6th

International Workshop on Micropiles Tokyo, Japan, 2004

Performance and Instrumentation of Micropiles for Paper Mill Expansion

Nadir Ansari & Matthew Janes, Isherwood Associates Jim Bruce, Geo-Foundations Contractors Inc.

INTRODUCTION

Expansion of the Bowater Pulp and Paper Mill in Thunder Bay, Ontario, Canada required the enhancement of column and pier loads at several locations. A new hopper bin placed on the roof of the structure resulted in significant increases to existing column loads. The present spread footings could not be increased to accommodate the new loads and provide acceptable settlements. The mill was to remain operating during the construction which, combined with cramped conditions and low overhead clearance limited the foundation options available. As a result micropiles were chosen to augment the vertical capacity of the piers.

A total of 10 micropiles, each 40m in length, were designed to augment the existing footings. The micropiles have capacities between 1100 kN and 1900 kN. The piles consist of grouted pipe and 655 MPa threadbar extending through 33 m of stiff clay into sound rock.

An instrumentation program was designed to determine the load distribution within the micropiles to provide quality assurance and enable improved designs in the future. The instrumentation consisted of installation of a contractometer to measure the relative displacement of the test micropile at 6 elevations. The relative strains were compared during a quick maintained

compressive load test. The results indicate the pressure grouting technique resulted in very high load transfers to the soil above the rock layer.

Soil Conditions

A soil investigation was conducted at the ‘A’ Mill Pre-Steaming Chip Bin location of the Bowater Pulp and Paper mill site by Thunder Bay Testing and Engineering Ltd. The results were provided in the Geotechnical Investigation Report, Reference Number 04-015, dated May, 2004, with accompanying borehole logs. Table 1 indicates the depths and strata thicknesses as reported in the soils investigation.

The soil consists of 2.5 m of fill over 30 m of stiff, varved, low to medium plasticity clays with moisture contents on the order of 20 to 35%. It is assumed this clay is normally consolidated. Below the clay lies silty sand tills with gravel over weathered rock. The till is approximately 3 m thick and the weathered rock varies to up to 2 m in thickness. Sound rock is found at a depth of 36m and consists of argillite, a very competent igneous formation. The engineering design parameters for the soils are estimated from experience gained in similar soils. The in situ soil stress condition was estimated assuming a Ko of 0.6.

© 2011 Geo-Foundations Contractors Inc.(www.geo-foundations.com)

2

Table 1 Soil strata elevations as modelled in FLAC

BH 1 Depth SPT Moisture Cu

m #/300mm % kPa

Concrete Slab 0-0.25

Fill 0.25-2.4 15

Clay varved, stiff 2.4-14.8 <10 35 60

brown, med plasticity

Clay varved, stiff 14.8-21.3 <10 25 >75

brown, low plasticity

Clay w/ sand & gravel 21.3-23.5

stiff

Clay varved, stiff 23.5-30.0 <20 28 28

brown, low plasticity

Till Silty sand gravel 30.0-33.0 58

gray, very dense

Weathered Argillite 33.0-35.0 RQD 40-74%, REC 79-86%

gray, fine grained, joints

Argillite Bedrock

Micropile Geometry Figure 1 indicates the design geometry for the high capacity, 1900 kN working load micropile (Type 1) as installed at the site. Two additional micropile geometries were designed for lower loads, including 1450 kN (Type 2) and 1100 kN (Type 3) requirements. The pile consists of a grout filled pipe section and a reinforcing bar within a rock socket. The pipe section is socketed into competent rock a distance of approximately 2 m. The micropile extends beyond the pipe section via drilling out the rock and grouting in a 63 mm diameter 655 MPa bar. The micropiles are designed based upon the FHWA Micropile Design and Construction Implementation Manuali. Figure 2 provides a cross section of the 3 regions of the Type 1 pile. The pile was installed by simultaneously advancing the boring bit and the pile casing (threaded sections of pipe). A combination of water and air was used to flush the cuttings

from the hole. The casing was fitted with a rock cutting shoe which integrated with the boring bit. When rock was encountered the bit and casing were advanced together to allow the casing to penetrate into the rock up to 2 m. The distance the pipe section was advanced into the rock is referred to as the plunge length of the micropile. The boring bit was then retracted and turned to release the casing. The boring bit was then advanced beyond the casing toe and developed the smaller (144 mm diameter) hole for the bar and grout. The Type 2 micropile, as tested, is identical to the Type 1 pile less the addition of the 44 mm top bar within the pipe section of the micropile. The micropiles were designed for loading of the steel sections (pipe and bar) with a factor of safety of 2.0 and for the grout section with a factor of safety of 2.5. This combination results in a total allowable working load for the Type 1 pile of just over 1900 kN.

