Fifty Feet of Peat: Design Challenges of Three Long HDD Crossings of the Sacramento River

Transcription

1 Fifty Feet of Peat: Design Challenges of Three Long HDD Crossings of the Sacramento River 3 rd Annual Rocky Mountain Regional No Dig Conference & Exhibition Westminster, CO November 8, 2012 David Bennett, PhD, PE Mary Asperger, PE Matthew Wallin, PE Project Background The Port of West Sacramento Accessed from the Pacific Ocean via the Golden Gate/SF Bay and the Sacramento River Deep Water Ship Channel US Army Corps of Engineers proposed increasing the dredge depth 1

2 Project Location Pacific Gas & Electric (PG&E) owns and operates four major gas transmission lines within the natural Sacramento River channel Three are in conflict with the proposed dredging River width 2,600 ft Geologic Setting Two primary geologic formations: Montezuma Formation Delta Mud Significant subsidence in the Delta Ground surface elevation is approx. 10 to 20 ft below sea level Delta Islands are protected by nonengineered levees Flooding is a major concern 2

3 Line 130 Geotechnical Conditions 8 borings: 4 land and 4 over water Two major concerns: Approximately 50 foot thick layers of very soft to soft peat and organic soils Gravel beds encountered in the river channel Line 114 Geotechnical Conditions 7 borings: 4 land and 3 over water Southeast land borings encountered approximately 50 feet of very soft to soft peat and organic soils 3

4 Line 400 Geotechnical Conditions 7 borings: 4 land and 3 over water Southeast land borings encountered approximately 50 feet of very soft to soft peat and organic soils Levee Concerns Settlement Risks Hydrofracture Risks Seepage risks US Army Corps of Engineers Engineering and Design Manual: Design and Construction of Levees (USACE EM ) Minimum 300 feet of setback from levee centerline 4

5 Settlement Risk Evaluation Settlement of the non engineered levees is a major concern Used approach as described by Bennett and Cording (2000), Wallin, Wallin, and Bennett (2008), and Bennett (2009) Systematic settlements are modeled as an inverted normal probability distribution curve, with the largest settlement predicted directly above the HDD bore centerline Bores sited within consolidated deposits outside of levee zone Lines 130 and 114 Settlement Evaluation Results 5

6 Line 400 Settlement Evaluation Results Hydrofracture Analyses Based on approach described in Bennett and Wallin (2008) Maximum allowable pressure (p max ) calculated using Delft Cavity Expansion Model Minimum required pressure (p min ) calculated using Bingham Plastic Model Hydrofracture risk is highest where p min exceeds p max Crossings were modeled using: Three to four layers Dramatically different geotechnical conditions on each side of the river led to using two sets of soil properties, one for each side of the crossing and combining the results into one graph 6

7 Line 130 Hydrofracture Analysis HDD Intersect Method Using a typical installation procedure the risk of hydrofracture was unacceptably high for the pilot bore. Instead, the intersect method will be used. Reduces minimum required drilling fluid pressure, P min, by 50% by using two rigs for pilot bore. 7

8 Line 130 Hydrofracture Analysis Line 114 Hydrofracture Analysis 8

9 Line 400 Hydrofracture Analysis Pipe Stress Analyses Steel pipe was specified for the replacement pipeline crossings, typical for gas transmission Pipe stress analyses were conducted in accordance with procedures described in ASCE Manual of Practice 108 (ASCE, 2005) Calculated stresses were well within maximum allowable stresses in all cases for the recommended pipe wall thicknesses Line 130: 16 Ø x Line 114: 12 Ø x Line 400: 36 Ø x

10 Bore Geometries Steel Pipe Diameter Approx. Length Bend Radii Conductor Casing Line inch 3,700 ft 1,600 ft 250 ft on each side Line inch 4,700 ft 1,200 ft 350 ft SE side Line inch 3,900 ft 3,600 ft 350 ft SE side Levee setbacks increased to 700 feet. Conductor Casing Needed Support unstable soils Contain drilling fluid within peat Long casings required, 250 to 350 feet Likely need to hammer, auger, telescope, etc. Soft soils should help installation 10

11 Contact Grouting Concern: Trenchless crossings beneath levees could create a preferential seepage path for flood waters Minimize the Risk: Seal the annular space by injecting grout into the end of the bore after pullback is complete Long conductor casings create a stable bore and will allow grout to be injected over the length of the casings The two uncased entry points must be grouted within a relatively short time after pullback is completed Concern about very weak soils collapsing around carrier pipe if grouting is performed while withdrawing casing Corrosion Concerns Need to be sure that the steel product pipe is electrically isolated from the steel conductor casing Where casing is located on the pipe pullback side insulated casing spacers can be used Line 114 has casing on the rig side: Requires special procedures to ensure electrical isolation and allow for grouting Specifications provide several suggested methods and an option for the Contractor to submit an alternative 11

12 Double Pull Head Holes for Grouting Double Pull Head Steel Product Pipe Non-Conductive Pipe Material Sliplining the Conductor Casing Holes for Grouting Conductor Casing Non-Conductive Pipe Material 12

13 Grouting Non- Conductive Pipe Material Steel Product Pipe Grout Conductor Casing Conclusions Three major HDD crossings of the Sacramento River required because of plans to increase the dredged depth of the Sacramento River Deep Water Ship Channel Geotechnical conditions are challenging, primarily due to deposits of approximately 50 feet of very soft to soft peat and organic materials Flood control and settlement of the flood control levees are significant concerns Design completed, construction on indefinite hold 13

14 Questions? Dave Bennett, PhD, PE Principal