Appendix C.pdf

Design Report Yonkers Avenue Village of Tuckahoe, NY 14 May 2010 Section 14 Streambank Protection Project Appendix C

Ocean and Coastal Consultants, Inc.

APPENDIX C

REPORT ON YONKERS AVENUE

REHABILITATION, TUCKAHOE, NEW YORK:

GEOTECHNICAL & HYDRAULIC CONCERNS

BY EARTHWORKS

Report on Yonkers Avenue Rehabilitation, Tuckahoe, New York:

Geotechnical, Hydraulic & Construction Concerns

Table of contents

1. Introduction ……………………………………………………………………….………….1

2. Site configuration ………………………………………………………………..…………..2

3. Sediment properties ……………………………………………………………….………….6

4. Rock properties …………………………………………………………………....…………8

5. Rock sockets ……………………………………………………………………..………..17

6. Rock bolts ………………………………………………………………………….……….18

7. Stream hydraulics ………………………………………………………………….………19

8. Toe erosion and riprap ………………………………………………………….…………20

9. Construction issues ……………………………………………………………...………….22

10. Conclusions ……………………………………………………………………...………….23

1. Introduction

The existing conditions along Yonkers Avenue in Tuckahoe have resulted from the long term and ongoing erosion and displacement of the retaining wall systems and rock mass along the eastern bank of the Bronx River. In addition, localized scour of the Streambank and the growth of woody vegetation (trees) has caused portions of the retaining wall systems to be displaced or partially collapse. Due to the confined site, variable rock surface and the need to maintain the existing stream hydraulics, the rehabilitation of the retaining wall system along Yonkers Avenue presents several obstacles to an effective rehabilitation of the roadway.

To support the evaluation of the site, a site investigation program consisting of two geotechnical borings and four auger borings was completed. This data was reviewed and used to estimate geotechnical properties of the sediment and rock.

Accurate determination of rock mass characteristics and properties will affect the design and cost.

The available information is limited and not complete; more data would improve the design and result in fewer possibilities for construction cost increases and delays. Nonetheless, there is data from which the sediment and rock properties can be estimated. The unknown will be determined as part of the construction.

Geotechnical concerns take four parts:

(1) Top of rock

(2) Compressive strength of the rock to hold piles in place

(3) Tensile strength of the rock to hold bolts and sockets

(4) Soil properties affecting rotational stresses on retaining wall, sockets, and piles

The top of rock is constrained by the borings and the auger holes. The profile is sufficient. The configuration of the fracture zones and weathering is also poorly defined. Some fabric and foliation are indicated in the drawings. The variation will affect the rock bolts. The contractor should evaluate the strength of the rock bolts emplaced during construction.

2. Site configuration

The surface and subsurface characteristics of the site are based on (1) surface examination at the site, (2) examination of rock core and sediment samples and (3) drilling reports, including previous lab tests, on 2 SPT borings and 4 auger borings. Table 1 shows the locations of the six borings measured by e4sciences at the site. The borings were drilled along Yonkers Avenue approximately 10 feet from the curb along the Bronx River. Figure 2 summarizes this data in a cross section along Yonkers Avenue and the Bronx River. Figure 3 shows photographs of the site.

The two SPT borings are referred to in documents either as B-1 and B-2, or as Tuck #1 and Tuck #2. Herein they will be referred to as Tuck #1 and Tuck #2. Tables 1 presents the locations of the borings.

Table 1. Boring locations measured by Earthworks keyed to survey benchmark.

Boring Boring type Label on street

Easting Northing

NY State Plane East

NAD83 (feet) NY State Plane East

NAD83 (feet) Tuck #1 SPT and NX core 6 677,302.1 772,064.9 Tuck #2 SPT and NX core 2 677,353.0 772,107.9 Tuck Aug. 1 Auger 5 677,315.0 772,077.0 Tuck Aug. 2 Auger 4 677,327.1 772,088.3 Tuck Aug. 3 Auger 3 677,340.5 772,097.5 Tuck Aug. 4 Auger 1 677,365.0 772,118.7

Figure 1. Location map of Yonkers Avenue along the Bronx River. The borings and auger holes are located. The locations are referenced to New York State Plane East NAD83 in feet. The surveyed elevations are in NGVD29.

