armour is used for river training to protect roads and bridges from erosion. ... put on maximising the outcome of the armour stone quarry and utilising this to the ...
Keynote lectures Nordisk Geoteknisk Möte-2000. Finish Geotechnical Society.
Quarry yield prediction as a tool in breakwater design Omar Bjarki Smarason STAPI Ltd - Consulting Geologists, Reykjavik, Iceland
Sigurdur Sigurdarson, Gisli Viggosson Icelandic Maritime Administration, Kopavogur, Iceland
ABSTRACT: Quarry yield prediction has played an important role in the design phase of harbour breakwater projects in Iceland since the early 1980’s. Preliminary designs are based on initial size distribution estimates from potential quarries, and the final design is tailored to fit the selected quarry. Quarry selection is a process which aims to provide rocks best suited to the wave conditions of the construction site and at the same time to minimise transport costs and environmental disturbance. The use of a small fraction of large (10-20 tonne) and extra large (2030 tonne) armourstone increases the stability and wave absorbing function of berm breakwaters and lengthens their durability. Moving stones break down faster than stable ones and voids get plugged, reducing the wave dissipation. Accurate size prediction of quarry yields, coupled with design adjustments, can make a considerable difference to the economics of the breakwater project.
1 INTRODUCTION Commercial rock quarries are relatively few in Iceland. New quarries are therefore often needed for new harbour projects although existing ones can sometimes be used. Selection of suitable quarries begins with inspection of geological maps and aerial photographs of the area adjacent to the planned breakwaters and the area is widened until successful. The search for suitable armour stone quarries is initially directed at any prominent thick lava flows that may be accessible on low ground or in accessible benches or hillocks. Promising sites are visited and inspected visually for geological features such as rock type, weathering forms, pores, pore fillings (amygdales), alteration and joint density. Further investigation is performed through pneumatic drilling and core sampling at promising sites before bidding. Quarries may be located right by the breakwater structure or up to 40 km away. However, the transport distance is commonly 5-15 km. Core material is sometimes produced more economically in poorer quarries closer to the structure or dredged from sediments on the sea floor. Possible quarry yield and rock quality is weighed against transportation distance in each case to optimise cost effectiveness and strength and durability of the structure. Environmental impact assessment is carried out for quarries exceeding 150,000 m3 in total volume in situ of rock, or 220,000 m3 of blasted rock, as the bulking factor is usually about 1.45 to 1.50. Armourstone is used for harbour breakwaters and for shore protection of roads and buildings around Iceland. Hydro-electrical power plants use armourstone to protect reservoir dams and small armour is used for river training to protect roads and bridges from erosion. The use of armourstone for harbour breakwaters and shore protection in towns and villages is about 200,000 m3 a year on average, including core material. Large harbour projects that are likely to go ahead in the next couple of years may increase this considerably. In 1996 to 1998 the use of armour for road protection will have been about 50,000 m3 a year and 60,000 m3 will be produced for reservoir dams of a power plant in 1997 to 1998. About 5 quarry projects (new or old quarries needing reassessment) where the total quarried armourstone material is at least 20,000 m3 are dealt with
every year, along with several smaller projects. A few preliminary quarry estimates are carried out each year as a part of feasibility studies for potential heavy industry sites. Scan-lines at horizontal and vertical rock exposures are used to measure the fracture density of the rock. Two scan-lines, at right angles to each other, are measured with a tape measure on the upper surface and also in vertical sections if possible. Pneumatic drilling is carried out to give an idea of the thickness of the rock, possible size of quarry and an idea of the soundness and alteration grade of the rock. Core drilling is usually carried out to give further information regarding spacing of discontinuities (joints and fractures), rock quality, specific gravity, absorption, point load index, freeze/thaw resistance and optical inspection in thin section. Measurements of discontinuity spacing in scan-lines and cores are used to establish an idea of the possible size distribution of blasted material from the rock. It is assumed, in the interpretation, that the shape of the stones is on average cubic.
2 BREAKWATERS Breakwaters are built mainly to reduce wave penetration into harbour areas. They are usually built as rubble mound structures consisting of an inner core of fine material of quarry run, covered with an armour layer of stones. The armour layer of a conventional rubble mound structure usually consists of two layers of armour stones, or concrete armour units if armour stones are not readily available, and a filter layer or an under layer to prevent the finer material from being washed out. The armour layer extends about 1.0 to 1.5 times the design wave height above the design water level and to the same distance below the lowest water level. The size of the armour stone needed to resist the wave energy is proportional to the wave height in third power. This means that for conventional rubble mound breakwaters very large stones are often needed in large quantities. This design method can be characterised as a “demand-based design” (Leeuwestein et al. 1995). Berm breakwaters were introduced in the early 1980’s. The design philosophy of berm breakwaters aims at optimising the structure not only with respect to wave load but also possible yield from an armour stone quarry, which can be characterised as “supply-based design”. The initial idea of berm breakwaters was that they should be wide voluminous structures, built of two stone classes with a wide size gradation, allowing a considerable reduction of armourstone size. These structures were allowed to reshape, with stones moving up and down the slope, into a Sshape profile, which was assumed to be a more stable profile and the structures are sometimes referred to as dynamically stable structures. Experience with a dynamically stable structure in Iceland has, however, demonstrated that when stones start to roll up and down the slope and hit each other, high abrasion and splitting of stones will occur. Voids will be filled up with smaller stones and the ability of the structure to dissipate wave energy will decrease. The forces acting on each rock unit on the slope will increase, which accelerates the dynamic movement of the stones and increases breaking and splitting even further. An “Icelandic type” of berm breakwaters has been developed, where the structure is less voluminous and more stable than the original berm breakwater concept anticipated. An emphasis is put on maximising the outcome of the armour stone quarry and utilising this to the benefit to the design. The goal in the design of the Icelandic type berm breakwater is that it shall be statically stable. Only some minor deformation of the berm is allowed under design conditions, but reshaping into an S-shape profile is prohibited. It is recognised that the reshaping will increase during the lifetime of the structure, where stone quality is insufficient and because of repeated wave action. It is not necessarily the approach of the design to fulfil certain prescribed stability parameters, Hs/Dn50 , but to look at the correlation between the armour stone quarry, size distribution and quality. It also takes into account the design wave, wave height, wave period and direction; water depth; function of the breakwater, for what purpose is it built, and whether wave overtopping is a problem or not. We have in many cases been able to design a berm with high stability of the armouring layer at no extra cost. Good interlocking by carefully placed stones is advantageous, especially on the front and the edge of the berm.
