsdorf Hall – Materials Science Engineering & Nanotechnology Building
University of Virginia
7.5
Conclusion
After reviewing the existing underpinning I have determined that the best
possible underpinning system was installed. The underpinning system is a
sensitive subject on the project and is a fairly new issue; therefore I was advised
against contacting any party directly involved in the installation or design of the
system. The reason the failure occurred is unclear and measures to keep the
failure from happening seem to have been unpreventable.
I have evaluated the installation of the underpinning system for MSENT
and came up with a number of factors that need to be determined prior to
installation. These conditions directly affect the selection and installation of any
underpinning system.
Also, through research of case studies I have developed a set of guidelines
construction managers must follow during the design of and installation of an
underpinning system. The number one guideline is to ensure that all safety
measures possible are being taken.
81
The Pennsylvania State University
AE Faculty Advisor: Dr. David Riley