Field Loading Test on Micropile Foundation regarding the Effect of Prestress IWM2003 in Seattle Kinya Miura: GeoMechanics Group, Faculty of Engineering, Toyohashi University of Technology

We conducted two series of loading tests for the effect of prestress. Model loading tests on the footings reinforced with Prestressed Micropiles got an idea of prestressing micropiles Field loading tests on the footings reinforced with Prestressed Micropiles made sure of the idea Details of the tests are in the two conference papers distributed

A measure for the improvement of bearing capacity by micropiles The ratio R was calculated from three types of loading tests: calling “Index of Network Effect”

Improvement observed in laboratory loading tests

Confining effect by micropile group Confining effect is enhanced by positive dilatancy of ground material, but not by negative dilatancy.

Prestress (tension) in MPs To overcome the disadvantage of negative dilatancy, the idea of prestress occurred.

Laboratory Loading Tests Model footing for applying prestress

Test results for medium dense sand ground Total Load carried by FT-PSMP Load by micropile group start with negative values Load by footing base base pressure is enhanced by the prestress.

Test results for medium dense sand ground Bearing capacity was improved even under small displacement and on non-dilative soil ground Ratio R was greater than 1.3

Field Loading Tests The boring log and the N-value of the upper 10 meters is presented The subsoils were fill, loam, cemented clay, sandy clay, and fine sand, respectively. The fill, loam and clay were soft and rather uniform mechanically; the N-values are less than 5.

Plan view of test site

Illustration of Micropile (MP) The micropile was 3 m long, 100 mm in diameter, with a steel core-rod of 32 mm in diameter (a) S-MP (b) FT-MP/FT-PSMP

Application of prestress with jacks

Loading in vertical direction (FT-MP/FT-PSMP test)

Loading in horizontal direction (sectional view)

Setup for horizontal loading

Loading test on FT and MP Pmax = 210 kN, Smax = 181.43 mm Pmax = 30 kN, Smax = 28.69 mm

Vertical loading test on FT-MP and FT-PSMP Pmax = 560 kN Smax = 243 mm Py = 240 kN Pult = 560 kN Pmax = 600 kN Smax = 227 mm Py = 240 kN Pult = 600 kN

Effect of prestress on bearing capacity of FT with MP group The bearing capacity of FT reinforced with MPs is remarkably increased. The bearing capacity of prestressed MP group (FT-PSMP) is 7% higher than FT-MP group. Settlement of MP group is reduced 16.4% due to prestress (FT-PSMP).

Horizontal loading test on FT-MP and FT-PSMP Pmax = 30 kN max = 27.45 mm Pmax = 30 kN max = 13.84 mm

Effect of prestress on horizontal movements The effect of prestress on horizontal movement control was significant. The lateral movement was increased rather linearly with load in the non-prestress group (FT-MP). y = 13.84 mm in the prestressed group (FT-PSMP).

Conclusions (Laboratory Tests) The bearing capacity of footing is improved by the interaction between footing, subsoil and micropile group, and induce confining effect on the subsoil. The prestress is effective to enhance the confining effect, even at the beginning of loading and on non-cohesive soil ground, such as loose sand and soft clay.

Conclusions (Field Tests) The effect of prestress on the improvement of bearing capacity is significant: vertical bearing capacity was increased by 7% in prestressed MP group (FT-PSMP) than in the non-prestressed MP group (FT-MP). Coefficient of subgrade reaction was remarkably improved. (more than twice in both vertical and horizontal directions) 1.86×104 kN/m3 (FT-MP), 3.97×104 kN/m3 (FT-PSMP) in Vertical 1.01×103 kN/m3 (FT-MP), 1.71×104kN/m3 (FT-PSMP) in Horizontal

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