3 DESIGNER AND GEOLOGIST COLLABORATION Close collaboration between the designer and geologist in the preparation of berm breakwater projects has proven very effective over the years. This has resulted in better designs and better use of the quarried material. This collaboration gives the designer the chance to fully utilise material from the quarry down to the smallest possible stone size and has often resulted in 100% utilisation of the quarries. Close co-operation between the geologist and the project supervisor with the contractor is often necessary to achieve maximum results in the quarry. Blasting and sorting of armourstone is by no means an easy task and slight alteration of spacing and tilt of drillholes may at times help to improve blasting results. It has to be realised that the contractor and the buyer should work as a team aiming for the same goal. Experienced contractors rely on the predicted yield curves in their bidding. Recent developments in berm breakwater design have aimed at using large to extra large stones (10-20 tonnes and 20-30 tonnes) in the more exposed parts of the structures, as many of the better quarries are found to produce 10 to 20% of armourstones exceeding 10 tonnes in size. As large backhoe excavators that can handle extra large stones have become readily available, we have started prescribing these large and extra stones to the advantage of the berm breakwater structures in some recent projects. A relatively low percentage (1-3%) of the largest stone class can be an advantage for most breakwaters. This is not only true for extreme wave conditions where these extra large stone classes are most needed but also applies to more moderate wave load conditions and where quarries with lower size distribution are used.
Grímsey Bolungarvík Siglufjördur
Thorshöfn
Ólafsfjördur Húsavík
Dalvík Skagaströnd
Akureyri Blönduós
Bakkafjörður Vopnafjörður
fjö ar nd ru G
Borgarfjördur-eystra
Gilsfjörður
ur rd
Djúpivogur Akranes Keflavík Helguvík
Reykjavík Keilisnes Thorlakshöfn
Hornafjörður Skeiðarársandur
Grindavík
Armourstone Project Sites 1984 - 1998
Figure 1. Armourstone quarries and breakwater projects in Iceland.
Legend Harbour pro jects Feasibility stud y Road protection Reservo ir dam
4 CASE HISTORIES Quarry investigation began in Iceland in preparation for the Thorlakshöfn harbour project in 1973, where about 600.000 tonnes of quarried stones were required (Hostrup – Schults & Sorensen, 1973). Hardarson (1979) carried out an investigation on Icelandic basalt in breakwaters. Preparation for the berm breakwater in Helguvik is described by Baird and Woodrow (1988). The present authors have been in charge of research and preparation of nearly all new or proposed breakwaters in Iceland since 1984. Table 1 gives an overview of some important geotechnical data on quarries for selected breakwaters in Iceland and Sirevåg in southwest Norway. It provides information on rock type, absorption, density, point load index (IS50), freeze/thaw resistance (Swedish standard SS 13 72 44) as well as the predicted maximum quarry yield of armourstone over 1 tonne and large and extra large armourstone over 10 and 20 tonne for the individual quarries. Location of the breakwater sites are shown in Figure 1. Table 1. Guidelines for quality control of armourstone of igneous rocks. Test Rock type
Specific gravity (SSD) (tonn/m 3) Water absorption (%)
Freeze/thaw test. Flaking in kg/m 2 Point Load Index IS(50) (MPa) Alteration of minerals Inner binding of minerals
Excellent (A) Gabbro, Porhyritic basalt, Dolerite
Good (B) Granite, Anorthosite Ol.-tholeiite, Alkali basalt
Marginal (C) Tholeiite basalt, Andesite
Poor (D) Rhyolite, Dacite, Hyaloclatite,
>2,9
2,65-2,9
2,5-2,65
10 tonnes >20 tonnes
5 13 32 34 21 24 23 51 25 31 47 35 48 10 32 33 34 54 47 51 40 25 65 60
0 0 9 5 4 2 5 20 6 5 21 10 13 0 0,1 tonne, 22% >0.5 tonne, 13% >1 tonne, 5% >2 tonnes, and 1% >5 tonnes. The actual outcome from the quarry during September, October and November 1983 was however 38% >0.5 tonne and 2.2% > 6 tonnes (Jónasson, 1983). The specific gravity of this rock is 2.87 and water absorption 0.9%. Flaking after 35 cycles in a freeze/thaw test was 3.6 kg/m2 . This was considered too much to continue the test. Deterioration of stones on the berm has accelerated a dynamic development of the profile. In the winter 1992/93 the breakwater is believed to have experienced waves close to the design load. The berm was eroded up to the crest and an unstable S-profile had developed. Repair took place in 1993 and in spite of the poor quality of the rock it was decided to use the local