Humboldt H-4140 GeoGauge - Manual (Page 20)

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Traditional subgrade compaction QC test methods do not evaluate in-place
material strength or the structural uniformity of each lift as placed. Evaluating
these two factors is essential if cost is to be held to a minimum while assuring
the performance needed for the roadways intended function and projected
life. This type of evaluation is essential if the industry’s trend towards modulus
based mechanistic design and performance specifications for roadways are to be
supported. Also, traditional methods do not provide contractors with sufficient
real-time feedback so as to optimize the balance of quality and cost.
Accordingly, a simple and precise modulus or stiffness based QC test method
for subgrades was needed by the Minnesota Department of Transportation
(MnDOT), District 2, Thief River Falls Construction Office that would evaluate the
required factors as compaction occurs. At the same time this method needed
to provide an index of percent compaction so as to fit within the framework
of traditional specifications. Finally, the method needed to provide an index
of resilient modulus to support the future use of mechanistic design and
performance specifications.
District 2 selected an in-place QC test method developed under FHWA Study
2(212) that did not interfere with or delay the construction process. Without
penetrating the ground, the method used the Humboldt GeoGauge to measure
the stiffness of each lift and thereby evaluate percent compaction. Using a
test section or strip of subgrade
material, lift stiffness at controlled
moisture content was measured
and spatially averaged as a function
of compactive effort. Initially this
data was compared to density
as a function of effort to confirm
the findings of FHWA 2(212)
that maximum stiffness occurs at
optimum compaction as constrained
by site conditions. The resulting
empirical relationship was used to
establish QC stiffness targets for the
subgrade that corresponded to the
traditionally specified levels of
percent compaction.
CASE STUDY:
Stiffness Based Compaction QC
of a Granular Subgrade
Minnesota DOT
District 2
423 West Zeh St.
Thief River Falls, MN 56721