Figure 2. Cross sections through Yonkers Avenue parallel to the Bronx River. The upper section (A-A’) shows the rock and sediment below the road. The lower section (B-B’) plots exposures on the surface above the road and on eastern bank of the Bronx River.

Figure 3. View from the south of the Yonkers Avenue site and Bronx River bank. Note the trees and utility pole.

3. Sediment properties

Table 2 and Table 3 list the soil characteristics obtained and measured in Tuck #1 and Tuck #2, respectively. The top 15 feet of sediment in Tuck #1 and all of the sediment in Tuck #2 is reported as an artificial fill composed of silty/clayey sand (SM/SC) with variable amounts of gravel, silt, and clay. The gravel content of these sands ranges from 25.5 to 28.95% in Tuck #1 and 1.4% in Tuck #2. The silt-sized fraction ranges from 8.6 to 9.9% in Tuck #1 and 33.8 % in Tuck #2. The clay-sized fraction varies from 3.2 to 4.0% in Tuck #1 and 10.9% in Tuck #2. The moisture content is 13-16%. The N-values range from 5 to 33, averaging 15. Based on the results of the site investigation program, the angle of internal friction is 28-30°.

Below the silty/clayey sands in Tuck #1 is 20 feet of poorly graded sand with gravel that is reported as fluvial sands. Gravel content of these sands ranges from 22.9% to 39.4%. The silt-sized fraction ranges from 0.6% to 3.4%. The clay-sized fraction varies from 0.3% to 1.0%. The moisture content is 9-16%. The angle of internal friction is 28-30°. The N-values range from 4 to 8, averaging 7.

Table 2. Soil characteristics in Tuck #1 (B-1).

Site Name Interval Sample Blows N-value Description Label on sample jars & test results

Water content

Specific gravity

USCS

classification

Lab Description

(ft) per 6” blows per foot Tuck #1 B-1 0-1 n.a. Pavement n.a. n.a. n.a. n.a.

Tuck #1 B-1 1-3 S-1 6,7,4,4 11 Brown sand, some silt, some gravel 0-2 n.a. n.a. n.a.

Tuck #1 B-1 3-5 S-2 4,2,3,2 5 Brown sand, some silt, some gravel 2-4 n.a. n.a. n.a.

Tuck #1 B-1 5-7 S-3 3,3,6,7 9 Brown sand, some silt, some gravel 4-6 13 2.748 SM/SC Brown & orange silty|clayy SAND with gravel Tuck #1 B-1 7-9 S-4 9,7,7,8 14 Brown sand, some silt, some gravel 6-8 16 2.882 SM\SC Brown & orange silty|clayy SAND with gravel Tuck #1 B-1 9-11 S-5 4,10,11,8 21 Brown sand, some silt, some gravel 8-10 n.a. n.a. n.a.

Tuck #1 B-1 11-13 S-6 22,21,12,5 33 Brown sand, some silt, some gravel 10-12 n.a. n.a. n.a.

Tuck #1 B-1 13-15 n.a. n.a. n.a. n.a. n.a.

Tuck #1 B-1 15-17 S-7 3,4,4,2 8 Brown fine to medium sand 15-17 9 2.777 SP Brown poorly graded SAND with gravel Tuck #1 B-1 17-20 n.a. n.a. n.a. n.a.

Tuck #1 B-1 20-22 S-8 3,3,3,4 6 Brown fine to medium sand 20-22 16 2.726 SP Brown poorly graded SAND with gravel Tuck #1 B-1 22-25 n.a. n.a. n.a. n.a. n.a. n.a.

Tuck #1 B-1 25-27 S-9 3,2,2,3 4 Dark brown sand 25-27 n.a. n.a. n.a. n.a.

Tuck #1 B-1 27-30 n.a. n.a. n.a. n.a. n.a. n.a.

Tuck #1 B-1 30-32 4,4,4,3 8 Dark sand 30-32 13 2.727 SP Brown poorly graded SAND with gravel Tuck #1 B-1 32-35 n.a. n.a. n.a. n.a. n.a. n.a.

