Soil Conditions and Geotechnical Solutions Adopted for Different Structures Constructed at Visakhapatnam Port Trust

Transcription

1 Indian Geotechnical Conference 2010, GEOtrendz December 16 18, 2010 IGS Mumbai Chapter & IIT Bombay Soil Conditions and Geotechnical Solutions Adopted for Different Structures Constructed at Visakhapatnam Port Trust Ramachandra Rao, K. Chief Engineer cevizagport@yahoo.co.in Engineering Department, Visakhapatnam Port Trust, Visakhapatnam ABSTRACT The soil conditions at Visakhapatnam Port at different locations starting from the Outer Harbour, the Inner Harbour and further in the vast extent land beyond the harbour within the port limits varies in nature since characteristics and type of soils are ranging from dense to very dense sands to silty sands and soft marine clay to ultra soft marine clay etc. with different depths followed by hard clays, soft/hard rock formations at depths of about 25 to 30 metres below the ground level. Different types of structures that were provided and methods adopted in the development of some of the supporting infrastructure like stacking areas deepening of harbour waters, road connectivity etc. to suit these soil conditions over a period of time and based on the technical advancements have been discussed in this paper. 1. INTRODUCTION Visakhapatnam Port comprising Inner Harbour and Outer Harbour was commissioned in the year 1933 by developing inner harbour initially in the estuary of River Megadrigedda. The Inner Harbour is a natural harbour protected by two hills viz., Dolphin Hill and Ross Hill HH with a leading entrance channel from the sea. The Outer Harbour was formed and commissioned in the year 1975 by providing three break waters to provide tranquil water conditions for handling deep draft vessels. Considerable variations in soil strata with different characteristics and the prevailing technology at the time when harbour development works were taken up governed the adoption of appropriate construction methods. The type of berthing structures adopted and related supporting infrastructure viz., deepening of harbour water, stacking areas, rail and road connectivity works etc., developed were based on the existing soil conditions and the functional requirements over a period of time keeping in view the future needs and costs and by adopting technological advancements available from time to time. 2. SOIL CONDITIONS AND EVOLUTION OF HARBOUR STRUCTURES AT VISAKHAPATNAM PORT TRUST Inner Harbour Inner Harbour was developed initially for vessels of about 10 m draft. A total number of 18 berths were developed in phases in the inner harbour for handling multi commodity cargo, POL products & fertilised raw materials. The soil profiles and structures adopted are detailed below: EQ-1, EQ-2, EQ-3 ( ) and EQ-4 (1955) The top soil up to a depth of 2.50 m below GL is fairly compact moorum and is followed by sand clay layer between depths of 2.50 m to 7.0 m and soft clay between 7.0 m to m. Between the depth of m to 32.0 m stiff clay was encountered with SPT N value of 16 to 32. Three berths (EQ- 1, EQ2 & EQ3) (1933) and one more berth EQ-4 (1955) were planned as gravity type structures and are constructed with the then available technology consisting of a series of Stone Masonry lined concrete Monoliths of size: 42 ft. x 26 ft. (with two pockets) With a Gap of 5 ft. between monoliths. The monoliths were sunk in position by scooping out material from the pockets. The construction is similar to the conventional well construction with cutting edge at the bottom. These berths were constructed to handle vessels of max. LOA of 500 ft. ( m) and the deck was designed for carrying a live load of 30 kn/m 2 with provision for quay cranes of capacity of 60 kn. These berths are designed for a dredged depth of m. The pockets were filled with sand from stability consideration. OR-I and OR-II (1957) The top soil up to a depth of 3.3 m below GL is filled with medium dense gravely sand. Between 3.30 m to m