3

Figure 1 Micropile geometry as designed for the 2900 kN test loading

4

Figure 2 Micropile section details The development of such a high load on a small pile diameter required the use of a heavy walled pipe, grout and an inner reinforcing bar. This required that the pipe section be embedded into the rock in order to dissipate a large portion of the load such that the steel section could be reduced to a single heavy reinforcing bar within the remaining rock socket. The plunge length was used to dissipate 704 kN (1408 kN ultimate) in the Type 1 micropile geometry and 254 kN (508 kN ultimate) in the Type 2 micropile

geometry. Establishing the design shear strength of the rock within this section realised some questions. It was anticipated that the rock would yield very high grout to rock shear values, estimated at 600 kPa for design. However, the problem became to establish whether or not the grout would adequately emerge from beneath the end of the pipe section, up around the annulus of the pipe and completely fill the void between the outside of the pipe and the cored rock. There existed high potential for the

5

overburden soils, consisting of soft clays and sand/gravel tills to fill the void with muck after the drilling was completed but prior to placement of the reinforcing bar and grout. A Type 2 micropile geometry was selected for the test pile. The as-installed data for the pile is provided in Table 2. Table 2 Pile data, as installed Pile C6-47-w Type 2 pile

Drill depth 41.1 m

Depth to weathered rock 32.3 m

Depth to sound rock 34.1 m

Depth of casing 36.4 m

Grout take 823.3 litre

Pressure Grout take 470.4 litre

Theoretical volume 801.8 litre

Total grout/theoretical 161% The procedures were augmented to include pressure grouting of the hole in order to enhance the potential for establishing good grout penetration around the pipe within the plunge length of the pile. Micropile Instrumentation An instrumentation program was designed to establish the load dissipation along the grouted micropile length. The program would establish the success of the pile design and installation method. As well it would indicate the actual bond strengths achieved along the weathered- and sound- rock to pipe sections, the pipe end bearing and the conventional anchor bar rock socket. The results could be used to assist in computer modelling of the various changes in pile section. Of

interest was the effectiveness of load transfer within the pile from section B-B to Section C-C. In this section, the plunge length, the load is transferred from the pile to the rock such that the steel section of the pipe is no longer required. Minimising the length of reinforcing bar within this section would present significant economies for future designs. A contractometer was supplied by Mine Design Technologies of Kingston, Ontario to monitor the relative displacement of the pile at 6 different depths. The contractometer permits the determination of the pile displacement at a given point relative to the top of the pile. This permits the calculation of the compressive strain between each of the monitoring levels and thus the load dissipation between each section. The contractometer supplied by Mine Design Technologies had sensors at the locations shown in Table 3. These locations were chosen in an effort to maximise the information that would be provided about the behaviour of the pile within the rock. The objective was to place the sensors above and below major changes in the micropile geometry or where significant load dissipation was anticipated. This meant that the sensors were concentrated immediately above the sound rock, on either side of the end of the micropile casing and within the rock socket. It was not anticipated that there would be significant load dissipation within the deep stiff clay layers above the tills and rock. At most the load loss was expected to be on the order of 200 to 250 kN of the 2900 kN ultimate test load.