Tuck #1 B-1 35-37 13,19,18,15 37 Weathered bedrock 35-37 15 2.813 SP-SM\SC Dark & light brown poorly graded SAND with silt/clay & gravel Tuck #1 B-1 37-40 roller bit down n.a. Weathered bedrock n.a. n.a. n.a. n.a. n.a.

Tuck #1 B-1 40-45 R-1 n.a. Bedrock – 7 pieces plus fragments n.a. n.a. n.a. n.a. n.a.

Table 3. Soil characteristics in Tuck #2 (B-2).

Site Name Interval Sample Blows N-value Description Label on sample jars & test results

Water content

Specific gravity

USCS

classification

Lab Description

(ft) per 6” blows per foot Tuck #2 B-2 0.0 - 1.0 n.a. n.a. Pavement Tuck #2 B-2 1.0 - 3.0 S-1 5,11,11,11 22 Brown sand, little silt, little gravel (Fill) 1-3 14 2.681 SM/SC Orange silty/clayey SAND Tuck #2 B-2 3.0 - 5.0 S-2 5,5,3,3 8 Brown sand, little silt, little gravel (Fill) Tuck #2 B-2 5.0 - 5.5 S-3 6 n.a. Brown sand, little silt, little gravel (Fill) Tuck #2 B-2 5.5 - 6.6 S-3 13,14,100/1” 27 Weathered bedrock Tuck #2 B-2 6.6 - 8.0 n.a. n.a. Bedrock ?

Tuck #2 B-2 8.0 - 13.0 R-1 n.a. n.a. Bedrock – 7 pieces plus fragments Tuck #2 B-2 13.0 - 18.0 R-2 n.a. n.a. Bedrock – 7 pieces plus fragments

4. Rock properties

The top of rock is defined by the two SPT borings, refusal in the 4 auger borings, and outcrop exposures on the northeast end of the project area. Rock is exposed on the riverside slope below Yonkers Avenue and above the road in the rock wall along the sidewalk. Table 4 lists the depth of top of rock below Yonkers Avenue determined by the borings. Table 5 shows the rock core properties. Figures 4 through Figure 8 show the rock core from Tuck #1 and Tuck #2. Figures 9 through 11 show the rock exposures at the site. The rock is the Inwood Marble. It is folded and has some layers with silicate minerals. The layers are dipping steeply and strike almost parallel to the road and river. The fractures are nearly vertical.

The rock ranges from soft to highly weathered decomposed rock along the top of surface encountered in both SPT borings. The weathered rock also occurs along some layers in the riverbank exposures. The SPT spoon was advanced through the decomposed rock to refusal; the N-values were reported to reach 27 and 37. The intact rock below the decomposed rock was cored for 5ft in Tuck #1 and 10ft in Tuck #2. In the boring logs, we have found errors in the RQD. We find RQD of 70, 50, and 40 in the three runs; where the report accompanying the core found 50, 35, and 50. The source of error seems to have been in confusing inches and tenths of foot. The correct values are 70, 50, and 40.

Table 4. Top of rock in the SPT and Auger borings. LCD = longest core segment; FPF=fractures per foot.

Boring name Top of weathered rock Top of intact bedrock

(refusal) depth below road

(feet) depth below road

(feet) Tuck #1 35.0 40.0 Tuck #2 5.5 6.6 Tuck Aug-1 n.a. 34.0 Tuck Aug-2 n.a. 27.0 Tuck Aug-3 n.a. 12.0 Tuck Aug-4 n.a. 5.5

Table 5. SPT core boring results: rock core properties.