2 144 K. Ramachandra Rao depth, there is soft marine clay with vane shear strength varying between 11 to 19 kn/m 2. The design of the berth is similar to the one adopted for EQ1 to EQ4. It is designed for dredged level of -9.45m. WQ-4 (1965) The top soil up to a depth of 2 m below GL is moorum fill followed by 3.5 m medium dense sand layer. Soft Clay exists between 5.5 m to 12 m with in-situ vane shear strength varying between 10 to 21 kn/m 2. Below this layer, the strata is similar to WQ-2 and WQ-3 as indicated in the following topics. WQ-5 (1965) The top soil up to a depth of 8.50 m below GL is made up of moorum and sandy layers, which are medium dense. Soft Clay was encountered between 8.50 m to 20 m with in-situ vane shear strength varying between 10 to 19 kn/ m 2. Below this layer, there is a stiff clay layer followed by a soft disintegrated rock. WQ4 and WQ5 berths were commissioned during 1965 to facilitate handling of iron ore. These berths were designed for dredged depth of m. The type of construction adopted is similar to EQ berths i.e., gravity type monoliths. The monoliths are of the size 42 ft. x 36 ft. with four pockets. The landside pockets were filled with metal and sand of 2:1 proportion. With cement concrete construction becoming more and more popular at this period of time, these berths were constructed with cement concrete staining with nominal steel at the centre of the staining. Fertiliser Berth (1967) The top soil up to a depth of 2.15 m below GL was medium dense gravely sand. Between 2.15 m to 6.10 m depth, there exists soft marine clay with vane shear strength of 14 kn/ m 2.and from 6.10 m to m it is stiff to very stiff clay with varying vane shear strength of 10.5 to 22 kn/m 2. This is a captive berth for handling of fertilizers in the Western arm and was constructed during Due to the advent of bored cast insitu piles by this period of time, this berth was constructed using four rows of 1 m. dia R.C.C. bored cast insitu piles with pre-cast cum insitu RCC deck. This berth was designed for dredged depth of m. EQ-5 and EQ-6 ( ) The top soil up to a depth of 5.9 m below GL was medium dense gravely sand. Between 5.90 m to 8.90 m depth, it is soft marine clay and Hard Strata sand at -22 m with SPT N value of 37. Construction consists of a series of Stone Masonry lined concrete Monoliths of size: 34 ft. x 30 ft. (with four pockets) with a Gap of 5 ft. between monoliths. It is designed for dredged level of m. WQ-1 ( ) and WQ-2 and WQ-3( ) The top soil up to a depth of 3.00 m below GL is dense filled up moorum and is followed by 3.00 m medium dense silty sand and soft marine clay between 6.00 m to 9.00 m with in-situ shear strength 9 to 11 kpa. Between the depths of 9.00 m to m loose to dense silty sand was encountered followed by 3.0 m soft to stiff marine clay with vane shear strength of 21 kpa. Between m to 18.0 m, dense silty sand followed by soft disintegrated rock, hard clay layer and soft disintegrated rock layers. Hard rock is encountered at a depth of m. The construction is with front 850mm vertical piles at 4.50m centre to centres spacing and rear Retaining Diaphragm wall of 800mm thick with Anchor Diaphragm of 600mm thick at 4.50m centre to centre. It is designed for a dredged level of -12 m. EQ-7 (1995) The top soil up to about 6 m below GL is fine sand with overlay of reclaimed material. From -2.7 m for a depth of -17 m below GL is grey soft marine clay having un-drained shear strength C u of 12 kn/m 2 SPT N value of only 5 and encountered hard rock m. The construction adopted is 600mm thick front T-Diaphragm wall, vertical piles of 850 mm diameter and raker piles 700 diameter at rear side. It is designed for dredged level of -12 m. EQ-8 and EQ-9 (2004) on BOT The soil profile is similar to that of EQ-7. The construction adopted is front Diaphragm wall with pre stressed rock anchor 2nos. in every 4.00m length, two rows of 1200 mm diameter bored cast in suit piles. It is designed for dredged level of m. For the backup area development ground improvement technique of band Drains and geofabric filters has been adopted. Outer Harbour Construction of Outer Harbour was taken up in 1970 to handle bigger size vessels upto DWT and maximum permissible draft of m. Protection to outer harbour was provided by construction of South Break water (1543 m) East Break water (1070 m), North Break water (475 m) with use concrete armour blocks. A total number of 7 berths were developed in the outer harbour for handling Bulk Cargo, Coal, Iron Ore, Oil Products, LPG / LNG and container cargo. The soil profiles and structures adopted are detailed below: Ore Berths (OB-1 & OB-2) (1975) The sub soil at ore berth is silty clay from -21.1m to m with SPT N value of 6. From -24.6m to m dark grey sandy clay of N value varies from 10 to 50 has been observed. Gravity type of structures were adopted for berth constructions due to available good bed profile at bed level.