quarry again. In the autumn of 1995, the structure was exposed to the design storm. Video recordings from the storm show breaking waves in front of and on the structure, resulting in heavy overtopping. Inspection of the reshaped profile showed that deterioration of the stones had caused filling and plugging of voids and the structure did not function as a berm breakwater any longer. The main conclusion that can be drawn from the Bakkafjördur breakwater is that in a dynamic structure stones will break and the voids will gradually be filled up with smaller stones. This will decrease the ability of the structure to dissipate wave energy. Inspection at the site led to the conclusion that the poor quality (highly altered basalt) of the stones in the Bakkafjördur breakwater only accelerated a development that would occur over a longer time period even if it was built of better quality stones of the same size grades. An inner rubble mound was constructed at Bakkafjördur in 1989. It was made of 24,000 m3 of blasted rock. It was decided to use the poor quality local rock and increase the armour stones size to about 4 times the design criteria to compensate for the poor rock quality. This was the only way the breakwater badly needed for protection of the local fleet of boats, could be built on the available budget. The project went ahead and the breakwater is still there 11 years later (Figure 3). The main breakwater at Bakkafjördur underwent a major berm repair in 1993, after 10 years of service. Armourstones from the local quarry were used for the repair of the berm, whereas a large part of it had been constructed from Pleistocene stones of olivine-tholeiite basalt which were quarried at Nonfell, about 35 km away north of Bakkafjördur. A forecast was done for the part of the quarry that was to be utilised for the inner breakwater
and repairs of the main structure, based on available scan-line data and previous work in the quarry. The predicted yield was as follows: 59% >0.1 tonne, 38% >0.5 tonne, 24% > 1 tonne, 10% >2 tonnes, 4% >5 tonnes and 2% >10 tonnes. The following was, however, the estimated actual outcome for the first 4,000 m3 : 47% >0.3 tonnes, 32% >1 tonne, 7% >4 tonnes. This rock was taken from the better part of the remaining quarry. This inner breakwater has withstood all storms since, including the storm of October 1995, which shows that it may be possible to use poor quality local stones for less exposed areas although they may not be sufficient for the more exposed conditions where stronger armour is needed. During the summer of 1996 the breakwater was repaired again as a result of damage in a severe storm in October 1995. This time it was decided to take the stones from a quarry at Vopnafjordur, some 33 km south of Bakkafjordur. This was made possible due to a new road between Bakkafjordur and Vopnafjordur. The rock is porphyritic basalt with specific gravity of 2.85 and water absorption of 0.5%. Flaking in a freeze/thaw test was found to be 0.11 on average for the three sides tested. The quality of the rock from Vopnafjordur is well known, as there is over 25 years experience in a very exposed breakwater. Quarry yield prediction based on surface scan-line data and experience indicated up to 70%>0.5 tonne, 55%> 1 tonne, 40%>2 tonnes, 23%>5 tonnes, and 10%>10 tonnes in the better parts of the quarry after the splitting of the largest stones over 16 tonnes. The surface scan-line data also indicated 10-20% >20 tonnes in a carefully blasted quarry prior to the splitting of the largest stones.
Figure 3. The Bakkafjördur breakwater in the summer of 1995, before the damage in the storm in October. The local poor quality tholeiite basalt quarry is to the left on the picture.
About 10,000 m3 were used in the repair in three stone classes, of which 7,500 m3 were over 4 tonnes. One of the restricting factors of the design was the decision to use readily available equipment. This lead to a design that made it possible to use 40 – 50 tonne excavators for the repair work. The design of the repair fully utilised all material over 4 tonnes. On the other hand a large overproduction of smaller stones and quarry run was expected, in the order of 15,000 to 20,000 m3. This was anticipated in the contract as the contractor was to set aside this material in two separate mounds, quarry run and class III of stones 0.5 to 4.0 tonnes, to be used in planned harbour projects
at Vopnafjordur. The repair work was started on the corner between the outer and inner part of the breakwater in station 115 and reached out to 120° on the breakwater head in station 196.
