EXPLOSIVE COMPACTION FOR THE SEYMOUR FALLS...

EXPLOSIVE COMPACTION FOR THE SEYMOUR FALLS DAM SEISMIC UPGRADE Len Murray, P.Eng., Klohn Crippen Berger Ltd., Vancouver, British Columbia, Canada Neil K. Singh, P.Eng., Klohn Crippen Berger Ltd., Vancouver, British Columbia, Canada Frank Huber, P.Eng., Greater Vancouver Regional District, Vancouver, British Columbia, Canada David Siu, P.Eng., Greater Vancouver Regional District, Vancouver, British Columbia, Canada ABSTRACT The Seymour Falls Dam is located in North Vancouver, British Columbia, and was built in the early 1960s. The dam is undergoing an upgrade to improve resistance to earthquake loading. The upgrade includes construction of a new earthfill dam immediately downstream and connected to the existing earthfill dam. An essential part of the work was compaction of natural loose very coarse granular bouldery soil to create a dense stable foundation for the new dam. This paper discusses the history of selection of EC at Seymour Falls. The results of the EC construction, which was completed in early 2005, are discussed and compared to available theories for design of EC. Available literature on which design of EC is based, mostly deals with fine grained soils, and is shown to be not completely applicable for the very coarse soils at Seymour Falls Dam. RÉSUMÉ Le barrage de Seymour Falls est situé à North Vancouver en Colombie Britannique et a été construit au début des années 60. Ce barrage est en cours de réhabilitation pour mise aux normes sismiques et ces réhabilitations incluent la construction d’un nouveau barrage en terre situé immédiatement à l’aval du barrage en terre existant. Une partie essentielle des travaux consistait à compacter le sol naturel très hétérogène composé de matériaux grossiers et de larges blocs afin de créer un sol plus dense favorable aux fondations du nouveau barrage. Cette publication présente le processus de sélection de la compaction par explosif (« EC ») pour Seymour Falls. Les résultats de l’application de l’ « EC » sur le site, complètèe au début 2005, sont discutés et comparés par rapport aux théories disponibles sur la conception de « EC ». La littérature disponible sur les méthodes de calcul relatives à l’ «EC» aborde seulement les sols à matériaux fins et montre que ces méthodes ne peuvent pas être directement appliquées aux matériaux grossiers présents au barrage de Seymour Falls. 1. INTRODUCTION The Seymour Falls Dam, see Figure 1, provides about one third of the GVRD water supply which services over 2 million people in the Lower Mainland of British Columbia. The existing Seymour Falls dam was designed to have an ultimate height of about 47 m above the riverbed. In the early 1960s, Stage 1 of the dam was constructed to a height of about 30 m with provision for future raising, Stage 2, which has not been built. The dam is a composite structure consisting of a 235 m long slab and buttress concrete section, a concrete gravity retaining wall, a 220 m long earthfill embankment and an extensive upstream impervious earthfill blanket. As the seismicity of the region became understood, GVRD commenced studies and began implementation of seismic upgrades to the earth and concrete dam. The

work on the current phase of the seismic upgrade is summarized on Figure 2. The Earthfill section of the dam is founded on a debris torrent type deposit know as the Cougar Creek Fan, which comprises very coarse granular material ranging from zero thickness at the East abutment to over 150 m deep in the West.

Sea to Sky Geotechnique 2006

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Figure 1. Location Plan of Seymour Falls Dam and the Lower Seymour Conservation Reserve

Figure 2. Extent of Proposed Seismic Upgrade. The top 30 m of the Fan deposit was naturally in a very loose state, while below about 30 m depth the Fan material is dense due to pre-consolidation by glacial action. The upper 18 m of the Fan is extremely coarse with huge, over 10m diameter, igneous boulders, floating and nested in a coarse cobbly to fine sand matrix. The deposit fines downward becoming a coarse sand below about 24 m depth. A typical cross section of the dam and foundation is shown on Figure 3, further details can be found in Murray et al 2005.

Figure 3. North-South Cross Section – Existing Earthfill Embankment prior to Seismic Upgrade. 2. DESCRIPTION OF THE SEISMIC UPGRADE

PROJECT Although the existing dam is in excellent condition, it does not meet current seismic standards. The seismic upgrade consists of excavating the ground immediately downstream of the existing dam to a nominal depth of about 10 m, then compacting below the excavated

surface to about 20 m depth, by Explosive Compaction (EC) and Dynamic Compaction (DC) followed by construction of a new earthfill dam tied into the existing dam, as shown on Figure 4. Extensive site investigations were completed between 1990 and 1998. The investigation concluded that the finer matrix in the EC compaction zone had an average


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(N1)60 of about 10 blows/0.3 m. A typical gradation envelope of the native fan matrix material is shown in Figure 5A, and typical boulders are shown in Figure 5B. Deformation and seismic liquefaction triggering analyses showed that a final (N1)60 varying from 18

blows/0.3 m at the top of the EC zone to 13 blows/0.3 m at the base of the EC zone was required to meet seismic design criteria.

