Report - Geotechnical_HWY_9_AC-827579.pdf

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Soil & Construction Materials Laboratory and Environmental Drilling Services

GEOTECHNICAL SUBSOIL EXPLORATION AND

REPORT ON LANDSLIDE AT PR-10, KM. 47.5

PROJECT I.D. ER-HWY-9 (PROJECT SITE 1)

UTUADO, PUERTO RICO

REVISION No.1

1.0 INTRODUCTION

This report presents the results of a Geotechnical Exploration performed at a site where slope instability problems have been observed in State Road PR-10, Km. 47.5, Utuado, Puerto Rico.

The work was performed at the request of Eng. Ray Morales, on behalf of the PRHTA, following the scope of work presented in our May 8, 2018's technical proposal to JOV &

Associates.

The purpose of the exploration was to establish the physical properties of the subsurface soils and review previous geoetchnical reports available to our firm, in order to present possible alternatives for remedial works. As discussed with Eng. Ray Morales, the intent of this study is to present geotechnical recommendations that permit the reconstruction and/or replacement of the drainage system that exists at the study area. To achieve this, the present soil mass (landslide toe) infringing the north shoulder of the PR-10 would need to be draw back away from the highway.

2.0 SITE LOCATION AND BACKGROUND INFORMATION

The specific evaluation pertains to the road sector around km. 47.5. The site comprises portion of the PR-10 alignment, approximately a 120 meters long, within the vicinity of Caguanas sector of Rio Abajo Ward in Utuado. Figure 1 shows the project location depicted on a copy of an aerial photograph. Figure 2 presents portion of the topographic plan with the particular area under study. Figure 3 is a cross section that shows how the toe of the slope infringes the north shoulder of the highway.

GEOTECHNICAL SUBSOIL EXPLORATION AND

REPORT ON LANDSLIDE AT PR-10 KM 47.5

PROJECT I.D. ER-HWY-9, UTUADO, PUERTO RICO

September 22, 2020

Figure 1. Site location

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Figure 2. Area under investigation

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Figure 3. Cross section showing how the toe of the slope (old landslide toe) infringes the north lane of PR-10.

3.0 PROBLEM DESCRIPTION

The highway alignment roughly parallels the Lares Cuesta Scarp, which is the steep cliff to the north side of the highway, rising in angles in excess of 50 degrees to elevations of up to 120 meters above highway level (see Figure 1). North of the scarp is a cone-Karsted Terrain characterized by North-South and East-West tending residual limestone ridges which rise 70 to 100 meters above similarly tending sinkholes and inter-ridge valleys.

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The Cuesta Scarp is the result primarily of relatively large landslides which have created a steep cliff by the breaking away of blocks of limestone. Several hundred meters north of the study area, limestone blocks tens to hundreds of cubic meters in volume can be observed separated by vertical joints, and often tilted. The ground surface between the bottom of the Cuesta Scarp and the roadway is a gently to rolling ground surface at angle that range from a steep 1:1 to 3:1 (H:V), often undulating ground surface. The same is typically littered with limestone blocks and boulders.

Numerous water seeps daylight irregularly within the study area.

Several landslides have occurred within this sector following PR-10 construction operations back in 1995. The main landslide is roughly oblate and elongated in plan view. It has a main scarp and several secondary progressive scarps have also been observed and traced all the way up to the bottom of the Cuesta Scarp. The toe of the landslide is well delineated as it pushed against the opposing highway cutslope. The latter is the reason why access along the highway under construction was truncated by the landslide mass itself.

Figure 4 is a picture of the frontal view of the main landslide taken in on August 1995. This failure had the characteristics of a "flow" of a large mass of the saturated colluvium soils "riding down" on the contact with the San Sebastián clayey formation. This failure was triggered by the excavations at the toe of the slope and, in this particular case, the "failed" mass extended as far as the toe of the limestone formation scarpment.

