Report_Geotechnical_ER-HWY-72.pdf

LUIS O. GARCIA & ASSOCIATES

GEOTECHNICAL ENGINEERING CONSULTANTS

Partners Luis Oscar García, MSCE, CE Carlos García Echevarría, MSCE

Associate Alejandro E. Soto, MS, PG

AMELIA DISTRIBUTION CENTER, EMMA STREET NO. 26A, GUAYNABO, PR 00968-8007

TEL. (787) 792-2626 FAX (787) 782-5990

Email: cgarcia@geocim.com

GEOTECHNICAL MEMORANDUM

TO : Eng. José Carro, P.E.

CMA Architects & Engineers jcarro@cmapr.com

FROM : Eng. Carlos García Echevarría, P.E.

Geol. Alejandro Soto, P.G.

RE : Landslide Evaluation PR-119, Km. 76.1, San Germán, PR

JOB NO. : 5144

DATE : December 10, 2018

This memorandum presents the results of a geotechnical investigation of a landslide that impacted State Road PR-119 at Kilometer 76.1 in Hoconuco Alto Ward of the municipality of

San Germán. The approximate location of the project site is shown in Figure 1 – Site Location

Map. The objective of the investigation was to evaluate subsoil conditions at the site in order to provide geotechnical recommendations for the design and construction of landslide stabilization alternatives that would enable the full reopening of PR-119. The Puerto Rico Highway and

Transportation Authority (PRHTA) are the project sponsors and CMA Architects-Engineers

(CMA) are the Project Designers. The work was performed in general accordance with our proposal dated April 25, 2018. Advanced results and preliminary conclusions and

Landslide at PR-119, Km. 76.1, San Germán, PR

December 10, 2018.

recommendations have already been discussed with and presented to CMA. This report provides a summary of the activities performed during the course of the investigation, a description of subsurface conditions encountered at the site, and geotechnical recommendations for the design and construction of landslide stabilization alternatives for the project.

The project site contains one (1) landslide area on the outside edge of the roadway where part of the roadway was affected and concrete barriers were installed for security measures.

The geotechnical investigation performed consisted of the drilling one (1) boring (BC-1) to a depth of 22 feet for a total of 22 lineal feet of drilling exploration; refusal to penetration of the auger was attained at this depth. The location of the boring at the site was established in the field by GeoCim by visual observation from existing local references; the presence of underground and areal utilities in the area and vehicle traffic requirements on the roadway were also taken into consideration in the selection of the location of the boring. The approximate boring location is shown on Figure 2 which uses a survey plan provided by the CMA; boring

BC-1 was located adjacent to the landslide on the outer edge of the roadway.

Geo Cim, Inc., our affiliate geotechnical testing services firm, performed the field and laboratory work. The borings were drilled with a CME-45 drill rig using the "Hollow Stem

Auger-Dry Sample" method of drilling and an Automatic SPT hammer. Soil samples were collected beginning at the ground surface, at 2.5-foot depth intervals to a depth of 10 feet, and at

5-foot depth intervals from a depth of 10 feet to the bottom of the borings. The samples were collected with a split-spoon sampler driven into the ground. The number of blows required to drive the split-spoon sampler into the soil was recorded in the field; the N-value of the SPT represents the number of blows required for the penetration of the last 12 inches of the sampler.

The samples were placed in tightly closed plastic bottles and transported to the GeoCim

Geotechnical Testing Laboratory in Guaynabo for unconfined compression testing (in cohesive samples using a spring tester and/or pocket penetrometer), natural moisture content determinations, and visual/manual soil descriptions following standard laboratory procedures. A total of two (2) Classification Tests, including Sieve Analysis and Atterberg Limits, were performed on selected samples from the borings. The results of the field operations, laboratory testing, and detailed descriptions of the soils encountered at each boring are documented in the boring logs included in Appendix A. Appendix B contains a description of the GeoCim, Inc.

drilling, sampling, and testing procedures. Results of the Classification tests are provided in

Appendix C.