Figure 4. Seismic Upgrade – Earthfill Section

GRAIN SIZE DISTRIBUTION

0

10

20

30

40

50

60

70

80

90

100

0.0010.010.11101001000

Grain Size (mm)

Perc

en

t F

iner

by W

eig

ht

COBBLESGRAVEL SAND

SILT OR CLAYcoarse fine medium finecoarse

SIEVE OPENINGS IN U.S. SIEVE NUMBERS

6 3 1.5 3/4 3/8 4 10 20 40 60 100 200l

816 12

Typical Native Material Envelope

(scalped at 12 inches)

Figure 5A. Typical Native Soil Envelope scalped at 12 inch Figure 5B. Typical Boulders in Cougar Creek Fan

3. OVERVIEW OF EXPLOSIVE COMPACTION Compaction by the EC method involves detonating explosive charges in controlled sequences to create shear strains and precipitate liquefaction. Following liquefaction, the soil mass re-consolidates to a denser state as pore water pressures dissipate. 4. EC TRIAL PROGRAM To assist in the final design, a trial compaction program was conducted in 1998. The trial included EC from depths of about 10 to 30 m below original ground level. The EC trial achieved (N1)60-CS > 20 blows per 0.3 m in zones in the range 10 m to 20 m depth below ground level but gave little or no improvement below about 25 m

depth. Test hole drilling indicated that arching, creating a loose zone, with voids totalling 0.4 m thickness, had occurred in the bouldery ground above the EC zone. 5. FINAL DESIGN OF GROUND IMPROVEMENTS Using experience from the trial program, the ground improvement for the new dam was designed as follows: · Pre- excavate the upper 10 m of the site; · Conduct EC from 10 m to 20 m depth below the

excavation base; and · After EC, conduct DC to compact from 0 m to 10 m

depth.


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Due to the difficulty in obtaining meaningful SPT results in the coarse bouldery deposit, and the potential for contractual conflict if SPT were used as a performance criteria, it was decided that the work would be tendered as a method specification, which required the Contractor to drill, install and detonate defined patterns of explosives, but left the risk of achieving performance with the GVRD. The success of the EC was evaluated by measuring surface settlement, with a target of 2% to 5% over the 10 m EC zone thickness, and with spot checks by SPT. The EC was designed to very strict limits of settlement and peak particle velocity at nearby structures. The nearest EC blast panel was offset about 300 m from a fish hatchery, and 18 m from both a 90 inch diameter steel water main and a chlorination building, and about 8 m from the existing earth dam toe. 6. IMPLEMENTATION OF THE GROUND

IMPROVEMENT Six piezometer bundles were installed at the toe of the existing embankment including pneumatic, electric strain-gauge, and electric vibrating wire piezometers at two depths in each of three holes. The piezometers were used to monitor pore pressure changes during and after EC blasting, for use in checking the stability of the existing dam. Temporary monitoring points (TMPs) were established on critical structures. These survey points were monitored before and after each blast to check that settlements were within acceptable tolerances. The TMP locations are shown on Figure 6.

Figure 6. EC Panel Layout and Location of TMPs Eleven deep settlement posts (DSPs) were installed in the foundation footprint, with the base of the posts at 12 m depth, just within the top of the EC improvement zone. The purpose of the DSPs was to allow monitoring of settlement at depth, and comparison against settlement at surface. Greater settlement at depth than surface might be indicative of arching of soils above the EC zone, as experienced during the EC trial in 1998. EC was conducted over approximately two-thirds of the footprint of the new dam in fifteen panels. EC proceeded from the highest bench (furthest west) on the right abutment down to the lowest bench, closest to the gravity wall. The staging was partially selected to allow the first few blasts to occur in the least critical area, allowing the project team the opportunity to fine-tune the blasting program. For each panel, holes were drilled, cased and blasted in three passes, with the number of holes per pass averaging about 18 holes, with each hole between 10 m and 23 m depth (average about 20 m). A typical blast hole layout is shown in Figure 7. Approximately 800 holes, totalling over 16,000 m were drilled. Holes were loaded with Iremite® TX, a cap-sensitive emulsion manufactured using high strength microballoons for use in applications with high transient over pressures. Up to 30 kg were placed at each depth with a minimum 2 m vertical separation between the top and bottom of charges in subsequent decks (see Figure 8). Electronic detonators capable of 1 ms delay


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precision were used. In total, over 33,000 kg of explosive was detonated in 56 blasts, over a period of 10 months, from March 2004 to January 2005.