The landslide toe presented in Figure 3 is several meters high and it is visually observed to consist of a colluvium mass (limestone and clay matrix), saturated, with some clay extrusion with in its front. Large blocks of limestone are evident within the clay matrix. The clay extrusion seems to be the result of a mechanism by which the sliding mass (colluvial deposit) is sliding over a pre-consolidated clay deposit (the San Sebastián Formation) and at the same time the same clay formation is being extruded from underneath. The latter is typical of failures within this project.

High water pressures are present from groundwater seeping from sinks located behind the

Cuesta Scarp into the colluvium. This sinks keep recharging the groundwater table.

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Figure 4. Frontal view of landslide back in year 1995

4.0 WORK PERFORMED

A total of two (2) boreholes were drilled to a depth of 40 feet in order to corroborate subsoil conditions at the area. The boring locations are shown on the site plan included as Figure 2.

Complementary subsoil conditions were gathered from previous borings performed by Vázquez

Castillo-Rodriguez Molina (VC-RM) at the area of interest. A comprehensive geotechnical report was prepared by our firm back in year 1995, part of which has been re-printed in the present report for the benefit of the reader.

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Various Atterberg Limit tests were carried out on selected samples to perform parametric correlations with shear resistance. A review of available geologic and topographic data was also performed. Site visits and geologic reconnaissances were conducted during the field work. Finally, all the information was used to prepare a subsurface model for the stability evaluation and for developing earthwork recommendations.

5.0 GEOLOGY ASSESSMENT

The site has been previously mapped as part of the Utuado Geology Quadrangle prepared by Arthur E. Nelson for the U.S. Geological Survey in 1967. For most of its alignment, the PR-10 is underlaid (in order of increasing age) by Quaternary Landslide Deposits (QL), the Lares

Limestone (TL), the San Sebastián Formation (TS) and a generalized volcanic rock basement (Ty, Ka).

Generally speaking, the area is characterized by sub-horizontal, middle Tertiary terrigenous sediment and limestone resting unconformable over steeply dipping, older volcanic rocks. The position and shape of the contact between these geologic rock groups is not known throughout most of the area. This deposition began when sea level rose and inundated low-lying coastal areas of the old island of Puerto Rico. High points remained as islands around which terrigenous sediment and eventually limestone were deposited. Eventually, these islands were submerged and also became sites of limestone accumulation. On them, however, the lower part of the middle Tertiary sequence is missing.

5.1 Quaternary Landslides (Ql)

The youngest map unit consists of Quaternary (relatively recent) Landslide deposits. These deposits are generally composed of colluvial, gravity-laid, loose, heterogenous and incoherent masses of soil material, organic matter, limestone fragments (gravel to boulder size) deposited at the base of the Lares Cuesta Scarp by sheetwash, landsliding and soil creep. In the quadrangle map these deposits are shown to underlie most of the bench area below the Escarpment.

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5.2 Middle Tertiary Sequence

The middle Tertiary sequence consists of muddy terrigenous sediment of the San Sebastián

Formation and limestone of the Lares Limestone and Cibao Limestone Formations. It underlies the

Escarpment above elevations of about 280 meters. The shape of the ground surface above this level generally reflects the outcrop of these two units: the San Sebastián Formation clays underlie the gently sloping bench at the base of the near vertical cliffs held up by indurated strata of the Lares

Limestone.

5.3 Lares Limestone

Resting conformably on the San Sebastián Formation and in places uncorformably over the volcanic basement rocks are the gently dipping beds of the Lares Limestone. These beds outcrop along the Escarpment cliff, in recent highway cuts along its base, and throughout the karstic terrain to the north of the Cuesta Scarp.

5.4 San Sebastián Formation

The most significant feature of the San Sebastián Formation is its muddy character. The unit consists of interbedded and lenticular masses of clay, silty to conglomeratic clay and clayey silt.