3 Soil conditions and Geotechnical solutions adopted for Different structures constructed Concrete caissons construction was adopted with caissons of size 28 m. x 18 m. (Pre-cast floated and sunk in position) for a design dredged depth of m. General Cargo Berth (1983) The GCB sub soil bed is dense medium sand from -4 m to m with N value from 91 to 17 and from m to m stiff clay with fine sand of N value from 16 to 21 followed hard rock encountered at -33 m. Bored piled structure construction with 4 rows of 1200 mm diameter piles (Cast- in-situ) founded on rock and RCC deck was adopted. The design dredged level is of m. Offshore Tanker Terminal (OSTT) (1985) The OSTT sub soil bed was sandy clay from m to m and from m to m medium to very dense sand of N value from 13 to 197 followed hard rock encountered at m. Gravity type of construction has been adopted. Concrete caissons with baby crib units were together joined to form a single crib of size 31.30m x 19.85m and were placed on levelled bed formed at 20.5 m level. LPG Jetty (1998) Sub-soil at bed at LPG Jetty is fine sand to medium dense sand from m to m the N value varies from 36 to 52 and from m to m clay moorum with kankers of N value 110 followed by hard rock encountered at m. Dolphin type structure with RCC deck supported on bored piles was adopted with a dredged level of m. Multipurpose Berth (2001) The sub soil bed is loose to very medium dense sand from m to m with N value from 7 to 41 and from m to m course sand with gravel. Hard rock exists at m level. Bored piled structures were adopted with 5 rows of 1200 mm diameter piles (Cast-in-situ) for the dredged level m. 3. GEOTECHNICAL SOLUTIONS Sand Wicks and Sand Drains at OHC Ore Handling Complex area was developed on the reclaimed area to cater to Iron Ore stacking and handling as a part of Outer Harbour development which was initially intended for iron ore export with a dedicated iron ore berth and with a mechanical conveyor system connecting from stacking areas to iron ore berth. The topsoil up to a depth of 3.30 m below GL is sand. Soft Clay was encountered between 3.30 m to m with higher vane shear strength values even up to 236 kn/m 2. Below this, it is stiff clay. From a recent soil investigation carried out near to this ore handling complex this soft clay with vane shear strength of 12 to 24 kn/m 2 is found up to a level of nearly -15 m. Since the condition of sub-soil at OHC area is weak in nature and it is required to use this area for higher stacks of iron ore, soil improvement technique of Sand Wicks & Sand Drains was adopted during OHC development done during early1970 s. This method of ground improvement facilitated stacking of Iron Ore up to 8 m height. Use of Geotextiles at MPB The MPB in outer harbour was constructed on piles structure as mentioned above with RCC deck and a rock bund behind the berth to retain the soil was provided with a slope 1:1.3 extending towards seaside. In order to maintain the slope stability of the rock bund and to avoid escape of filled up soil by piping action, to relieve the pore water pressure by allowing the seepage water through the bund and to avoid withdrawal of soil by suction due to wave run up a Non-woven Poly Propylene Geotextile filter is provided. This Geotextile membrane provided on the seaside has been performing well in meeting the functional requirement as envisaged. Use of Prefabricated Vertical Drains and Preloading Port Connectivity Road Road connectivity from V.P.T. to NH-5 was developed as a joint venture project by V.P.T. and NHAI at a project cost of Rs.116 Crores commissioned in the year The alignment of this port connectivity road is typically through marshy area with deposits of soft marine clay extending to a considerable depth of about 10 to 15 Mtrs. below the ground level. The situation called to adopt suitable ground improvement technique to achieve required baring capacity with tolerable limits of differential settlements. Accordingly upon examination of available technique band drains also called Polymeric vertical drains (PVD) with preloading techniques was adopted polymeric vertical drains were installed in a triangular 1.3 m c/c. Two rows of pvd s were laid beyond the toe of the embankment, so as to help in relieving excess pore pressure. The embankment has been designed for achieving 70 percent consolidations and 20 per cent consolidation was expected to take place during the course of construction of pavement layers like granular sub-base and WMM base courses. The constructions of these layers required about 3 months and by the end of this period the sub-soil has achieved 90 per cent consolidation. The Port Connectivity Road with the above ground improvement technique adopted for the sub-grade is performing satisfactorily with marginal uniform settlements as expected, thus proving the viability of Geo-technical investigations carried out vis-à-vis the actual behaviour of soil and the success of the ground improvement technique adopted with Polymeric Vertical Drains.