GEOLOGY AND SUBSOIL CONDITIONS

Figure 3 reproduces the portion of the San Germán Quadrangle geologic map prepared by the U.S. Geological Survey (USGS) wherein the PR-119 Km 76.1 landslide is located; the geologic units present in the area are distinguished by different colors and letter symbols. The site sits within the outcrop of map unit Ky (dark green-colored) which is identified as the Yauco

Formation of Cretaceous age (over 65 million years old). According to the USGS, the Yauco

Formation consists of “thin- to medium-bedded variously interbedded calcareous siltstone, argillaceous limestone, claystone,” and “fine-grained tuff”. These old strata have been folded and faulted and presently exhibit widely varying orientations throughout their outcrop. There is no information on bedding orientation near the Km 76.1 landslide site, but the nearest strike-and-dip symbols to the south show steep southwesterly dips whereas those to the west show moderate to steep northwesterly to northeasterly dips.

The USGS map shows three (3) other map units near the project site. These include two other Cretaceous units that are in fault contact with the Yauco Formation several hundred meters south of the site: the predominantly volcaniclastic strata of the Sabana Grande Formation (unit

Ks) and basaltic-andesitic lavas of a Basaltic Andesite unit (labeled Kba). The bounding fault is oriented to the northeast and its trace is marked by a dashed line that indicates that the location shown is approximate; no other geologic faults are shown near the project site. The third other map unit occurs just north of the Km 76.1 landslide site and consists of Quaternary (last 2.5 million years of Earth history) alluvium (unit Qa) deposited within the Río Hoconuco flood plain, which sits some 30 to 40 feet below the level of the PR-119 pavement. The USGS describes the alluvium as mixtures of alluvial “clay, silt, sand, and gravel” that mixes with colluvial debris along the valley margins.

The subsoil profile seen in boring BC-1 consist of an upper Fill layer underlain by a

Silt/Clay strata which in turn is underlain by Weathered Rock (Volcanic Sandstone). The Fill layer extends from the existing ground surface to about 2 feet depth and is described as a Gravel with little sand (probably pavement base course locally called mogolla) in a loose to medium dense condition as indicated by N-value of 9 blows per foot (bpf). Underlying the Fill layer and extending to about 17 feet depth is a Silt/Clay stratum which is described as a Silt with little to some fine sand and which is encountered in a medium stiff to hard consistency as indicated by

N-values of 8 to 28 bpf and Unconfined Compressive Strengths (using spring tester) of 3 to over

4.5 tons per square foot (tsf). Results of Classification tests performed on soil samples from this strata obtained from 2.5 to 4 feet and 7.5 to 9 feet depths indicate that it classifies as a low plasticity Clay (CL) to clayey Sand (SC) with 36.5% to 53.6% fines and Liquid Limit (LL) of

41% to 49% and Plasticity Index (PI) of 15% to 23%.

Underlying the Silt/Clay strata and extending to the bottom of the boring at 22 feet depth is a Weathered Volcanic Sandstone strata which is encountered in a very dense condition as indicated by N-values of over 100 bpf indicating virtual refusal to the penetration of the sampler and finally refusal to the penetration of the auger at the bottom of the boring. Groundwater was not observed during the drilling operations.

A detailed description of the soils encountered is provided in the boring logs in Appendix

A. Groundwater observations were made during drilling operations or within the open borehole immediately after drilling operations were completed and the augers were removed from the ground. For more accurate determination of groundwater levels or to determine groundwater level variations under different conditions (at times of heavy rain, etc.) and on a long term basis, then installation of piezometers and observation wells and their monitoring during extended periods of time would be required.

ANALYSIS, CONCLUSIONS AND RECOMMENDATIONS

This section of the report presents our analysis, conclusions and recommendations based on the results of the geotechnical investigation and site reconnaissance performed for each of the landslide areas within the site. It is important to note that the main objective of these recommendations is to provide stability to the reconstructed roadway in the landslide areas identified.