Figure 7. Typical Blast Hole Geometry for Panels

Figure 8. Typical Blast Hole Loading – Loading Pattern for Panel 2 Shown Significant settlement was achieved during the EC work, meeting the project requirements, while staying within the restrictive limits of settlement, PPV and site water quality. The average post EC SPT results achieved the specified targets. 7. COMPARISON OF SEYMOUR FALLS EC

PROGRAM WITH CASE HISTORIES The Seymour EC work is judged to have been highly successful and a comparison of the EC design

parameters and values quoted in the literature is provided below to assess whether readily available design tools are applicable to very coarse soil as encountered at Seymour. EC design normally focuses on hole spacing and charge weight, with secondary parameters of number of passes, detonation sequence and delay timing. The following discussion looks at how the above parameters, as used at Seymour, compare with available case histories, which primarily deal with fine sandy soils. 7.1 Hole Spacing The conventional range of the hole spacing based on case histories (Narin VanCourt 2003) is between 4.5 m to 11 m, with the optimum range being 4.5 m to 6 m spacing. At Seymour the spacing per pass was a 6 m to 8 m chevron, see Figure 7, with three offset passes. This gave the maximum distance between holes at about 7.2 m in each pass and a final spacing of about 4.5 m. This falls within the normal range of case histories. 7.2 Charge Weight and Depth Typically decks of 19 kg at 11 m depth and 25 kg at 18 m depth were used. Charge weight and depth was primarily selected based on the requirement to produce liquefaction at all points in the EC explosive pattern. Liquefaction at Seymour was defined by a pore pressure rise (∆Ru) equal to about 90% of the initial effective over burden stress. Methods of selecting this charge size include: Hopkinson’s number; Energy density; and Powder factor. 7.2.1 Hopkinson’s Number (Hn)

Hn relates charge weight (W kg of TNT) to radial distance (R) in metres and is expressed in the form

mkgR

WHn /3/1

3

=

Wang et al (2006) give the relationship for the maximum radius Rmax (metres) to liquefaction for large explosions in the kilotonne range (W in Kg): Rmax = 1.21 W1/3 i.e. Hn = 0.8 for liquefaction Charlie et al (1992) suggest Hn = 0.33 for liquefaction in saturated shallow silty soil for engineering scale


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charges. For Seymour a typical value of Hn at the mid point between holes is:

mkgHn /75.06.3

20 3/13

==

The required Hn, to achieve Ru=0.9, was measured at Seymour in 1998 as between 0.33 and 0.6 Kg1/3 /m, with Hn = 0.6 being selected for design. 7.2.2 Energy Input Attenuation Energy input attenuation relates points within the blast zone to charge weight (W) and vector distance (Ri) from the various charges by the relationship.

å= 2/2)(

mg

iR

iWEi

Ei has the advantage that if calculated for many points in the densified mass it can demonstrate that energy is being evenly distributed. Narin van Court (2003) review of case histories indicated that 750 to 1,500 g/m2 per pass and 1,500 to 3,000 g/m2 final energy was a normal range of Ei. At Seymour the estimated Ei was in the range of 8,000 to 12,000 g/m2 or up to 4 times the normal practice limit. The Seymour Ei values indicated relatively even distribution of energy in the improved zone. 7.2.3 Powder Factor Powder Factor (PF) is a conventional blasting parameter which gives a crude estimate of the energy delivered to the blasted mass and can be expressed as g/m3. The volume to be used in the calculation of PF for EC is not obvious, but at Seymour we used the plan dimensions of the blasted area plus 3 m beyond the perimeter holes

and an average 12 m thickness over the zone of improvement. For Seymour PF per pass ranged from 80 to 65 g/m3 with a total PF of about 225 g/m3. A review of case histories (KC 1998) suggested that reported total PF is in the range of 10 to 60 g/m3. Hence Seymour used 4 to 20 times the PF quoted in case histories. 7.3 Secondary Parameters 7.3.1 Number of Passes

