Meetng_Document_-_Soils_Investigation_Report.pdf

Soil Investigation and Helical Test Probes

Project: Big Four Boardwalk By: Luke Silvis with input from Rene’ Renteria

Project Summary The project consists of replacing a 760 ft long by 6 ft wide boardwalk adjacent to the Big

Four/Ice Caves trailhead for on the Darrington Ranger District. Currently the boardwalk sits on a variety of mudsills and other foundation structures. Due to the saturated soils in the project area, the boardwalk has become very uneven. The new boardwalk would be elevated 2-3 ft off the ground to enhance the wetland connectivity, will be increased to 8 ft wide, and the bent spacing is expected to be 8-10 ft except for two locations that would have 20 ft spans. The proposed boardwalk would utilize a deeper foundation to better support the design loading of 175 psf. Due to the ease of installation and limited environmental effects, the Helical Pile is the preferred foundation choice for this project.

Other foundations still in consideration include micropiles, mudsills, diamond piers, and a mattress foundation.

Soil Auger

On June 28, 2017 a soil auger was used to collect a soil sample by the Forest Service Regional Geotech. The sample was approximately taken at Station 2+00 on the left side of the boardwalk. A hand soil auger was easily pushed to 4 feet of depth and some minor effort to a depth of 6 feet. The field classification of the soil is a Clayey Sand (SC), brown or gray, wet, above the plastic limit (APL), very soft to soft. Even though it is classified as cohesive, there is a significant amount of sand in the sample, so for design purposes it may be acceptable to consider the material “granular”.

Williamson Drive Probe On August 8, 2017, a Williamson Drive Probe was used to further identify soil characteristics conducted by the Forest Service Regional Geotech. The Dive Probe setup used a 1/2 inch steel water pipe driven with an 11-lb hammer dropped from 4 feet. The probe was completed at Station 1+50 on the right side of the boardwalk. The Drive Probe blows per foot were correlated to the Standard Penetration Test (SPT) blows per foot using data developed by the Forest Service San Dimas Technology Development Center. The interpretation from the findings is there were four different soil units penetrated before hitting refusal: 0-3.5 ft Very Loose, 3.5-8 ft Loose, 8-11.5 ft Medium Dense, and 11.5-12.5 ft Medium Dense to Dense (see Table 1).

Helical Test Piles On December 5, 2017, four helical test piles were installed. The intent of the test piles was to determine the torsional resistance during installation which could then be correlated for an estimated load capacity of the piles as well as determine the length of piles that would be required.

The pile arrangement used was a 2-7/8 in. diameter pile with a single 8 in. diameter helix. The pile was driven in 5 ft increments using a Bobcat 324 excavator with a 7 kip drive head. The drive head was equipped with a Pro-Dig Intelli-Tork device that monitored the torque while installing the piles.

The four test piles were installed at locations shown on the map (Figure 2). Each test pile location experienced similar results for a relatively consistent data set. The top 5 ft of material experienced low torque values (45-60 ft-lbs) indicating it is loose low bearing material. The torque gradually increased with the increased depth below 5 ft with an average final torque of 800 ft-lbs before the pile would reach a point of refusal. The refusal condition of all four piles was determined to be the result of encountering a gravel layer with cobbles/boulders too large for the helix to pass. As the piles would come into contact with the obstruction, the torque would spike likely as the cutting edge of the helix was coming into contact with the obstruction, and would then pop free with a drop in the torque. In only the first hole did the pile advance beyond the initial encounter with the obstruction, and in all four locations the pile would appear to grind on rock before spinning out at the point of refusal. After the pile was removed from each hole, a small amount of gravel material (angular rock particles up to 1/8 inch) was found packed into the open end of the pile tip which confirmed the presence of the gravel layer below. A table showing the results and a sketch of each test pile are included (Table 2 and Figure 1).

Conclusions The data collected with the drive probe and the helical test piles appear to be very consistent with one another which indicates the data is fairly representative of the project area. It can reasonably be concluded that the top 5 ft of soil provides very little structural strength. As the depth increases below 5 ft, the properties of the material begin to improve incrementally with the depth. At a depth of 12-15 ft it can be expected that a cobble formation will be encountered.

Table 1 - Williamson Drive Probe to SPT Correlation

Blows / 0.5 ft Depth

(ft)

WDP

BPF

Corr.

SPT BPF

Avg Unit

SPT BPF

Granular Compaction

Cohesive Consistency

0 -0.50 0 0

0 Very Loose Very Soft

0 -1.00 0 0 0 -1.50 0 0 0 -2.00 0 0 0 -2.50 0 0 0 -3.00 0 0 0 -3.50 0 0 7 -4.00 14 6

6 Loose Firm

7 -4.50 14 6 6 -5.00 12 6 5 -5.50 10 5 8 -6.00 16 7 7 -6.50 14 6 8 -7.00 16 7 7 -7.50 14 6 8 -8.00 16 7

17 -8.50 34 13

12 Medium Stiff

17 -9.00 34 13 17 -9.50 34 13 16 -10.0 32 12 16 -10.5 32 12 17 -11.0 34 13 15 -11.5 30 12 30 -12.0 60 20

25 Medium+ Very Stiff 50 -12.5 100 29

Table 2 - Summary Table of Helical Test Pile Installations

TEST PILE #1

STATION 2+20 RIGHT

TEST PILE #2

STATION 4+10 LEFT

TEST PILE #3

STATION 4+90 LEFT

TEST PILE #4

STATION 6+20 LEFT

Depth (ft)

T (ft-lbs)

Qu (lbs)

Depth (ft)

T (ft-lbs)

Qu (lbs)

Depth (ft)

T (ft-lbs)

Qu (lbs)

Depth (ft)

T (ft-lbs)

Qu (lbs)

5 45 405 5 50 450 5 60 540 5 60 540

10 180 1,620 10 150 1,350 10 300 2,700 10 200 1,800

15 800 7,200 12 700 6,300 12 900 8,100 11 700 6,300

17 1,500 13,500 - - - - - - - - -

NOTES:

Pile began encountering rock at about 15ft deep. Between 15- 16ft the torque spiked to 5500 before dropping to 1500 at 16.5ft. Pile spun out at 17ft.

NOTES:

Pile started to encounter rock at 11ft deep. At 12ft the pile spun out and the torque would bounce around between 700 and 1500.

NOTES:

Pile started to encounter rock at 10ft. At about 11ft the torque spiked to 3600 several times.

Pile spun out at 12ft deep.

NOTES:

The pile started to encounter rock at 9ft. Between 9-10ft the torque spiked up to 3600 before spinning out at 10.5ft.

Formula for Torque/Pile Load Capacity Qu = T x Kt

Qu = ultimate capacity (lbs) T = Torque (ft-lbs) Kt = empirical torque factor (ft-1); value is 9ft-1 for round 2-7/8" piles

Figure 1

Figure 2 - Map Showing Location of Test Piles