EMBANKMENT CONSTRUCTION ON HARBOUR MUD AND ORGANIC SILT Arvid Landva, GEMTEC Limited, Fredericton, New Brunswick, Canada Donald C. Yeamans, Fredericton, NB, Canada Kevin Kerr, City of Miramichi, NB, Canada Csaba Kazamer, Maritime Infrastructure Technologies Inc., Miramichi, NB, Canada ABSTRACT Part of the retaining berm for the Miramichi wastewater treatment lagoon rests on a soft and weak layer of diatomaceous organic silt. The construction of the berm over the soft silt was started by placing a seawall to el.+2.0 m on the el.+0.5 m river bottom, using coarse rockfill. The rockfill sank into an almost liquid mud, which was underlain by a very soft dark grey organic silt extending to depths of up to 4 m below the river bottom. The silt had a compression index in the range 0.14-0.23, a coefficient of consolidation of 0.01 m2/day and a coefficient of hydraulic conductivity of about 1x10-7 cm/sec. Its shear strength parameters were c = 3.5-6 kPa and φ = 23º. Piezocone tests gave qc-values as low as 150-300 kPa. The lagoon berm was built up to el.+8.2 m during a total construction time of 18 months, including a break of 7 months over the winter and spring seasons. At the end of this break, when the berm had reached el.+5.3 m, porewater pressures had just dissipated and consolidation of the silt layer under the berm had also just come to an end. RÉSUMÉ Une partie de la berme de rétention pour la fosse de décantation des eaux usées à Miramichi repose sur une couche molle et faible de limon organique diatomée. La construction de la berme au dessus du limon faible a été commencé en installant une digue de gros remblai rocheux jusqu’à une élévation de 2.0 m sur le fond de la rivière (+0.5 metres élévation). Le remblai rocheux a foncé dans une boue presque liquide soutenue par une couche de limon organique très molle et gris foncé s’étendant une profondeur de jusqu’à 4 m dessous le fond de la rivière. Le limon avait un indice de compression dans la gamme de 0.14 à 0.23, un coefficient de consolidation de 0.01 m2/jour et un coefficient de conductivité hydraulique d’approximativement 1x10-7 cm/sec. Ces paramètres de résistance au cisaillement étaient c=3.5-6 kPa et φ = 23º. Des essais de piezocone ont donnés des valeurs de qc aussi bas que 150-300 kPa. La berme de rétention a été construite jusqu’à une élévation de +8.2 m pendent une période de construction de 18 mois, qui inclus une pause de 7 mois pour l’hiver et le printemps. À la fin de cette pause, la berme avait une élévation de +5.3 m, les pressions de l’eau interstitielle venaient de dissipé et la consolidation de la couche de limon sous la berme venait aussi d’être complet. 1. INTRODUCTION The Miramichi wastewater treatment lagoon is located within the Miramichi City limits. About half the length of the lagoon water-retaining berm is located on land, partly natural and partly in-filled, and the other half along the Miramichi River seashore, 50 to 100 m outside the high tide line. The seashore portion of the berm rests on a soft and weak layer of diatomaceous organic clayey silt of thickness up to about 3.0 m. The construction of the berm over the soft silt was started by placing a seawall to el. +2.0 m on the el. +0.5 river bottom, using coarse sandstone rockfill. The rockfill settled to el. -0.8 m, a combination of consolidation and displacement, forcing part of the disturbed silt into the rockfill voids and part of it outside the seawall as a mudwave. The foundation soil displaced a sponge-like behaviour during construction. Sampling of the soil with a

backhoe from the top of the seawall showed that the foundation soils immediately below the river bottom consisted of 0.6 m of extremely soft almost liquid mud, underlain by a very soft dark grey silt extending to depths of up to 4 m below the river bottom. 2. GEOTECHNICAL PROPERTIES OF SILT

The silt had an organic content of 8-13% and a water content in the range 80-120%. Its density was as low as 1300 - 400 kg/m3, corresponding to a porosity as high as about 70 to 80%. Yet its specific gravity was still as high as about 2.66. This apparent anomaly was found, as observed under a scanning electron microscope, to be the result of the existence of a very high content of diatoms (hollow silica shells), as shown in Fig. 1. With respect to the organic constituents, these could not be

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detected under the scanning electron microscope, indicating that they would be colloidal (<0.001 mm size) and therefore probably located both in the voids within (intraparticle) and in the voids between (interparticle) the microfossils or the solid silt mineral grains.