4.3
Vopnafjördur
A 40,000 m3 breakwater was constructed at Vopnafjörður in 1989. Blocks from a porphyritic lava were used for this construction. The specific gravity was 2.85 and water absorption 0.5%. The flaking in a freeze/thaw test was found to be 0.11 on average for the three sides tested. The rock is therefore class C (marginal) with respect to resistance to freeze/thaw action. The rock is fairly altered with laumontite, calcite and green clay. Two scan-lines were measured and the average joint spacing was found to be 87 cm in the NVSV scan-line and 179 cm in the NA-SV scanline. The average predicted quarry yield was 85% >0.1 tonne, 72% >0.5 tonne, 66% >1 tonne, 59% >2 tonnes, 42% >5 tonnes, 31% >10 tonnes, and 20 % >20 tonnes. The largest loose blocks by the cliff face were measured to be 9-80 m3, or 25-225 tonnes. The design criteria was class I (>1 tonne) 4,000 m3 (10%), class II (0.3-1 tonne) 11,000 m3 (27.5%), class III (core) 25,000 m3. The quarry yield was in fact far to good for this breakwater and this caused problems in the blasting as the shots had to be bigger than usual and the drillholes less closely spaced to shatter the rock as much as possible. The quarry yield of this intentionally mismanaged blasting turned out to be about 33% over 1 tonne. A conventional low crested porous rubble mound structure was constructed at Vopnafjörður in 1968-1969. The main armour layer consisted of 10-15 tonne stones. When inspected in 1989 the following was observed (Viggosson 1990): Surface flaking and splitting of stones causes gradual deterioration of the rubble mound. Weathering is lesser where the wave action is most severe at and below mean sea level and where the stones are saturated. Freeze/thaw weathering is most severe where aggravated by sunshine on the landward side and top of the structure, where the rock is relatively dry. The loss in thickness on each side of stones above the tidal zone is estimated 0.5-1.0 cm/year on average. The weight loss of a 10 tonnes armour stone is therefore 1.8-3.4 tonnes on average, reducing the stone size to 6.6-8.2 tonnes in 20 years. Deterioration below a critical weight limit in relation to the wave action causes stone movement and accelerates their breakdown. 4.4
Hornafjördur
Hornafjordur is the biggest town on the Southeast coast of Iceland, characterised by a sandy shore with a heavy littoral drift, Figure 1 (Viggosson et al. 1994). Hornafjardaros is a tidal entrance channel to the town of Hornafjördur, an active fishing harbour. The entrance has a rock headland on its west side and rock reefs which shelter the entrance from southerly waves are located about 2 km to the south, Figure 2. Although the entrance has been stable in its present location for about 100 years, heavy shoaling has occurred in the entrance at 10-15 year intervals. The South coast of Iceland is exposed to a most severe wave climate. In January 1990, an extreme storm hit the coast, with a measured offshore significant wave height of 16.7 m and a peak period of 19.4 seconds. The storm caused large shoalings to encroach upon the entrance channel from both sides. In March a break through occurred between the rock headland Hvanney and the South Barrier, as shown in. The total amount of material that was flushed over the East Barrier into the navigational channel was estimated to be about 200,000 m 3 and about 500,000 m3 flushed over the South Barrier. The inlet was closed for coasters for several weeks and large quantities of fish and other products had to be sent by land to other ports.
Measures to prevent further breakdown and to restore the barrier to its former shape began in June 1990. The gap between the South Barrier and the rock headland Hvanney was about 300 m long with an average depth of about 3 m. The gap was closed with sandbags, each with a volume of about 1 m3. To fill most of the gap 9 to 10 bags were used for every meter, but near closure 80 to 100 bags were needed for every meter. Totally about 8,000 sandbags were used in this action which took about six weeks, Figure 4 (Thorsteinsson and Gudmundsson, 1994).
Figure 4. Stabilisation of the Hornafjördur tidal inlet. Layout of the shore protection on the South Barrier, the curved jetty on the East Barrier and the weir east of the entrance which is yet to be constructed. The Quarry for the East Barrier was at the foot of the mountain in the distance, some 11 km away.
An extensive field investigation programme commenced during the summer of 1990 and continued to 1994. It included several bathymetric surveys, water level measurements, bottom sampling, seismic refraction surveys, hydraulic measurements, aerial photography, offshore wave measurements, weather observations, geological assessment and research for selection of an armour stone quarry. For the design of improvements to the entrance a hybrid model was used. It consisted of properly interpreted field data, numerical models and a physical model. 4.4.1 The South Barrier During the summer of 1991, in the early phase of field measurement and modelling, a 665 m long conventional rubble mound shore protection was constructed at the South Barrier to replace the temporary sandbag protection (Figure 4). A detailed study was carried out for the proposd main quarry at Smyrlabjorg. This included percussion and core drilling, detailed mapping and a magnetic survey to locate dykes and possible faults in the proposed quarry area. The rock in the main quarry for the South Barrier is an 8 m thick porphyritic basalt lava of Late Tertiary age, situated about
30 km from the construction site. It is fairly altered and all amygdales are filled with zeolites. It has a specific gravity of about 2.85 (ssd) and water absorption is about 1-1.5%. The Point Load Test Index (IS(50)) is 5.1 MPa and the Schmidt impact hammer test gave a value of 37 for tests on NQ cores. The rock is good in quality according to (CIRIA/CUR 1991 and Table 1). The predicted quarry yield was as follows: 75% >0.1 tonne; 55%>0.5 tonne; 40-45% >1 tonne; 30-35% >2 tonne; 15-20% >5 tonne; 5-10% >10 tonne and 2-5% >20 tonne. A less thorough surface study of the alternative quarry at Hjallanes in the same rock type and perhaps the same lava flow, north of Kolgrima River, indicated that similar results could be expected there regarding size grade and rock quality. The shore protection has a front slope of 1:2 and a +9.0 m crest elevation, which is determined according to an estimated uprush assuming that all voids between stones are filled with sand, Figure 7 (Sigurdarson et al. 1994b). It is built of three layers of stones and a filter layer of quarry run. The outermost layer is made of stones from 3 to 8 tonne, the middle layer of stones 0.5 to 3 tonne and the innermost layer consists of 0.2 to 0.5 tonne stones. On the inner side, facing the tidal entrance, a thick layer of small stones 0.2 to 0.5 tonne blended with quarry run was used to prevent scouring. A total of 60.000 m3 of material was needed from the quarry. Material over 0.5 tonne in size was critical for the quarry production, as