4 146 K. Ramachandra Rao Development of EQ8 & EQ9 Similar ground improvement technique using Polymeric vertical drains (PVD) with pre-loading as discussed above was also adopted in the development of two multipurpose berths i.e., EQ-8 & EQ9 in the Inner Harbour of Visakhapatnam Port where the need of the ground behind the berths is to support high intensity loads of the bulk cargoes. The salient features of the method adopted is as follows: The pre-loading is carried out from the existing ground level of +2.7 Mtrs. up to Mtrs. by providing murum fill in layers. Necessary drainage facility was created by placing perforated HDPE pipe lines wrapped with filter fabric (geo fabric) at +4.5 mtr. level pre-load of murum fill from 4.5 to 12.1 is done in stages allowing for consolidation to take place for 9 weeks in each stage. Total settlements of the order up to 1.5 mtrs. have occurred with the release of pore waters along the drainage arrangements made at +4.5 Mtr. level. After attaining the desired settlement and escape of pore water, pre-load of murum fill was removed and the ground is levelled and concrete pavements with CC Concrete blocks was done. The ground improvement technique yielded satisfactory result since the intended purpose of catering to higher stack heights is being served. Strengthening By Grouting / For Eq - 5 & 6 Berth In the Inner Horbour As a part of development, Visakhapatnam Port Trust has taken up the deepening works to accommodate 12.5 m draft vessels (requiring dredged depth up to (-) 13.5 m). In order to take up these deepening works, the existing berths were required to be strengthened. Two existing EQ-5 & EQ-6 berths were originally constructed by providing gravity type monoliths structures founded at about m level and were deigned for vessel of about 10 m draft. Deepening in front of the berth upto 13.50m cater to 12.50m draft vessel was necessitated. Hence upon exploring feasibility to strengthen these berths with out resorting to demolition & reconstruction which is not a practical solution in an operational Port. Cement / lime grouting technique is being adopted there by deepening in front berth is feasible. Accordingly grouting with cement, water and lime, water at a ratio of about 1:3 with a pressure up to 400 to 500k N/ m 2 is carried out in sand and clay layer respectively. After the treatment significant improvement in soil characteristics is noticed. Slope Protection for Deepening Water Channel The existing hill slopes adjacent to South Bank of Inner harbour entrance channel are slipping day by day due to deepening and widening of the Inner Harbour entrance Channel being done to bring deep draft vessels through the inner harbour entrance channel which may lead to slope failure causing soil / boulder slippage inside the channel causing hindrance to the navigation of vessels into the channel. To prevent this slope failure, a slope protection work has been taken up by providing one row of 1200 mm diameter RCC cast-in situ bored piles and intermittent anchor piles with capping beam / slab and rock / gravel filling behind piles. Stone Column Technique at OHC As stated in the earlier topic keeping in view of constraint of Sub-soil characteristics in the OHC ground improvement with stone column methods was suggested. It is found that the soil between depths of 3.0 m and 20.0 m, in general, is soft marine clay. The liquid limit values reported is between 65 to 88% and the plastic limit is between 30 to 40%. The natural moisture content values are between 50 to 62%. Hence, the consistency index value is in the range of 0.18 to 0.40 and based on this, it is reasonable to assume and un-drained strength, Cu of 20 kn/m 2. Compression index, Cc of 0.55 to 0.60 can be assumed and from the results of consolidation tests. Considering the poor nature of the subsoil, stone column method of ground improvement method of technique was suggested. Upon a preliminary study based on the requirement to stack iron ore stacks of the order of 9 10 mtrs with a required bearing capacity of the order of 20 t/m 2, it was advised to provide 800 mm dia. stone column with crushed rock of 75mm down to 3mm in a triangle grid 2.50m x 2.50m well compacted gravel of min. thickness 1.50m shall be placed above ground with drainage channels connected to manholes where the water collected and pumped out. The stone columns can be designed based on I.S:15284 (PART I):2003. Load Transfer is mainly through the skin friction and the load transfer through the end bearing is not a specific requirement. The soil near the ground surface has a dominating influence on settlement and ultimate bearing capacity of stone columns. Thus the bearing capacity of the treated area is proposed to be improved multi-fold by adopting technique of providing stone columns and other novel options of providing shallow treatment using geosynthetcs are also being explored. 4. ROAD CONSTRUCTION PROBLEMS AND CRRI RECOMMENDATIONS Upon detailed study of the conditions of the existing roads by Central Road Research Institute (CRRI) the following conclusions were evolved. The VPT roads are having a failure mechanism due to the uneven settlements of soft subsoil along the entire length, due to the presence of marine clay deposits to a depth of about 15 m with minimum SPT N values of 1 or 2 with very low values of angle of internal friction. This requires faster consolidation of subsoil and increasing the bearing capacity of the subsoil. Provision of

5 Soil conditions and Geotechnical solutions adopted for Different structures constructed a cement concrete pavements may develop cracks due to non uniform settlement. Hence, the methodology of treatment prescribed by M/S.CRRI was to implement the ground improvement methods like stone column, stage construction of road and band drains which minimise the time of consolidation. The stone column technique involves the improvement of subsoil upto a depth of 10 mtrs upon reaching the ground level, a continuous sand layer of 150 mm thick followed by two layers of WBM (Grade II and Grade III) and over this 100 mm Dry lean concrete and 260 mm of pavement quality concrete is to be adopted. Alternatively, stage construction prescribed by CRRI involves soil replacement upto sub grade level, followed by sand blanket, geotextile layer, GSB layer and subsequently two layers of WBM followed by wearing course with premix carpet with seal coat, then allowing the traffic to ply for two years, record the settlement and then upon seizure of settlement, to provide layer of DLC (200 mm thick) and PQC (26 cm thick). Visakhapatnam Port is in the process of implementation of the above recommendations. 5. CONCLUDING REMARKS The soil conditions at Visakhapatnam Port Trust are typically dense sand to silty sand and soft marine to ultra soft marine clays followed by stiff clay and weathered/hard rock. These types of soils pose a great problem in designing different structures at Port. Some of the problems faced and geotechnical solutions adopted for different types of structures are summarised in this paper.