Question
If I am reading the new bridge diagram pdf correctly, question 1 relates to TP10 and questions 2 relates to PP6. Question one asks us to calculate the pressure to -35 feet which is the extent of the support at TP10. But question 2 asks us to calculate the force down to -65 feet when the support at that location only goes to roughly -20. Am I interpreting this wrong? Does the depth of the support not matter in terms of the vertical pressure at the point and depth? And are approximate estimations for where one layer begins and the other ends acceptable?
Response
You are interpreting the locations correctly.
We ask you to calculate the pressure at a particular depth, not necessarily at the bottom of the support pier.
I’d note that at Location 2 we’re slightly below the bottom layer at –65’.
- A reasonable way to interpret that last few feet is that it’s boring into rock, which would not add to the total pressure. Only the water would act.
- Another way is to assume that the drill rig stops at that layer and say that it cannot get to 65’
We’ll be lenient in how we evaluate student interpretions of those last few feet.
To solve Q3 you should keep in mind that:
ReplyDelete• When you calculate the effective stress in one layer and switch to the next layer you should restart the new depth for that layer then add the effect stress of previous layer. Because, each layer has its own unit weight (gamma); so, if you don’t restart the depth it means that you consider the whole depth from top to that point as the same material which is wrong and your calculation couldn’t be correct.
Suppose that you calculate the effective stress for the first layer with unit weight of 15 and depth of 20ft with intervals of 5ft and your effective stress would be 300 in the depth of 20 ft. the next layer have the unit weight of 20. If you calculate the effective stress with the whole depth 25ft time the unit weight of the new layer your number would be 500. That is wrong!!!! Because you calculate the whole depth as the same material. While you should first calculate the effective stress of the new layer then add up the effective stress of the last layer that your number would be (5*20)+(300)= 400
• Always look at the figures to see if there any water effect!!!!
Effective stress= total stress - pore water pressure