Based on the results of the boring drilled and the site reconnaissance performed we understand that the landslide on the outside edge of the roadway is occurring within the upper

Fill horizons which extend to about 2 feet depth and the underlying Silt/Clay strata below (visual observations of the landslide area are limited due to heavily vegetated and steep slope conditions). This landslide is probably due to erosion of the fill and underlying soils due to uncontrolled surface water runoff over the outside edge of the roadway during the heavy rainfall associated with Hurricane María. Any stabilization alternative in this area will have to extend below the upper Fill layer and into the underlying stiff to very stiff Silt/Clay strata. The stabilization alternatives may include the following:

1. Gravity retaining wall such as a Gabion or Concrete wall – this will require an excavation of a minimum of about 5 feet in order for the wall to bear on the underlying stiff to very stiff Silt/Clay strata. The location of the wall shall be established such that the top of the proposed wall extends to the edge of the reconstructed roadway; the final depth of the wall shall also provide for the outside bottom edge of the wall to be located a minimum of 3 to 5 feet from the outside edge of the Silt/Clay strata on the existing slope which may be approximately established by the Designer using the topographic survey information.

At the required excavation level an Allowable Soil Bearing Pressure of 4,000 pounds per square foot (psf) may be utilized for wall design; PRHTA standard drawings for these walls may be utilized.

For a detailed design, the lateral pressures acting on the retaining wall shall be determined using a triangular earth pressure diagram acting on a vertical plane passing through the heel of the wall and extending from the bottom of the wall to the proposed ground surface. This triangular earth pressure diagram shall be calculated using an

“active” lateral earth pressure coefficient, Ka, and an angle of internal friction (Ø) of 32 degrees for the backfill material to be utilized; a unit weight (γ) of 130 pounds per cubic foot (pcf) may also be used for the backfill material behind the walls. Any other surcharge loads acting on the ground surface at or near the walls must be added to the previously mentioned earth pressures to determine the final design lateral loads acting on the walls.

The above lateral earth pressure recommendations assume that the backfill material behind the wall will consist of an engineered compacted fill constructed as described under the Earthwork Recommendations section of this report. The backfill material within 10 feet from the wall shall be placed in 4 to 6 inch thick layers and compacted with light, hand operated compaction equipment to 90% of the maximum dry density as obtained in the Modified Proctor Test (ASTM D 1557). The backfill material beyond 10 feet from the wall shall be placed in 8 to 12 inches thick layers and each layer compacted to 95% of the maximum dry density as obtained in the Modified Proctor Test

(ASTM D 1557) using conventional compaction equipment.

We have also assumed that the design of the retaining wall will include a drainage system that will provide a fast and efficient way to collect and dispose of any water that may infiltrate behind the wall in order to prevent the development of unbalanced water forces behind the wall; a typical drainage system would include a free draining crushed stone layer behind the retaining wall separated from the backfill material by a geotextile filter fabric (such as Mirafi 140N or equivalent), and a perforated drainage pipe placed at the bottom of the wall and connected to a proper outlet.

Alternatively, other types of drainage composites specifically manufactured for drainage behind retaining walls may be used.

Prior to beginning construction of the retaining wall, the foundation bearing surface for the wall shall be cleaned and any soil disturbed during the excavation operations (or any time thereafter) shall be compacted or removed to provide an undisturbed soil bearing surface. We recommend that a Geotechnical Engineer and/or

Engineering Geologist be present at the site when the landslide area is excavated in order to observe and confirm that the proposed excavation depths extend below the landslide sliding surface and that a firm bearing surface is reached.

2. Reinforced concrete retaining wall – in this alternative a reinforced concrete retaining wall may be designed following the same recommendations provided above for the

Gravity Retaining Wall, including bearing on the Silt/Clay strata and the inclusion of adequate drainage measures behind the wall.

3. Mechanically Stabilized Earth Wall – in this alternative this retaining wall may be designed following the same recommendations provided above for the Gravity Retaining

Wall, including bearing on the Silt/Clay strata and the inclusion of adequate drainage measures behind the wall.

Earthwork Recommendations

The following general earthwork recommendations are provided for construction of any fill or backfill that may be required within the project in the landslide areas.