the predicted quarry yield over 0.5 tonne was 55%. Also in the case of a shore protection the supply based design method is used where most size grades are utilised. When possible the design aims at having the critical stone size for the quarry production rather low. Two quarry locations, at 25 and 30 km distance both with rock of porphyritic basalt, were considered for this project (Smarason 1994). The main-quarry was further away and at 160 m altitude. The alternative quarry, closer the construction site, was on the wrong site of a glacier river Kolgrima with an average annual flow rate of 33.5 m3 /s and a higher variable summer flow rate of over 80 m3 /s in July and August and sometimes from mid-June to mid-September. The river Kolgrima has a history of summer floods exceeding 1000 m3 /s. The lowest alternative bidder chose to bridge the river to use the quarry closer to the construction site. The crossing was designed to deal with flow rates up to 80 m3 /s. The river exceeded this flow rate during July and August and occasionally in the latter part of June and early September. One flood of 1,130 m3 /s destroyed the crossing completely. The construction period was from June to October. Work was halted for 6 weeks from late June to early August due to the high flow rate of the river. The latter part of the work was carried out on two 12 hour shifts to ensure completion before the autumn storms. Equipment used for the construction included three excavators, one 75 and two 45 tonne, one 30 tonne wheel loader, and four 35 tonne construction trucks. Three trailers were used for the latter part of the construction period. Between twenty to thirty personnel worked on the project. A recent inspection shows that voids have filled up with sand up to a elevation of +7.0 m. And in heavy seas, waves reach the crest of the structure with limited overtopping. No displacement of stones has been observed so far. 4.4.2 The East Barrier The goal of the project was to improve the stability and the navigational conditions in the tidal inlet of Hornafjördur. The plan for construction of shore protection and stabilisation of the inlet consisted of three elements. Firstly a rubble mound shore protection on top of the South Barrier was built to prevent overflowing of material. Secondly a curved jetty was laid out from the tip of the East Barrier to stabilise the tip and the shoal in the entrance. The aim of the curved jetty was also to improve the current conditions, by deepening and channelling the entrance and thus improving the navigational conditions. And thirdly a weir has to be built about one kilometre east of the entrance to minimise sediment transport from east. The jetty is 330 m long in addition to the 200 m long shore protection along the tip of the East Barrier. A rational design was needed to construct the first phase of the jetty within 14 days to eliminate the expected tip erosion caused by current as predicted by the mathematical model. After
the construction of the jetty the mathematical model predicted erosion along the jetty and especially at the tip of the jetty although the bottom consisted of coarse material. The design condition of the wave height and current and erosion were evaluated by the numerical and the physical models. Design conditions for the jetty were established as follows: • The bottom material in the tidal entrance consists of coarse lava sand with particle size of 2-30 mm in diameter with some larger gravel. • The barrier material is 1-5 mm with some gravel. • The maximum current velocity was estimated 3.0 m/s at ebb tide and 3.5 m/s at flood tide. • At spring tide the maximum discharge through the inlet during ebb tide was estimated 3,440 m3 /s and 4,420 m3 /s during flood tide. • At spring tide the water level is about +2.1 m and the design water level is +3.5 m. • The offshore significant wave height with 100 year return period is about 17 to 19 m with peak period 18 to 20 s • At 30 m water depth outside the entrance the 100 year significant wave height is about 12 m with peak period 18 to 20 s • During the design storm the significant wave height just outside the jetty is 3.8 m. The design of the jetty had to take into account strong currents and moderate storms during the construction time. This led to a berm type breakwater of several stone classes with large toe protection as shown in Figure 9. Stone classes in the range of 2 tonne up to over 10 tonne were used. The berm it self consists of two classes of mean weight 6.7 and 3.0 tonne, the larger on top of the other, which corresponds to a stability numbers of 1.5 and 2.0. In front of the berm there is a toe protection of 0.2 - 2 tonne stones 20 m 3 per meter length of the structure. The predicted quarry yield over 2 tonne was 25 - 30 % which the design aimed to utilise completely to the advantage of the berm structure. Construction of the berm breakwater was carried out in four phases: Phase I: Building of construction road of coarse quarry run with up to 1 tonne stones to get control of the current in the inlet and dumping of the toe protection. Phase II: Excavating the upper part of the “road“ and completing construction of class IV and III at the inlet side. Phase III: Construction of class VI and V at the lee side and class II at the inlet side. Phase IV: Construction of the super structure. The quarry for the shore protection of the East Barrier is in a gabbro intrusion at Halsendi, about 11 km east of the construction site. The gabbroic rock is of excellent quality although it is highly altered with epidote, chlorite and pyrite. It has a specific gravity of 3.00 (ssd) and water absorption is 0.32%. The Point Load Test Index (IS(50)) is 10.8 MPa and the Schmidt impact hammer test gave a value of 35 for tests on NQ cores. Weight loss in a freeze/thaw test was 0.02 kg/m2 in 56 cycles in accordance to the Swedish test standard SS 13 72 44. This is an excellent freeze/thaw resistance (Smarason 1994). Quarry yield prediction for the quarry was 70-75% >0.1 tonne; 45-50% >0.5 tonne; 35-40% >1 tonne; 25-30% >2 tonne; 15-20% >5 tonne; 10-15% >10 tonne; 5-10% >20 tonne; and 2-5% >50 tonne. This prediction was somewhat lower than the calculated average for the surface and core measurements, as the eastern margins of the quarry consisted of more fractured rock than the main body of the gabbro intrusion (Figure 5). A weakness in rock quality was also noted in the uppermost 1-1.5 m of gabbroic rock. This damage is thought to be due to coastal weathering when the rock body was submerged in seawater at the end of the last glaciation of the Ice Age. In the summer of 1994 six contractors were pre-qualified to bid for the construction of the East Barrier. Lowest bidder was Sudurverk hf with tender price of 1,3 million US$ or 60,5 % of cost estimate. The contract was signed in November 1994 and work commenced in February 1995 with mobilisation and road construction. A total of 11 km of road was needed to connect the quarry to the construction site. At peak production 20 personnel where working on site.