These vary greatly in color from green, brown, reddish-brown, red and brownish to dark gray. The lower part of the units is textually coarser, being predominantly conglom ratic. Clasts are composed of weathered gravel to cobble-sized red, green, purple and dark volcanic rock fragments surrounded by a clayey matrix. Clasts are mostly soft to friable because of intense weathering. In places, the volcanic fragments are completely altered to clay and except for their color they are indistinct from the matrix. The upper part of the formation is dominated by varicolored silty clay, clay, calcareous clay and organic clay. Thin carbonaceous layers (lignite) occu.r at the top of the unit.

Clays within the unit have low to very high plasticity. They are stiff and fissured to soft and massive. Fissuring is due to previous slide deformation and overconsolidation. by desiccation and unloading. Smooth, shiny break surfaces generally smaller than 0.5 cm are locally abundant. These are similar to surfaces commonly found in creep-prone clayey soils.

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The thickness of the San Sebastián Formation varies between 0 meters to about 15 meters.

5.5 Hydrology

Geologic factors favorable to landsliding are related to the presence of weak clayey material overlain by more competent strata outcropping along a major topographic escarpment. Groundwater flow from sinkholes behind the escarpment into the weak materials at its base. This fosters landsliding by keeping the clayey sediments wet and inducing large pore water pressures.

The area receives between 80-90 inches of rain per year with most of it falling during the wet months of May through November. From the report prepared by VC-RM it is known that many springs along the PR-10 alignment discharged between 50-110 gpm. During PR-10 construction, a weir was constructed near the study area to estimate groundwater flows emanating from the burst out seep that developed after the landslide. During dry periods the flow was estimated at 200 to 300 gpm, while during intense rainy periods the flow was estimated at 2,800 to 3,000 gpm, and even then some water was lost by spilling sideways before flowing into the weir.

6.0 SUBSOIL CONDITIONS

Compared with the subsoil information obtained from the VC-RM report, the two (2) borings recently performed for the present report revealed fairly consistent subsurface conditions.

Most important of all, the subsurface conditions revealed from the old and new borings are in accordance with the geologic conditions observed at the site. That is, an upper horizon of colluvial deposits lays over the San Sebastián Formation, followed by volcanic rock. The general description of these horizons are presented next:

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6.1 Upper Horizon-Colluvial Deposits

The colluvial deposits consists of heterogeneous or landslide debris composed of brown, yellowish brown, calcareous sandy clayey silt matrix with angular to rounded limestone blocks and boulders. The blocks vary in size from a few inches to several feet. The colluvial deposits developed as falling debris from the Lares Cuesta Scarp. The silty soil matrix mostly has a medium to very hard consistency. Occasionally this horizon seems to be indistinguishable from an earthy limestone horizon, so for geomorphological reasons it has been discerned to be a colluvial, rather than an in-situ limestone horizon.

The colluvium has been found to be from several feet to over 120 feet in depth, the larger thickness being the borings drilled closer to the Cuesta Scarp.

6.2 Middle Horizon -San Sebastián Formation

In its main part, the San Sebastián basically consists of a purple, reddish brown and green, clayey silt and silty clay of a hard consistency. Most of the samples reveal a shiny luster on exposed surfaces upon breaking the samples. The formation is fissured and sometimes contains traces to subordinate amponts of sand and faint clast, an end result of a highly weathered conglomerate/alluvium.

According to borehole information the thickness of the San Sebastián Formation generally varies in this sector from 10 feet to approximately 55 feet. Standard Penetration Test Values (N-

Value) recorded for the formation range from 19 to in excess of 100 blows per foot. Tested soil samples from the formation exhibited unconfined compressive strength values from 1.0 to in excess of 5 tons per square foot, which is an indication of the hard consistency of these deposits. Natural moisture content generally ranges between 19 and 40 percent, but it is suspected that through the actual failure planes (or slip surfaces) the water content is higher.

