5.0 Ground Failure

5.1 Introduction

Observed ground failures, defined as permanent ground deformations induced by the earthquake, are considered relatively minor. As enumerated below, ground failures are related mainly to slope instability, rock falls, and seismic compression of unsaturated soils. Minor occurrences of localized lateral spreading may possibly be associated with some slope deformation features. In addition to these well known types of ground failure, some memorable but rather uncommon features of seismic slope instability are summarized. Figure 5.1 is a reference map with the locations of specific sites labelled (as cited in the text).

The observations presented herein do not include co-seismic ground deformation and the possibility of co-seismic surface fault rupture, which are described in Chapter 2.

Figure 5.1. Reference map with locations of specific sites.

Mountains surrounding the L'Aquila area are rugged; however, only limited occurrences of significant slope instability were observed. Typically, the failures are localized and minor, with modes including ravelling and sloughing of road cuts, quarries, and natural outcrops, permanent displacement of fill embankments, and rock falls. Examples of typical slope failures are summarized below, together with an interesting case of permanent displacement of saturated sediments around the margin of Lake Sinizzo.

Typical ravelling-type failures of cut slopes made into the strong and fractured limestone bedrock dominating the local geology are shown in Figures 5.2 and 5.3. The failures involve the uppermost weathered blocks that are bounded by soil-filled joints. These examples are located along SP38 approximately 9.5 km south of L'Aquila. In general, the performance of cut slopes into the limestone bedrock was excellent, and minor surficial failures such as shown were not widespread.

Figure 5.2. Shallow ravelling-type failure of weathered limestone blocks (42.278N, 13.467E).

Figure 5.3. Shallow ravelling-type failure of weathered limestone blocks (42.278N, 13.468E)

Minor ravelling of weakly cemented sand and gravel, as depicted in Figure 5.4, also occurred locally but significant or deep-seated failures in such deposits was not observed. Similar geologic materials were observed along the southern flank of Castelnuovo, with Figure 5.5 depicting a natural outcrop exposed by a structural collapse. It is not clear if failure of this slope contributed to the structural failure.

Figure 5.4. Minor ravelling of weakly cemented sand and gravel; dashed lines indicate the extent of freshly deposited talus along the base of road cut (Via Opi N42.261, E13.581)

Figure 5.5. Outcrop of cemented granular sediments exposed by structural collapse along southern flank of Castelnuovo (N42.294, E13.629).

Cuts made into clayey regolith also generally performed well, and only minor instances of shallow sloughing as shown in Figure 5.6 was observed.

Figure 5.6. Shallow sloughing of regolith along road cut (southern flank of Castelnuovo N42.294, E13.628).

Several quarries in the region surrounding L'Aquila have been developed in cataclastic geologic units formed along the base of the bounding mountain ranges. Cataclasites are created by the mechanical breakdown of bedrock materials resulting from past major fault activity, and in the region they are typically white to light gray, well cemented, with strongly interlocked clasts ranging from clay to boulder sizes. Typical characteristics of the cataclasite, as exposed along the Pettino fault approximately 4.0 km northwest of L'Aquila, are depicted in Figure 5.7.

Figure 5.7. Cataclasite exposed along the Pettino Fault (left) and detail of cemented and interlocked particulate structure (right) (42.376N, 13.365E).

Quarries in the cataclasite were observed in Fossa (Figure 5.8) and northeast of the town of Colle (Figures 5.9 and 5.10). These quarries performed very well, with only localized and insignificant minor ravelling.

Figure 5.8. Quarry in Cataclasite with estimated 600 slopes up to approximately 35 m high, located in Fossa (42.300N, 13.485E)

Figure 5.9. Quarry in Cataclasite with near vertical slopes segments up to approximately 15 m high, located northeast of Colle (42.435N, 13.330E).

Figure 5.10. Quarry in Cataclasite with estimated 500 slopes up to approximately 60 m high, located northeast of Colle (42.437N, 13.328E).

Approximately 300 m east of the quarry depicted in Figure 5.8, and continuing along the base of the northwest-trending mountain range, are natural outcrops of the cataclasite. The outcrops are recognizable by their characteristic white to light gray erosional scars (Figure 5.11). Although the natural slopes also experienced only minor and localized raveling-type failures, their occurrence (Figure 5.12) is greater than observed in the nearby cataclasite quarries. This is potentially attributable to the relative absence of weak and weathered near-surface material in the quarries, having already been removed by mining operations.

Figure 5.11. Characteristic erosional scars developed in cataclasite (42.434N, 13.334E).

Figure 5.12. Characteristic ravelling-type failure in natural outcrops of cataclasite; partially buried pine trees evidence recent movement (42.434N, 13.334E).

Few observations of side-hill fill embankment failures were made, with the exception of failures within a series of tight switchbacks along an unpaved rural access road located approximately 8.0 km north of L'Aquila. Headscarps of the failures, as depicted in Figures 5.13 and 5.14, appear to coincide with the approximate cut-fill contact. Corresponding maximum lateral and vertical displacements are estimated at about 2.0 m and 1.0 m, respectively.