Boring name

Alternative Name Run

Interval (ft)

LCS

(ft) FPF

Tuck #1 B-1 1 0 to 1 0.48 3 Tuck #1 B-1 1 1 to 2 0.38 3 Tuck #1 B-1 1 2 to 3 0.53 5 Tuck #1 B-1 1 3 to 4 0.80 0 Tuck #1 B-1 1 4 to 5 0.54 3

Tuck #2 B-2 1 0 to 1 0.80 1 Tuck #2 B-2 1 1 to 2 0.43 2 Tuck #2 B-2 1 2 to 3 0.58 2 Tuck #2 B-2 1 3 to 4 0.75 5 Tuck #2 B-2 1 4 to 5 0.23 3 Tuck #2 B-2 2 0 to 1 0.45 4 Tuck #2 B-2 2 1 to 2 0.35 3 Tuck #2 B-2 2 2 to 3 1.25 4 Tuck #2 B-2 2 3 to 4 1 Tuck #2 B-2 2 4 to 5 0.30 3

Figure 4. Inwood Marble core run 1 from Tuck #1. The left hand side indicates the depth of the core below Yonkers Avenue.

Figure 5. Inwood Marble core run 1 and 2 from Tuck #2. The left hand side indicates the depth of the core below Yonkers Avenue.

Figure 6. Inwood Marble in core box. The upper foot (fifth foot from bottom) of the core box. Tuck #1 (B-

1) run 1 is in the bottom row of the box; Tuck #2 (B-2) run 1 and run 2 is in the middle and top row respectively.

Figure 7. Inwood Marble in core box. (Above) Second foot from top (fourth foot from bottom) of box.

Tuck #1 (B-1) run 1 is in the bottom row of the box; Tuck #2 (B-2) run 1 and run 2 is in the middle and top row respectively. (Below) Third foot from top and bottom of box. Tuck #1 (B-1) run 1 is in the bottom row of the box; Tuck #2 (B-2) run 1 and run 2 is in the middle and top row respectively.

Figure 8. Inwood Marble in core box. (Above) Second foot from the bottom (forth foot from top). Tuck #1 (B-1) run 1 is in the bottom row of the box; Tuck #2 (B-2) run 1 and run 2 is in the middle and top row respectively. (Below) The bottom foot (fifth foot from top) of the box. Tuck #1 (B-1) run 1 is in the bottom row of the box; Tuck #2 (B-2) run 1 and run 2 is in the middle and top row respectively.

Figure 9. Inwood Marble exposed along the riverbank in the northeast end of the project area. Note the rock exposures on the riverbank and in the rock wall across the road. North is to the left.

Figure 10. Inwood Marble exposed along the riverbank in the northeast end of the project area. Note the steep dip of the layers and varying degrees of weathering. North is to the left.

Figure 11. Highly weathered Inwood Marble exposed along the riverbank in the northeast end of the project area. The tree roots may have contributed to the degradation of the rock. North is to the left.

5. Rock sockets

The design requires the installation of pipe pile rock sockets into the rock that underlies the site.

Based on our observations at the site and the cores retrieved, some of the bedding planes in the rock mass approach vertical (Figures 9 and 10). The variable nature of the rock and the observation that some of the weathered joints are in the same orientation as the rock sockets may cause the embedment depth to be adjusted as the construction progresses.

The design has been developed to be slightly conservative in an effort to limit the potential cost increases and construction time delays.

It is recommended that additional site investigations be conducted to limit possible delays in the project schedule and possible increases in cost of construction.

6. Rock bolts

Along the northern end of the site, the existing weathered rock mass requires the installation of a retaining structure that relies on rock bolts to maintain the stability of the system and limit further erosion of the slope. The design of the rock bolts is based on the assumption that the rock mass adjacent to the site is massive enough to support the design loadings. Based on our site visits and review of the core samples retrieved from the site, it is possible that the rock mass is severely weathered and may effect the rock bolt pullout strength accordingly.

The design and contract documents have been developed assuming that the rock mass is competent and that testing of the rock bolts will be conducted during the construction. The contractor should be required to load test each bolt and verify the integrity of the system. The contractor will be required to test each bolt using ASTM D4435-08, the standard method for rock bolt anchor pull test. The bolt shall be tested to two thirds of the failure strength of the bolt itself.

If the rock bolt does not develop adequate strength, a replacement bolt will be installed and tested. This procedure will continue until an installed bolt achieves the required strength. It is possible that a more regular pattern or an increase in the embedment length will be required to support the retaining wall system.

Figure 11 shows the effect of tree roots on the rock weathering. The roots can penetrate between the layers and then expand cracking the rock.