100 Joint space averages
90
Design Curve Used from quarry
80
Quarry yield prediction A 70
Quarry yield prediction B
60 50 40 30 20 10 0 0,01
0,10
1,00 W e ight of stone s (tonne s)
10,00
100,00
Figure 5. Quarry yield prediction, design curve and production results for the Halsendi quarry for the East Barrier, Hornafjördur. Yield prediction B assumed that only the better parts of the quarry were used, whereas prediction A allowed for the poorer part of the quarry for core and road construction.
A curved breakwater of the berm type was constructed during the summer 1995. Due to the severe wave action and strong current a berm structure with toe protection was chosen (Sigurdarson et al. 1997). A rational design made it possible to construct the first phase of the jetty within 14 days to eliminate the expected tip erosion caused by current as predicted by a mathematical model. The breakwater is 330 m long and has a volume of 100,000 m3 with several stone classes from 0.2 up to over 8 tonnes. The estimated quarry yield was 25-30% over 2 tonnes and the design aimed to utilise that completely to the advantage of the structure. This was successful and a 100% utilisation of the quarry was achieved. The measured bulk factor from solid rock to blasted material in the structure was 1.53. All drilling was done with a drilling rig using a 3” drillbit. Depth of holes was 10 to 16 m. A typical drill pattern was 3 m by 2 m. ANFO explosives were used approx. 350 g/m3 . Loading in the quarry was done with a 46 tonne excavator, quarry run was transported with trailers, smaller rocks with 3 and 4 axle lorries, larger armour stones where transported with 20 tonne trucks. At the early stage of the project rocks were stockpiled at the land-end of the breakwater. Placing of armour stones was carried out with a 49 tonne excavator. A total of 99,700 m3 were excavated from the quarry, 27,400 m3 were over 2 tonne. 4.5
Sirevåg in SW-Norway
In 1998 Stapi Ltd. - Consulting Geologists and the Icelandic Maritime Administration (IMA) were commissioned by the Norwegian National Coastal Administration to investigate quarries and design a berm breakwater in Sirevåg, which is located at the west coast of Norway, about 50 km south of Stavanger. The breakwater should be designed as a stable Icelandic type berm breakwater for a wave height with a 100 years return period. It should also withstand a wave height with 1000year return period, which is referred to as the worst-case scenario, without total damage. Sirevåg is exposed to heavy waves from the North Sea. The design wave with 100 years return
period for the outer part of the breakwater was established by SINTEF as Hs = 7.0 m with Tp=14.2 s (SINTEF, 1999). Wave measurements were started in the beginning of December 1998 at the location of the breakwater head at 17 m water depth. Measurements are taken every half-hour. Two large storms with waves close to the design storm were recorded during the winter 1998 to 1999, on December 27th with Hs = 7.0 m and Tp = 14s and on February 4th with Hs = 6.7 m and Tp = 15s. SIREVÅG - QUARRY B
100 Quarry B: Yield prediction A - optimum yield of quarry Quarry B: Yield prediction B - after splitting of largest stones
90
Test Blasts 4-6
Percentage of mass heavier (cumulative %)
Design Curve
80
70
60
50
40
30
20
10
0 0,01
0,10
1,00
10,00
100,00
Weight of stones (tonnes)
Figure 6. Quarry yield prediction, design curve and first results of test blasts in quarry B at Sirevåg. Blast results prior to splitting of the largest stones should lie between prediction A and B.
To establish a design wave height along the breakwater IMA has performed wave refraction analysis from offshore into the location of the Sirevåg breakwater (IMA, 1999). The HISWA wave model was used for this purpose. The breakwater will partly be constructed on a rocky bottom and partly on fine quartz sand. The depth of the rocky bottom is variable from 3 m to 22 m with steep slopes. Under the outermost 150 m is a flat sandy bottom. The breakwater is about 400 m long. The equivalent head-on wave height for stability calculations is estimated by the incoming wave height, 50 m, or half wave length outside the berm, multiplied with a cosine of the wave obliquity in a power of 0.4 (Lamberti and Tomasicchio, 1997), Table 3. During the preparation phase for the Sirevåg project various model tests were performed at SINTEF. An interesting study was made to compare wave damping for different configuration of berm breakwaters to a conventional rubble mound breakwater (Jacobsen et al. 1999). The analysis shows that berm breakwaters reduce the wave energy penetrating around the breakwater head and into the harbour more efficiently than a conventional rubble mound breakwater of equal length.