1. Prior to commencing construction the site shall be cleared and grubbed of vegetation cover and topsoil that may exist at the ground surface and any existing asphalt/concrete pavement from the existing roadway shall be demolished and removed from the site. After the clearing, grubbing, and demolition operations of the existing facilities are completed, and after any cut excavations are performed, the exposed surfaces shall then be proof-rolled with several passes of a heavy vibratory roller to detect any soft soil spot that may have to be removed and to densify any surface soil that may have been disturbed during the clearing and grubbing, and/or cut excavation operations.

2- Excavations into the fill and existing soils near the ground surface may be performed with conventional excavating equipment, including bulldozers and/or excavators, as indicated by the results of the borings.

3- Materials from required cuts at the site shall not be used as Fill at the site. Fill and/or backfill material to be used at the site shall consist of a non-expansive A-2-4 soil or better in accordance with the AASHTO (American Association of State Highway and Transportation

Officials) Soil Classification system. All fill within the site shall be placed as engineered compacted fill in 8 to 10 inch thick layers and each layer compacted to a minimum of 95% of the Maximum Dry Density as obtained in the Modified Proctor Test (ASTM D-1557); in excavations of limited extent the thickness of the fill layers placed shall be reduced to about 4 to 6 inches in order to attain the above compaction level. Fill required on sloped surfaces shall be placed in horizontal layers starting from the lower elevations and each horizontal layer shall be keyed into the in-situ soils so as to prevent the development of a preferred surface of sliding along the contact of the natural soils and the engineered compacted fill.

4- Temporary cut excavations deeper 4 feet shall be performed with cut slopes not steeper than

1(H):1(V); alternatively, the cut shall be properly braced and supported. Since bracing and support of cut excavations is a temporary construction feature, the Contractor shall design and select the most suitable support system for the excavation equipment and technique that he will use in the project.

5- We recommend that final cut and fill slopes (if any) be constructed at inclinations not to exceed 1.5(H):1(V) and 2(H):1(V), respectively. All cut and fill slopes shall be properly and promptly vegetated in order to prevent excessive erosion of the soils that may cause instability of the slopes.

6- The project should be provided with adequate surface drainage measures to collect and conduct runoff to the project storm water system. Under no circumstance should water be allowed to accumulate anywhere within the project or to spill over the recommended retaining walls and/or slope embankments.

We request to be consulted should there be any questions on the intent and general scope of our recommendations. Likewise, we request to be informed of any changes in the scope of this project that may require our revision of these recommendations or if additional recommendations are needed.

N:\0000-project files\5144 Landslide Evaluations CMA\PR-119 Km76.1 San German\PR-119 Km 76.1 Landslide Report.doc

Figure 3: Portion of the San Germán geologic quadrangle map that shows the general geology of the PR-119

Km 76.1 landslides area (the site sits along the north map boundary). The various map units are identified by different colors and letter symbols. The Km 76.1 landslide section is located near the tip of the arrow within the outcrop of the Yauco Formation, the dark green-colored unit labeled Ky. Other map units shown include the Sabana Grande Formation (unit Ks in light green), a Basaltic Andesite unit (gray-colored unit Kba), and alluvial sediment along the Río Hoconuco (unit Qa in yellow). See report text for more information.

Ky

Ks

Kba

Qa

APPENDIX A

BORING LOGS

APPENDIX B

FIELD AND LABORATORY TEST PROCEDURES

GEO CIM, INC.

Appendix B

Field and Laboratory Test Procedures

I. DRILLING METHODS

A. Hollow Stem Auger Boring - Dry Sample method (ASTM-D-1452)

In the auger method, borings are advanced by turning a hydraulic powered auger into the ground in 5 feet increments or less. As the auger penetrates the cuttings rise to the surface on the spirals. The depth from which the material comes, however, cannot be accurately determined. Therefore, soil samples are not taken from the spiral augers. With the use of a hollow stem auger, a sampling apparatus may be inserted in the hollow stem to the bottom of the auger, eliminating the need for casings. The sampling procedures are described in the following paragraphs. Geo Cim, Inc. normally uses CME (Central Mining Equipment) drilling rigs which are trailer mounted and pulled with a 4 x 4 wheel drive F-350 or equivalent truck. The drilling rigs are either CME-45 or CME-55 models.