6

Table 3 Contractometer sensor positions

Identification Depth Distance Description

from top above toe

m m

Top, level 1 0.6 40.5 Pile top, reference

Level 2 31.5 9.6 0.8m above weathered rock

Level 3 32.4 8.7 0.1m below top of weathered rock

Level 4 36.6 4.5 0.2m below end of casing

Level 5 37.1 4.0 0.7m below end of casing

Level 6 39.1 2.0 midpoint of rock socket

Compression Micropile Test

0

500

1000

1500

2000

2500

3000

-5 5 15 25 35 45 55 65MOVEMENT (mm)

LO

AD

(k

N)

Averageof Dialgauge 1and 2

Slope 2

Slope 1

Note: Slope 1 shows the theoretical compression based on the micropile length in soil and weathered rock. Slope 2 shows the theoretical compression based on the

micropile length in soil, weathered rock and half the length in rock. The calculations take into account the stiffness of the bar, casing and grout.

Figure 3. Compression load test results for Type 2 micropile. Results The test micropile was installed as per the dimensions provided in Table 2. The tremie grouting program provided a grout take almost equal to the theoretical grout volume. The pressure grouting program indicates a grout take of almost 60%

more than the theoretical volume. The pressure grout program was deemed a success because it produced grout outside the pipe, within the pipe-soil annulus, right to the surface. This would indicate that sufficient grout was placed outside the pipe within the plunge zone of the pipe.

7

Figure 3 provides the results for a quick maintained compression load test, conforming to ASTM standard D 1143, using increments of 0.25P (where P = design load). The results confirm the pile was able to carry the test load. The deflection at peak load was greater than the theoretical compression in the soil and weathered rock (Slope 1), and well below the theoretical compression in the soil, weathered rock and half the sound rock (Slope 2). This would indicate that the micropile load is being translated into the rock below a depth of 32 m. The results of the contractometer testing are provided in Figure 4. The results show that the there was no movement of the contractometer sensors below top of the pile. This would indicate that no load

is translated to the pile below the Level 2 contractometer sensor, placed just above the weathered rock (9.6 from the pile toe). This result contradicts the predicted theoretical elastic stiffness and deflection measurements witnessed in the test. The contractometer test results indicate downward movement spikes in the sensor readings during or immediately following the offloading of the micropile during the load cycling. The spikes are seen in orange, purple and green and occur first during the offloading between the 1811 kN and the 2192 kN load cycles, and again following two offloading cycles. The interpretation of these spikes is that the sensor is moving down relative

Displacement of Micropile During Cyclic Loading

-70

-60

-50

-40

-30

-20

-10

0

10

00-Jan 10-Jan 20-Jan 30-Jan 09-Feb 19-Feb 29-Feb 10-Mar

MO

VE

ME

NT

(m

m)

2 m from bottom

4 m from bottom

4.9 m from bottom

8.7 m from bottom

9.6 m from bottom

top node in micropile

Dial Gauge Reading

0.00 2906 kN

2525 kN

2192 kN

1811 kN

1429 kN

1096 kN

Figure 4 Contractometer test results during compression load test

to the top of the pile. It does not make sense that such a physical phenomenon would occur. It is possible to interpret that the sensor is moving relative to the top sensor during the unloading of the pile. As the pile unloads the top of the pile moves upward. If the other filaments in the pile are slightly adhered to one another then they will move as a unit, stretch and show no movement at the sensor. If during unloading they come free they would show a relative downward movement (when remaining stationary) relative to the top sensor which is moving upward. The results of the pile creep testing are provided in Figure 5 and indicate that the pile was performing well within the standard of 2 mm per log cycle. This indicates that the pile is transferring the

load to competent strata within the elastic range of performance. Thus the normally consolidated clays that are now suspected of carrying significant load are behaving elastically. These clays could be carrying as much as 500 to 800 kN of load, which corresponds to a shear strength of 30 to 50 kPa. Similarly the sandy till layers overlying the weathered bedrock are performing very well. It is estimated the high pressure grouting takes resulted in significant cementation of the till layers and high bond between the micropile pipe and the till soils. Estimating the bond at 1000 kPa would result in a load transfer of 1800 kN over the estimated 3 m depth of till soils. However, it remains inexplicable how no load could be transferred to the first contractometer sensor located just above the weathered rock.