Atterberg limits were determined for several soil samples from this formation. Basically the

Liquid limit range from 39 to 57 percent. From previous explorations by others, the San Sebastián

Formation is known to be over-consolidated.

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6.3 Third Horizon - Volcanic Basement

This horizon was only probed by deep boreholes previously performed near the study area and penetration into the rock basement was minimal. The latter based on previous knowledge that the failures are controlled by the colluvium-San Sebastián contact zone.

Below the San Sebastián, the borings revealed a volcanic basement consisting of greenish gray to light-gray, dark gray, with and dark mottles, volcanic breccia with white and dark mottles of quartz and feldspar crystals.

SPT values within this horizon wherein in variably higher than 100 bpf, while Natural

Moisture Content were lower than 10 percent.

7.0 GROUNDWATER LEVELS

As part of the investigation, groundwater observations were conducted from the open holes after drilling completion and at 24 hrs after completion. The groundwater level appears in boring

#2 at 20 ft feet, were the soil samples were retrieved wet.

8.0 DISCUSSION

We are of the opinion that the infringement of the landslide toe into the guard rail of PR-10 has evolved as a consequence of landslide creep and toe extrusion, combined with soil saturation and heavy erosional forces against the slope surface. It is evident that the water flowing out from the top of the slope, has added instability to the slope.

Considering that the extruded landslide toe remains at a steep angle and shows loose to medium density, it is conceived that severe weather conditions and extraordinary precipitation events would tend to decrease the safety factor of the current slope. This could trigger additional instability problems which could jeopardize the integrity and safety of the highway.

The following sections discuss possible remedial measures to improve the stability of the landslide toe and at the same time permitting the reconstruction and/or replacement of the drainage system that exists at the study area.

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9.0 SLOPE STABILITY ANALYSES AT REPRESENTATIVE CROSS SECTION

(STATION 211+80) AND REMEDIAL MEASURES

Detailed stability analyses were performed in order to investigate the present factor of safety and also the future factor of safety of the landslide toe for two possible remedial measures.

The position of the groundwater table was considered in our analyses, as previously discussed in this report. Water pressures and water underground flows are one of the principal causes of instability in most zones throughout this project and very much indeed at the area under study. Therefore, the success of this project lies mostly on the capacity of lowering and controlling groundwater pressures.

Soil parameters used for stability analyses are consistent with the results of the lab test program previously conducted for this site. Figure 5 identifies the parameters assigned to soil layers.

10.0 FIRST ALTERNATIVE

The results of the stability analyses for the present conditions are presented in Figure 5. The present factor of safety during high watertable conditions is around 1 (incipient failure and movement). Based on this result it is conceived that landslide movement occurs during periods of extraordinary rain events when groundwater seeping from sinks located behind the Cuesta Scarp gets into the colluvium, recharging the groundwater table.

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Figure 5. Slope stability analysis for existing conditions (Sta 211+80)

Additional slope stability analyses were performed in order to increase the factor of safety and at the same time provide the necessary earthworks to draw back the soil mass, away from the shoulder and guard rail of the highway. The analyses demonstrate that by lowering the groundwater level, a cut slope can be performed as shown in Figure 6. As observed from Figure 6 (static model), a cut slope 2H:1H with an intermediate bench and 2 trench drains make up the first and our preferred remedial alternative. Refer to Figure 6A for the seismic model.

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Figure 6. Slope stability analysis results (static)

Figure 6B. Slope stability analysis results (seismic a=0.2g)

The use of deep trench drains are preferred over sub-horizontal drains. After the observation of the water seepage flow regime back in year 1995, in this zone of the project, where partially monitored discharges from spring flows have been measured with rates in the order of 3,000 gpm, we have concluded that the implementation of sub-horizontal drains, should not be used. As the present water regime is considered conduit-controlled, our best recommendation is to use a series of deep drainage trenches built from different elevations in an attempt to intercept any underground flow of water from any direction it may come, and with ample drainage capacity to handle large volume of water.