7. Stream hydraulics

The Bronx River is constricted upstream and downstream from the construction site. At the southern end of the project area the river has a nearly 90° turn (Figure 12). The river flow will naturally try to round out this unnatural configuration.

The existing streambed varies strongly in the vicinity of the work. The variations include sandy shoals to riprap and shot rock lined areas with various debris and outwash from upstream reaches.

In addition, the stream banks include stone lined channel areas to masonry walled reaches to unprotected and eroding soil or rock surfaces.

The designs have been developed to limit the disturbance of the existing streambed and, as a result, limit the changes to the hydraulics throughout the project area. It is likely that during the course of the work, portions of the stream banks and surrounding retaining features will be damaged or displaced. Provisions for protecting surrounding features and restoring the streambed must be included in the contract documents.

Figure 12. View from the north of the sharp bend in the Bronx River at southern end of the project area.

Flow is to right. Erosion has occurred on the outside of the curve, whereas some material has deposited on the inside of the curve. The photograph is taken facing southwest and so west to the right. Note the drainpipe (red arrow). The double red arrow marks the point where the stream hits the wall nearly straight

on. This 20ft section of the wall is severely undercut and deformed.

8. Toe erosion and riprap

Even though the design limits the disturbance of the existing streambed, there is a potential to affect the streambed by concentrating streamlines and channeling flows. Erosion of the sand and sediment deposits along installed sheeting or retaining features may occur if the sheeting below the streambed is exposed by scour. To prevent further localized scour, additional erosion control should be considered. One approach would be to set large riprap below the streambed so that it does not extend above the present bottom. By installing the riprap into the streambed, as opposed to on top of the streambed, the hydraulics of the area will remain unchanged. This may be a conservative approach, but it would guard against a nonlinear scour event.

As an alternative to installing toe erosion control in the streambed, the sheet pile system can be designed to include localized scour and loss of toe support accordingly. The designs for this option should consider that the streambed may erode, causing the steel sheet pile system to support an increased soil load. As a result, the installed system will include slightly stronger sections and a deeper embedment depth for the sheet pile

Figures 12 and 13 show the wall at the bend in the river. Recent erosion has occurred at the bend in the southern end resulting in erosion and removal of the cemented riprap and sediment underlying the retaining wall. The undermined retaining wall is close to collapse. In this area, the existing mortared stone wall will be replaced with a steel sheet pile wall system designed to protect the Streambank and the adjacent parking areas at the top of bank.

To permit the construction and installation of the sheet pile retaining wall system in this area, some of the trees along the top of wall will have to be removed. In addition, the removal of the stumps and restoration of vegetation along the slope should be considered.

Figure 13. Wall at southern end of project area at the turn of the river. The river flow has eroded and removed the original cemented riprap at the base of the wall particularly over the 20ft section. It is highly likely that this wall will fail completely and collapse over the next year or so. Note the drainpipe just to the north of the endangered section (red arrow).

9. Construction Issues

During the construction activities, several issues need to be considered and addressed, these include the following:

• Traffic Control and Maintenance.

• Partial collapse of existing retaining walls, and

• Vibrations due to construction activities.

Traffic Control:

The areas surrounding the site are congested and have limited access and areas available for the construction. In particular, Yonkers Avenue is a main thoroughfare for commercial traffic through the Village. Due to the volume and size of the vehicles that will be affected by the construction, the Village of Tuckahoe will be required to implement a site specific traffic control plan.

Partial Collapse of Retaining Walls:

Along the alignment of the proposed pipe pile and sheeting retaining walls, an existing stone retaining wall system is in place. Portions of this wall have subsided and rocks have been displaced. During the construction activities, it is likely that sections of this wall will be further displaced or partially collapse into the Bronx River. The contractor will be required to remove the debris as required and restore the areas accordingly.

Vibrations due to Construction Activities:

During construction of the retaining walls, the contractor will be drilling sockets into the underlying bedrock to receive the pipe piles, driving steel sheeting to bedrock or selected depths, and drilling rock bolts into bedrock. Each of these activities, will result in vibrations that may affect the buildings and structures surrounding the work area.

