GEOTECHNICAL INVESTIGATION FOR PROPOSED WATER SUPPLY PROJECT IN HINNAVARU ISLAND, LHAVIYANI ATOLL, MALDIVES Prepared for: M/s. United Nations Office for Project Services (UNOPS) Document No: OVE 092 Geotechnical Investigation 0 01/08/2014 in Hinnavaru Island R. M.Wasantha G.D.P.De Zoysa Prof. H.S.Thilakasiri Ratnayake Recommendation Rev Date Description Prepared By Approved By given By Engineering & Laboratory Services (Pvt) Ltd 62/3 Neelammahara Road Katuwawala, Boralesgamuwa, Sri Lanka Tel: 011-4309494, 011- 2517365; Fax: 011-2509806 Email: [email protected] Web site: www.elslanka.com Quality is our Trademark ELS & AMIN GEOTECHNICAL INVESTIGATION FOR PROPOSED WATER SUPPLY PROJECT IN HINNAVARU ISLAND, LHAVIYANI ATOLL, MALDIVES 1.0 Introduction The project involves the construction of a sewerage treatment plant, water pumping station and rainwater storage tanks of 165 to 714m3 capacity in Hinnavru Island. The detail summary of the geotechnical investigation as follows; Table 1: Summary of the Geotechnical Investigation in Hinavaru Island Location Borehole No Proposed Structure Site ‐01 BH‐01, BH‐02 and BH‐03 Sewerage Treatment Plant, Water Pumping Station and Rainwater Storage tank of 714m3 capacity Site ‐02 BH‐07 Rainwater Storage tank of 165m3 capacity Site ‐03 BH‐04 Rainwater Storage tank of 270m3 capacity Site ‐04 BH‐05 Rainwater Storage tank of 668.5m3 capacity Site ‐05 BH‐06 Rainwater Storage tank of 432m3 capacity M/s. ELS & AMIN International (Pvt) Limited was authorized the M/s. United Nations Office for Project Services (UNOPS), to carry out the necessary geotechnical investigations and to carry out the soil investigation at proposed development area and prepare the report of soil investigation. 2.0 Site Description The Maldives is situated on the Central Indian Sill, running from the southern point of India via the Kerguelen – Gauberg – Ridge to the Antarctic continent. When considering the climatic characteristics, Maldives experience a monsoonal climate, as the northeast monsoon is from January to March; hot days, cooler nights and relatively dry periods are common feature during this season. The wet, southwest monsoon prevails from mid‐May to November. Gales and heavy rainfall occur during this season. Page 1 ELS & AMIN The investigated site is in Hinnavaru Island of Lhaviyani atoll. Lhaviyani Atoll is an administrative division of the Maldives and is located between 5° 15" and 5° 35" N and between 73° 20" and 74° 40" E. The capital of the atoll is Naifaru. There are a total of 54 islands in the atoll of which only 5 are inhabited, namely Naifaru, Hinnavaru, Kurendhoo, Olhuvelifushi and the recently settled Maafilaafushi. Hinnavaru has a population of more than 4000 people. There are around 715 houses registered but people live in only around 480 houses. Geotechnical Investigation was carried out to get the bearing capacities values of the soil underneath. Seven boreholes were drilled at five sites as summarized in Table 1. Investigation area is shown in Figure 1(a) and 1(b). Page 2 ELS & AMIN Figure 1(a): Hinnavaru Island, Lhaviyani Atoll Page 3 ELS & AMIN Figure 1(b): Proposed Development Area and Borehole Locations Page 4 ELS & AMIN 2.1 General Geological and Sub Grade Characteristic of Site Area When the underneath geological formation of the site is considered, the coral reef formation is predominated at all the Maldivian islands, it may be stated that a classic atoll chain and the reef limestone, of which they are built, have accumulated on a volcanic ridge foundation associated with a transform fault on the floor of the Indian ocean which is now inactive. As mentioned above the Male Island also had been formed under volcanic ridge foundational phenomena. When describing the soil conditions in the Maldives, it has been observed from previous investigations that the structure of the reef flats generally consists of either coral sand, soft or hard coral rock and is usually overlaid with a relatively thick layer of coral sand. On the lagoon side of the reef edge the reef is mostly covered with dead corals and a few colonies of live corals. The cavities between the coral heads are constantly being filled up with coral sand and pieces of broken and dead corals and will ultimately become a substantially hard cemented material. 