(a)

(b)

Figure 1. Scanning electron micrographs of diatomaceous organic clayey silt. (a) 400x (b) 1250x The consistency limits of the silt are typically 30-35% (plastic limit) and 80-90% (liquid limit). These are high limits and again reflect the presence of the microfossils and the organics. The silt size content (in % of inorganic constituents) is typically 80%, clay size 10% and sand size 10%. The soil is therefore classified as a diatomaceous organic clayey silt. Consolidation tests showed the soil to be very compressible and to have a coefficient of permeability of the order of 1x10-7 cm/sec, a coefficient of consolidation cv of about 4 m2/year (about 1E-3 cm2/sec) and a compression number Cc’ in the range 0.14 – 0.23.

A piezocone investigation showed a cone resistance of 150-300 kPa, i.e. an extremely low resistance to penetration. The very high contents of water, organics, microfossils and silt size, combined with the high consistency limits and the extremely low piezocone resistance values, showed this to be a deposit of extremely low strength and extremely high compressibility. These index properties suggested that a relatively rapid application of loads (i.e. rapid placing of the fill) would lead to high pore pressures and hence only a limited increase in the strength of the silt. A stability analysis of the berm would therefore have to allow for the existence of pore pressures of the same order of magnitude as the applied load during and immediately after application of the load. The results of triaxial and simple shear tests are shown in Fig. 2. The geotechnical parameters of the silt were determined to be c’ = 3.5 - 6 kPa and φ’ = 23°. This may be compared with the results of the piezocone tests on the basis of which the angle of friction φ’ was estimated to be in the range of 19 to 22°. These are all low values, but not unexpectedly low when considering the index properties reported above and the behaviour of the soil in the field.

Figure 2. Triaxial and simple shear test results (CAU = anisotropically consolidated, undrained, triaxial test; CCV = anisotropically consolidated, constant volume simple shear test; CD = anisotropically, consolidated, drained simple shear test) An examination of the test results in Fig. 2 showed that at increasing normal effective stresses (σ) the shear strength (τ) tended to increase. It was concluded that at effective stresses lower than about 25 kPa the shear strength could be represented by c’ = 3.5 kPa and φ’ = 21°. At effective stresses higher than about 100 kPa the shear strength could be represented by c’ = 6 kPa and φ’ = 23°. 3. BERM CONSTRUCTION

100µm

10µm

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The berm was constructed to approximately el. +5.3 m in 1996. Piezometer readings taken in June 1997, just before construction was resumed, showed that the pore pressures induced by the rockfill load had dissipated more or less completely. Test pit excavations during the infilling of the area inside the seawall, after construction, showed that the rockfill voids were partly filled with mud. A considerable portion of the mud was displaced in the form of mudwaves during rockfill placing. Some penetration of the rockfill also occurred into the organic silt. There is little doubt that some disturbance of the upper portion of the silt was caused by the placing of the rockfill. The construction of the berm was resumed in June 1997 and was completed in August of the same year. Porewater pressures and settlements continued to be monitored during this period. The settlement of the lagoon floor on filling the reservoir varied between 37 and 71 mm, close to the predicted settlement. 4. SETTLEMENT AND PORE PRESSURES The settlement and pore pressures recorded after installation of the settlement plates and piezometers are shown for the first year of construction in Fig. 3. During this period the berm was constructed to a total thickness of about 5 m. It will be seen that both settlement and pore pressures were completed by the end of the season, i.e. after about 200 to 250 days. The curves in Fig. 3 (as well as separate plots of settlement and pore pressures vs. the logarithm of time and the root of time) indicate a normal consolidation behaviour for inorganic normally consolidated clay, suggesting that no further settlement would take place and therefore that the coefficient of consolidation cα ≈ 0. Normally, cα increases with increasing organic content. For this soil, however, it is possible (as indicated earlier) that the organic content is amorphous or colloidal (completely decomposed organics) and located within the hollow diatom shells and in the voids between the shells or the solid silt grains. In such a case, the organics would not affect the long-term behaviour. This would mean that long-term compression of this silt is insignificant.