In order to address these issues, the contractor will be required to develop a site specific vibration monitoring program that addresses the existing buildings in the areas adjacent to the site. In addition, the contractor will be required to sequence the construction activities and select the equipment suitable to prevent damage to the buildings and property.

10. Conclusions

1. Provisions for protecting surrounding features and restoring the streambed are included in the contract documents.

2. The designs for the entire system have been developed to accommodate localized erosion or scour at the base of the installed pipe pile and sheet pile systems. The embedment depth presented in the design provides sufficient cover to prevent the loss of support due to localized erosion or scour of the streambed. As a result there is no need for the installation of large rip-rap armor in the streambed.

3. Accurate determination of rock mass characteristics and properties will affect the design and cost. The available information is limited and not complete. More data would improve the design and result in fewer possibilities for construction cost increases and delays.

4. Core borings and the auger holes constrain the estimate of the top of rock horizon. The profile is sufficient, but should be evaluated critically to verify.

5. The rock is Inwood Marble. It is a folded marble that has some layers with silicate minerals.

The layers are dipping steeply and strike almost parallel to the road and river. The fractures are nearly vertical.

6. The rock ranges from soft to highly weathered decomposed rock along the top surface and along some layers in the riverbank exposures. Below the surface, the rock varies from highly weathered to highly fractured to intact.

7. Due to the variable nature of the rock and the observations that some of the weathered joints are in the same orientation as the rock sockets; it is possible that the embedment depth may have to be adjusted as the construction progresses.

8. The design has been developed to be slightly conservative in an effort to limit the potential cost increases and construction time delays.

9. It is recommended that additional site investigations be conducted to limit the potential for delays in the project schedule and cost of construction increases.

10. The concept of using rock bolts is based on the assumption that the rock mass adjacent to the site is massive enough to support the design loadings. Based on our site visits and review of the core samples retrieved from the site, it is possible that the rock mass is severely weathered and may effect the rock bolt pullout strength accordingly.

11. The design and contract documents have been developed assuming that the rock mass is variably competent and that testing of the rock bolts will be conducted during the construction. The contractor will be required to load test each bolt and verify the integrity of the system.

12. The contractor will use ASTM D4435-08, the standard test method for rock bolt anchor pull test. If the rock bolt does not develop adequate strength, a replacement bolt will be installed and tested. This procedure will continue until an installed bolt achieves the required strength.

It is possible that a more regular pattern or an increase in the embedment length will be required to support the retaining wall system.

13. Each rock bolt will be tested to two thirds of the tensile strength of the bolt itself.

14. The designs have been developed to limit the disturbance of the existing streambed and, as a result, limit the changes to the hydraulics throughout the project area. It is likely that during the course of the work, portions of the stream banks and surrounding retaining features will be damaged or displaced. During the construction activities, the Contractor must remove any debris and materials that fall into the streambed.

15. Recent erosion has occurred at the bend in the southern end resulting in erosion and removal of the cemented riprap and sediment underlying the retaining wall. The undermined retaining wall has locally subsided and is close to collapse. Since the adjoining construction activities were expected to further damage or cause the collapse of this wall system, the design has incorporated the replacement of this retaining wall with a steel sheet pile retaining wall. The proposed sheet pile retaining wall system will be installed along the same alignment as the existing mortared stone retaining wall to maintain the existing stream hydraulics.

16. The top 15 feet of sediment in Tuck #1 and all of the sediment in Tuck #2 is reported as an artificial fill composed of silty/clayey sand (SM/SC) with variable amounts of gravel, silt, and clay.

17. The sediments have a friction angle estimated at 28-30° and zero cohesion.

18. Below the silty/clayey sands in Tuck #1 is 20 feet of poorly graded sand with gravel that is reported as fluvial sands.

19. There is no data on soil properties for the southern portion of the project site, as a result, the design of the steel sheet pile retaining wall at the bend along the southern portion of the site is based on the closest soil boring, Tuck #1.

20. Trees, stumps and root systems on the riverbank along the construction site should be removed to permit the construction activities and prevent further degradation of the site.