2.2 Formation of Coral in the Region With respects to coral formation in general corals are preserved as calcareous skeletons, originally secreted by a simple animal known as polyps. Reef building polyps avoid deep water more than 25m deep and grow optimally at depths within 10m. The sea water temperature should be between 250C‐290C. Emersion or exposure above water could be tolerated only for short periods during tidal cycles. Salinity levels should generally be between 2.7% and 4.0%. The water turbulence is desirable in order to disperse carbon‐dioxide to bring in plank tonic food and oxygen. A moderate fall out of fine sediments from the water can be tolerated because corals have self cleansing mechanisms but burial beneath sediment for lengths of time could result in an asphyxiation and death. The polyp sack like body had an internal cavity which acted as its stomach. There was only a single opening to the out side, surrounded by tentacles. The polyp sat in a cup like depression on to pot its calcareous skeleton, or corallites, which is built upwards to form a support as it grew. Coral are classified according to this internal structure, which cannot often be observed directly. 3.0 Field Investigation Field investigation consisted of advancing seven boreholes and five field permeability tests at the locations marked in Figure 1(b). The field investigation was commenced on 17th of July 2014 and completed on 25th of July 2014. Page 5 ELS & AMIN 3.1 Borehole Investigation The boreholes were advanced by means of rotary drilling machine and the drilling was carried out with overburden cutting tools and the wash boring process was adopted to remove the cuttings from the bottom of the borehole. During the drilling operation the walls of the boreholes were supported by 82 mm dia. NX type flush coupling casings. In order to achieve better alignment of borehole NWY flush coupling drill rods were used. Details of the depths of drilling are indicated in the Table 2. Table 2: Summary of the Borehole Investigation Ground water Overburden Rock Drilling Borehole No. Total Depth(m) level (m) Drilling (m) (m) BH‐01 0.60 11.45 ‐ 11.45 BH‐02 0.60 11.50 ‐ 11.50 BH‐03 0.65 11.50 ‐ 11.50 BH‐04 0.60 11.50 ‐ 11.50 BH‐05 0.60 11.45 ‐ 11.45 BH‐06 0.82 11.45 ‐ 11.45 BH‐07 0.90 12.45 ‐ 12.45 3.1.1 Standard Penetration Test (SPT) In this investigation the SPT was carried out in regular intervals in the overburden, at each of the borehole. The performance of this test is based on the test method specified in BS 1377. Disturbed samples of soil were collected from SPT tube. 3.1.1.1 Test Procedure • SPT sampler (Split spoon sampler) inserted in to the boring and it has been connected via steel rods to 63.5kg hammer. • Using automatic safety hammer mechanism, hammer was raised a distance of 760mm and allowed it to fall freely and the energy drives the sampler in to the bottom of the boring. The process was repeated until the sampler has penetrated a distance of 450mm. The numbers of blows were recorded for first 150mm (Seating drive) and then two consecutive 150mm intervals (Test drives). • The N value was computed by summing the blow counts for the two 150mm intervals of penetration. The blow count for the first 150mm is retained for reference purposes but not Page 6 ELS & AMIN used to compute N value because the bottom of the boring is likely to be disturbed by drilling process and may be filled with loose soil that fell from the side of the boring. • The SPT samples were extracted from the sampler and saved the obtained soil samples in appropriate manners. • Boring to the depth of the next test been done with the above procedure. 3.2 Field Permeability Test Five numbers of field permeability tests were carried out in the Boreholes BH‐01, BH‐04, BH‐05, BH‐ 06 and BH‐07. The results of the field permeability test are attached as Annexure II. The method used was Variable Head Method (Falling Head Method) as specified in Section 4: 25.4 of BS: 5930:1999. Table 3: Summary of field permeability test Depth –Test Borehole No Test No Permeability (m/Sec) Section (m) BH‐01 3.00 Test 01 3.47 x 10‐4 BH‐04 3.00 Test 01 2.08 x 10‐4 BH‐05 3.00 Test 01 2.75 x 10‐4 BH‐06 3.00 Test 01 4.46 x 10‐4 BH‐07 3.00 Test 01 3.02 x 10‐4 4.0 Laboratory Investigation Laboratory investigations were taken place in order to the sub surface assessment in geotechnical investigation. In connection with the entire laboratory testing the performance has been made as per BS 1377 part 4: unless otherwise stated. The detailed results of the laboratory investigation are presented in Annexure III. 4.1 Chemical Analysis of Soil • Ph, Sulphate & Chloride Content • 5.0 Sub‐Surface Conditions The results of the borehole investigation are given in Annexure I. Using this, profiles of subsurface conditions across the boreholes have been constructed and these are shown in Figure 2(a) to 2(g). ELS & AMIN Construction (Pvt) Ltd., 01st of August 2014 Page 7 6.0 INTERPRETATION OF THE RESULTS OF THE SITE INVESTIGATIONS 6.1 Properties of the materials in the subsurface and bedrock The energy method of SPT correction (Bowles, 1996) was used to estimate the soil strength parameters of the soil layers.