B. Wash Boring - Dry Sample Method

For inaccessible locations a portable drilling unit is used which consists of a tripod mounted motorized cathead which is used to drive casings and sampling rods to conduct this method of drilling. Borings are advanced into the ground by the wash-boring-dry sample method. The borings are normally cased through most of this length by the drop of either a 350 or 140 lbs. hammer from a height of 30 inches. The casing diameter is 2-1/2 inches. The number of blows for every foot of penetration the casing advanced is recorded and is reported in the boring logs. When driving of the casing becomes too difficult due to the hardness of the soils encountered, and the hole does not cave-in, the boring may be advances without casing by continuous washing out of the soils with the drill rods. The color and nature of the soils washed out (gravel, sand, silts or clay) is examined and recorded in order to determine the extent of each soil strata this is complemented with the sampling operations which are regularly made every five feet.

C. Core Borings (ASTM-D-2113)

Core Borings are used when necessary to penetrate into rock and obtain a continuous rock sample. The sample is obtained by means of a core barrel which is attached to the drill rods. At the end of the core barrel is a special bit studded with industrial diamonds which cuts into the rock. The drill rod, and hence the core barrel and diamond bit are rotated as downward pressure is applied. As the bit cuts into the rock, the rock core is free to move into the inner core barrel head, which is suspended on a swivel and therefore does not follow the motion of the outer core barrel with its bit. Cooling water or bentonite slurry is circulated through the drill rod and the core barrel. Penetration depends on the length of the core barrel and the quality of the rock (amount of joints or fractures). Rock discontinuities such as joints and fractures affect the length of penetration in a given core run. The runs are longer as the rock quality increases. As the core barrel is withdrawn, the core lifter, located inside the diamond bit, wedges itself around the bottom of the rock core and thus permits it to be pulled free from the underlying rock. The two most common core sizes are NWM which produces a core of 2.15 inches diameter and HWM which produces a core 3.00 inches in diameter.

II. SAMPLING IN SOILS

A. Standard Penetration Test (ASTM-D-1586)

The standard penetration tests are made with a split-spoon sampler 2.0 inches/O.D.

diameter, 1,375 inches I.D. diameter, 18 inches long. The sampler is driven into the ground by the drop of a 140 pounds hammer from a height of 30 inches. The number of blows from the three consecutive six inches of penetration of the sampler is recorded and the number of blows for the last foot of penetration is reported as the N-value. The samples are stored and sealed in glass jars for future classification tests in the laboratory.

The standard penetration test has been correlated with the consistency of fine grained soils and with the angle of internal friction or the relative density of sands. Such correlations can be used for preliminary estimates and to aid in the stratigraphic classification of the soil strata at a given site.

In the case of fine grained soils, the correlations of the SPT with the undrained shear strength of medium and stiff silts and clays of low sensitivity have been found to be fairly good; however, in the case of the soft silts and clays the SPT gives poor estimates of the undrained shear strength. Testing in undisturbed samples and vane shear tests are recommended in such cases.

B. Undisturbed Sampling

Undisturbed Samples are obtained using thin wall brass or zinc coated steel shelby tubes 2" to 3"O.D. by 24" to 30" long. The sampler is forced to penetrate the soil by static force or downward pressure and is pulled out also using static pull. The samplers are sealed in the field and shipped to the laboratory where the samples are extruded at the time of testing by pushing in the same direction that the sample penetrated the sampler. Special care is taken in packing and handling these samples to avoid disturbance.

III. LABORATORY TESTING

A. Atterberg Limits (ASTM-D-2166)

The Atterberg limits and related indices are very commonly used in soil mechanics works mainly for soil identification and classification purposes. They are also used in connection with some semi-empirical methods of design for preliminary estimates.