Figure 3. Typical settlement and pore pressure behaviour under embankment 5. STABILITY OF BERM Figure 4 shows a cross section of the berm as finally built to el. +8.2 m, i.e. a final thickness of about 8 metres. The geotechnical properties assigned to the various zones were as follows:

Zone Unit weight (kN/m3)

Friction angle φ'

(degrees)

Cohesion c (kPa)

Mud 12 0 0 Sandstone / garbage

22 36 0

Sandstone fill

19 42 0

Sandstone / silt

18 36 0

Silt 13 23 3.5 to 5.0 Sand 18 33 0 Bbar = ∆u/∆σ was taken to be a maximum of 0.8. With reference to Fig. 4, the computer program GSLOPE was used to calculate the stability as follows: (i) Seawall construction, el. +2.0 m, allowing an additional 0.4 m to account for the weight of loaded trucks and construction equipment, Bbar = 0.8, F = 1.05. (ii) Sandstone / garbage fill to el. +5.3 m, Bbar = 0.8, F = 1.31. (iii) Sandstone / garbage fill to el. +5.3 m, Bbar = 0 (full dissipation), F = 1.51. (iv) Sandstone fill to el. +8.2 m, Bbar = 0.8, F = 1.50. The stability of the seawall (case i above) during construction was not satisfactory, as evidenced by the very considerable shaking of the berm during construction. A factor of safety of 1.05 – 1.1 was considered to be representative of this type of behaviour, and this value of F was used to “calibrate” the laboratory test results and the piezocone results. On that basis it was concluded that c and φ' should be taken to be 3.5 kPa and 21° respectively. This would give a factor of safety of 1.05 for the seawall if Bbar = 0.8, which is close to the value reported in Fig. 3 for a somewhat lower rate of construction. The raising of the berm to el. +5.3 m was carried out under conditions of a relatively low factor of safety (F = 1.31, c’ = 4 kPa, φ' = 22°). However, the allowable minimum factor of safety for temporary conditions was F = 1.30, and in any case the F = 1.31 condition represented a significant improvement from the conditions under the seawall construction. The main reason for this is that the berm to el. +4.0 m and the lower porewater pressures under the seawall provided some support for the new fill. The geometry of the silt-sand stratigraphy forced any potential failure zones to the outer portion of the berm, i.e. to the portion with the lower porewater pressures.

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The excess pore pressures before construction resumed the following construction season had dissipated almost completely, and the factor of safety thus increased to F = 1.51 (c’ = 5 kPa, φ' = 22°). The factor of safety for the completed structure after full dissipation of all excess porewater pressures was F =

1.59 for the static case. For a 0.1 g earthquake and a porewater pressure corresponding to 0.05 times the weight of each slice superimposed on the 0.1 g forces, F was determined to be 1.16.

Figure 4. Cross section of main berm 6. CONCLUSIONS Part of the retaining berm for the Miramichi wastewater treatment lagoon rests on a soft and weak layer of diatomaceous organic clayey silt. The strength of this silt is extremely low. It extends to depths of up to 4 m below the river bottom. The construction of the berm over the soft silt was started by placing a seawall to el.+2.0 m on the el.+0.5 m river bottom, using coarse rockfill. The rockfill sank into an almost liquid mud overlying the silt, partly displacing the mud and partly forcing it into the rockfill voids. The underlying silt displayed a sponge-like behaviour during construction, shaking very considerably under the construction equipment. The factor of safety was assessed to be in the range 1.05-1.10. Because of tight scheduling, the berm had to be raised to el.+4.0 m before any field instrumentation could be installed. Simple observation did not disclose any distress of the berm, but the spongy behaviour continued and was still a matter of much concern. The factor of safety was assessed to be around 1.3. The lagoon berm was built up to el.+8.2 m during a total construction time of 18 months, including a break of 7 months over the winter and spring seasons. At the end of this break, when the berm had reached el.+5.3 m, porewater pressures had just dissipated and consolidation of the silt layer under the berm had also just come to an end. The final factor of safety was determined to be 1.6 for the static case and about 1.15 for a 0.1 g earthquake and a porewater pressure corresponding to 0.05 times the weight of each slice superimposed on the 0.1 g forces.

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