AlaaElnahas
Geotechnical
- Dec 7, 2005
- 12
I am amending a geotechnical report and making geotechnical design for a an integral bridge (one span of 30m) in a motorway to cross railway line. The abutment is 10 m high above the ground surface and founded on bored piles (mostly CFA piles but may be the 27 m piles will be constructed by rotary auger), as it is 6 m away from the rail lines. The ground is 4-5 m of soft and weak recent alluvium soils (clay, peat, silty sand), underlain by about 4 m of sand and gravel, underlain by firm to stiff clay then stiff silt. The stiff silt and clay layers are underlain by about 3-4 m weathered sandstone. Below the weathered sandstone, come the intact weak to moderately weak sandstone (qu=12MPa and RQD=0-38%, with inclined discontinuities). The rock head depth below the ground level ranges from 8.5 m in one side of the 200m-long abutment (with no stiff silt and clay layers between the sand and gravel layer and the bedrock)to 26 m on the other side (with the stiff silt and clay layer existing for 15 m depth). The 10 m high embankement (behind the abutment)will be carried by embankment piles+tension membranes (not ideal solution but will be used). The fill behind the abutment will be light weight fill (Maxit with unit weight=5kN/m3).
The structure engineer want to found the integral bridge on one row of end bearing piles (0.9m diameter), that are socketed in the intact rock. I would like to ask about:
1- In the structural engineer solution, The piles and abutment will work like a vertical beam, encastre at the bottom and not fully fixed at its top (connection with the bridge deck). In this case, the pile row will be loaded with vertical loads (100kN/m), lateral loads, and bending moment. Am I correct in seeing that this is not acceptable solution, as the piles should either be axially or laterally loaded, and that at least a second pile row is needed to avoid a bending moment at the pile cap?
2- If the answer is yes, what should we do (we need, if possible, to stick to the piles solution for non-engineering reasons)?
3- If the answer is No, how to consider the combined loading effects in estimating the allowable pile load?
4-I used the Goodman (1980) relations, mentioned in Prof. Das Book, to estimate the allowable bearing capacity for the end bearing piles. I used a safety factor 6 to get the allowable compressive strength for the bedrock samples, estimated the ultimate bearing stress at the pile tip using Goodman relation, and a safety factor 3 to get the allowable bearing stress for the pile using the uniaxial strength of the bedrock.For a Pile diameter of 0.9, the allowable point load is 1055kN but Tomlinson book says that New York Foundation Code restirict the carried load by any single pile to 600kN for weak rock, and 1500 for medium strong to strong rock, and 1000kN for hardpan over laying rock. Is my choice of allowable load acceptable or unsafe?
5-Are there regulations/guidelines to choose the number of piles to be tested in the field, and the number of piles for which pile integrity tests should be conducted? If the answer is yes, would you tell me where to get such information?
Thank you
The structure engineer want to found the integral bridge on one row of end bearing piles (0.9m diameter), that are socketed in the intact rock. I would like to ask about:
1- In the structural engineer solution, The piles and abutment will work like a vertical beam, encastre at the bottom and not fully fixed at its top (connection with the bridge deck). In this case, the pile row will be loaded with vertical loads (100kN/m), lateral loads, and bending moment. Am I correct in seeing that this is not acceptable solution, as the piles should either be axially or laterally loaded, and that at least a second pile row is needed to avoid a bending moment at the pile cap?
2- If the answer is yes, what should we do (we need, if possible, to stick to the piles solution for non-engineering reasons)?
3- If the answer is No, how to consider the combined loading effects in estimating the allowable pile load?
4-I used the Goodman (1980) relations, mentioned in Prof. Das Book, to estimate the allowable bearing capacity for the end bearing piles. I used a safety factor 6 to get the allowable compressive strength for the bedrock samples, estimated the ultimate bearing stress at the pile tip using Goodman relation, and a safety factor 3 to get the allowable bearing stress for the pile using the uniaxial strength of the bedrock.For a Pile diameter of 0.9, the allowable point load is 1055kN but Tomlinson book says that New York Foundation Code restirict the carried load by any single pile to 600kN for weak rock, and 1500 for medium strong to strong rock, and 1000kN for hardpan over laying rock. Is my choice of allowable load acceptable or unsafe?
5-Are there regulations/guidelines to choose the number of piles to be tested in the field, and the number of piles for which pile integrity tests should be conducted? If the answer is yes, would you tell me where to get such information?
Thank you
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