Micropile Load Transfer Micropile Load Transfer Rates and Bond Stresses Rates and Bond Stresses in the Puget Sound in the Puget Sound Region of Washington Region of Washington State, U.S.A. State, U.S.A. Carole Mitchell, P.E. Carole Mitchell, P.E. Wendy Mathieson Mathieson, P.E. , P.E. Wendy Seattle, Washington, U.S.A. Seattle, Washington, U.S.A.
Background Background � S&W began in S&W began in- -house research project in house research project in � 2004 2004 � Advance state of practice for load transfer Advance state of practice for load transfer � rates and bond stresses rates and bond stresses � Compare to soil types/densities and Compare to soil types/densities and � geologic units geologic units
Research Process Research Process � Collect information from S&W and partner Collect information from S&W and partner � contractor files contractor files � Extract ultimate or maximum frictional Extract ultimate or maximum frictional � capacities, soil types, etc. capacities, soil types, etc. � Assign geologic unit and soil type based on Assign geologic unit and soil type based on � bond zone location and elevation bond zone location and elevation � Enter into database Enter into database �
Soil Classification Soil Classification � Unified Soil Classification System, ASTM Unified Soil Classification System, ASTM � D2487 D2487 � Majority of data: silty sand with varying Majority of data: silty sand with varying � amounts of gravel (SM); low plasticity clay amounts of gravel (SM); low plasticity clay and silt (CL and ML); slightly silty sand and silt (CL and ML); slightly silty sand with varying amounts of gravel (SP- -SM SM with varying amounts of gravel (SP and SW- -SM) SM) and SW
Major Geologic Units and Major Geologic Units and Soil Types Soil Types � Deposits during glacial retreat: Recessional Deposits during glacial retreat: Recessional � Outwash (Qvro Qvro) ) - - sand and gravel sand and gravel Outwash ( � Overridden deposits: Till ( Overridden deposits: Till (Qvt Qvt) ) – – sand, clay, and sand, clay, and � silt; Advance Outwash (Qva Qva) ) – – sand and gravel sand and gravel silt; Advance Outwash ( � Lean silt and clay, silty sand, and sand and Lean silt and clay, silty sand, and sand and � gravels gravels
Simplified Stratigraphy Stratigraphy Simplified
Frictional Capacity Frictional Capacity � Test to failure (ultimate) versus test to Test to failure (ultimate) versus test to � maximum maximum � Ultimate frictional capacity: engineering Ultimate frictional capacity: engineering � judgment, evaluated load/unload curves, judgment, evaluated load/unload curves, permanent set, equipment influence, etc. permanent set, equipment influence, etc. � Plotted both as load transfer rate and as Plotted both as load transfer rate and as � bond stress bond stress
Data Data � Not in a controlled lab environment Not in a controlled lab environment � � Some bond zones isolated, many were not Some bond zones isolated, many were not � � Grout mix design not evaluated Grout mix design not evaluated � � No instrumentation on bond zones No instrumentation on bond zones � � No sacrificial bit, hollow No sacrificial bit, hollow- -core tests core tests �
Drilling Methods Drilling Methods Method Drilling General Description Method Drilling General Description Number Method Number Method 1 Single- -tube tube Casing with drill teeth, flush with air 1 Single Casing with drill teeth, flush with air Advancement and/or water. Advancement and/or water. 2 Rotary Duplex Simultaneous rotation and 2 Rotary Duplex Simultaneous rotation and advancement of casing plus internal advancement of casing plus internal rod, carrying flush. Rod typically has rod, carrying flush. Rod typically has down- -the the- -hole hammer. hole hammer. down 3 Rotary Percussive Same as 2, except casing and rods 3 Rotary Percussive Same as 2, except casing and rods Concentric Duplex percussed as well as rotated. as well as rotated. Concentric Duplex percussed 8 Open Hole with Air rotary down- -the the- -hole hammer. hole hammer. 8 Open Hole with Air rotary down Air Rotary Casing may or may not be used in Air Rotary Casing may or may not be used in Hammer upper no- -load zone. load zone. Hammer upper no
Grouting Methods Grouting Methods Micropile Grouting Drill Casing Micropile Grouting Drill Casing Type Method Type Method Type A Gravity grout only Varies from temporary Type A Gravity grout only Varies from temporary casing/unlined (sub- casing/unlined (sub -type A1) to type A1) to permanent casing in upper shaft permanent casing in upper shaft only (sub- -type A3). type A3). only (sub Type B Pressure Pressure- -grouted grouted Varies from temporary Varies from temporary Type B through the casing casing/unlined (sub- -type B1) to type B1) to through the casing casing/unlined (sub during withdrawal permanent casing in upper shaft during withdrawal permanent casing in upper shaft only (sub- only (sub -type B3). type B3). Type D Primary grout placed Varies from temporary Type D Primary grout placed Varies from temporary under gravity head casing/unlined (sub- -type D1) to type D1) to under gravity head casing/unlined (sub (Type A) or under (Type A) or under permanent casing in upper shaft permanent casing in upper shaft pressure (Type B), then only (sub- -type D3). type D3). pressure (Type B), then only (sub one or more phases of one or more phases of secondary pressure secondary pressure grouting grouting
Plot Legend Plot Legend
Drilling Method SM- -SC SC Drilling Method SM 1 Drilling Method 2 3 8 0.0 0.2 0.4 0.6 0.8 1.0 Measured Maximum or Ultimate Bond Stress [MPa]
Grouting Method SM- -SC SC Grouting Method SM A Grouting Method B D 0.0 0.2 0.4 0.6 0.8 1.0 Measured Maximum or Ultimate Bond Stress [MPa]
Groundwater Presence Groundwater Presence SM- -SC SC SM Above Ground water Below Ground water 0.0 0.2 0.4 0.6 0.8 1.0 Measured Maximum or Ultimate Bond Stress [MPa]
Conclusions Conclusions � Groundwater appears to lower friction for Groundwater appears to lower friction for � SP- -SM and SW SM and SW- -SM, but not CL/ML or SM SM, but not CL/ML or SM SP and SC and SC � Evaluate USCS classification and geologic Evaluate USCS classification and geologic � unit unit � Published values differ for same soil types Published values differ for same soil types �
Conclusions Conclusions � Glacially overridden soil Glacially overridden soil – – HIGH frictional HIGH frictional � capacity! capacity! � Pressure/Post Pressure/Post- -grouting increase frictional grouting increase frictional � capacity in non- -overridden soil overridden soil capacity in non � Some currently published ultimate values Some currently published ultimate values � are conservative for soil in the Puget Sound are conservative for soil in the Puget Sound Area Area
Future Research Future Research � Test to Failure Test to Failure � � Expand Database Expand Database �
Acknowledgements Acknowledgements
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