Figure 5.13. Failure of side-hill fill embankment (42.420N 13.376E).

Figure 5.14. Failure of side-hill fill embankment (42.420N 13.376E).

Along the Aterno River and south and west of Onna are two bridges that suffered significant damage and complete collapse, respectively. In both cases, evidence of ground failure was observed in the abutment areas. The failures were represented as fissures oriented approximately parallel to the river alignment, developed in the alluvial floodplain at distances up to about 150 m from the river (Figure 5.15). Additionally, significant ground cracking was observed along the approach fills and foundation abutment/flood protection levee, with orientations tending parallel to the local strike of slope.

Figure 5.15. Ground fissures developed in alluvial floodplain directly south of central Onna and approximately 150 m from the Aterno River.? Orientation of fissures is approximately parallel to the river (42.325N, 13.480E).

Along the approach road to the bridge directly south of Onna are a series of about ten cracks oriented parallel to the river alignment and at distances ranging in the range of about 15 to 160 m from the Aterno River (Figure 5.16). The cracks are primarily tensional, extending through the asphalt but not into the sub-base or approach fill embankment (Figure 5.17). These cracks, together with those such as depicted in Figure 5.15, record permanent ground displacement that is considered consistent with the possibility of minor lateral spreading of the alluvial floodplain sediments toward the Aterno River.

Figure 5.16. Characteristic crack developed in bridge approach road directly south of Onna, at distances of 15 m to 160 m from the Aterno River (42.324N, 13.478E).

Figure 5.17. Typical tensile condition of road cracks such as depicted in Figure 5.15, not extending into neighboring sub-base or approach fill embankment (42.324N, 13.478E).

Adjacent to the bridge abutment south of Onna, significant but localized cracking of the flood protection levee was observed, as shown in Figures 5.18 and 5.19. The observed cracks are oriented subparallel to the Aterno River and are consistent with permanent displacement toward the free faces of the levee.

Figure 5.18. Ground cracks up to 1.0 m deep, 20 m long, and with maximum 0 15 cm vertical displacement (down-dropped toward river) developed directly adjacent to bridge abutment (42.324N, 13.478E).

Figure 5.19. Ground cracks developed along levee crest adjacent to bridge abutment (42.324N, 13.478E).

Approximately 1.3 km upstream and east of the ground failures summarized in Figures 5.14 through 5.18 is the site of a bridge collapse, north of the town of Monticchio (Figure 5.20). The approach fill exhibits linear cracks along the road shoulder that indicate on the order of 10-15 cm of permanent lateral displacement toward the local free face condition (Figure 5.21).

Figure 5.20. Bridge collapse along the Aterno River, approximately 450 m north of Monticchio (42.325N, 13.463E).

Figure 5.21. Approximately 15-20 cm lateral displacement of approach fill (42.325N, 13.463E).

Although the flood protection levee has side slopes up to about 1.25:1 (horizontal to vertical) and appeared to be in a rather loose condition at the surface, the levee generally performed well in the area of the bridge failure (Figure 5.22). An exception is the western abutment on the northern side of the river. As depicted in Figure 5.23, directly beneath the abutment a levee crack having a maximum width of about 6 cm and down-dropped 2-3 cm toward the river developed. From the observations made, it is not clear if the levee damage occurred first and possibly contributed to the collapse, or if the deck collapse enabled localized failure of the levee.

Figure 5.22. Flood protection levee in vicinity of bridge collapse (42.325N, 13.463E).

Figure 5.23. Ground cracking in levee directly beneath western abutment along northern side of Aterno River (42.325N, 13.463E).

Within the cataclasite quarry northeast of Colle and depicted in Figure 5.9, two parallel shear failures in clayey sand fill materials were observed (Figure 5.24). The shear failures are oriented perpendicular to the free face of the fill slope, have approximately 15 m separation, with vertical displacement systematically increasing to a maximum of about 40 cm toward the free face. A block of soil between the parallel shears has dropped down, forming a graben structure (Figure 5.25), but a distinctive headscarp has not developed. These structural relations would not be expected for usual soil slope failure modes, and further inspection indicates the shear failures resulted from differential seismic compression of the fill.

Figure 5.24. Shear failure developed in fill material, with vertical displacement increasing to approximately 40 cm at free slope face (42.435N, 13.330E).

Figure 5.25. Graben structure (between lines) created by parallel down-dropped shears (42.435N, 13.330E).

Figure 5.26 reveals that the fill material has been placed in a tight bedrock notch, and considering the nature of mining operations it was likely loose dumped and poorly compacted. The boundary conditions, with very steep bedrock sidewalls, promote differential settlement due to seismic compression. In this case the resulting shear strain appears to have been sufficient to cause shear rupture through the soil.

Figure 5.26. Graben structure (between vertical lines) and bedrock boundary conditions (42.435N, 13.330E).

