The theoretical research on the behavior of pile subject to lateral load has several problems due to the uncertainties of pile-soil boundary elements, non-homogeneous ground and underground water. Strain wedge(SW) model is one of the models which was suggested to overcome such problems; however, the suggested SW model is a theory based on the earth pressure theory and the experimental verification of SW model is performed only for a single pile.
The depth of rotation-point, wedge failure angle, and wedge wing angle are generally required in determination of the lateral resistance of pile group. The model experimental is performed by varying the width, spacing and number of piles and the relative density of sand in this study.
The results are as followings:

1. According to the observation of the rear ground surface deformation of the pile in series on lateral load, rotation point ratio, wedge failure angle, and wedge wing angle of the front row were similar with those of middle row; however, those were relatively smaller in back row.

2. Even the rotation point ratio, wedge failure angle and wedge wing angle of pile in parallel were similar to those of a single pile, the resistance of a single pile was greater than that of pile in parallel. It indicates that the resistance decreases due to overlap effect in case of pile in parallel.
The lateral resistance of the pile group can be evaluated under the condition of determining the shape of the wedge as the pile in series and take account of the lateral overlapping area in the calculation.

3. The lateral resistance in SW model is composed of earth pressure of wedge rear, the shear resistance on the side of the wedge, and the frictional resistance between pile and ground. Based on those components, it is proposed the implicit formulas to calculate the lateral resistance of the pile in series, pile in parallel, and pile in a square array.

4. The ultimate lateral resistance determined by the proposed formula are compared with model test, field test and finite element analyses. As the results, the resistance of the pile located in the middle row are closely matched each others. The resistance of front row is predicted as slightly bigger and that of the rear row is predicted as slightly small when they are compared with experimental results. However, the predicted total resistances are well matched with those of experiments.