Compression Micropile

Creep Plot - Log Time

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

1 10 100

Time (min)

Cre

ep

(m

m)

1429 kN

1mm percycle

2 mm percycle

1850 kN

2525 kN

2906 kN

Figure 5 Plot of the micropile creep during the quick maintained compression load test

9

Figure 6 Installation of micropiles indicating cramped conditions

Figure 7 Completing the compression load test preparations

Conclusion

The instrumentation program designed for the micropiles at the Bowater Pulp and Paper Mill was successful in establishing the integrity and quality assurance of the piles. It is unfortunate that the designers did not foresee the potential positive effects of the pressure grouting program and the enormous increase in shaft capacity that it would provide. The shear stresses calculated for the clay and till soils, while very high, are within a plausible range (50 kPa for the stiff clays and 1000 kPa for the cemented tills). These results are contradicted by the elastic compression analysis routinely conducted as part of the compression load test. The theoretical elastic compression of the pile lies well within the range of performance for a pile transferring load to the rock socket. The placement of contractometer sensors was aimed at establishing rock performance which never materialised. In the future designers should make safeguards to ensure they collect data along the shaft of pressure grouted piles despite the low strength nature of overburden soils. The behaviour of the contractometers leaves some questions unanswered. While these instruments have provided exceptional data of great economic benefit in the past, in this instance there may have been a malfunction that limited their performance. Commentary A review of the FHWA recommendations for micropile design reveals that attention is paid to providing adequate factors of safety against yield for the steel and

concrete (or grouted) sections of the pile. However, the guideline does not take into account strain compatibility between the material sections. Upon investigation it is witnessed that the actual stress condition within the pile is dependent upon the ratio of the areas of the designed steel and concreted cross section. Calculation of section stresses while adhering to the condition of strain compatibility within the steel and grouted section reveals the following: Adjusting the equation δ= PL/AE to δ= σL/E, where: δ = deflection, P= load, L = length, A = area, E = Young’s modulus and σ = stress (or P/A), and assuming that deflection compatibility is the governing condition for stress, we can calculate (via iteration) the actual load distribution, and thus the stress, within the steel and grouted sections of the pile. The stress in each material is dependent upon the relative area of each section and the Young’s modulus of each material. It turns out that to achieve stress compatibility, which is to say the equivalent ratios of stress as a fraction of yield stress in each material the designer must provide an area of concrete that is equal to: Esteel / Econcrete times the area of steel. Further if the relationship is to be maintained as a ratio of the Factor of Safety against yield we must introduce the relative FOS for each material as well. For the recommendation of the FHWA this ratio of area of concrete to steel becomes: 2.5*Esteel / (2.0*Econcrete) such that the area of concrete is increased to accommodate the desired higher FOS for that material.

11

Applying the FHWA design implemented for the Type 1 pile above within the upper section (Section A-A) which includes pipe, grout and reinforcing bar, we see that the total area of steel is 92 cm2 and the are of concrete is 217 cm2. The ratio of steel to concrete area is 217/92 or 2.36. Calculation of the stress within each section according to strain compatibility indicates that at 2 times working load, or 3800 kN, the stress in the steel is 295 MPa and in the concrete is 50.3 MPa. Thus the steel is at 53.6% of yield and the concrete is at 126% of specified yield. Yet the design complies with the FHWA criterion. In order to meet the intention of the FHWA criteria and maintain strain compatibility the area of concrete to steel would have to be raised to 550*2.5/(40*2.0) or 17.2. While achievable, this ratio may not be the most economical. It is important to understand the ramification of violating

the strain compatibility equation when designing these piles. At yield what is the behaviour of grout? What is the high stress behaviour of well confined grout (i.e. grout contained within a steel casing)? If the plastic behaviour of grout is such that it continues to increase in strength (and or does not fail and yield at a lower stress level) then the ramification is that the steel section quickly ‘catches up’ from its stress holiday. If there are undue circumferential stresses placed on the confining steel under grout yield, perhaps these assumptions are non- conservative. If the grout is unconfined (other than by the soil) achieving the proper relationship between steel and grouted section areas is both more important and more easily attainable. In summary, this is an area where Isherwood Associates will continue to invest in field instrumentation and numerical modelling activities.

i MICROPILE DESIGN AND CONSTRUCTION

IMPLEMENTATION MANUAL, US Department of

Transportation, Federal Highways Administration,

Publication No. FHWA-SA-97-070, June 2000.