Test 2 for Specimen collected from Depth of 56.5 ft (see above description)
3) Lateral displacements of the facing blocks of the MSE walls (see Figure C.1, facing
5.6 Structure E-17-PR @ I-76 at 136 th Ave (Flowfill Abutment Backfill)
Mr. Ilyess Ksouri (2001) from CDOT Soil and Foundation office completed a subsurface geotechnical investigation at the east and west approaches of the subject structure. The purpose of the investigation was to identify causes of the approach slab settlement problems and to provide recommendations for remedial measures. The report prepared by Ksouri (2001) was reviewed by Mr. C.K. Su from CDOT Soil and Foundation office. The materials presented next are extracted from that report.
5.6.1 Site Conditions:
The Structure is a two span bridge at 136th Ave. and carries traffic over I-76. A site visit revealed
that the approach and roadway concrete slabs and the slope paving settled quite noticeably at both sides of the bridge. The amount of damage is much more severe at the southeast (SE) bridge approach. The joint between the side walls and the approach roadway allowed water to get into the embankment materials. This resulted in localized erosion under the approach slab.
5.6.2 Subsurface Geotechnical Investigation
A total of six test holes were drilled at the east and west approaches.
At the west abutment, subsurface conditions in boring B2 at the approach slab consists of a 12- inch thick layer of reinforced concrete underlain by 8 feet of flowable fill (cemented gravelly sand material) and 10 feet of natural clayey sand material (foundation soil). The weathered claystone bedrock was encountered at a depth of 19 feet below grade. The clayey sand material (foundation soil) varies in density from very loose to medium dense, with a moisture content varying from 19 to
22% and a plasticity index varying from 23 to 26. Drilled holes B1 and B6 at the roadway
approach consist of a concrete slab varying in thickness from 12 to 14 inches, underlain by 8 feet of clayey sand fill material and 5 to 10 feet of natural sandy clay material. The weathered claystone bedrock was encountered at a depth of 19 feet below grade. The clayey sand fill material
is very loose to loose, and the sandy clay natural material (foundation) is very moist and plastic with moisture content varying from 22 to 25% and a plasticity index of 35.
At the east abutment, drilled hole B4 at the approach slab consists of a 12-inch reinforced concrete slab underlain by 8 feet of flowable fill and 7 feet of clayey sand fill material. Beneath the fill material, 17 feet of sandy clay to clayey sand natural materials were encountered. The weathered claystone bedrock was at a depth of 31 feet below grade at this location. The fill material is loose
to medium dense with a moisture content of 13%. The sandy clay natural material (foundation
soil) is medium stiff to stiff with moisture content of 21% and a plasticity index of 28. The clayey sand natural material (foundation soil) is loose to medium dense. At the roadway approach, drilled holes B3 and B5 consist of 3 feet of flowable fill underlain by 7 to 10 feet of clayey sand and sandy clay fill material. Twenty feet of sandy clay to clayey sand natural materials were underneath the fill material. The weathered claystone bedrock was encountered at a depth of 31 feet below grade. The fill material at the SE bridge approach is soft to medium stiff and very moist with a moisture content of 22% and a plasticity index of 26.
5.6.3 Causes of the Bridge Approach Settlement Problem
Ksouri (2001) indicated that the approach settlement resulted from the compression and erosion of the embankment fill, and consolidation of some of the foundation soil layers.
It is very common for water to infiltrate at the expansion joint and along the bridge wingwall into the embankment fill. The infiltration of water can have different degrees of effect on reducing strength and increasing deformability of the embankment materials. The lab results showed very soft and excessive moist embankment material at the SE bridge approach that gets stiffer and drier with depth. This indicates infiltration of water from the surface into the embankment materials. The combination of excessively moisture in the embankment and repetitive traffic loading, induced settlement of the embankment material.
Water from surface run off could also wash out the embankment material creating voids underneath the approach and roadway slabs. Localized erosion underneath the slab at the SE
wingwall area was observed during the site investigation. Erosion of the embankment material will lead to further settlement of the approach and roadway slabs.
The embankment fill material is predominantly clayey sand and sandy clay with SPT blow counts between 2 and 7 measured during the subsurface investigation. Ksouri (2001) stated that the relative low SPT blow counts (4 or below) of the embankment fill resulted from: 1) excessive moisture, and 2) varying and low degrees of compaction efforts at some locations of the embankment during construction.
5.6.4 Recommendations
Based on the above findings and the limited subsurface investigation, Ksouri (2001) suggested the following measures to be considered for solving the approach slab settlement problem:
(1) Drainage Improvement:
Deck drains to collect the water before it reaches the approach slabs should be installed. Water collected by the deck drains should be piped to the bottom of the embankment fill. The new drainage system should discharge water to the bottom of the concrete apron areas. All the gaps between the wingwall and the concrete pavement should be properly sealed to eliminate infiltration of the surface water into the embankment material.