B. Unconfined Compression Test (ASTM-D-2166)

The best quality samples recovered in the split-spoon sampler are subjected to unconfined compression to failure. These samples cannot be regarded as “undisturbed” and the strengths obtained are lower than the “true” in-situ value due to the effect of disturbances and sensitivity of the soil. Furthermore, increased brittleness of friability of the soil structure results in strength values lower than the in-situ undrained strength.

Therefore, the unconfined strength values measured in the split-spoon samples are used only as an index property for classification and identification purposes. When more accurate values of strength are needed, undisturbed samples from thin-wall shelby tubes are used.

C. Natural Moisture Content (ASTM-D-2216)

The natural moisture content is the water content of the soil in-situ. The soil sample is obtained from the SPT sampler or shelby tubes. Approximately 40 gms. of soil are weighed and then placed in an oven for 24 hours at a temperature of 105� - 110�C. The natural moisture content, “Wn” is obtained from the difference between the weights in the natural state and after oven drying, divided by the dry weight of the sample and expressed in percentage.

IV. SOIL DESCRIPTION (ASTM-D-2488)

The description of the soils include the type (gravel, sand, silt, clay, organic), consistency (if a fine grained soil), size and roundness (if a coarse grained soil), color, and some other special characteristic which can aid in the identification and classification of the soil such as presence of rubbish, organic matter, shells and fossils, stratification and structure, cementation, mineral composition, relict structures, stains and others.

To aid in the preparation of these descriptions some simple tests are made such as those recommended by the Unified Soil Classification system for field classification (dilatancy, dry strength, shine, toughness). To approximate the consistency of fine grained soils (soft, medium, stiff, hard), a simple test is performed with the hand: a hard fine grained soil is difficult to indent with the thumbnail, stiff soils are readily indented with the thumb, medium soils can be penetrated by moderate thumb pressure and soft soils are easily penetrated with the thumb.

The description of coarse grained soils (sands and gravel) includes size (fine, medium coarse) and roundness (angular, sub-angular, sub-rounded, rounded and well-rounded).

The relative amount of coarse fractions in fine grained soils is estimated by placing a representative sample or some 50 gms. in a graduated cylinder filled with water. The mix is shaken and allowed to settle. Particles of a size larger than a fine sand are visible to the naked eye while silts and clays are not. In this manner, estimates of the relative amounts of the coarse fraction are made and reported as:

Trace 1 10% Little 10 20%

Some 20 35% Sandy, gravelly, silty or clayey 35 50%

The relative density of sands has been correlated with the results of the Standard

Penetration Test, as follows:

For Granular Soils N-Blows/ft. Relative Density

0 – 4 Very loose 4 – 10 Loose

10 – 30 Medium 30 – 50 Dense over 50 Very dense

These are very approximate correlations which vary with, among other factors, overburden pressure (Gibbs and Holtz, 1957, and Peck and Bazaraa, 1967). These correlations are meaningless in soils with a significant amount of gravel or cobbles.

The relative amounts of the fine grained soils is estimated based on the reaction of the soils to the dilatancy, shine, dry strength and toughness tests, with adjective indicating the less active fraction i.e. a sandy clay behaves more like a clay than a sand.

The consistency of cohesive soils has also been correlated to the results of the Standard Penetration Test, as shown below. The correlation, however, is greatly affected by the clay structure and factors such as sensitivity.

For Cohesive Soils

Unconfined Compressive Strength (T.S.F.)

N-Blows/Ft.

Consistency

Less than 0.25 Less than 2 Very soft

0.25 – 0.50 2 – 4 Soft

0.50 – 1.00 4 – 8 Medium

1.00 – 2.00 8 – 15 Stiff

2.00 – 4.00 15 – 30 Very stiff More than 4.00 More than 30 Hard

Once the soil samples have been tested, they are stored for three months after the date of our final report, and then destroyed, unless required in writing by the client to store them for a longer period.

APPENDIX C

CLASSIFICATION TEST REPORTS