Trenchless Crossing Evaluation on the Trenchless Crossing Evaluation on the Martha Lake Gateway Sewer

An Introduction to Trenchless Methods and Evaluation for the Installation of New Pipelines

Introduction

• Purpose− Provide engineers and owners with one example of a trenchless crossing evaluation.

• Outline−

•− Trenchless Considerations− Trenchless Methods

HDD, microtunneling, pipe jacking, auger-boring, pipe ramming.

−Martha Lake Sewer Project Overview−Martha Lake Trenchless Evaluation− Conclusion of Evaluation− Next Steps

Trenchless Considerations

• Ground Conditions− Soil Type−Groundwater− Rock / Boulders−−Mixed Face Conditions−Wood−Man-made Debris (concrete, steel, VW Bugs, etc.)

• Crossing Length• Pipe / Casing Diameter• Alignment (Horizontal and Vertical) • Depth (cover over excavation)

Trenchless Methods

• Horizontal Directional Drilling (HDD)• Microtunneling• Pipe Jacking• Auger Boring• Auger Boring• Pipe Ramming

Horizontal Directional Drilling (HDD)• Pilot hole• Reaming pass(es)• Spoils removed by recirculation of drilling mud•• Pipe insertion

Horizontal Directional Drilling (HDD)• Strengths− Performs well in groundwater− Drilling mud mix can be alteredfor ground conditions

− Very long crossings−−− Curved alignments

• Weaknesses− Elevated risk of frac-out or settlement− Smaller diameters (< 36”)− Difficult to control line and grade− Requires a 2% minimum slope for gravity applications

− Large area for pullback layout

Microtunneling

• Jacking shaft• Spoil Removal via slurry lines• Slurry pressure provides •excavation face support• Hydraulic jacks advance pipe string and MTBM

Microtunneling• Strengths− Performs well above or below groundwater− Slurry pressure can be varied based on support needed at excavation face

− Longer Drive Lengths−Good line and grade control−Good line and grade control

• Weaknesses− Has difficulties excavatingobstructions larger than 1/3 the diameter

−Most do not allow face access to remove obstructions− Rescue pit required if stopped by an obstruction− Can be deflected by mixed-face conditions

Pipe Jacking

• Jacking Shaft• Open face allows manned access• Spoil Removal via mechanical or manned methods• Hydraulic jacks provide advance pipe string and • Hydraulic jacks provide advance pipe string and pipe jacking machine

Pipe Jacking• Strengths− Face access allows removal of obstructions− Longer Drive Lengths−Good line and Grade Control

• Weaknesses− Difficult to control groundwater− Difficult to control flowing soils− Can be deflected by mixed-face conditions

Auger-boring• Jacking Shaft• Spoil Removal via auger flight though jacking pipe• Hydraulic jacks advance pipe string and auger flight

Auger-boring• Strengths−Good in cohesive soils−Minor line and grade control

• Weaknesses− Difficult to control groundwater−Max. drive length of around 300’

Pipe Ramming

• Excavated pit• Pneumatic tool advances pipe• Minimal Spoil Removal• Face support provided by soil plug• Face support provided by soil plug

Pipe Ramming• Strengths−Minimal risk of ground settlement

− Excavation continuously supported

−− Performs well above orbelow groundwater

• Weaknesses− Difficult to control line and grade

− Short drive lengths (< 300’)

Martha LakeProject Overview• Alderwood Water & Wastewater District • Lynnwood, WA

Site

Martha Lake Project Overview

• I-5 and 164th St. SW• 3,400 LF of 12” to 15” gravity sewer• Residential Septic• Residential Septic• Commercial Pump Stations• 1700 LF of 24”water main added• Currently at 60%

Martha Lake Trenchless Crossing• Ground Conditions− Soil

§ Dense Glacial Till overridden by fill−Groundwater

§§ Perched groundwater on top of Till§ Pockets within Till

− Rock / Boulders§ Boulders: 1.5’ to 7’

−Mixed Face Conditions – None Known−Wood – None Known−Man-made Debris – None Known

Martha Lake Trenchless Crossing

TILL

• Drive Length− 535’

• Pipe Diameter(s)− 54” - 60” Casing

Martha Lake Trenchless Crossing

− 54” - 60” Casing− 12” Ductile Iron Sewer− 24” Ductile Iron Water− Six 4” Conduits

• Alignment− Vertical profile lies completely within Glacial Till

• Depth− 14’ minimum, 25’ maximum

Martha Lake Trenchless Evaluation

Trenchless Trenchless Trenchless Trenchless MethodsMethodsMethodsMethods

Soil Types

(Glacial Till)

Ground Water

(Gravel Pockets)

Rock / Boulders

Settlem

ent Risk

Drive Length

(535')

Line and Grade

Control

TOTA

LTOTA

LTOTA

LTOTA

L

(Glacial Till)

Ground Water

(Gravel Pockets)

Rock / Boulders

Settlem

ent Risk

Drive Length

Line and Grade

HDD 6 8 5 6 9 4 38

Microtunnel 7 8 6 8 8 8 45

Pipe Jack 8 5 8 6 8 8 43

Auger-bore 5 4 5 6 1 4 25

Pipe Ramming 4 8 2 8 1 3 26

Conclusion

• Microtunneling or Pipe Jacking− Two options increases number of eligible contractors− Contractor can chose machine he believes will work best for ground conditions

− Three jacking pipe options give more flexibility− Three jacking pipe options give more flexibility§ HOBAS§ Reinforced Concrete§ Steel (PermaLok)

• GO BIG!!! = 60” Casing

WSDOT

PUD

WSDOT

Boulders!!

Boulders!!

Next Steps

• Specifications− Specify MTBM best suited for ground conditions− Specify options for rescue shaft to free MTBM− Require mitigation plan for encountering groundwater with a pipe jacking machinewith a pipe jacking machine

− Require obstruction removal plan for pipe jacking• GBR−Must apply to both pipe jacking and microtunneling operations

Trenchless Tips

• Use in combination with other methods• Pay a premium for short crossings• Talk with machine manufacturers and local contractors!contractors!

Questions?