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Figure 7. Gabion wall alternative

11.0 SECOND ALTERNATIVE

A second alternative consisting of a gabion retaining wall was investigated. This alternative is presented in Figure 7.

For this alternative, the results of global stability analyses shown unsatisfactory factor of safety under seismic loads after construction (a=0.15g, F.S<1.2). In addition, global stability analysis for the cut slopes need during construction resulted inadequate (F.S=0.76). This is schematically presented in Figure 8.

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Figure 8. Gabion wall alternative. Inadequate solution

12.0 RECOMMENDATIONS

Based on the previous sections, our preferred recommendation is slope regrading combined with affective groundwater drainage. The regrading of the landslide toe, using a 2H:1V geometry and incorporating an intermediate bench is presented in Figure 9. Due to equipment and construction limitations, we are recommending that the interceptor drainage trenches are built to a depth of 25 feet below ground surface elevation, and to have an average width of 8 feet. A relatively wide trench (~8 ft wide) is considered necessary because of the possibility of encounter blocks of limestone during the excavation. Upon occurrence, these block would need to be demolished from inside the excavation, work that could be performed with small impact hammers positioned inside a large trench box.

The trenches are to be dug and built from each of the berm plateaus indicated Figure 9. The backfill for the trench drains shall be crushed stone of 3-inch minus gradation with not more than

5 percent by weight of non-plastic fines passing the No. 200 sieve.

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Figure 9. Recommended remedial measures

The trenches should be built with an impervious 60 mil HDPE synthetic liner ( equal or better) on the bottom and downhill side of the excavation and a heavy permeable synthetic geofabric on the uphill side. The contractor will be responsible for the safety of personnel in performing this operation. Box trenching will be needed. The trenches shall be opened in segments no longer than

30 ft.

A 18 inch perforated pipe should be installed on the bottom of the trench with a sufficient slope to collect and drain water to adequate discharge locations. The final slope of the pipe shall be selected by the designer. The drainage trenches should be built as early as possible during the work.

The earlier the trenches are installed; the safer the rest of the work will be. Therefore this operation shall be incorporated in the schedule as one of the primary activities, preferably the first.

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The characteristics of the non-woven fabric are listed in Table 1.

TABLE 1; Geofabric Properties

Geotextile Property Test Method Minimum Values

Grab tensile strength ASTM D 4632 180 lb

Burst strength ASTM 3786 290 psi

Puncture strength ASTM D 4833 OR 3787 80 lb

Apparent opening size ASTM D 4751 600 ìm sieve or smaller opening

Permittivity ASTM D 4491 0.01 s-1

The base of the trench should be sloped so that water does not pond in the trench. A minimum slope is 2%, but steeper slopes are better.

12.1 Field Monitoring

As it is always the case for this type of geotechnical problems, field monitoring must be conducted during and after construction of the long term remedial measure to carefully evaluate its effectiveness and detect any problem that may occur during construction. The field monitoring will consist of periodical measurements from a control bench marks, and inclinometers installed. During construction the instrumentation must be read at least three (3) times per week.

The results of this monitoring program will provide any need to have to incorporate additional control measures. This monitoring program needs to be evaluated weekly during construction and monthly thereafter.

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13.0 ADDITIONAL CONSIDERATIONS

The long term remedial measures presented in this report require to strictly enforce closely-sequenced construction methods, where the excavation for benching and filling in any part of the hillside area occur in a close sequence such that the length of open excavations is left to a practical minimum at all times during the work day.

It is imperative that all reconstruction and stabilization works must be accompanied by a careful monitoring program, during and after construction, that will help in assessing the effectiveness of the recommended interim remedial measures. Although long term remedial measures given herein should be effective in minimizing future landslides at the hillside, variations in the site soil conditions, may require some changes in the stabilization program that can be promptly implemented with the help of the field instrumentation measurements in the hands of a well experienced geotechnical engineer. Therefore, it is recommended to adopt the field monitoring program as a component of the remedial measures suggested for this project.

