Humboldt H-4140 GeoGauge - Manual (Page 21)

21
The Humboldt GeoGauge is a 10” diameter,
11” tall, 22 lb. electro-mechanical instrument
that when placed on the surface of the ground
evaluates the stiffness of the top 9” to 12” of
material. It vibrates the ground over a range of
discrete frequencies, applies force, measures
the resulting deflection and displays the results
in about a minute. It was chosen by MnDOT
District 2 because measurements could be made at a rate greater than the rate
of compaction, it has no licensing or safety requirements and its performance
(reliability, precision & bias) had been proven by FHWA Study 2(212) &
TRB NCHRP Project 10-65.
In the summer of 2004, District 2 chose road TH200 in Ada, MN for its initial
use of this QC test method. The method was contractually specified. This was
the only way District 2 thought that sufficient data could be collected for a
comprehensive evaluation of the method. The subgrade was an AASHTO A-1-b
material, placed in two 12” lifts over two miles of 2-lane roadway. Stiffness was
measure approximately every 100 ft. on each lane for each compacted lift, one
1,000 ft. section at a time. Based on test strip measurements at the start of the
project, a stiffness value of 23 klb/in was assigned as a target corresponding to
the specified 90% compaction. Moisture was measured approximately every
500 ft. by either time-domain-reflectometry or field oven. Density was measured
randomly as a check on the method and took precedence in judging quality if
there was a conflict with stiffness.
The over 1,000 stiffness QC tests made on the TH200 project indicated that the
level of compaction was from 87% to 97% (18.2 klb/in to 32.2 klb/in for 95%
of the data). This was better than the best quality traditionally possible for the
material in District 2’s experience. Moisture content was typically 3.5% below
optimum, varying from about 6% to 12%. This was consistent with the best
quality traditionally possible.