Table 3. Design Wave Height and the Worst Case Scenario. Station number along the breakwater
Design wave height 100 year return period
Worst case scenario 1000 year return period
(m)
Hs (m)
Hs (m)
0 to 70
4.8
5.3
75 to 125
3.5
3.9
145 to 210
6.2
6.8
215 to 240
6.4
7.3
245 to 275
6.2
6.8
280 to 400
6.7
7.4
Breakwater head
7.0
7.7
In the preliminary design three sets of stone classed were considered. Based on the overall utilisation of all quarried material according to a preliminary quarry yield prediction and fulfillment of stability criteria for all sections of the breakwater, one set was chosen, table 2. Three possible quarries (A, B and C) were assessed for the Sirevag breakwater. A quarry yield prediction was carried out for the three quarries for a 640,000 m3 breakwater (Stapi Consulting Geologists, 1999). The armourstone material is gabbroic anorthosite rock of good quality (Tables 1 and 2). The quarry yield prediction for a carefully worked quarry is about 50% over 1 tonne, about 30% over 3 tonne and about 15% over 10 tonne. This will result in about 6% in stone class I, 20 to 30 tonne, 10% in stone class II, 10 to 20 tonne, 14% in class III, 4 to 10 tonne, and 19% in class IV, 1 to 4 tonne (Figure 6 and Table 4). Table 4. Stone classes and quarry yield. Stone class
wmin-w max
w mean
wmax/ w min
dmax/ dmin
Predictedted Quarry Yield
I
20.0 – 30.0
23.3
1.5
1.14
5.6%
II
10.0 – 20.0
13.3
2.0
1.26
9.9%
III
4.0 – 10.0
6.0
2.5
1.36
13.7%
IV
1.0 – 4.0
2.0
4.0
1.59
19.3%
A computer model of the breakwater is shown on Figure 7, and a cross section of the outer part is shown in Figure 9. Figures 8 and 10 are from quarry B on the northern side of Sirevåg, The design fully utilises all quarried stones over 1 tonne and a 100% utilisation of all quarried material is expected for the project. Six contractors were pre-qualified to bid on the project. The lowest bidder was E. Phil & Søn of Denmark. They draw on experience gained by their subsidiary company Istak of Iceland, which has experience in construction of berm breakwaters. The construction of the breakwater began last January.
Figure 7. The Sirevåg breakwater southewest Norway.
Figure 8. Loading of the barge from quarry B on the northern side of Sirevåg.
Figure 9. Sireåg berm breakwater, showing a cross-section cross section for the outer part. Note the two layers of extra large stones of class I (20-30 (20 30 tonnes) on the sea-side. sea side. The berm should be statically stable.
Figure 10. The results of size reduction of an oversized block in quarry B at Sirevåg. The charge c harge was too large and as a result too much material was wasted to lower size grades.
5 DISCUSSION In his paper, Jensen (1984) states: “ In many projects, in which DHI has been involved in recent years, the lack of knowledge of available stone sizes in the t he quarry has turned out to be decisive for the breakwater profile at a very late stage, namely after initiation of the construction work. In some cases it has been necessary to modify the profile to fit the actual stone classes available.” And later “It is for the above reasons extremely important for a breakwater project that information on the specific quarry is available at an early stage. It may therefore be necessary that test blastings are carried out at a very early stage to ensure that the stone classes that can actually be obtained from
the quarry are used in the project design.” We agree with this although we maintain that core drilling in a 25 to 50 m grid in the quarry coupled with thorough investigation of joint spacing and quality assurance programme of the cored material gives more reliable results than test blasting in a limited part of the quarry. It will always take some time to learn how it is best to work each quarry and the results of initial test blasts may be misleading for the bulk quarry. Suitable excavators and other equipment may not be readily available for the test quarry, especially for projects using large to extra large stones. Often the owner/designer has to rely on the contractor for information on the maximum quarry yield. But dedicated armourstone production is not common and therefore there are not many contractors that have much experience in this field. Guidelines for blasting for armour stones are insufficient and only a few contractors’ employees have much experience in drilling and blasting for breakwater construction. We have been trying to change this situation and are gradually training our contractors to work the quarries to our specifications. Many contractors are now familiar with the quarry yields prediction and rely on the in their bids. We have demonstrated in manyprojects that although contractors complained at the beginning of the work that it would not be possible to obtain the predicted quarry yield. However, after we have worked with them on a change of the blasting design, the quarry yield prediction has been fulfilled. Furthermore, increased knowledge through quarry yield prediction and in the production of armourstone from various quarries has allowed us to specify large (10-20 tonnes) and extra large (20-30 tonnes) stones, typically to improve the stability of the edge of the berm. By increasing the size of the stones at the edge of the berm by a factor of two, the design wave height may increase by 25%. The percentage of large stones produced in the quarry can be as low as 2-5 % of the total quarried volume to allow for this 25% increase in design wave condition. Large hydraulic excavators and front loaders (75 to 110 tonnes) that can handle these large to extra large stones have become readily available. These large machines will raise the cost of the projects by 1-2%. We have in recent projects utilised these large to extra large stones to the advantage of the stability and strength of the berm structures. A relatively low percentage of these largest stone classes can be of a greaat advantage for most breakwaters. This is not only valid for high to moderate wave conditions but also applies to lower wave load conditions where quarries with relatively low size distribution are used. For the same design wave condition and stability of the berm, the additional cost of the larger hydraulic excavator is compensated for by smaller berm width. Table 5. Quarry yield prediction for some recent breakwater projects. Breakwater site Bolungarvik Blonduos Hornafjördur,.Sudurfjörutangi Hornafjördur,.Austurfjörutangi Husavik Sirevåg, Norway Vopnafjördur