Effects of differential compaction were also observed in boulder fill placed along the sidewalls of a cut-and-cover tunnel structure located about 1.5 km northeast of L'Aquila. As shown in Figure 5.27, the boulder fill is of fairly uniform particle size, and Figure 5.28 shows the amount of settlement (10 to 15 cm) relative to the spanning parapet wall foundation. Some of this settlement may have existed prior to the earthquake. Fresh cracks in the overlying asphaltic surfaces and a narrow settlement trend along the sidewall of the cut-and-cover structure may indicate the boulder fill is continuous and not just at the portals (Figure 5.29).

Figure 5.27. Boulder fill placed along side wall of cut-and-cover structure (42.358N 13.415E)

Figure 5.28. Settlement trough reaching 10 to 15 cm maximum, relative to the spanning parapet wall foundation (42.358N 13.415E).

Figure 5.29. Cracks in the overlying asphaltic surfaces and a narrow settlement trend along the sidewall of the cut-and-cover structure (42.358N 13.415E).

Minor instances of utility trench backfill such as depicted in Figure 5.30 were observed sporadically.

Figure 5.30. Seismic compression of trench backfill in the town of Paganica (42.365N, 13.465E).

Ground shaking was sufficient to destabilize loose (and perhaps somewhat precarious) surficial rock blocks that subsequently traveled downhill as rock falls. In the mountainous terrain traversed, little evidence of significant rock fall events (for example impact marks on road surfaces) was observed, with only a single case of a rock fall blocking a roadway (Figure 5.31). Additionally, a significant high-impact rock fall event occurred at the Caves of Stiffe (Grotte Stiffe). In this case, a large block was liberated from the slope high above this popular tourist area and impacted a concession building (Figure 5.32). The last impact mark of the block prior to impacting building is at a distance of about 15 m (Figure 5.33), indicating a very high velocity event. Additional impact marks are indicated by an alignment of fresh scars progressing up the hillside (Figure 5.34)

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Figure 5.31. Large block of a rock fall event (42.266N, 13.585E).

Figure 5.32. Rock fall impact at the Caves of Stiffe (42.255N 13.547E).

Figure 5.33. Last impact mark prior to impacting building at the Caves of Stiffe; the block trajectory was above the small tree (42.255N 13.547E).

Figure 5.34. Additional impact marks as indicated by an alignment of fresh scars (42.255N 13.547E).

Lake Sinizzo is situated in a natural karstic depression located east of San Demetrio ne Vestini. The lake is roughly circular in plan view, with an average diameter of approximately 120 m. The lake appears to be partially impounded by a small embankment located as shown in Figure 5.35. A bathymetry survey by Tete et al published in 1984 is shown in Figure 5.36, indicating maximum side slope relief of about 10 m.

Figure 5.35. Overview of Lake Sinizzo with location of impounding embankment indicated by vertical white line (42.291N 13.576E).

Figure 5.36. Bathymetric survey of Lake Sinizzo; impounding embankment at upper left corner (Tete et. al, 1984).

Significant ground cracking was observed along approximately 70-80 percent of the lake perimeter, such as depicted in Figures 5.37 and 5.38. Soils exposed in the sidewalls of the ground cracks are visually classified as clayey gravel (GC) to gravelly clay (CH), with notable high plasticity of the fines. These materials may have a mixed alluvial/lacustrian origin, and artificial near surface fill may exist locally. A series of tape-and-compass survey transects oriented perpendicular to the local shoreline indicate maximum lateral displacements (above shoreline) of about 1.2 m, with significant cracking occurring at distances up to about 10 m from the shoreline.

Figure 5.37. Ground cracks along the northwestern perimeter of Lake Sinizzo (42.291N, E13.576).

Figure 5.38. Ground cracks along the eastern perimeter of Lake Sinizzo (42.291N, E13.576).

Several meters of local slope displacement are evidenced by submerged trees and a prominent arcuate landslide scar near the western margin of Lake Sinizzo (Figure 5.39). Pre-and-post earthquake imagery, shown in Figures 5.35 and 5.40, respectively, indicate that submergence occurred as a result of slope displacement during the earthquake.

Figure 5.39. Submerged trees located several meters from the western margin of Lake Sinizzo; picnic table within arcuate landslide scar at shoreline. (42.291N 13.576).

Figure 5.40. Post earthquake satellite imagery showing submerged trees within small circle and rock slope failures within large circle; compare to pre-earthquake imagery shown in Figure 5.35 (42.291N 13.576).

LIDAR imaging of the shoreline was performed 11 days after the mainshock. This data is being processed as of this writing. Post-earthquake bathymetry is being planned. These data sets will be released in subsequent publications.

Along the hillside directly northeast of Lake Sinizzo are fresh scars and deposits resulting from rock slope instability (Figure 5.41). The failures involve a prominent and locally overhanging ledge of limestone, but the precise detachment mechanism was not evident from distant observations. The pre-and-post earthquake imagery (Figures 5.35 and 5.40, respectively), indicate these instabilities were triggered by the earthquake.

Figure 5.41. Fresh scars and rock debris deposits along the hillside directly northeast of Lake Sinizzo (42.292N, 13.580E).