At Seymour three passes were selected based on the Trial EC program (KC 1998). However, there was considerable uncertainty whether three passes was sufficient. Surface settlement measurements during EC (see Figure 9) suggested that there was no trend to decreasing settlement with increasing number of passes, possibly because later blasts were partly breaking down arching and causing previously induced settlement to be reflected at the surface. The presence of the 1998 EC trial area within the blast mass provided a means of comparing an area which had received 6 passes (3 in 1997 and 3 in 2004) with areas that had received only 3 passes. Inspection of the EC settlement plan, Figure 9, shows the trial EC area as a zone of lower than average nearby settlement. After removing the 400 mm arching settlement caused during the 1998 EC trial (not reflected at the ground surface until 2004) the reduction in settlement is even more apparent. From this we concluded that 3 passes was sufficient at Seymour. Case history data suggests two passes is common, however at Seymour where arching was a concern, three passes were needed.


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Figure 9. Measured EC-induced ground surface settlement. 7.3.2 Detonation Sequence The ability to experiment with detonation sequence at Seymour was restricted by the nearby operating water supply storage dam. To reduce the possibility of stacking vibrations, blasts proceeded in rows from north to south, away from the dam. There was no obvious impact on settlement distribution due to the blast sequence. Other authors suggest blasting in a spiral from the centre out may be advantageous but we are unable to assess this. 7.3.3 Delay Timing The delay timing in the blast is important to the effectiveness of EC (Narin van Court, 2003) and is related to the processes by which EC is thought to densify the ground. Compaction by EC is believed to result from a number of processes, but principally from shearing, due to the blast, causing a volume reduction, subsequently realized during pore pressure dissipation under the weight of the overlying ground. Green (2001) notes that each explosion causes about two to four cycles of loading as gases expand and contract to reach equilibrium, subsequent blasts at later delays add to the cycles. Typically at Seymour if one assumes that the closest eight holes contribute most to the shear strain at any point, with two decks per hole and three passes, that

equates to about 48 explosions each of two to four cycles or roughly 100 to 200 cycles. Vucetic (1994) suggests that threshold shear strains of at least 0.1% are needed to cause liquefaction, but once liquefaction has occurred further shear strain will not occur until pore pressures dissipate. At Seymour, recorded vertical settlement averaged about 0.5 m in zones of deep bedrock and showed a maximum of almost 2 m in the north east of the blast area, assuming a thickness of liquefied ground of 10 m to 16 m, this gives a vertical strain of 5% to 20%. The large settlement suggests that collapse of the ground occurred, consistent with the geologic model of very loose ground including nested boulders. 8. CONCLUSIONS The seismic upgrade of the Seymour Falls Dam included explosive compaction at depth. The work was completed under very tight restrictions of settlement, vibration and groundwater chemistry appropriate to a major operating water dam. EC was effective in achieving the required ground compaction. In general the EC design followed published practice, except that the charge weight per unit volume was much higher than commonly used in finer ground.


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References Charlie, W.A., Jacobs, P.J., Doehring, D.O. (1992).

“Blast-Induced Liquefaction of an Alluvial Sand Deposit”. ASCE Geotechnical Testing Journal, GTJODJ, Vol. 15, No. 1. pp 14-23.

Green R.A., Mitchell J.K., Via C.E. (2001). “Energy-Based

Evaluation and Remediation of Liquefaction Soils” Ph.D. Dissertation, Virginia Polytechnic Institute and State University, 397 pages

Klohn Crippen Consultants Ltd., (June 1998). “Case

Histories of Explosives Densification”. Memo from A. Wightman to F. Huber.

Murray, L., Singh, N., Huber, F., Siu, D. (2005). “Seismic

Upgrade of the Seymour Falls Dam Using Explosive and Dynamic Compaction”. Association of State of Dam Safety Officials, Proceedings of Dam Safety Officials, Proceedings of Dam Safety 2005, New Orleans LA (conference moved to Orlando FL).

Narin van Court, W. (1997). “Investigation of the

Densification Mechanisms and Predictive Methodologies for Explosive Compaction (Blast Densification)”. Dissertation for PHd. University of California at Berkeley.

Narin van Court, W. (2003). “Explosive Compaction

Revisited: New Guidelines for Performing Blast Densification”. Soil and Rocks 2003, 12th Panamerican Conference on Soil Mechanics and Geotechnical Engineering. Vol. 2, pp. 1725-1730.

Rollins, K., Lane, J., Nicholson, P., Rollins, R. (2004).

“Liquefaction Hazard Assessment Using Controlled-Blasting Techniques”. 13th World Conference on Earthquake Engineering, Vancouver, B.C. Canada August 2004


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