0.00000005 % of soil volume is explored. Soil tested is even smaller percent of soils Soil disturbance lowers soil strength. Soil and bedrock conditions are random
Subsurface Exploration of Soil and Bedrock Conditions Borings CPTs Test shafts Geophysical Exploration Drive Indicator Piles Dynamic Testing Load Testing
Large project where small savings on length is a large monetary savings. Project cost is critical. Structure is very sensitive to settlement. Critical Structure with high loads i.e. bridge. Engineer lacks experienced in the area. Piles must resist uplift.
Analytical capacity is often conservative. Pile length increased for unknowns. High factor of safety used.
Some engineers use soil property estimates from a Design Manual which are generally conservative. The size of the geotechnical exploration budget can affect how conservative the design is. Most agencies are conservative.
Some engineers will arbitrarily increase the pile lengths just to be on the safe side. Added pile length is the most expensive part of the pile when driving into dense or hard materials.
The size of the factor of safety reflects the uncertainty of the soil conditions and the pile capacity. If the uncertainty is reduced it is appropriate to reduce the factor of safety. i.e. from 3.0 to 2.0
Uncertainty is reduced with multiple tests. Soil Parameters used in Calculations checked. Factor of safety may be reduced. Pile lengths may be reduced. Cost savings from reduced pile lengths and driving time.
Hydraulic Clamp Leads 25 kg Weight Damping Springs Test Pile The RLT Drop Weight Rapid Load Tester
MOBILE TESTING UNIT CAPABLE OF 6 TO 10 COMPRESSION TESTS/DAY UP TO 800 KIP LOADING CAPABILITY IMMEDIATE REPORTING SELF-SUFFICIENT TESTING UNIT Rapid Load Test System
25,000 kg mass dropped on pile from varying heights Deflection Measured @ Point of Impact Force applied to pile top for 200-ms duration Energy transmitted to pile via anvil and dampened via springs Springs recoil and push load up to unload pile
Excess Load Demand Load Test Workmanship Test Pile Location Time After Pile Driven
Column Load = 175 kips Allowable Pile Load = 110 kips Two piles required Capacity of pile cap = 220 kips Excess Capacity = 220 – 175 = 45 kips Actual ultimate capacity needed 175 / 2.0 = 87.5 x 2.0 = 175 kips/pile instead of 220 kips/pile
Many Factors Can Effect Test Results Is top of Pile level? Was test stopped and started for some reason? Were readings taken accurately? Was pile damaged during driving? Have gauges been calibrated?
Readings taken at 100 kip increments only. Data between last 100 kip reading and ultimate load missing. Always take as much data as possible. Run test past design load to Davisson Criteria ultimate capacity
APPLIED LOAD Test Stopped Here Actual Ultimate Capacity DEFLECTION
Determine if there is an anomaly in the location of lower capacity test(s). If so assign a lower allowable load in that area only.
1 10 100 1000 Hours After Installation 0 50 90 Percent of Capacity
The term “test failure” indicates the pile is useless or design values are wrong. Analytical capacities are “best estimates” Better to Use “Load Test Results” The owner may be able to live with a lower design capacity
Inexperienced inspectors tend to overdrive piles looking for blow counts. Load tests can minimize reliance on blow counts Have a preconstruction meeting with engineer and agree on blow count criteria.