The exact amount of this instrumentation will be determined by the Soil Engineer in charge of the geotechnical inspection during construction, as the site conditions dictate the requirement of the instrumentation. A typical monitoring station shall consist of control bench marks, observation wells and inclinometers.

The control of surface run-off waters in this project is of paramount importance. An effective surface drainage system will have to be provided in this project, including (but not limited

to) paved ditches, swales and interceptors, in order to properly collect and dispose surface and groundwater to properly designed outlets, both during construction and after the hillside has been repaired. Run-off shall not be permitted to run freely during nor after the remediate has been completed. Road run-off shall not be disposed on the ground. This run-off shall be diverted by meas of the required piping, ditches, culverts, and other hydraulic works into properly designed outlets out and way from the project.

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Weak fills or incompetent soils are present on the current sloping ground (landslide debris).

The contractor shall follow all cleaning, stripping and benching recommendations presented by

Suelos, Inc., full time soil technician and geotechnical engineer field supervisor. The actual depth and extension of removal shall be identified at the field during actual earthworks.

14.0 LIMITATION OF REPORT

This report has been prepared taking into consideration the design factors presently known to us. The project designers shall be alert of any item that might have been overlooked, that could require clarification or that may need additional recommendations to those discussed herein.

The study area is a dynamic and unstable environment with continued movement features such as new scarps, tension cracks, seeps and overall landslide features. That said, the inspection or supervision of excavations, earthworks and trench construction is, in general, is a very delicate matter in this particular scenario. In some cases this service is not rendered by the Soils Engineer who made the subsoil investigation and recommended the specific foundation alternatives for a given project. In a large number of projects these operations are successfully completed due to the prevalence of ideal and uniform conditions and to the fact that they are being performed by a competent and responsible contractor. But there are cases, where the lack of proper construction techniques and the lack of adequate supervision has given rise to the occurrence of geotechnical problems and failures.

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Conscious of the above exposed problem, we wish to state that the validity of our recommendations about global stability and trench drain effectiveness are subordinated to the inspection of the earthwork operations by us. If the contract is awarded to another experienced Soils

Engineer, he shall receive a copy of this report, evaluate the same and adopt it as his own, or request additional soil data to verify our foundation recommendations or modify them in accordance to his personal knowledge and judgement, thus assuming full responsibility for his recommendations and the report itself.

Respectfully submitted, IVAN JACKSON MADURO, P.E., M.S.C.E., CWI

Partner mgn

Reference No. 4834-PS1.rep

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TABLE OF CONTENTS

1.0 INTRODUCTION

2.0 SITE LOCATION AND BACKGROUND INFORMATION

3.0 PROBLEM DESCRIPTION

4.0 WORK PERFORMED

5.0 GEOLOGY ASSESSMENT

5.1 Quaternary Landslides

5.2 Middle Tertiary Sequence

5.3 Lares Limestone

5.4 San Sebastián Formation

5.5 Hydrology

6.0 SUBSOIL CONDITIONS

6.1 Upper Horizon-Colluvial Deposits

6.2 Middle Horizon -San Sebastián Formation

6.3 Third Horizon - Volcanic Basement

7.0 GROUNDWATER LEVELS

8.0 DISCUSSION

9.0 SLOPE STABILITY ANALYSES AT REPRESENTATIVE CROSS SECTION

(STATION 211+80) AND REMEDIAL MEASURES

10.0 FIRST ALTERNATIVE

11.0 SECOND ALTERNATIVE

12.0 RECOMMENDATIONS

12.1 Field Monitoring

13.0 ADDITIONAL CONSIDERATIONS

14.0 LIMITATION OF REPORT

BORING LOGS

DRILLING APPENDIXES

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