Geotechnical Investigation Reports and Foundation Recommendations - Scope for Improvement - Examples

Transcription

1 Geotechnical Investigation Reports and Foundation Recommendations - Scope for Improvement - Examples Prof. V.S.Raju (Formerly: Director, IIT Delhi & Professor and Dean, IIT Madras) [email protected] Prof. V.S. Raju 1 CHOICE OF APPROPRIATE FOUNDATION AND EXECUTION Optimum foundation design should ensure: technical adequacy, cost effectiveness and ease of execution. This is not easy, because of many variables including insufficient and inaccurate information at the time of design. Variation in strata and changes in project requirement during execution 2 1

2 TOPICS COVERED Inadequacy of the Investigations and Inappropriate Foundation Recommendations with Illustrative Examples 1. Fertilizer Plant on the East Coast 2. Fertilizer Plant in Gangetic Belt 3. Office cum Residential Complex at Jodhpur 4. University Campus at Adilabad District 5. L&T Serene County (Residential Campus) 3 FERTILIZER PLANT ON THE EAST COAST The average soil strata consists of : top layer of dense fine sand 4 to 8 m thick. followed by soft marine clay up to a depth of 14 to 18 m below ground level. very stiff clay up to 30 to 40 m below ground level. 4 2

3 5 PROBLEMS DUE TO THE PRESENCE OF SOFT CLAY LAYER For site grading, 1-2 m fill is required - results in a settlement of 12 cm of soft clay. - Along with clay, sand layer also moves down. In case of pile foundation, large negative drag on piles from soft clay and sand layer. Considerable reduction in pile capacity (upto 50%), and hence increase in the number of piles. For structures like bulk storage with large area loads, the stability of soft clay layer and lateral flow to be checked. Large settlements of floors and lateral forces on neighbouring pile foundations. 6 3

4 FOUNDATIONS ADOPTED Precast driven piles were chosen, with bitumen coat in the top layers to reduce the negative drag. All light structures, not sensitive to settlements, are supported on shallow foundations. For structures having distributed loads over large areas (silos, water storages), surcharge provided on the periphery to achieve reduced shear stresses on soft clay. 7 CHOICE OF THE PILE The original recommendation was to go for bored piles, 45 cm in diameter. Two alternative pile types have been tested. a) Precast driven piles 40 cm x 40 cm, 22 m long (to be able to drive). b) Bored cast in situ piles, 45cm dia, 22 m long Full scale tests for a final decision on the choice of the pile type. 6 test piles, 3 each of the 2 types, installed in locations in close proximity. 3 different Bored cast in-situ piles were installed by bailer boring method. 8 4

5 RESULTS OF PILE LOAD TESTS 9 Table 1: WORKING LOAD IN TONS AS PER IS 2911 Pile No. (1) 2 / 3 of Load at 12 mm settlement (2) Failure Load (3) Half of Failure Load (4) Working Load (smaller of 2 and 4) (5) P (100) * B P B P B The pile length is 19.5 m, and failure load extrapolated for 22 m length is 100 tons. 10 5

6 CONCLUSIONS Precast driven piles with design load of 70 to 100 tonnes adopted for the following reasons : For comparable dimensions, the precast pile has 50 to 80% higher capacity than a bored pile. Precast pile offers better protection to reinforcement. This is particularly important under the present saline ground water conditions. The jetting of precast pile up to 12 m will reduce the friction over this depth, and consequently the effects of negative drag. In case of precast piles, the negative drag can be reduced by applying a slip layer of bitumen. 11 Conclusions (Contd.) Bored piles not preferred for the following reasons: Lower Capacity Boring operations through stiff fissured clay will result in softening of this layer, thereby limiting the load carrying capacity of piles. There is no possibility of applying any slip layer and to reduce the negative drag. 12 6

7 ANTICIPATED NEGATIVE DRAG 40 tonnes based on theoretical consideration. Field load tests on i) Short piles resting on soft clay, ii) Instrumented piles, where the load distribution with depth has been measured. Model tests for the proper choice of bitumen coating to reduce negative drag. i) Precast concrete 10 cm dia and 50 cm long piles were used. ii) Bitumen coatings using different grades of bitumen were applied, and results showed 80 to 90% reduction in friction. iii) Finally SAE 80 grade bitumen was adopted. 13 FERTILIZER PLANT IN GANGETIC BELT The Soil Strata: Silty sand with low N values (<10) upto 10 m depth, N > 20 beyond 20 m depth. N-values ranging between 10 to 20 for the layer between 10 to 20 m depth. High water table with possibility of liquefaction during earthquake. 14 7

8 PHASE I : INITIAL FOUNDATION DESIGN RCC Piles, Driven Cast-in situ, 400 mm dia Sand Compaction Piles, 2-3 Rows Around RCC Piles, Pile capacities Vertical downward : 50 t, Tension : 5t, Lateral : 2.5t, Result Total Requirement 16,000 RCC Piles 32,000 Compaction Piles Problem of execution on time 15 REVIEW AND FURTHER INVESTIGATION (Pile Load Tests) Revised Pile Capacities Vertical : 65t Tension : 25t Lateral : 3.5t Reduction in RCC Piles : 40% Increase in spacing of compaction piles from 3d to 4 d ; reduction in Compaction Piles : 50% Saving in construction time : 6 months Substantial cost savings as well. 16 8