Predicted Quarry Yield >20 tonnes 2 4 2-5 5-10 3-4 15-17 10-20
>10 tonnes 5 9 5-10 10-15 7-10 22-25 20-30
>5 tonnes 11 14 15-20 15-20 12-16 30-33 30-40
Volume >1 tonne 34 32 40-45 35-40 24-32 47-54 50-60
(m3 ) 265,000 100,000 60,000 100,000 300,000 640,000 40,000
Table 5 shows the results of a few quarry investigations where large and extra large stone have been required. Blast design is the most important factor for a successful breakwater project. It is the deciding factor in securing that the desired fragmentation of the rock is obtained. It is absolutely vital that the blasting engineer is prepared to adjust his blasting pattern to suite each particular quarry and he may have to adjust his pattern several times within the same quarry to maximise his results. We usually find that a drill pattern with a 3” drillbit close to 3-4 m burden (b) and 2-2.5 m spacing (s) for a bench height of 9-12 m gives the best results in sound porphyritic basalt lavas. The ratio b/s
should lie between 0.6 and 0.7. For best results it should preferably never exceed 0.7. A new blasthole row should not be drilled until after the clearing of the bench face and quarry floor is completed. Only then can the blasting engineer decide on his drill pattern and tilt of holes. It is important that the holes be drilled parallel with a dip of 70-80°, for best results and minimum damage to the blasted rock. This causes minimum throw of the blasted rock as only the bottom part of the bench is trown out and the upper part falls into the blasted pile. A low specific charge should be used, generally 200 g (+/-50 g) per cubic metre of solid rock, depending on rock soundness and desired block size. Contrary to CIRIA/CUR (1991) we maintain that explosives with a high shock energy and lower gas content give better results. We also prefer explosives with higher detonation velocities, close to the sound velocities of the rock mass. Otherwise the sound wave may be reflected from the quarry face back to the blasted wall before the explosive have opened up the blast line, causing unnecessary additional damage to the blasted armourstone. Production of large and extra large armourstone requires a coarse drill pattern than generally used in armourstone production. For optimum results it may be necessary to produce a significant amount of blocks that may be two to three times the largest desired armourstone for the project. These oversized block will have to be split afterwards using a single 2”-3” hole or a row of narrower hand drilled holes for more accurate splitting into two pieces. A single hole can not be recommended unless the quarry yield is somewhat better than the design requires. A steel ball of 67 tonnes is sometimes used but it can only be recommended in quarries exceeding the demand of the design. It should be emphasised here that the size reduction of the largest block is the area where the contractor can make his biggest earnings on a breakwater project. Unprofessional approach to this part of the work can lead to considerable overproduction in the quarry, which should by no means be rewarded. Contractors may in the past have been able to claim on quarries where limited preparation was carried out, as the buyer had not got the means to prove that overproduction could have been caused by mishandling of the quarry. Our thorough quarry investigation and quality assurance programme has freed our clients from compensation to the contractors in this area. If, however, the quarry investigation is not carried out in accordance to our recommendations, unforeseen defects have appeared in some of our quarries, which has lead to overproduction as some of the substandard armourstones have been rejected and those quarries have dissected unforeseen fractures. Our quality assurance programme aims at finding out the weaknesses of the quarried rock at an early stage. This makes it possible to delay its deterioration as much as possible, as we realise that all stones will with time be fragmented into smaller stone and eventually end up as pebbles and finally sand on the beach. It is therefore important to know the material and its properties, i.e. rock type, discontinuity spacing for quarry yield prediction, density and absorption, strength (point load index), freeze/thaw resistance in cold climates, and resistance to abrasion in abrasive conditions. No test, however, can replace the eye of the experienced engineer or geologist.
6 CONCLUSION A thorough quarry investigation has proven to be a valuable part of the design process in preparation for successful breakwater projects in Iceland. A preliminary study of possible quarries is carried out simultaneously with the preliminary design process to establish an estimate of quantities of material needed. Decisions regarding stone classes are halted until a preliminary quarry yield prediction has been made. A final design is completed after a thorough study of a selected quarry has been undertaken. This may include detailed geological mapping, percussion and core drilling. Test blasting can be avoided as quarry yield predictions have proven to be just as accurate as test quarries. This minimises damage to the environment, as the potential quarry sites remain untouched until the quarry is opened. An accurate quarry yield prediction leads to a more precise design and fewer problems in the execution of the construction, which in turn leads to fewer claims and thus a lower overall project cost.
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