Pullout Tests of Grouted Piles in Stiff Clay
Authors: Cox W.R. , Reese L.C. 
Published: December 2007

This paper describes pullout tests that were conducted in stiff clay near Austin, Tex., to determine the tension capacity of grouted piles and to assess the effect of lateral loading on pullout resistance. In some tests, failures occurred at the pile-grout interface with maximum shearing stresses of 30 psi. In most tests, failures occurred at the grout-soil interfaces with shearing stresses of 50 percent of the soil-shear strength.

Introduction

Pullout tests on grouted piles in stiff clay were run at a site Pullout tests on grouted piles in stiff clay were run at a site 5 miles northeast of Austin, Tex., adjacent to U.S. Highway 290. The pullout tests followed a series of lateral-load tests on 6- and 24-in.-diameter pipe piles. Four W-piles were routed in drilled holes to serve as anchors for a reaction frame used during the lateral-load tests. After these tests, three W-piles were pulled in the investigation of the strength of grouted piles. In addition, pull tests were made on two 16-in.-diameter pipe piles pull tests were made on two 16-in.-diameter pipe piles that were grouted in drilled holes. The tests on pipe piles were made before and after lateral loadings.

Soil Investigations

Nine soil borings were made at the test site to provide information for the lateral and pullout test. Details of these borings and of the preparation of the test pit were reported previously and are not repeated here. The bottom of the test pit was approximately 6 ft below the original ground surface. The pit had been inundated 9 months before the first pullout tests.

A composite profile of soil types, Atterberg limits, and shear strengths of the stiff clay is given in Fig. 1. To represent the in-situ properties of the clay, the following distribution of shear strength with depth was selected.

Undrained Depth Shear Strength (ft) (tons/sq ft) --------- -----------------------

0 0.1 1 0.8 13 3.5 21 3.5 31 12.4

Installation of W-Piles

A plan of the test pit is given in Fig. 2, indicating locations of the three W-piles (W-1, W-2, and W-3) and of the two pipe piles (P-1 and P-2). In addition, locations are indicated for five of the nine soil borings. Boring 9 located 2 ft from Pile P-2 was taken 3 months after the pullout tests. The 24-in.-diameter pipe piles used for pullout tests. The 24-in.-diameter pipe piles used for the lateral-load tests are designated as Pile 1 and Pile 2 in fig. 2.

The W-piles were 14 in. deep with a 12-in. flange, and weighed 78 lb/ft. Two piles (W-1 and W-3) were installed in holes that had been drilled wet with a fishtail bit and Pile W-2 was installed in a hole that had been dry augered. The holes were approximately 25 in. in diameter and 30 ft deep,

After the holes had been drilled, the W-piles were lowered into the holes through a template to assure proper positioning. Concrete with a mix ratio of 1:2:2 by weight positioning. Concrete with a mix ratio of 1:2:2 by weight of cement, sand, and gravel, respectively, then was placed into the holes using a 20 ft-long tremie pipe. For placed into the holes using a 20 ft-long tremie pipe. For ease in placing concrete, the maximum gravel size was 3/4 in. The placement of concrete continued until returns were received at the ground line. The concrete had a 7-day compressive strength of 3,020 psi with an estimated 28-day compressive strength of 4,530 psi.

Concrete was not placed along the total 30-ft penetration of the W-piles. The grouted lengths for Piles W-1, penetration of the W-piles. The grouted lengths for Piles W-1, W-2, and W-3 were 20, 27, and 24 ft, respectively. These grouted lengths were determined at the end of pullout tests when the piles were salvaged.

JPT

P. 349