Zhang Zhizhong's initial plan was to have the numerically superior Chinese forces attack the Japanese by surprise and push them into the Huangpu River, then blockade the coast to deny the Japanese the opportunity to land reinforcements at the Huangpu wharves between Yangshupu and Hongkou. The 88th Division was to attack Japanese army headquarters near Zhabei, and the 87th Division was to attack the reinforced Kung-ta Textile Mill, where the Japanese naval command was located. Zhang estimated it would take one week to achieve these objectives; however, the operation ran into trouble when his troops were fought to a standstill just outside the Shanghai International Settlement. Japanese strongholds were fortified with thick concrete and were resistant to 150mm howitzers, the only heavy weapons the Chinese possessed. Chinese troops could only advance, under the cover of machine gun fire, by getting close enough to the emplacements to kill those within with hand grenades. The Chinese advance was greatly slowed and the element of surprise was lost.