9 SETTLEMENT (mm) PHASE II OF THE PLANT No RCC Piles. 900 mm dia Vibro-Stone Columns with varied spacing (2d,2.25d and 2.5d) to suit the foundation requirement. Full scale field trials. Several Footing tests for confirmation. Substantial savings in time and cost. 17 LOAD kg / m 2 x LOAD SETTLEMENT CURVES FOR SINGLE COLUMN LOAD TEST 18 9

10 SETTLEMENT (mm) LOAD kg / m 2 x Aonla (Compressor House) Single column Test Aonla (Benefield) Aonla (Prill Tower) Three Column Test LOAD SETTLEMENT CURVES FOR SINGLE AND THREE COLUMN TEST 19 Office cum Residential Complex at Jodhpur 2-3 Storeyed Buildings Investigations Done 8 Bore holes drilled to 6 m depth each. As per bore logs Rocky strata. Strata starts at Ground level (GL) in all bore holes except in Bore hole 5, where it starts at 1.5 m. BH 5 is at the extreme corner of the plot where nothing is planned to be built. Bore logs do not give the core recovery, which is a must to be given

11 Recommendations by Investigation Agency 1. Open foundations (footings) 2. Unconfined compressive strength of rock range given 650 to 850 t/m 2 3. Calculated safe bearing capacity (SBC) 80 to 100 t/m 2 21 FOUNDATION RECOMMENDATIONS (Contd ) 4. Recommended SBC is 40 t/m 2 at 1m below GL, except in BH 5; where SBC is 10 t/m 2 at 1.2 m depth and 40 t/m 2 from 2 m depth onwards. 5. The SBC adopted in Design is not known. It should have been written on the drawings

12 WHAT HAPPENED AT SITE 1. Foundation Depth adopted: 1.5m. 2. Foundation sizes 1.5m x 1.5m to 2.3m x 2.3m 3. For excavation Rock Blasting has been done 4. Instead of excavating / blasting individual pits for each footing, the entire foot print of all the buildings has been blasted and excavated. 23 What Happened at Site (Contd ) 5. This resulted in excavated rock material ranging from huge boulders to rubble of volume of about 25,000 m 3 (Actually needed 10% of this). 6. Additional issues: (a) How to dispose of the excavated material. (b) Huge quantity of soil material for plinth filling needed

13 Prof. V.S. Raju s Investigation of the Situation and Foundation Recommendations Basis : Site visit Inspection of the strata in the excavated pits Study of the soil report 1. The investigation is not as per the relevant Indian Standards. 25 Prof. V.S. Raju s Recommendations (Contd ) 2. The recommendations in the report are wrong and are less by a Factor 3 to The correct SBC values are 150 t/m 2 (on a conservative side) with a minimum size of footing as 0.8m x 0.8m. Depth of footing 0.5m to 1m

14 Prof. V.S. Raju s Recommendations (Contd ) 4. There is absolutely no justification to stipulate a foundation depth of 1.5m for the entire site. 5. There was no need at all to make the footing sizes so big as given (1.5m x 1.5m to 2.3m x 2.3m) 6. There is no need to blast the strata over the entire foot print of the building. 27 Prof. V.S. Raju s Recommendations (Contd ) 7. No need to blast for the individual footings also as the required likely sizes are 0.8m x 0.8m to 1.2m x 1.2m, depth 0.5 m to 1.0m only. Pavement breakers (jack hammers) will do the job

15 Photographs of Blasted Rocks 29 rap Photographs of Blasted Rocks 30 15

16 5.8 m to 25m soft rock University Campus at Adilabad District Total area ~ 300 acres For Phase I development, Only 7 boreholes Bore logs improper and inadequate. First SPT at 10 m below GL. Pile Foundations recommended and executed, which are not at all required. Pile safe capacity for 600 mm dia, 12 m length : 83 tons (Very low) - Settlement of pile up to 10 % of pile diameter could not be attained by 3 times the design load. Prof. V.S. Raju to 1.80 m Brown clay 1.80 mto 5.80 m yellow clay & Murrum First SPT at 10m below G.L, Required every 1m to 1.5m Typical Bore Profile Prof. V.S. Raju 32 16

17 Pile Load Tests: Five Initial Load Tests At 3 times the design load, settlements are only 2 mm to 27 mm as against permitted 45 mm to 60 mm. Pile capacities are not revised. Piles, which are not required in the first place are grossly over designed. Prof. V.S. Raju Acres site L&T Serene County Strata Rocks and Boulders with local depressions, highly uneven. 10 towers between 11 and 14 floors Recommended SBC by the soil Investigation Agency 30 t/m 2 - One value for the entire site? 34 17

18 30 m L&T Serene County Prof. V.S. Raju 35 Extreme Issue was with one of the 10 towers built Tower Dimension : 75 m x 30 m Highly variable strata at Founding Level. 75 m Hard Rock SBC 400 t/m 2 Hard Murrum Soft Clay 80 t/m 2 4 m thick 0 t/m 2 Prof. V.S. Raju Tower Foot Print 36 18

19 Important: These type of variations do occur in rocky and bouldry strata The soft clay is due to a old pond, which normally gets covered up during site grading. 37 All these examples reaffirm the requirement of high quality Geotechnical Investigation, Interpretation by a qualified Foundation Engineer in close collaboration with the Structural Designer

20 THANK YOU JAI HIND 20